Manual - Stanford Research Systems
Contents
1. The time to draw gas through this The example above assumes that the region in the volume determines the response time large diameter tube is well mixed with the gases present in the main chamber The PPR system was designed with the intention that it is directly attached to the chamber being measured as shown in Figure 5 Space limitations can force the use of an extension on the inlet Improper additions to the inlet can catastrophically degrade the response time of the system As an example consider a vacuum furnace where the insulation forces remote SRS PPR System 16 Design Principles location of the PPR system Figure 6 shows an PPR system connected with a bellows tube to a length of 1 4 inch tube which extracts gas from a chamber operating at 1 mbar The bellows tube The internal volume of the 1 4 inch tube 0 028 wall is 19 ml Ata flowrate of 30 ul s it will take over 600 s to travel the length of the tube Once it exits the tube it mixes into a dead volume of over 250 ml The resulting time constant is over 2 hours Figure 7 shows a better approach which accomplishes the same result with a time constant of about 16 seconds Much of the improvement of the system is due to moving the pressure reduction closer to the source The small tube accomplishes the first two decades of pressure reduction and the aperture is chosen to accomplish the remaining pressure reduction In general a system will perform faster if the pressure reducti2. through the turbo pump into the main system The composition of the backstream will most likely be air The user can isolate the PPR system from the system by closing both valves but this will prevent the RGA from being operated Users of ultra high vacuum systems need to consider adding an isolation valve to the PPR system at the inlet of the turbo pump Closing this third valve and opening the hi C valve allows the RGA to be operated and eliminates backstreaming SRS PPR System Troubleshooting 19 7 Troubleshooting Turbo pump does not start when button is pressed The controller has an external interlock that may be necessary in some applications Normally the interlock is bypassed with a jumper connected to P1 on the back of the controller If the display of the controller shows PUMP WAITING INTERLOCK check that the connector is installed Pumps operate hot to the touch In confined spaces or hot environments fans will be needed to cool the pumps A cooling kit is available for the turbo pump Any fan is suitable for the diaphragm pump Only a small airflow is necessary to keep the pumps cool The turbo pump will not reach full speed Check that the controller is not in low speed mode Check the time interval over which the controller is set to ramp to full speed The interval could have inadvertently been set to an extremely long value up to hours A high backing line pressure will slow the rate at which the turbo pump ram
3. an equivalent rebuilt pump immediately in exchange for the users pump Contact SRS or your distributor for details B Diaphragm Pump Typically the PPR system operates at high vacuum and places very little load on the diaphragm pump Lifetimes of several years are possible under these conditions Frequent start ups and shut downs increase the load on the pump and decrease the lifetime Keeping the pump cool will also extend its operating life The performance of the pump can be diagnosed by tracking the ultimate vacuum of the PPR system To measure the ultimate vacuum isolate the RGA by closing both valves With the system isolated measure the spectrum with attention to the nitrogen and oxygen peaks New systems will show nitrogen at approximately 2 x 10 mbar and oxygen at 1 4 that value These gases are present because air backstreams through the turbo pump The pressure at the RGA will increase directly with the pressure at the turbo exhaust Thereby the ultimate vacuum at the RGA is a direct indication of the vacuum in the backing line and the performance of the diaphragm pump As an example a new system will have a backing line pressure of 1 mbar and at ultimate vacuum will have oxygen present at 5 x 10 mbar One year later another ultimate vacuum test might show oxygen present at 1 x 10 mbar This value indicates that the backing line pressure has doubled to 2 mbar The turbo pump can easily tolerate exhaust pressures up to 5 mbar so
4. high pressure it will generate more heat than it can dissipate This situation will lead to an over temperature error at the bearings which will shut off the pump Both of these protection measures must not to be relied on to shut off the RGA and turbo pump They only provide some defense against serious damage to the equipment SRS PPR System 6 Measurement Techniques 4 Measurement Techniques In general the PPR system only scales the pressure and does not change the function of the RGA The techniques discussed in the main manual and other texts about RGA s are applicable to this system The following sections discuss issues unique to the use of the RGA in the PPR system A Operating at Pressures Other Than the Design Point Each system is specified for one inlet pressure the design point The aperture is chosen to reduce the inlet pressure from the design point to about 5x10 mbar at the RGA As the inlet pressure increases or decreases from the design point the pressure at the RGA will vary proportionally The RGA will operate well to pressures of 1x10 mbar and will operate with loss of linearity to 1x10 mbar above which it will turn off Based on these values the inlet pressure can increase about 1 decade above the design point The ultimate vacuum of the PPR system is 2 x 10 mbar The ultimate vacuum causes a background spectrum on which the sample spectrum is superimposed Thereby at the design point the ratio of sample t
5. ion gauge s reading as the reference value The sensitivity factor calculated by the software will be the product of the accurate sensitivity factor times a factor that accounts for the different effective pumping speed that the two gauges are exposed to SRS PPR System Measurement Techniques 9 Method 3 Some systems may not have a high vacuum gauge as assumed in the method above but instead only a medium vacuum gauge e g capacitance manometer Pirani or thermocouple The RGA software allows the sensitivity factor to be calibrated with the pressure reduction inlet in use and thereby allows the RGA to be aligned with the system gauge First setup the system for making measurements at medium vacuum as discussed above Make the pressure in the main chamber such that the pressure at the RGA ionizer is between 7 x 10 and 7 x 10 mbar For example if the pressure reduction factor is 1 x 10 then the main system pressure must be between 0 007 and 7 mbar Make sure to enable the pressure reduction factor and then calibrate as usual In the sensitivity dialog box enter the reading of the main system gauge in the Reference Pressure edit box the value must be in Torr Typically an ion gauge reading would be entered in the Reference box but with the Pressure Reduction factor enabled the software will allow and compensate for higher values Next click the Measure button and accept the results When the RGA is calibrated with this method an i
6. is specified for power line of either 110 V 60 Hz or 220 V 50 Hz The diaphragm pump will only operate on the specified voltage Operating at other voltages will damage the motor The turbo pump controller is factory preset for the specified voltage refer to the controller manual for information about changing the line voltage The SRS RGA power supply has a universal input which automatically detects the power line voltage but requires different fuses for 110 or 220 V operation For 110 V operation use one 2 A fuse For 220 V operation two 1 A fuses must be used in the power entry module The SRS RGA power supply is configured at the factory for one of these options Exhaust As shipped this system exhausts to the atmosphere If the PPR system is attached to a vacuum chamber that contains hazardous gases the user must make provisions to handle the exhaust from the diaphragm pump Ventilation Do not block the air inlet to the cooling fan of the RGA Components will fail without this cooling The PPR system requires forced air cooling to operate at a reasonable temperature Although the pumps can operate without forced air cooling they will do so at a high temperature Place fans near the pump bodies to lower their operating temperature and extend their life Elastomer Seals Silicone has been reported to react adversely and irreversibly with the glass contained in an electron multiplier In systems containing an RGA w electron mul
7. not be operating When the sample valve is first opened there is a small volume of high pressure gas trapped between the aperture and valve that is suddenly released into the turbo pump The pump will slow momentarily and then recover Measurement can now be made with the RGA software To make the RGA software account for the pressure reduction occurring across the aperture choose the Pressure Reduction item under the Utilities menu In the dialog box which appears enter the pressure reduction factor from the calibration sheet and check the Enable box All the partial and total pressure values are now multiplied by this value This is only a scaling operation the data from the RGA software is valid with or without the pressure reduction factor enabled When the sample valve is not open the pressure reduction factor can be disabled by un checking the Enable box The pressure reduction factor is saved with the RGA files so once the value has been entered the user need only check or uncheck the enable box to activate the scaling The button provides quick access to the dialog box C Measuring at High Vacuum Measurements at high vacuum are made by opening the hi C valve There is no need to close the sample valve The low conductivity of the bypass loop will cause it to have no effect on the measurements Also if the sample valve is closed it is possible that a small volume of high pressure gas is trapped between the apertu
8. the results may not be what is expected When the RGA is back in the PPR system it will not agree with a gauge located in the main vacuum system This is because the two instrument are exposed to different pumping speeds and gas throughputs they should disagree The RGA software contains a scaling factor that allows the values reported by the RGA to agree with system gauges The scaling factor is present for all the software modes and is limited to values near 1 0 01 to 100 and is intended for correcting for systematic differences without adjusting the accurate sensitivity factor stored in the RGA ECU The scaling factor is very useful if the RGA is moved from system to system Instead of recalibrating the sensitivity factor a different scaling factor can be used for each system The scaling factors are stored in the RGA files used by the software Method 2 If the goal of calibration is to make the RGA agree with another gauge in the main system a practical solution is to align the RGA with that gauge By align we mean that the RGA and main system gauge will show the same numbers but the RGA is not strictly accurate To align the RGA with an ion gauge in the same system open the hi C valve on the PPR Leak a calibration gas into the main vacuum system so that the composition is largely one species and so that the pressure is between 7 x 10 and 7 x 10 mbar Use the RGA software to calibrate the sensitivity factor using the main system
9. turbo pump will have reached full speed as indicated on the controller It should show NORMAL OPERATION By default the turbo pump is programmed to reach full speed in 8 minutes The startup time can be changed to shorter or longer values if needed The controller manual discusses how to change this value Once the system is at speed either of the two inlets can be opened depending on the pressure in the main vacuum system SRS PPR System 4 Operation The turbo pump controller tracks various data on the pump Pressing the PUMP CURRENT TEMPERATURE POWER button displays these value and the rotation speed in the display window The temperature is measured in the pump at the bearings and will be 30 35 C with good cooling The CYCLE NUMBER CYCLE TIME PUMP LIFE button displays history data about the pump The cycle number is the count of the times the pump has been started The time to the right of CYCLE shows the duration of the current cycle The time to the right of PUMP LIFE show the cumulative time the pump has been operated The historical data is remembered by the controller even when the unit is not connected to line power B Measuring at Medium Vacuum When the system is up to speed it is ready to make measurements When the main vacuum system is at medium vacuum gt 10 mbar measurements are made by opening the sample valve Presumably the hi C valve is already closed otherwise the turbo pump would
10. GA will the high vacuum values that occur at the ionizer The reading of the system gauge provides the medium vacuum pressure reference The new pressure reduction factor is the system pressure gauge reading divided by the RGA reading Enter this value in the pressure reduction dialog box and check the Enable box The RGA and system pressure gauge will now agree This pressure reduction factor is stored in the RGA files and is recalled when the files are reopened SRS PPR System 10 Care amp Maintenance 5 Care amp Maintenance The PPR system is designed to require low maintenance No maintenance schedule is recommended components can be used until they fail At such time factory service or kits are available to rebuild the system The sections below discuss methods for diagnosing the performance of the major components of the system the RGA is discussed in its manual A Turbo Pump The turbo pump is permanently lubricated It requires no maintenance for the life of the bearings The life of the bearings is highly dependent on the gases pumped and the environment the pump is used in Under normal environments high vacuum and no corrosive gases the pump can be expected to work continuously for many years Under higher gas loads corrosive environments or repeated shock forces the life of the bearings is degraded Bearings in the pump can be replaced by the factory An exchange program is available that minimizes down time by shipping
11. User s Manual Process Pressure Reduction system S RS Stanford Research Systems 1290 D Reamwood Ave Sunnyvale CA 94089 408 744 9040 408 744 9049 fax info thinkSRS com www thinkSRS com Version 1 20 Jul 01 Warranty This Stanford Research Systems product is warranted against defects in materials and workmanship for a period of one 1 year from the date of shipment Service For warranty service or repair this product must be returned to a Stanford Research Systems authorized service facility Some components may be serviceable directly from the supplier Contact Stanford Research Systems or an authorized representative before returning this product for repair Trademarks Ultra Torr and vcn are registered trademarks of Swagelok Co Tygon is a registered trademark of Norton Co All other brand and product names mentioned herein are used for identification purposes only and are trademarks or registered trademarks of the respective holders Information in this document is subject to change without notice Copyright Stanford Research Systems Inc 1996 All rights reserved Stanford Research Systems Inc 1290 D Reamwood Avenue Sunnyvale California 94089 408 744 9040 Printed in USA iii Table of Contents KAO 0721 11 0 Eee CEP 3 AOD cei orae nte e pH OR ne ocn Rad otetib A S 3 B Measuring at Medium Vacuum cescessecssecescceceeceeceeceaccesceecsaeeesceecsaeeesaeecsaeceeaeecaeeseat
12. activate the ionizer Click the GO button on the tool bar and a scan will begin from 1 to 50 amu During initial operation monitor the operating temperature of the pumps to be sure that they are sufficiently cooled They should only be warm to the touch SRS PPR System Vil Specifications Performance Gas flow Response time Startup time Connections Inlet Inlet to controller 3x 10 mbar 1s P V with pressure reduction inlet active 20 s at 1 mbar inlet pressure scales linearly with pressure 8 minutes standard 2 3 4 inch CF flange rotatable with through holes 6 foot cable provided Inlet to backing pump 6 foot 1 4 ID x 7 16 OD flexible hose provided Computer Power Pumps High Vacuum Backing Cooling General Power requirement Dimensions Weight RS 232C 28 800 baud 9 pin D connector 3 pin grounded cables hybrid turbomolecular drag pump 70 liter s ultimate pressure 2 x 10 mbar diaphragm pump with ultimate pressure less than 1 mbar protection class IP44 requires forced air cooling either 110 V 60 Hz or 220 V 50 Hz not field selectable less than 300 W total vary with configuration inlet mounted on chamber 7 kg 16 Ibs diaphragm pump 7 5 kg 16 5 Ibs controller 7 kg 16 lbs SRS PPR System viii Materials List SRS receives many requests for information about corrosion compatibility It is our policy not to state the compatibility of our sy
13. and higher Extensions on the high vacuum side are much less likely to cause problems because the volumetric flowrate at the RGA ionizer is 30 liter s Even a large tube will have a short response time Following the general advice of keeping the pressure reduction as close to the source as possible leads to the arrangement shown in figure 9 An inlet assembly has been attached to each chamber Only the RGA and pumps move from system to system With the extension on the high vacuum side it is now practical to add a flexible bellows and clamp flanges This allows such possibilities as mounting the RGA on a cart which rolls up to each chamber when needed WE ih S T ANNANN Harrow j 4 SOY R j FSN Figure 9 Multiple chambers can be serviced by splitting the PPR system after the inlet valve Flexible tubing and KF flanges allow easy connections C Operation at Ultra high Vacuum The ultimate vacuum of the pump package is 2 x 10 mbar The PPR system is not designed to operate at or below these pressures When the main vacuum system is at a pressure lower than 2 x 10 mbar and the PPR hi C valve is opened gas will backstream
14. at level Carbon dioxide from the ionizer will be present at levels from 10 to 107 The RGA software contains a background subtraction feature that allows the chamber background to be removed from the mass spectrum The background spectrum is correctly measured with both valves closed To obtain a background subtracted spectrum follow these steps SRS PPR System Measurement Techniques 7 1 Measure one analog or histogram with both valves closed If the RGA is scanning continuously you can select Stop at End from the Scan menu The data displayed must be a complete scan and be measured with the same parameters as the scans to follow 2 Under the Utilities menu select Background and select Scan Data Background from the dialog box Check the box next to Enable and select OK to close the dialog box This makes the current spectrum the background and all spectra displayed subsequently will have this spectrum subtracted from it 3 Open either the sample flow valve or the hi C valve and start the scans The displayed spectra are the background corrected result The ability to subtract background is limited by signal proportional noise which is typically present at between 1 10 of the signal magnitude Because this noise originates in the ionizer of the QMS subtraction can only remove 90 of the background This limits the ability to see small changes of less than 1 at the same masses as the backgrou
15. d for vacuum systems that nominally operate at room temperature Under these conditions any species that is a gas in the vacuum system can be expected to travel through the PPR system without condensing Without a heat input a gas will cool as it expands through a pressure reduction aperture according to its Joule Thompson coefficient In the PPR system the absolute pressure difference across the aperture is small and the flow rate is small under these circumstances the interior metal surfaces can provide sufficient heat to the expanding gas to keep it from condensing If problems due to condensation are suspected the aperture and sample valve can be wrapped with heating tape to test for condensation When the vacuum system being measured is significantly hotter than the PPR system condensation is likely and presents a problem If the species in the hot vacuum system are gases only at an elevated temperature they will condense when they reach the PPR system The condensed material will continually build up in the PPR system and cover the valve seats and aperture Two approaches can prevent this problem control the location of condensation or prevent condensation The first approach can be very simple place screens or metal plates in the inlet to provide sacrificial surfaces for condensation The sacrificial surfaces should have good thermal connections to the outer walls These surface will act like a trap and prevent the unwanted materials from
16. e included KF16 clamp and o ring Connect the cable between the turbo pump controller and pump body and connect the power cord The controller display should show READY FOR LOCAL SOFT START Close both valves to isolate the RGA chamber The small valve is a 1 4 turn valve that is closed when the handle is perpendicular to the small tube Plug in the diaphragm pump to start roughing the RGA chamber Press the start button on the turbo pump controller There is no need to wait until the RGA chamber has reached rough vacuum Both the diaphragm pump and turbo pump can be started at the same time The turbo pump will reach full speed in about 8 minutes The controller display will show Normal operation when the pump has reached speed Connect the power cord to the RGA power supply and turn on the power switch Connect the serial cable between the RGA and an available COM port on the computer typically COM2 Install the RGA software on the computer by inserting the first disk and executing the SETUP program Start the RGA program Under the Utilities menu choose RS232 Setup In the dialog box that appears choose the COM Port that the RGA is connected to and then press the Connect button After a short initialization the RGA is ready To confirm communications under the Head menu choose Get Head Info A box will appear showing information about the RGA Click the filament button p on the toolbar to
17. e valve Last to double check the result pump the main chamber back down If a valve seat was leaking the pressure measured by the RGA will drop Contact SRS or distributor for information about factory service and field service kits The metal seals will last indefinitely under normal usage The integrity of these seals can be assured using the leak testing mode of the RGA software Helium or any other gases can be used e g argon or tetrafluoroethane The CF flanges and VCR fittings have leak testing ports Spray the test gas directly into these ports and look for any increase in the level measured by the RGA Leaks should be immediately detected in this manner Standard 2 3 4 CF and 1 4 VCR gaskets are used in the PPR system The backing line can be visually inspected or more rigorously tested with helium If helium leaks into the backing line it can backstream through the turbo pump to the RGA and be detected Setup the leak testing mode to detect helium and spray the hose fittings well Smaller leaks can be detected by placing a bag over the fittings to produce a pure helium atmosphere around the suspect leak This process is not as fast as with the metal seals It can take several minutes for the helium to be detected F Replacement Parts Some of the parts discussed in the previous sections are widely available For users who wish to obtain replacement parts directly the following manufacturers part numbers will be needed Nupro and Ca
18. ecsaeeeneecsues 4 C Measuring at High Vacuum cece ee E E E E E A A a 4 D AN EE E E A E E E E AE E E 4 E Sh td W s epeei eorr espe Moraes 5 FOVELPLESSULS eneee aE UR SEE E O EE qe E PUES E E E I ER E EE R E ESS 5 4 Measurement Techniques ccceseeeeceesseeeeeessneeeeenseeeeesnseeeeeenseaeeesaseeeeeseseeneesesseeeeeseseenensnseeeess 6 A Operating at Pressures Other Than the Design Point esee 6 B Correcting for the Chamber Background sees 6 C Calibrations diee EP RP ene deine Bebe orl ee NI 7 5 Care amp CI CEU X 10 As Turbo PUMP estre oe etre Row deor ridens 10 B Diaphragm Pump n ttt perte rie e e Pe Spei DH AO n reb Re Ete egere tree g eere EEES 10 C Bake out 5 one oU n dite ode eeu e etn eho eau 11 D Operation with Condensable Gases ener nent nest nennen trennen 12 E Leak Testing aue reete ena Up Mr Ep Ei M pp REM 12 EF Replacement Parts eoe ei te Detienen ttes 13 6 Design Principles 14 A Aperture Selection cette heed onte p ete edite ie detener diee ite rut 14 B Response TIME sinioro merene ror Esa E cap aaeivastsTeactees i D Ret ru ber ete cipere tree Ree EEE Ee 15 C Operation at Ultra high Vacuum uite e on eese stet ope 18 OE M 19 SRS PPR System Safety Line Voltage The PPR system
19. ed replace it with a standard o ring for a KF16 flange either Viton or buna N Only the elastomer needs to be replaced the metal centering ring can be reused C Bake out Periodic high temperature bakeout can be used to clean the interior surfaces of the PPR system Several components limit the highest temperature that can be safely used The hi C valve contains an Viton seal and is rated to 200 C at the valve body The plastic handle cannot tolerate this temperature The sample valve is rated to 120 C at the valve body The plastic handle cannot tolerate this temperature SRS PPR System 12 Care amp Maintenance The flange of the turbo pump cannot exceed 120 C The bearings cannot exceed 60 C The RGA cannot exceed 100 C while operating or 250 C if the ECU is removed Given these a safe strategy is to bake the entire system at 100 C If heating tape is used wrap the system stopping at the turbo pump flange and about 1 inch before the RGA ECU flange For more aggressive bake outs careful control of temperature is required A multipoint thermocouple monitor e g the SR630 or temperature controllers will be necessary Cooling the body of the turbo pump with air or water may be required to keep the flange below the 120 C limit If contamination is a recurring problem constant operation at elevated temperature may be more efficient than periodic bakeouts D Operation with Condensable Gases The PPR is designe
20. hese pumps results in a compact and completely oil free system There is no danger that improper operation of the PPR system will contaminate the user s vacuum system This manual is an addendum to the full RGA manual and discusses the PPR system and aspects of the RGA unique to the PPR system This manual assumes the reader has general familiarity with RGA s those who do not should read the RGA manual first All users should read the Operation and Measurement Techniques sections describes situations specific to the PPR system The RGA manual contains an appendix which discusses additional measurement techniques The PPR system requires little maintenance the section on Care amp Maintenance describes what is required Users are referred to the full RGA manual for details of the RGA its maintenance and programming The PPR system is offered in many geometries one of which is shown in Figure 1 The system consists of three main groups the inlet a small chamber formed by the tee and the pump group The components shown in Figure are attached to a flange on the user s vacuum system Also contained in the system are a controller for the turbo pump and a diaphragm pump both of which can be placed up to 1 meter away hi C valve NONEM sample Z un RGA valve D N y eee bypass turbo loo
21. ight As an example if the inlet is at 1 mbar the volumetric flowrate is 30 ul s Assuming that only the small tube is not mixed with the gas in the main chamber the time constant is 20 s At 1 mbar gas in the small tube travels via viscous flow If the composition at the inlet suddenly changed we could expect to wait at most 20 seconds before the change is seen by the RGA This delay is the amount of time it takes to drain the dead volume and replace it with the new composition Transport on the low pressure side of the aperture is very fast approaching sonic velocities and will not contribute significantly to the time constant pe M s The volumetric flowrate and thereby response time 10 mbar scale linearly with the inlet pressure If the inlet E pressure was decreased to 0 1 mbar the time constant NA in the above example would be 2 s and at 0 01 mbar would be 0 2 s Increasing the inlet pressure leads to m long time constants Although it is possible to volume construct apertures with small enough holes to allow inlet pressures above 10 mbar we have not done so because the response time is unacceptable At such pressures a bypass pumped system is recommended aperture 10 nbar which will have response times under 1 s This system Figure 4 The small volume on the high is discussed in SRS Application Note 8 included in pressure side of the aperture is not the appendix of the RGA manual well mixed with the main chamber
22. jon parts are carried by your local Swagelok distributor diaphragm pump diaphragm rebuild kit contact SRS Sample valve entire valve Nupro SS DLVCR4 Kel F stem kit 6L 3AK DS KF diaphragm gasket kit SS 3DK DS VCR gaskets 1 4 stainless steel w silver coating Cajon SS 4 VCR 2 RS gaskets BSP ISO parallel thread 1 8 28 Cajon S 2 RS 2V Hose Tygon or similar 1 4 inch ID x 7 16 inch OD SRS PPR System 14 Design Principles 6 Design Principles The PPR system has been designed to suit the general needs of users Some users will choose to modify the system to suit unique applications This section describes the principles of the system so that users may better understand how to make modifications A Aperture Selection The aperture is chosen to reduce the inlet pressure from the design point to about 5x10 mbar at the RGA This value allows the RGA to operate with the inlet pressure one decade above or below the design point Some users may specific applications which would be better suited if the RGA were operating at different pressure The pressure at the RGA ionizer is determined by the throughput of the aperture Q and the effective pumping speed S4 Q in mbar s P in mbar S n s 1 The effective speed at the ionizer is 30 liter gs Changing the diameter of the hole in the aperture effects the throughput and thereby the operating pressure One motivation to increase the operating pre
23. mped into by people or equipment During initial operation monitor the operating temperature of the pumps to make sure they are receiving sufficient cooling Under average conditions only small fans are required to cool the pumps They should operate warm to the touch 35 C or lower If the pumps are hot they will require additional cooling SRS PPR System Operation 3 3 Operation Figure 2 shows a schematic of the inlet system The system has two routes to the RGA which are controlled by a high conductivity hi C valve and the small sample valve The pressure in the main vacuum system determines which valves are opened The hi C valve is opened by turning counter clockwise for several turns The sample valve is a 1 4 turn valve It is opened by turning counter clockwise until the mechanical stop is reached The handle provides a positive indication of whether the valve is opened or closed it is closed when the handle is perpendicular to the tube Hi C Valve Main Chamber P gt poe gt RGA i Eo TN Aperture hs go de PIE 7 Hybrid Turbo Pump Sample Valve i _ _ Diaphragm Pump A LA E Em w Exhaust Figure 2 Flow schematic of the PPR system A Startup Prepare by closing both valves on the inlet Next start both pumps There is no need to wait between starting the diaphragm pump and turbo pump simply start them both at the same time After a few minutes the
24. mplicit assumption is that the pressure reduction factor is accurate The RGA sensitivity factor is only as accurate as the pressure reduction factor The software has no method of determining whether an accurate value was entered for the pressure reduction factor Calibrating the Pressure Reduction Factor If the RGA sensitivity factor was calibrated by methods 1 or 2 above the pressure reduction factor can be calibrated Calibrating the pressure reduction factor is done by comparing the RGA reading to that of a second system gauge known to be accurate at medium vacuum pressures Because the RGA was calibrated above we know it is accurate and thereby can use its reading as a reference for the high vacuum pressure If the RGA was calibrated with method 3 above the pressure reduction factor cannot be calibrated That method assumed the pressure reduction factor was already accurate and the procedures below would simply recalculate the same factor To calibrate the pressure reduction factor make a measurement through the aperture but with the pressure reduction factor disabled in the software Leak a calibration gas into the main chamber such that the composition is largely pure 29096 and at the pressure you need to operate the main system Use any software mode to obtain the partial pressure of the calibration gas table mode is easiest Do not use the total pressure reading its precision is low and varies with composition The readings of the R
25. nd C Calibration Calibration is not necessary on a frequent interval Two calibration factors are used in the PPR system the standard RGA sensitivity and one additional factor the pressure reduction factor Determination of these factors requires comparing the RGA with a known accurate pressure gauge and calculating a factor that makes the two agree The RGA is calibrated at the factory for partial pressure of nitrogen and the pressure reduction factor was measured for nitrogen using a capacitance manometer The RGA sensitivity is stored in the ECU 9 0E 05 unit the aperture factor is included in the test report The RGA sensitivity will age in the same manner as high vacuum ion gauges and may need periodic recalibration The aperture is stable but 7 0E 05 its performance varies slightly with the pressure at the inlet and the gas composition at the inlet A typical 8 0E 05 Reduction Factor One performance curve is shown in Figure 3 If your measurements need high precision 5 0E 05 to account for this slight difference the 0 5 10 15 pinhole must be calibrated at the exact pressure the main system operates Also you can recalibrate the pinhole to account for gases with different transport properties e g helium In general though as long as the aperture has not physically changed e g plugged or corroded the pressure reduction factor will not need recalibration Procedures for calibration of these factors is described in the
26. nnection While the system in figure 7 works well at high pressure it is unable to monitor the main system at base pressure When the main chamber pressure is below 10 mbar gas will not flow through the small bore of the 1 16 tube Other designs for extension tubes will trade sensitivity at low pressure for response at high pressure To get performance at both pressure ranges both paths the high conductivity and the low must be extended to the source Figure 8 contains one method of solving the problem of the example above Extension to Chamber i aan im MAI ER RIP C4AMBER ww i P ee P ea 1 mbar i o J E ame Ems 1 16 OD x 0 010 wall tube Qo Figure 8 An optimum extension which provides both high and low conductivity paths A valve is not needed on the small tube Use with Multiple Chambers Users with multiple vacuum chambers may need to use one PPR system moving it from chamber to chamber as needed The most obvious method would be to add a clamp type flange KF to the ports on the main chamber This allows the system to be easily be connected and disconnected As seen above an extension can degrade the response time Adding large volumes to the inlet should be carefully evaluated especially at inlet SRS PPR System 18 Design Principles pressures of 1 mbar
27. o background is 1000 1 As the inlet pressure drops the sample to background ratio drops The background is spectrally unique i e not broadband and will present different problems to different users For example measurements of an argon environment containing helium will not be strongly effected by the background of water nitrogen and oxygen But if the measurements where looking for trace oxygen and water the sample to background ratio must be kept high Thereby the inlet pressure can decrease 2 decades or more below the design point as long as the user is aware of the degradation of the sample to background ratio B Correcting for the Chamber Background Even with the sample flow and hi C valves closed their will be a noticeable background in the mass spectrum This background in the RGA chamber is caused by outgassing from the surfaces backstreaming through the turbomolecular pump and gas production from the ionizer of the RGA These three processes account for the ever present background of hydrogen water nitrogen oxygen and carbon dioxide seen in high vacuum The outgassing of water can be minimized by extensive pumping with both valves closed typically the system should achieve water partial pressures around 1 x 10 mbar The other two process backstreaming and ionizer are fundamental and cannot be reduced The ultimate vacuum of the turbo pump causes nitrogen to be present at no lower than 2 x 10 mbar and oxygen at 1 4 of th
28. on occurs as close to the source as possible ag 7 A i x C4AMBER d j J 10 mbar 1 mbar n 7 gt cem MUSS Nes EN Figure 5 The inlet of the PPR system typically is attached directly to a chamber as shown here Extension to Chamber ane Vd s an des a Eu e bx 1 VA Aie CyAMBER lt gt 10 mbor v Jp mbar Not Nu Ae Age A TIES ae XI Ej y y A 1n 1 1 2 bellows a T 1 4 OD 0 028 wall tube PQ Figure 6 An obstruction e g insulation forces an extension to be used at the inlet A flexible bellows has been used to allow the PPR system to be mounted on a rolling cart SRS PPR System Design Principles 17 Extension to Chamber E m N Ma nm Y C AMBER A n e s 10 mbor 1 mbar LUS 7 Jt 1 16 OD x 0 010 wall tube ih d WSS Figure 7 In this faster system the 1 4 inch tube has been replaced with small bore tube The flexibility of the smaller tube allows compensation for misalignment between the two systems therefore the bellows has been removed An Ultra Torr fitting allows easy connection and disco
29. p pump Figure 1 Inlet system components SRS PPR System 2 Installation 2 Installation Check to make sure you have received all of the parts of the system The system attaches to an available 2 3 4 CF flange on you vacuum system The CF flanges can support the weight of the system but it is necessary to use high strength bolts High strength bolts are typically sold by vacuum hardware suppliers Do not use standard 1 4 28 bolts or other substitutes The system is shipped in a clean state and is ready to attach with no preparation Installation is much easier if a second person is available to hold the system while you tighten the bolts Once the system is attached make the electrical connections from the turbo pump to its controller and connect the power cord to the controller Connect the diaphragm pump to the turbo pump with the KF16 o ring and clamp The o ring does not need grease Inspect both faces of the KF flanges for dirt or scratches If they are clean place the o ring assembly between the two flanges and place the clamp around the flanges Tighten the clamp Connect the cable from the RGA to the computer and the power cord to the RGA The system is now ready to operate The RGA manual contains a section describing installation and use of the software The CF flange can support the weight of the PPR system Because the system is a significant lever arm be wary of placing the system where it is likely to be bu
30. passing The second approach involves operating the entire PPR system above the condensation temperature of the condensable material This may be feasible if the operating temperature is below 100 C The previous section lists the temperature limits of the various components This high temperature approach is difficult because every surface of the system must be maintained above the condensation temperature and below its temperature limit The first approach is recommended because of its ease E Leak Testing The seals in the system will have a long life The valve seat seals in the hi C valve and sample valve will eventually require replacement The hi C valve seat is the most critical a small leak could easily be equal to the amount of gas that is pumped through the aperture If a leak across the hi C valve seat is suspected perform this quick test SRS PPR System Care amp Maintenance 13 Start with the main chamber at high vacuum Open both valves briefly and then close both valves tightly Setup the RGA software to monitor the species present in the main vacuum system in the pressure vs time mode Next increase the pressure in the main chamber If the pressures on the RGA side of the valves increase gas is leaking across the seat of one of the valves If the rate of rise directly follows the pressure rise in the main chamber the leak is across the hi C valve seat If the rise is delayed the leak is likely across the seat of the sampl
31. ps The turbo pump is stopped by pressing the START STOP RESET button there is no need to unplug the controller After several minutes the turbo pump will coast to a stop The diaphragm pump should not be stored under vacuum for long periods Because of this different procedures are recommended when the system is shutdown for short or long periods For short periods simply turn off both pumps For shutdown periods longer than about 15 minutes it is advisable to vent the system Venting is accomplished by opening the vent valve on the body of the turbo pump If the diaphragm pump is stored under vacuum for extended periods the pressures internally will reach a state that prevents the pump from starting When this happens the diaphragm pump will not start This locked state is cured by venting the system the diaphragm pump will then start up Storage under vacuum also temporarily degrades the ultimate pressure of the diaphragm pump Once the pump is operating the performance will return over several hours F Overpressure If the user forgets to close the hi C valve as the main vacuum system transitions from low pressure to high pressure the pressure in the RGA will increase to undesirable levels Above about 10 mbar the RGA and turbo pump cannot operate The RGA has a protection check that will shut off the filament if the pressure in the chamber is too high The turbo pump will loose speed as the pressure increases When it operates at
32. ps to full speed After each step the controller waits for the current drawn by the pump to decrease before proceeding to the next step With high exhaust pressure the current remains high and delays the acceleration of the pump If the exhaust pressure is too high the controller can stop at less than full speed Check the performance of the diaphragm pump The turbo pump is drawing more current than usual A high exhaust pressure will increase the current drawn by the turbo pump Inspect for damage which might cause leaks in the backing line Remake the hose fitting at the inlet of the diaphragm pump Degradation of the performance of the diaphragm pump will cause the backing line pressure to increase it may be time to service the diaphragm pump The diaphragm pump does not start when power is applied The diaphragm pump does not always start against full vacuum If the system was not vented when last turned off the chambers of the pump can reach a state which prevents the pump from starting This locked state tends to occur about 1 2 hour after pump was turned off and persists for days as the system slowly leaks back to atmospheric pressure The locked state is immediately cured by venting the system to atmospheric pressure SRS PPR System
33. re and valve seat This trapped volume will bleed out slowly through the aperture and may interfere with the measurements By leaving the sample valve open this possibility is eliminated If the pressure reduction factor was enabled disable it now D Idle If measurements are made with the system every day the system can be kept running 24 hours a day and idled when not in use Only for long periods of down time is it recommended to turn off the system When the main vacuum chamber is not being measured for short periods e g several hours shut both valves This state allows the SRS PPR System Operation 3 turbo pump to continue pumping on the interior surfaces of the system thereby reducing the background Also with the system isolated the load on the pumps is decreased The system is immediately available to begin measurements from this state by opening either of the valves For longer periods e g overnight or weekends the RGA filament can be turned off to extend its life The turbo pump can be set to spin at half its nominal rate by pressing the LOW SPEED button on the controller This idle state places the lowest load on the RGA and pumps but the system is not immediately available to make measurements To restore the system press the LOW SPEED button a second time and activate the RGA filaments The system will be ready in a few minutes E Shutdown To shutdown the system close both valves and turn off both pum
34. sections Pressure high side mbar Figure 3 An example of aperture performance at difference pressures SRS PPR System 8 Measurement Techniques below The type of gauges available on the main vacuum system determine the type of calibration that can be performed The RGA intrinsically measures an ion current which is proportional to partial pressure The partial pressures reported by the RGA software are calculated by the formula P pressure reduction factor x sensitivity factor Torr A X ion current A To determine both the factors the RGA sensitivity must be calibrated first Once the sensitivity factor is calibrated the pressure reduction factor can be calibrated The RGA sensitivity factor is set using the Utilities Sensitivity Tuning menu item in the RGA software The use of this feature is fully described in the section RGA software Head Calibration amp Security in the main manual The user should read that section before proceeding below Calibrating the Sensitivity Factor Method 1 The most accurate method of calibrating the RGA sensitivity requires removing just the RGA from the PPR system and attaching it to a calibration test stand The test stand should ensure that the RGA and reference gauge are exposed to the same effective pumping speed and test gas throughput Once the RGA is calibrated it can be returned to the PPR system While the sensitivity factor determined by this calibration is accurate
35. ssure is to increase the signal to background ratio Outgassing produces its own throughput that produces pressures governed by equation 1 The signal to background ratio is simply the ratio of the two throughputs Because the background is fixed the only route to improving this ratio is to increase the signal i e aperture throughput Examples would be measurements of water or hydrogen in the main chamber A low pressure of water background is 2x10 mbar If a pure argon atmosphere was being monitored in a typical PPR system the pressure at the RGA would be 5x10 mbar Therefore water would be seen at 4000 ppm due to the background To improve the detection limit for water in the argon the aperture can be reselected such that the operating pressure was pushed near its limit e g 9x10 mbar The new aperture improves the detection limit of water to 220 ppm This improvement does not occur without cost If the inlet pressure increases slightly the overpressure circuit of the RGA can trip and turn off the filament The detection limit has been improved a factor of 20 but the tolerance for inlet pressure fluctuations is lost A second motivation to modify the pinhole is to increase or decrease the throughput If the main chamber contains a small or fixed volume the throughput of the aperture might draw more gas from the main chamber than desired To decrease the perturbation to the main chamber the throughput can be decreased This change will directl
36. stem with various corrosive environments We simply cannot test the myriad combinations of environments that our customers use We do provide a list of all the materials exposed to the gas being introduced into the system Our expectation is that users who need to measure corrosive environments already have some type of system that creates handles and contains the corrosive gases Given that they have designed and operate said system they are the best people to decide the compatibility of the materials in our system with the specific corrosive environment The PPR system contains the following materials Body e 304 stainless steel high vacuum tube e 316 stainless steel quarter inch tube and fittings e molybdenum electrical feedthrough ceramic electrical feedthrough e AgCuln braze material on feedthroughs e alumina contained in the RGA e aluminum body of diaphragm pump Replaceable Components e glass if an electron multiplier is installed in the RGA e chromium surface of the electron multiplier e rO ThO filament of RGA Seals e copper seals in the CF high vacuum flanges e 316SS major component of VCR seals e silver a thin layer on the VCR seals to prevent gauling e Viton o ring seal in the KF flange e buna N seal in the high conductivity valve e Kel F seal in the isolation valve e TFE 316SS body seal in the isolation valve e neoprene diaphragms in diaphragm pump e nitrile b
37. the performance of the diaphragm pump is still acceptable in this example A second indicator to track is the current drawn by the turbo pump and its operating temperature The turbo pump works harder as the backing line pressure increases and will cause the current and temperature to increase Operating at higher currents causes no problems as long as the turbo pump is sufficiently cooled More aggressive cooling may be needed to operate when the diaphragm pump performance has decreased SRS PPR System Care amp Maintenance 11 Good statistical lifetime data is not currently known for diaphragm pumps The pumps contains two components that are most likely to fail valves and membranes The valves tend to wear resulting increased backing line pressure decreased compression ratio The membranes tend to fail suddenly by tearing and result in the pump being unable to achieve usable vacuums Kits are available that allow the pump to be serviced by the user or the pump can be returned for service A kit of replacement seals is available from SRS or the manufacturer If the backing pressure increases the first check should be to remake the connection between the 1 4 inch tube and the diaphragm pump The connections to the ends of the 1 4 inch hose can be quickly remade Twist the aluminum sleeve counter clockwise to remove it from the hose barb e Slide off the flexible hose and either cut off the end or replace the entire hose e A thin layer amo
38. tiplier do not use silicone greases or oils on seals use only hydrocarbon based materials SRS PPR System Installation Check List Your PPR system is shipped assembled Only a few electrical connections and the turbo pump foreline connection need to be done once the system is installed The system is shipped with the following parts e the assembled inlet valves turbo pump body and RGA if ordered e turbo pump controller and power cord e diaphragm pump and attached power cord e KFl6clamp and o ring e this manual and manufacturer s manual for turbo pump and controller if RGA ordered e RGA attached to system e RGA software and serial port cable e RGA manual Parts Needed to Install e one copper gasket for 2 3 4 CF flange e high strength nuts amp bolts for the 2 3 4 CF flange and wrenches e recommended a second person to hold the inlet during installation SRS PPR System Vi Fast Start Attach the PPR system to an available 2 3 4 CF flange Attach the electronic control unit ECU to the RGA probe Match the hole pattern in the ECU with the two alignment rods on the probe and push the ECU The electrical connections should be easy to make If the ECU does not slide easily onto the RGA probe back off and try slightly rotating the ECU Once the ECU has been attached turn the two knobs on the back of the ECU to secure the ECU to the probe Connect the hose from the diaphragm pump to the turbo pump body with th
39. unt of grease Apiezon or other hydrocarbon based vacuum grease can be applied on the hose barb to improve the seal e Reattach the sleeve When remaking the connection wet the outside of the hose with water to ease installation of the sleeve After re attaching the hose start the pumps and watch the current drawn by the turbo pump If the current is not as low as your records indicate the next check is to replace the o rings at each end of the hose The hose barb at the diaphragm pump has a BSP ISO 1 8 28 228 1 thread which uses an elastomer seal retained within a metal ring Cajon part number S 2 RS 2V Simply turn the fitting counter clockwise to remove the seal This fitting is a face type seal and thereby does not rely on the threaded portion of the body to make a seal The fitting has a finish that prevents gauling Teflon tape is not required nor helpful The smooth faces on the diaphragm pump and on the body of the hose barb must be clean and free of nicks or scratches Before replacing the seal wipe the faces clean with a lint free cloth which has been wetted with a minute amount of grease Excessive force is not required when tightening the fitting to the body of the pump The metal ring limits the compression of the elastomer seal to its ideal value Once metal to metal contact is reached tighten the fitting only slightly further The o ring sealing the backing line to the turbo pump body is unlikely to leak In the event it is damag
40. utyl rubber NBR diaphragm pump valves Tygon connections to diaphragm pump can be substituted SRS PPR System ix Calibration Log SRS serial number In the table below are the results of the factory calibration The factor is entered in the pressure reduction factor dialog box under the Utilities menu in the RGA software Although the RGA software will store the value for you a written record is recommended iii by FACTORY SRS PPR System Introduction 1 1 Introduction RGA s can only operate in vacuum at pressures below 10 mbar The PPR system allows the RGA to analyze gases in vacuum systems that operate above 10 mbar The system contains two inlet paths that provide a high conductivity path and a low conductivity path The high conductivity path is used when the user s vacuum system is at pressures below 10 mbar At high vacuum typical applications are leak testing and monitoring the ultimate vacuum of the chamber The low conductivity path is used when the user s vacuum chamber is at pressures above 10 mbar This path contains an aperture that reduces the pressure several decades to a level suitable for the RGA Different apertures are used depending on the pressure in the users vacuum system A pair of pumps are required to draw the gas through the aperture to the RGA The pump group used is a hybrid turbo molecular drag pump referred to as turbo pump and a high performance diaphragm pump The use of t
41. y reduce the partial pressure of the gas being measured Other than the loss of signal to background ratio reducing the pressure will require making measurements at slower speeds to compensate for the loss of signal to noise ratio Alternatively increasing the throughput may be desired to modify the response time which is the topic of the following section SRS PPR System Design Principles 15 B Response Time The response time of the PPR system is determined by the throughput of the aperture and the amount of dead volume on the high pressure side of the aperture The aperture is contained in the VCR fitting at the inlet of the sample valve The volume of the tubing before the aperture is about 0 6 ml and is considered an unmixed volume dead The throughput is nominally 3 x 10 mbar liter s which is the product of the pressure P and volumetric flowrate V The time constant tc is determined by the dead volume Vaead and the volumetric flowrate t Vyeaa V 2 The time constant is a measure of how quickly a change in concentration in the main chamber will be detected by the RGA The response can be experimentally measured by introducing a step change in concentration in the main chamber Depending on the type of flow the response to a step change will either be a delayed step or an exponential response In both cases the value of t is determined by equation 2 but represents either the delay or time to reach 63 of the step he
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