I-Site Manual - version 2
Contents
1. Most important of all take a base reading and note the time date and ambient temperature User Manual 12 Section 4 Testing and Wiring the Strain Gauge 4 01 Testing Test each sensor before installing it Use a readout and an ohm meter to conduct these tests e Connect a readout see manual readout instructions Pull but do not twist gently on the ends of the gauge The Hz reading should be seen to increase as the ends are pulled and decrease as they are released e The thermistor reading should be near ambient temperature e Resistance between the red and black leads should be about 180 ohms e Resistance between white and blue leads should be about 3k ohms at 25 C 4 02 General Considerations Sensor Handling Do not twist pull hard or attempt to remove the pads on the ends of the sensor This may cause non repairable damage to the sensor Sensor Identification Mark cables before installation so that sensors can be identified after installation Wiring The gauge wires are as follows Identification VW Blue Thermistor ve 4 03 Strain Gauge Considerations Orientation Position the strain gauge so that it is parallel with the axis of loading Bending The strain gauge should be installed along the neutral axis of the structural member when possible Bending will increase strain on one side of the neutral axis and decrease strain on the opposite side Axial strain can be isolated from bending stra2. The description of surface preparation and safety instructions must also be well understood and adhered to to ensure the gauge provides a high level of data quality for satisfactory results The instructions given in this manual form basic guidelines to help install and read the vibrating wire strain gauges Modifications to these procedures demanded by varied situations can be based on sound engineering judgement with due regard to safety 1 01 Introduction The Vibrating Wire Spot Weldable Strain Gauge is a simple device for measuring strains in the steel elements of a structure Common applications are found in steel elements of bridges tunnel linings pile foundations retaining wall reinforcing structures building structures etc The most common means of installation of these gauges is by spot welding directly onto the steel member In certain applications bonding using suitable adhesives can be employed for mounting these gauges For more details of this and suitable bonding agents please contact Soil Instruments This manual describes the operating principle basic components and construction of the gauge the procedure to prepare install and read the gauge together with basic troubleshooting suggestions 1 02 Operating Principle The Strain Gauge employs the Vibrating Wire principle A high tensile steel wire when held under tension between two points and plucked will vibrate at its natural resonant frequency The frequency of
3. can be set for tension compression or at mid point full details are in Section 3 03 User Manual The Quick Set Clip pictured below in place on the gauge is used to set the gauge to its approx mid point before installation please see section 3 06 for details of its use 2 06 General Purpose Tools The tools required can vary from application to application and with the number of gauges to be welded The following tools will help in a quick and efficient preparation e Electric Angle Grinder with grinding and sanding discs together with safety gloves and goggles Hand Tools files wire brush emery strips etc Cleaning and Degreasing solvents and materials Silicon Sealant Silastic 732 RTV or Cyanoacrylate Adhesive Super Glue A flat piece of metal measuring 90 x 45mm for use as template to check size and flatness of the gauge mounting surface area This could be cut from a 150mm steel rule 2 07 Spot Welder This is a portable battery or mains powered spot welder capable of welding up to 0 010 thick stainless steel shim sheet to base metal of structure Units are available for hire from Soil Instruments It is advisable to use the length of 0 15mm thick stainless steel strip similar to strain gauge and coil mounting flanges for practising and testing of spot weld strength Use of the spot welder is quite simple Connect the earthing cable to the main body of the structure Wear eye protective goggles Switch the we
4. encompasses the current sensor wire frequency If not readings will not be stable Similar to the Units button the Sweep Range button should be pressed and immediately released The LCD display will briefly indicate the Sweep Range number 1 to 4 and the range frequencies 450 to 1250Hz 800 2000Hz 1400 to 3500Hz and 2400 to 6000Hz and then start sweeping the sensor wire with the current Sweep Range approximately every 4 seconds For sweep excitation readout units set the sweep range to 1000 3000Hz 15 User Manual Section 6 Data Reduction 6 01 Converting from Hz to microstrain Data from strain gauges is generally presented in micro strain ue where strain is the ratio of the change in length per unit length Practical K factor 3920 Gauge calibration constant Conversion of Period and Linear Units to microstrain is carried out using either of the formulae detailed below Period Units 107 a ug Jop Toy Where pe Change in strain in micro strain k Gauge Calibration Constant Ty Base reading in Period units x 10 T Current reading in Period units x 10 107 107 PI note when ease note en cy ny is positive the resultant strain is tensile Linear Units ue K F Fg x 10 Where ye Change in micro strain k Gauge Calibration Constant Fy Base reading in f 1000 units F Current reading in f 1000 units Please note when bal is positive the resultant stra
5. vibration is a function of the tension of the wire As two end blocks gripping the ends of the wire are fixed to a steel structure the tension in the wire will change as the distance between two blocks varies as a result of strain in the steel member The changes in strain can be used to calculate load and stresses in the structural member User Manual Section 2 Sensor Pick up and Spot Welder Description 2 01 Components The components of the system are identified below Pickup Coil Assembly Retaining Straps for Pickup Coil Strain Gauge Assembly P Strain Gauge Installation Jig Inset Quick Set Clip 2 02 Spot Weldable Strain Gauge The gauge consists of the following components Two end blocks fitted with circular welding flanges to which each end of the vibrating wire is securely fixed The wire is enclosed into an outer tube made of User Manual stainless steel Two O ring seals between the outer tube and the end blocks provide water proofing rated to 7 bar external pressure The wire is tensioned by fitting a Strain Gauge Installation Jig over the gauge before welding it to the prepared surface of the member to be monitored The gauge must be connected to a readout during this operation so that the correct tension can be applied 2 03 Electromagnetic Coil Assembly with Cable The Electromagnetic coil assembly in conjunction with a suitable readout is used to pluck the wire and read the frequency
6. DOXY TESSINS usce arcet tet ERE AOI dct adip teda paw a ER AR eine ufu lint a mtra 11 Place Coil Assembly Over Gauge Check Reading and Secure cceseennn nme nnn 11 Complete Instala iON erare aa aA AAE E EAE 12 Testing and Wiring the Strain Gauge reeeeeeee nennen nnn nnn 13 E Re e pe T rcm 13 Genera Considerato s cave bep e Rakion Y bacio eoLER GRUT D UPC d Ron X ba V a eL P A 13 Strain Gauge Consideratiloris EE 13 Taking Readings Using Soil Instruments Readout een nnn 14 EECHER 14 Selecthg Displayed JhlEs icio vices eege eege 14 Selecting SWeeDRaLde xsaxeerdeakalvr reader br wr rd na Pc eot er Ee paco E wp D RC wc o Aw 15 Data ReducLibDD EE 16 Converting TRON HZ t9 TilerostEalb esse eee aq CORE ac Ed a CR UE EX Re CPG OE ER a ac DERE Ries 16 Teniperature Effects iissseisuevssessvesetvssvevevsseseussessuevs essvesetessvevevesevesevesvevseesevesssess vsves 18 2 Section 1 Foreword Introduction amp Operating Principle It is essential that this User Manual is read and understood by suitably qualified technicians before undertaking the installation of Vibrating Wire Spot Weldable Strain Gauges Although no specialist welding skills are required for spot welding the gauge familiarisation with the equipment and some initial practice to develop a feel of clean spark free spot welding must be acquired by following the instructions given in this manual
7. SCH INSTRUMENTS Vibrating Wire Spot Weldable Strain Gauge User Manual Mani26 6 0 2 04 08 14 Chris Rasmussen Phil Day Phil Day Manual No Revision Date Originator Checked Authorised for Issue User Manual Contents Section 1 1 01 1 02 Section 2 2 01 2 02 2 03 2 04 2 05 2 06 2 07 Section 3 3 01 3 02 3 03 3 04 3 05 3 06 3 07 3 08 3 09 Section 4 4 01 4 02 4 03 Section 5 5 01 5 02 5 03 Section 6 6 01 Section 7 User Manual Foreword Introduction amp Operating Principle rr rre nnnm 3 l gidgersiP adio n Tr Tet 3 ODepating Dalle e TEE 3 Sensor Pick up and Spot Welder Description ee eeee eene eene n nnn n nnn 4 G elpnlsleln RICE Pt 4 Spot Weldable Strain Gauge EEN 4 Electromagnetic Coil Assembly with Cable NNN nnn 5 Vibrating Wire Readout Data E ole Le DEE 5 Strain Gauge Installation Jig amp QUICK Set Clm 5 General le vie TOOL E I LUTTE 6 SDOE WEE ee EE 6 Le CIE d Le EE EEN 8 GR P E 8 Clean 7 Gnd Surfaces cho d eee ene E iae cr ELM a E S Mal e 8 Ser Gauge With Tnstalldtlon JIG siriene r Fabr EE A FE PEFEEKRODVPOD FPE FEY FEVER FE PEE VERE PVEE A Fin 8 Tack Wed GANG EE 9 Remove Jig and Complete Welding NN 10 Set Gauge with QUICK Set CIID iiisisees it uua aka kx ka REX ERR ERKENNEN EEN EEN EE ENEE EN E ENEKN KEEN EN EEN 10 Gluing Gauges USE of E
8. at which it vibrates The assembly consists of an electromagnetic coil and a thermistor The coil and thermistor are connected to a four core shielded cable The complete assembly is encapsulated in a plastic mounding to fit snugly on top of the welded gauge Once positioned over installed gauge the coil assembly can be secured to the base metal structure by spot welding retaining straps cable ties and quick setting epoxy resins and in some cases pipe clips Coil Assembly 2 04 Vibrating Wire Readout Data Logger A Vibrating Wire Readout is required to test install and read the Vibrating Wire Spot Weldable Strain Gauges Detailed instructions to use the readout logger are given in the equipment s own operating manual The thermistor gives an analogue output in ohms which can either be read using a digital voltmeter or alternatively an Soil Instruments Handheld Readout or Logger 2 05 Strain Gauge Installation Jig amp Quick Set Clip Two methods are available for pre tensioning the strain gauge the Strain Gauge Installation Jig and the Quick Set Clip Typically the Jig is used where the installer accurately wishes to place the gauge at a known position to allow for tension or compression The Clip provides a quick and easy way to set the gauge to its approximate middle point The Jig pictured below is used to pre tension the gauge prior to installation Dependent on the expected strain change dpe in the steel to be monitored the gauge
9. case and is not in the scope of this manual Guidance may be sought from Soil Instruments for such individual application To install a gauge using Epoxy or other glues follow the guidelines in 3 01 and 3 02 above ensuring that the gauge and the surface it is to be attached to are free from any dust grease oil or any deposits which may prevent the glue from coupling the gauge rigidly to the surface 3 08 Place Coil Assembly Over Gauge Check Reading and Secure Once the welding or gluing is complete place the Coil Assembly over the gauge connect it to a readout and check that the reading is a stable and b broadly the same as when the Installation Jig was in place You may need to hold the Coil Assembly in place to do this If all okay then secure the Coil Assembly by the desired method For steel such as I beams it is recommended that the stainless steel securing straps are welded in place to secure the assembly on reinforcing bar cable ties or tape may be used Coil Assembly placed over strain gauge on reinforcing bar to check readings 11 User Manual Coil Assembly with securing straps in place prior to welding Caas Exec a Den e DL E x pa GEL a e at j e 3 09 Complete Installation Depending on the site circumstances it may be necessary to protect the completed installation with a cover or by wrapping in the case of reinforcing bar with a large quantity of protective tape or other material
10. connecting to a switchbox via the switchbox flylead insert the connector into the switchbox socket after careful alignment and select the required sensor via the rotary channel switches The Handheld Readout is now switched on by depressing and holding the On Off button for about 1 second until Soil Instruments appears on the LCD screen release the on off button and Handheld VW Readout scrolls across the lower row There will then be a pause of 5 seconds or so whilst the Readout self tests and checks after which the upper row will display the vibrating wire reading in frequency and the lower row the sensor temperature in C If no thermistor is connected the message NO THERMISTOR will be displayed which is just a prompt and maybe ignored if not applicable Battery compartment amp flylead socket 5 02 Selecting Displayed Units When switched on the Readout always defaults to sensor units of Frequency display If Period or Frequency Squared 1000 display is required the Units button should be pressed and immediately released The unit type may take up to 2 seconds to change Pressing the Units button again will scroll to the next unit displayed in the order Frequency Period Frequency 1000 14 User Manual 5 03 Selecting Sweep Range For accurate and stable sensor readings we need ensure that the sensor is correctly connected to the Readout and that the selected sweep range
11. e in a secure place ready for the next installation Place a minimum of 15 welds on each pad at either end of the gauge evenly spaced to ensure that all of the change in stress of the steel being monitored is transmitted evenly to the gauge An example of a correctly welded end pad on reinforcing bar D A L i du H e i i fi Es pre MY IW IT nw 3 06 Set Gauge with Quick Set Clip To use the Quick Set Clip in place of the Installation Jig place the Quick Set Clip over the gauge and then glue or weld the gauge to the surface to be monitored Once the glue has fully cured or immediately after making tack welds if welding remove the Quick Set Clip and proceed as set out in 3 08 and beyond Note Use of the Quick Set Clip will place the gauge approximately at its mid point 10 User Manual 3 07 Gluing Gauges use of Epoxy resins In certain applications where spot welding is not feasible i e gauging of concrete cast iron or nonferrous materials possibilities of bonding by epoxy resins such as HBM X60 may be considered A successful installation by bonding will depend on a number of factors such as e Nature and physical characteristics of structure material e Selection of epoxy resin e Operating temperature ranges e Operational life Due to complexities of temperature effects on composite epoxies and changes in these long term characteristics each application requires to be treated as a special
12. in by installing gauges on opposite sides of the member and averaging the change in strain reported by both gauges Irregularities Avoid installing strain gauges near irregularities in the member or near the ends of the member since readings from these locations may not adequately represent strain in the other portions of the member Sunlight Try to shield gauges from direct sunlight If the gauge is heated faster than the steel beneath it it may report changes in strain that are not representative of the steel User Manual 13 Section 5 Taking Readings Using Soil Instruments Readout 5 01 Sensor connection Remove the dust cap from the connector socket located at the top end of the readout Store this cap safely Take the required fly lead and carefully aligning its connector with the socket push the two together until they click The other end of the lead may now be connected to their sensor or switchbox If using the crocodile clips ensure that red is connected to red and the black crocodile clip to the black or sometimes blue sensor wire The polarity of the thermistor wires if fitted to the sensor usually green and white is not important but for convention white connect to white and green connect to green Make sure that the wires and crocodile clips are lying so that they do not touch each other If the vibrating wire circuit should short the VWH Readout will shut down to protect the circuitry If
13. in is tensile The calculation of Load in a member using data from strain gauges is often complex The fundamental problem is determining the composite Young Modulus E of the member since it is often difficult to accurately determine the properties of the insitu materials Once a Young Modulus is calculated the following equations can be used to calculate the loading on the structural member at the location of the Strain Gauge Force F 2 Stress S x Area A Where A Cross sectional area in m Where F units Newton s Where S units N m Stress S Young Modulus of Elasticity E x Strain e Where E units N m 16 User Manual Example calculation Steel pipe outside diameter 1 016m Steel pipe inside diameter 0 984m Calculated change from the strain gauges 54 688 ye Young Modulus of Elasticity of the steel pipe 200 000 000 000 N m Stress Exe 200 000 000 000 x 0 000054688 10937600 N m2 Area mr m X outside diameter 2 2 x x inside diameter 2 mt X 1 016 2 2 1 x 0 984 2 mt X 0 508 a x 0 492 mt X 0 258064 m x 0 242064 0 810732 m 0 760466 m 0 050266 m Force SxA 10937600 x 0 050266 549789 4N User Manual 17 Section7 Temperature Effects It is best practice to record temperature when you record strain readings You can then use the temperature data as well as strain data to analyse the behaviour of the structure The steel used for the wire in the st
14. lder on For battery operated AVT unit ensure that battery is fully charged Select medium energy level User Manual Place a piece of shim strip on the prepared surface and place the welding probe in its centre Apply gentle but firm pressure The welder will operate when it is triggered achieving one spot weld Pull the strip off If the strip tears off leaving a hole in it the weld is satisfactory otherwise increase the energy level and repeat test Once energy setting is correct weld the strip using three or four spot welds in a line and do a pull test Switch off the welder and disconnect earth connection Note Loose and unclean earthing may result in poor quality welds Unclean and too pointed tips will cause their fusion to the shim metal If a spark is seen when making the weld reduce the energy setting on the welder Keep the tip clean and slightly radiused by rubbing it on fine emery sheet User Manual Section 3 Installation 3 01 Overview e Clean grind surface gauge is to be installed on ensuring in the case of circular material such as reinforcing bar that the surface is flat Place Strain Gauge Installation Jig over gauge and adjust to desired tension Tack gauge to steel using 3 welds at each end of the gauge or glue gauge in place Remove Installation Jig Complete welding Place Coil Assembly over gauge check reading is stable and secure Protect gauge coil assemble as necessary These steps are covered in m
15. ore detail below 3 02 Clean Grind Surface It is essential that the gauge tube is not deformed by the end pads being welded to material which has more than a very slight curvature On flat structural members such as I beams this is not normally a concern but for round elements such as pile casings or reinforcing bar an angle grinder should be used to gain as flat as a surface as possible If a flat surface cannot be created then another form of gauge such as an Arc Weldable Strain Gauge should be considered The surface can be cleaned using emery cloth and if needed a proprietary degreasing agent suitable for steels Once the surface is prepared position the gauge and check for orientation and surface suitability as in the photograph below Gauge positioned on cleaned surface ready for installation 3 03 Set Gauge with Installation Jig Place the installation Jig over the strain gauge and connect to a readout device set to read in Hz Using the machine screw at the right hand end to tension the gauge to the desired tension depending on whether the steel member is likely to go into tension compression or if this is uncertain to the gauge mid point The table on the next page gives an idea of the Hz values to achieve this Note Take extreme care when turning the machine screw to tension the gauge after more than approx two turns from the non tensioned low Hz position the wire may well break rendering the gauge unserviceable E
16. rain gauge has a thermal coefficient of expansion similar to that of structural steel Thus if the gauge and the steel have the same thermal coefficient of expansion and are subjected to the same temperature change corrections for temperature change are not required If the gauge is heated by direct sunlight so that its temperature increases faster than that of the structural steel you may see large changes in apparent strain It is difficult to correct for this so try to shield gauges from direct sunlight using thermally insulated covers If the steel in the structure has a thermal coefficient that is quite different from that of the gauge the following temperature correction might be appropriate Aue corrected Aue TCm T x Temp Tempo Where Aue is the change in strain TC is the thermal coefficient of the member TC is the thermal coefficient of the gauge 11 0 ue C Temp is the current temperature Temp is the initial temperature SCH INSTRUMENTS Bell Lane Uckfield East Sussex t 44 0 1825 765044 e info itmsoil com TN22 1QL United Kingdom f 44 0 1825 744398 w www itmsoil com Soil Instruments Ltd Registered in England Number 07960087 Registered Office 5th Floor 24 Old Bond Street London W1S 4AW 18 User Manual
17. xpected due Turn Machine Screw To Set Approx Gauge Reading 8 User Manual Tension i e steel lengthens 1660Hz 1640Hz 2755 2680 f 1000 20 RATIO SETTING Compression i e steel shortens 2715Hz 2695Hz 7371 7263 1000 20 RATIO SETTING Unknown midpoint i e steel may move 2250Hz 2230Hz 5062 4973 f 1000 either way Note e It may be necessary to hold the Installation Jig in place over the gauge in order to get a stable reading e The reading should be stable to within 1Hz e Never set the gauge to a range with more than a 80 20 ratio Installation Jig placed over strain gauge note machine screw for adjusting tension on right hand side 3 04 Tack Weld Gauge With the Installation Jig remaining in place and still connected to the readout apply 3 welds to each end of the stain gauge to tack it in place Once you are confident that these are good welds slacken the machine screw and remove the Installation jig Turn the machine screw to minimum tension and place the jig back over the gauge so that the jig is now applying zero tension to the gauge check the reading in Hz is roughly the same as before the jig was removed and that the reading remains stable The photos below indicates the areas to make the tack welds User Manual Tack weld where marked 3 position Tack weld where marked hidden under cable in photograph 3 05 Remove Jig and Complete Welding Remove the Installation Jig and stor
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