HR-33T Dew Point Microvoltmeter Instruction/Service Manual
Contents
1. Description EA ADJUSTABLE LEFT HAND GRIP LATCH W STEEL KNOB EA 6 32 HEX NUT STANDARD EA SPACER 6 x 250 LG x 1 4 OD ALUMINUM EA 4 40 x 3 4 LG PANHEAD SLOTTED SCREW ZINC PLTD EA CONNECTOR 3 PRONG MALE EA HR 33T 5109 POWER SUPPLY PCB ASSEMBLY 230 VOLT EA 470 uF 50 V ALUMINUM ELECTROLYTIC EA 4 40 HEX NUT STANDARD EA 4 40 x 1 4 LG PANHEAD SLOTTED SCREW ZINC PLTD EA 10 OHM 5 1 2W EA 1N4004 1 AMP RECTIFIER DIODE EA POWER TRANSFORMER 50 60 HZ 7 5 6A EA HR 33T 5109 POWER SUPPLY PC BOARD EA SWITCH DPDT ON ON MINI BAT TOGGLE EA FUSE 1 8 AMP 250 VOLT 3AG TYPE T EA FUSE HOLDER EA IEC CONNECTOR BOTTOM FLANGE W SOLDER TERMINA U M Description EA HR 33T 5103A BATTERY PACK BRACKET EA STANDOFF 6 32 THD X 1 4 OD X 1 25 L EA 6 32 X 3 8 FLATHEAD PHILLIPS CAD EA 6 32 X 3 8 L PANHEAD PHILLIPS ZINC PLATED EA ADJUSTABLE RIGHT HAND GRIP LATCH W STEEL KNOB EA ADJUSTABLE LEFT HAND GRIP LATCH W STEEL KNOB EA 6 32 HEX NUT STANDARD EA 6 WASHER FLAT STEEL EA CONNECTOR 3 PRONG MALE EA BATTERY CLIP 9 VOLT EA RIVET 0 118 DIA 0 156 L 0 218 HEAD BRASS EA BATTERY 9 VOLT ALKALINE EA SWITCH DPDT ON ON MINI BAT TOGGLE U M Description EA CASE SUBASS RUBBER BUMPER 5 8 SCREW EMBLY DIA X 5 16 H2. should give the same water potential The dew point method of water potential measurement is affected much less by changes in ambient temperature than the psychrometric method The sensitivity coefficient of the dew point method is approximately twice that ofthe psychrometric method For these reasons the dew point method is often preferred Either method may at different times appear to be the better choice depending upon the circumstances and conditions of a particular experiment Procedures for both methods are detailed in the following 2 1 Dew Point Mode Be sure that the battery pack or the power supply is supplying the proper voltage see section 6 The following step by step procedure will enable you to obtain consistent and meaningful results from any type of thermocouple psychrometer including thermocouples mounted in sample chambers 1 Always set the FUNCTION switch on INPUT SHORT while connecting or disconnecting to the input terminals If the sensor is equipped with a SUREFAST M connector plug the connector into the SUREFAST receptacle on the front panel If the sensor does not have a connector the leads should be connected to the binding posts The wire from the chromel side of the hygrometer psychrometer should be connected to the black binding post and the wire from the constantan side of the hygrometer psychrometer should be connected to the red binding post The constantan wire from the temperature measurement thermocouple s
3. 930245 EA HR 33T 5106 POWER SUPPLY PC BOARD S6 39 0102 EA SWITCH SPDT ON OFF ON 39 0113 EA FUSE 3 16A 250V SLO BLO 1 4 DIA X 1 1 4 LONG 39 0150 EA FUSE HOLDER 39 0235 EA IEC CONNECTOR BOTTOM FLANGE W SOLDER TERMINAL 5109 230 Volt AC Power Supply Ref No Part No Qty Description 05 0116 2 CAPACITOR 005 uF 1000 W VDC 20 CERAMIC DISC 120364 d EA HR 33T NICAD BATTERY PACK amp POWER SUPPLY HOLDER 120370 1 EA HR 33T 5106 amp 5109 POWER SUPPLY FRONT PANEL 17 0101 2 EA 6 32 NUTSERT ALUMINUM 17 0124 2 EA 4 40 HEX NUT STANDARD 17 0128 2 EA STANDOFF 6 32 THD X 1 4 OD X 1 25 L 17 0154 2 EA 6 32 X 3 8 FLATHEAD PHILLIPS CAD 17 0160 6 EA 6 32 X 3 8 L PANHEAD PHILLIPS ZINC PLATED 17 0226 1 ADJUSTABLE RIGHT HAND GRIP LATCH W STEEL KNOB 41 Ref No Part No 17 0227 17 0245 17 0250 17 0314 Pl 17 0390 340337 14 15 05 0117 17 0124 17 0244 R61 CR12 CR13 2 35 0301 36 0116 44 0100 940184 56 39 0104 39 0136 39 0150 39 0235 Battery 5103A Ref No Part No 130678 17 0128 17 0154 17 0160 17 0226 17 0227 17 0245 17 0323 Pl 17 0390 17 0464 17 0585 30 0114 55 39 0104 Case Ref No Part No 340472 17 0339 17 0160 17 0245 17 0407 42 Qty n n Qty
4. Depending upon the position of the IL set knob the meter will tend to 1 fall back toward zero 2 hold steady or 3 climb upscale Rotate II SET until the meter holds relatively steady and the reading is between 15 and 30 on the bottom scale If the needle is rising or pegged upscale rotate the control clockwise d When the II button is depressed the meter will give a reading proportional to the position of the II SET control The numerical value of the reading on the 100 uvolt scale is the cooling coefficient IL of the thermocouple being used See Section 4 Theory of Operation 3 Ifthe thermocouple is in a sample chamber insert the sample to be tested and allow sufficient time for thermal and vapor equilibration If using a soil psychrometer or a similar sensor allow time for equilibration after installation Paragraph 3 2 4 With the RANGE switch set to the anticipated range and the FUNCTION switch on READ bring the meter reading to zero by adjusting the ZERO OFFSET control 5 Rotate the FUNCTION switch to COOL long enough to ensure that water has condensed on the junction Cooling times required will vary from 5 seconds or less for very wet samples to 30 seconds or more for dry samples Once the necessary cooling time has been determined for a given water potential range and sample type this time should be used for all measurements within that range including calibration measurements 6 The temperature of the th
5. HOLDER 120570 1 EA HR 33T 5112 NICAD BATTERY PACK FRONT PANEL 120802 1 EA HR 33T NICAD BATTERY PACK BOTTOM PAD INSULATION 14 0138 1 EA CONNECTOR JACK FOR 312 HOLE W SILVER PLATED LUG 17 0101 2 EA 6 32 NUTSERT ALUMINUM 17 0128 2 EA STANDOFF 6 32 THD X 1 4 OD X 1 25 L 17 0154 2 EA 6 32 X 3 8 FLATHEAD PHILLIPS CAD 17 0160 4 EA 6 32 X 3 8 L PANHEAD PHILLIPS ZINC PLATED 17 0226 1 EA ADJUSTABLE RIGHT HAND GRIP LATCH W STEEL KNOB 17 0227 1 EA ADJUSTABLE LEFT HAND GRIP LATCH W STEEL KNOB 17 0390 1 EA CONNECTOR 3 PRONG MALE 30 0121 1 EA NICAD BATTERY PACK 39 0104 1 EA SWITCH DPDT ON ON MINI BAT TOGGLE 45
6. WESCOR Model 52 Sample Chamber 0 5514 molal NaCl 25 C Water Potential 25 2 bars 2 3 4 3 6 7 Time 1 minute major division Dew Point Psychrometric Combined Output For Dew Point Psychrometric and Combined Modes Figure 2 2 4 Temperature Measurements Use the following procedure to measure temperature with a copper constantan thermocouple Connect the hygrometer psychrometer using the SUREFAST connector if it is so equipped If a SUREFAST connector is not provided on the sensor connect the constantan wire to the blue binding post and the copper wire to the red binding post It is not necessary to disconnect the hygrometer psychrometer sensor leads while making temperature measurements nor is it necessary to disconnect the temperature measurement thermocouple while making psychrometer measurements Set the C u V switch to and the RANGE switch to 30 for measurement of temperatures between 0 and 30 Celsius Above 30 use the 100 scale Read the temperature in degrees Celsius directly from the meter For optimum accuracy correct the reading using Figure 3 10 2841 JAI oqnjeJoduio T sn sj99 soe1Dap eunjejeduie 2 01 0 s isiao 2 10113 11 SECTION 3 ADDITIONAL OPERATING INFORMATION The measurement of water potential using the dew point method is relatively straightforward and eliminates errors that might otherwise occur in reading the
7. and the conditions of measurement Some leaves equilibrate in 15 minutes or less but others may require several hours Repeatable water potential readings indicate that equilibration has occurred Care must be taken that the period between measurements is long enough to detect small changes especially where long equilibration times are involved 14 3 3 Effect Zero Drift on Reading Given the condition of water condensed on the junction at the dew point temperature the direct reading of the instrument in dew point mode will continue indefinitely However the accuracy of the reading will be directly affected by any thermally induced zero offset that occurs in the thermocouple or intermediary connecting points or in the instrument itself Zero drift in the instrument will normally be negligible but other effects may become substantial Therefore sustained continuous readings over several minutes in duration should not be relied upon for accuracy unless extreme care has been taken to allow complete temperature equilibration and unless ambient temperature variations and thermal gradients have been eliminated 3 4 Ancillary Instrument Functions In addition to the operational functions used to measure water potential as described in Section 2 the HR 33T also features three additional functions that are incorporated in the RANGE switch and the FUNCTION switch HEAT supplies heating current to the thermocouple to dry the junction w
8. generally not rise more than 10 pvolts It the leads are reversed the needle rapidly rises to 20 uvolts 2 to 3 seconds It will continue to rise to and beyond 100 uvolts on COOL and DEW POINT Open Thermocouple Measure the thermocouple resistance with an ohmmeter It the resistance is less than 2 ohms the thermocouple is probably shorted If it is greater than 20 ohms it is probably open or has a bad connection Loose Connection If the meter can be controlled with the ZERO OFFSET controls when the FUNCTION switch is in the SHORT position but not in the READ position then the binding posts may be loose from the meter or the psychrometer leads loose from the binding posts These should be cleaned and tightened Wet Measuring Junction If the measuring junction is wet readings will generally be near zero Wet junctions may occur if readings are repeated without waiting for the junction to dry completely this may require 5 minutes or more at water potentials approaching zero Wet junctions apparently also persist for long periods in soil psychrometers perhaps resulting from temperature gradients Detection and avoidance of wet junctions differs with the type of psychrometer and the way in which it is being used 1 Wescor C 52 Sample Chamber If a wet junction is suspected turn the FUNCTION switch to READ and observe the meter needle while the chamber is opened and the sample pulled partway out to allow dry air to enter the chambe
9. so that the meter drifts downward then while a dew point reading with a wet thermocouple is being taken water slowly evaporates from the junction Conversely if II is such that the meter drifts upward then the junction will slowly accumulate water In other words failure to correctly adjust II SET to match the II of the thermocouple being used will result in the thermocouple being held at a temperature slightly above or slightly below the dew point If the junction gradually loses its water until it is dry then the reading will become meaningless and decay toward zero The cooling coefficient determines the ultimate limit of the thermocouple s range in measuring water potential since it represents the maximum temperature depression that can be reached by the junction through Peltier cooling If the dew point is below this temperature no measurement can be made in the dew point mode since it will not be possible to condense water from the sample onto the junction For example if a given junction has II 50 uvolts the lowest potential that can be measured by the junction will be 50 u volts 66 6 bars 0 75 uvolts bar Using the psychrometric method this same junction would produce a maximum signal of approximately 31 uvolts Wescor junctions typically exhibit cooling coefficients of 50 to 80 uvolts with some units as low as 40 uvolts or as high as 90 uvolts Researchers have reported measurements as low as 3000 bars using a
10. the output signal will change dictating a corresponding change in the cooling duty cycle thus maintaining a balance of thermal energy being conducted into and out of the junction To further illustrate Figure 6 depicts the temporal relationship between the various signal and control waveforms and levels as they will appear during initial cooling and as the junction temperature converges to the dew point The height of the sawtooth waveform is determined by the II SET control to match the thermocouple cooling coefficient For another junction having a different cooling coefficient a different sawtooth height will accordingly be set in the instrument At output levels below approximately 0 75 uvolts 1 bar the cooling duty cycle becomes so small that the frequency response characteristics of the processing circuitry begin to affect the linearity of the system such that automatic dew point operation cannot be maintained in this range 23 aand IPOJA UOMA LEE IH 1081 02 37949 10400 204 9 esind uooj DS 19487 0v3 1044009 040 indino 19830 087 MEE ejdnoo 924048 4988045 noa 002 e 31000N ebuoy 26 9 amdu SA SULIOJOA
11. will actually reduce the cooling effect Furthermore microscopic differences in the geometry and alloy makeup of the junctions influence the value of R and hence the net cooling effect Thus while the thermocouple voltage temperature sensitivity P and the Peltier Coefficient IL will be identical in every junction made from the same two materials the maximum cooling capability of a junction will vary somewhat from one junction to the next When the temperature of the junction is depressed by means of Peltier cooling heat will begin to flow into it from its surroundings This transfer of energy will limit the maximum realizable temperature depression depending upon numerous factors including chamber geometry volume and materials When these factors are fixed however the maximum realizable temperature depression of the junction is also fixed This maximum temperature depression is of concern in thermocouple psychrometry and of particular interest where the dew point method is used From the foregoing it is clear that the Peltier Coefficient does not by itself determine the maximum temperature depression of the junction For our purposes therefore we shall define a slightly different coefficient which we shall call the cooling coefficient It shall be a characteristic parameter of a given thermocouple psychrometer representing its maximum junction temperature depression resulting from the passage of a nominally optimum cooling cur
12. 1 EA 1 0M 5 1 4W R231 35 0251 1 EA 2 0M 5 1 4 R251 35 0500 1 EA 10 0 OHM 1 1 4W R259 35 0568 1 EA 9 09K 1 1 4W R939 35 0625 1 EA 27 4K 1 1 4W R257 35 0802 1 EA 976K 1 1 4W R934 35 0810 1 EA 1 00M 1 1 4W R211 35 1001 1 EA 20K 1 TN 1 4 DIA TOP ADJ R295 35 1003 1 EA 100K 1 TURN 1 4 DIA R294 35 1030 1 EA 5K 1 TURN 1 4 DIA Q221 36 0102 1 EA DUAL GATE MOSFET 5 3991 CR215 CR217 36 0103 3 EA 1N4148 SWITCHING DIODE CR207 36 0103 1 EA 1N4148 SWITCHING DIODE CR209 CR212 36 0103 4 EA 1N4148 SWITCHING DIODE CR103 CR106 36 0103 2 EA 1N4148 SWITCHING DIODE CR104 CR105 36 0108 2 EA 1N4743A DIODE 1 WATT 13V Q220 36 0109 1 EA 2N3904 NPN TRANSISTOR GENERAL PURPOSE 0101 0204 36 0109 2 EA 2N3904 NPN TRANSISTOR GENERAL PURPOSE 0206 0208 36 0109 2 EA 2N3904 NPN TRANSISTOR GENERAL PURPOSE 0205 0207 36 0110 2 EA 2N3906 PNP TRANSISTOR GENERAL PURPOSE Q222 36 0110 1 EA 2N3906 PNP TRANSISTOR GENERAL PURPOSE 0102 0203 36 0110 2 EA 2N3906 PNP TRANSISTOR GENERAL PURPOSE CR101 CR102 36 0116 2 EA 1N4004 1 AMP RECTIFIER DIODE Q211 36 0159 1 EA FET P CHANNEL GENERAL PURPOSE DEPLETION MODE 930297 1 HR 33T MOTHER PC BOARD 44 5112 Battery Ref No Part No Qty Description 110070 2 EA HR 33T NICAD BATTERY PACK END PAD INSULATION 120364 1 EA HR 33T NICAD BATTERY PACK amp POWER SUPPLY
13. 5106 110 Volt AC Power Supply Ref No Part No Qty Description C T6617 05 0116 2 EA CAPACITOR 005 uF 1000 W VDC 20 CERAMIC DISC 120364 1 HR 33T NICAD BATTERY amp POWER SUPPLY HOLDER 120370 1 EA HR 33T 5106 amp 5109 POWER SUPPLY FRONT PANEL 17 0101 2 EA 6 32 NUTSERT ALUMINUM 17 0124 2 EA 4 40 HEX NUT STANDARD 17 0128 2 EA STANDOFF 6 32 THD X 1 4 OD X 1 25 L 17 0154 2 EA 6 32 X 3 8 FLATHEAD PHILLIPS CAD 17 0160 6 EA 6 32 X 3 8 L PANHEAD PHILLIPS ZINC PLATED 17 0226 1 EA ADJUSTABLE RIGHT HAND GRIP LATCH W STEEL KNOB 17 0227 1 EA ADJUSTABLE LEFT HAND GRIP LATCH W STEEL KNOB 17 0245 2 EA 6 32 HEX NUT STANDARD 17 0250 2 EA SPACER 46 x 250 LG x 1 4 OD ALUMINUM 17 0314 2 EA 4 40 x 3 4 LG PANHEAD SLOTTED SCREW ZINC PLTD 17 0390 EA CONNECTOR 3 PRONG MALE 330698 EA HR 33T 5106 POWER SUPPLY PCB ASSEMBLY 115 VOLT C14 C15 05 0117 2 470 uF 50 ALUMINUM ELECTROLYTIC 17 0124 4 40 HEX NUT STANDARD 17 0244 4 40 x 1 4 LG PANHEAD SLOTTED SCREW ZINC PLTD R60 35 0308 1 EA 20 OHM 5 1 2W CR12 CR13 36 0116 2 EA 1N4004 1 AMP RECTIFIER DIODE T1 44 0100 EA POWER TRANSFORMER 50 60 HZ 7 5V 6A
14. A battery pack A charger is supplied with the 5112 battery pack The charger supplies 25 mA current to the batteries A discharged battery can be completely recharged in 14 to 16 hours The charger should not be left connected to the power supply for longer than two days or the batteries may deteriorate and become inoperative A fully charged power supply will provide approximately 20 hours of operation 29 SECTION 7 TROUBLESHOOTING If the instrument fails to operate satisfactorily check the batteries and replace when needed as outlined in Section 6 If the batteries are satisfactory but the instrument does not operate properly one of the following circumstances may be the reason a b c d Leads Improperly Connected to the Binding Posts Make sure that the leads are connected to the binding posts as outlined in Section 2 It may be impossible to tell from inspection of the lead wires which wire is connected to the constantan and which to the chromel wire To determine if the leads are connected properly set the RANGE to 100 If the lead wires are connected properly the meter needle should rise from 0 to 30 uvolts with the thermocouple in dry air in about 7 seconds after the FUNCTION switch is rotated to COOL The meter will then approach a maximum value asymptotically When the FUNCTION switch is rotated to DEW POINT the needle will gradually rise of fall depending on the II setting Paragraph 2 1 2c but will
15. BOARD 43 Mother Board Ref No Part No Qty U M Description C101 C102 05 0100 2 EA 10 uF 25VDC 20 TANTALUM DIPPED 213 214 05 0107 2 OluF X7R 10 1 LEADS 50V c219 05 0109 1 EA CAPACITOR CERAMIC RADIAL 0 2 0 047uF X7R 50V C204 05 0113 1 EA 0 1 uF 100 V 10 METALLIZED POLYESTER U205 21 0106 1 EA 4011 QUAD 2 INPUT NAND GATE U204 21 0144 1 EA F355N 0202 21 0190 1 EA M741CN OP AMP U201 U203 21 0191 2 EA M301N OP AMP 320777 1 EA 5500 AMP BOARD DC M7 ASSEMBLY R241 R242 35 0124 2 EA 10 OHM 5 1 4W R101 R103 35 0140 2 EA 47 OHM 5 1 4W R223 35 0177 1 EA 1 6K 5 1 4W R102 R104 35 0184 2 EA 3 3K 5 1 4W R210 R213 35 0189 2 EA 5 1K 5 1 4W R240 35 0193 1 EA 7 5K 5 1 4W R215 R221 35 0196 2 EA 10K 5 1 4W R205 35 0201 1 EA 16K 5 1 4W R262 35 0203 1 EA 20K 5 1 4W R207 R220 35 0203 2 EA 20K 5 1 4W R218 35 0205 1 EA 24K 5 1 4W R209 R212 35 0213 2 EA 51K 5 1 4W R222 R224 35 0213 2 EA 51K 5 1 4W R225 R228 35 0213 2 EA 51K 5 1 4W R260 R261 35 0213 2 EA 51K 5 1 4W R216 R217 35 0220 2 EA 100K 5 1 4W R226 35 0220 1 EA 100K 5 1 4W R284 R293 35 0220 10 EA 100K 5 1 4W R230 35 0227 1 EA 200K 5 1 4 R219 35 0235 1 EA 430K 5 1 4W R214 R229 35 0244 2 EA 1 0M 5 1 4W R208 35 0244
16. FOR 6 EAD PHILLIPS ZINC 6 32 X 3 8 L PANH PLATED 6 32 HEX NUT STANDARD 225 6 X 1 4 PANHEAD SLOT CAD SCREW Front Panel Ref No Temperature Board Ref No R301 R302 R306 R310 R308 R304 R309 R307 R303 R305 CR301 CR302 Q301 CR303 Part No 13 0101 13 0102 17350125 17 0160 17 0240 17 0241 17 0243 17 0248 17 0251 17 0304 17 0407 17 0480 17 0493 17 0657 17 0720 18 0107 310329 340324 340541 35 0904 35 1055 239 0001 39 0103 39 0226 Part No 35 0165 35 0213 35 0508 35 0614 35 0680 35 0720 35 1000 35 1004 35 1009 35 1071 36 0103 36 0110 36 0204 910027 Qty NN TYN HNH HNN N N Qty U M EA Description KNOB WITHOU KNURLE 1 4 SHAFT 4 40 HEX NU Xx 379 ATED NDING POST B NDING POST R D ECEPTACLE 8 WASHER INTERNAL STAR STE 3 8 32 NUT DIA X 3 32 CAD WASHER IT2T 6 32 x E
17. R 1203815 pue jonuop peord T 1NIOd M30 1002 535104 LN3YYND 9 7002 301 0 WHO33AVM TOY LNOI LINDYID JAWS WOH3 LNdLNO 015538430 JYNIVITANIL 27 The FUNCTION control switch establishes appropriate interconnections among the instrument s control and measuring circuits for the operational mode selected by the operator The cooling duty cycle and instrument output indication for each operational mode are set forth in Table 1 Table 1 Instrument Operation Versus Function Switch Position Operational Mode Cooling Duty Cycle Meter Reading Notes HEAT No cooling Not meaningful Psychrometer junction heated to drive off moisture INPUT SHORT No cooling Controlled by the offset Allows OFFSET to be set to zero Protects meter while changing connections READ No cooling Not meaningful Microvolt output from the psychrometer thermocouple COOL 0 95 Microvolt output from Maximum cooling the psychrometer thermocouple DEW POINT Automatically Microvolt output from Junction converges to 28 controlled between 0 0 95 the psychrometer thermocouple dew point temperature SECTION 6 MAINTENANCE 6 1 General Maintenance The instrument is constructed using solid state circuitry and high quality components throughout There is no requirement for general periodic maintenance other than battery replacement in the 5103A power supply module As with any precision equipment
18. T COOL 10 volts DEW POINT 10 aab DEW POINT 10 volts Cool es le 10 volts Read Cooling duty cycle varies with signal and DP GAIN ADJ setting Frequency approximately 30 to 60 Hz HEAT COOL 10 volts 0 95 0 05 10 volts Frequency approximately 30 to 60 Hz INPUT SHORT READ WE EL A 0 volts Duty cycle and frequency same as 6 12 volts READ HEAT INPUT SHORT DEW POINT 11 5 volts 16 5 volts COOL 12 volts DC This waveform only appears during on portion of duty cycle Test Point Data amp Waveforms Figure 11 wal ATENISSV en Sumus an Iv Govea HALON ARSS HOLS3NNOS Levens Me 500 ex tod tos pomis 52220 Tuve av 9 15 ATBAacEV w zc ge xame rrecsv 1604 ue ASE 181016 E a IT Wod LU TARE pif LINDAS Salon 10 6 NOLE AEVINZNON aowa Tan E pitoa tod LYM C mna fared des Hollis 3 28 1564 SNICKET Front Panel Assembly Figure 12 39 Chassis Wiring Schematic Figure 13 40 SECTION 8 PARTS LIST
19. This manual is a facsimile of the original DO NOT SELL FOR REFERENCE USE ONLY CHECK DATABASE FOR CURRENT REVISION HR 33T Dew Point Microvoltmeter Instruction Service Manual 1978 1986 2001 Wescor Inc M2820 1 Wescor Inc 459 South Main Street Logan UT 84321 USA Phone 435 752 6011 Toll Free 800 453 2725 Fax 435 752 4127 Email wescor wescor com Email Service service wescor com CONTENTS SECTION 1 INSTRUMENT SPECIFICATIONS 1 44 3 SECTION 2 BASIC OPERATING PROCEDURE 5 2 Weed BY AEN Point aee EE hehe en ed e eet dd 5 2 2 Psychrometric Mode i ov AA e RU oS 6 2 3 Combined Dew Point Psychrometric 0 0 cc tenes 6 2 4 Temperature Measurements 2 1 1 8 SECTION 3 ADDITIONAL OPERATING INFORMATION 11 pr De dp ute cte e Rr ca bet 11 3 2 Thermal and Vapor Equilibration 414 4 4 12 3 3 Effect of Zero Drift on Reading cese e 13 3 4 Ancillary Instrument Functions 7 13 3 5 IL Versus Temperature eoe 13 SECTION 4 THEORY OF OPERATION 1 15 4 1 Ther
20. X ALUM WASHER INT BINDING POS 2 56 HEX 2 56 X 3 PR LACK SKIRT FOR 1 4 D KNOB W ARROWHI SMALL L PANHI NG SHAFT EAD ON SKIRT FOR EAD PHILLIPS ZINC GR 1 2 ID 1 212 RNAL STAR ST EEN NUT STANDARD 1 4 PANHEAD SLOT CA D SCREW EL 8 BRASS 3 8 OD X 1 THICK STEEL 16 THICK X LG FEMALE S ANALOG 0 1 1 SCAL R 33 R 331 331 H METER H H RECEPTACLE 5103A FRONT PANEL F Di MTG PL SCRE 100K 1 50K 1 SWITCH 3 POLE SWITCH DPDT SNAP ACTION SWITCH CAP SMALL MSP 5 TURN PANEL TURN PANEL B MOUNT ON NON PUSH BU A RY PACK 2 UNT E ON PLAS Description 510 OHM 5 51K 5 41 2 OHM 20 0K 49 9K 80 6K 10K 1 1 4W 1 4W 1 1 4W 1 4W 1 1 4W 1 1 4W TURN 1 4 1 DIA 10 OHM 1 TURN 1 4 DIA 100K 20 TURN SIDE WITH 7 IN 1N4148 SWITCHING 2N3906 PNP TRANSISTOR GENERAL PURPOSI DIODE ZENER 6 4V 400 100K 1 1N4570A 1N4571A MILLIWATTS D HR 33T EMPERATUR F ADJUST ANDOFF 1 4 SUBASSEMBLY ASSEMBLY NED WATT TTON MOMEN ARY TIC FOR ALCO LEADS THERMISTOR IODE Gl PC
21. age at the input terminal The microvoltmeter signal is compared to the sawtooth ramp voltage by this circuit Since the ramp voltage starts at zero and assuming that some finite signal is present from the thermocouple thermocouple temperature depressed from ambient the comparator output will initially be high and the thermocouple will receive cooling current through the electronic switching circuits When the ramp voltage becomes higher than the signal voltage the output of the comparator switches to low and the cooling current is discontinued The microvoltmeter indicates the thermocouple voltage during this interval and the sample hold circuit maintains the signal level from one reading interval to the next The negative pulses produced by the sawtooth pulse generator circuit are added to the thermocouple signal at the input of the comparator These pulses represent a 0 05 duty cycle and limit the cooling duty cycle to a maximum of 0 95 ensuring a minimum read interval of at least 0 05 to maintain the output signal in the sample hold circuit The height of the sawtooth waveform is adjusted by the II SET control such that for a given thermocouple the cooling duty cycle as dictated by the thermocouple temperature depression signal will remove precisely that amount of heat that is flowing into the junction from its surroundings If the water on the junction then causes a movement of the junction temperature toward the dew point
22. bove 10 uvolts often preclude reliable determinations Solution Place the psychrometer perpendicular to the temperature gradient and wait until its temperature is uniform with the offset near zero to make readings Routine checks of zero offset are highly recommended Contaminated Chamber or Thermocouple The thermocouple or the chamber in which it is mounted may become contaminated making accurate water potential measurements impossible One method of detecting a contaminated thermocouple is to compare the psychrometric plateau with the plateau produced by a clean psychrometer for the same water potential and the same cooling time Figure 7 shows typical curves for a clean and for a contaminated thermocouple The fall off from the plateau begins earlier and is more gradual for a contaminated psychrometer 30 Contaminated 30 Clean OUTPUT Microvolts 20 20 10 10 0 0 30 60 90 120 0 60 90 120 TIME Seconds Output Characteristics for Contaminated and Clean Thermocouples Figure 7 33 Often a soil psychrometer can be cleansed by running clean water over the unit for a period of several hours It this is not successful the case must be removed The thermocouple and mount can be cleansed by rinsing in distilled water several times and drying by blowing with clean air Distilled or deionized water used for rinsing must have a resistance of 1 megohm mL or better Rinse water of lower quality may preclude successful cleaning of t
23. discovered it in 1834 dW 2 dt dW dt is the rate of energy transfer The coefficient is known as the Peltier Coefficient For very small currents a linear relationship exists between the thermoelectric e m f and Peltier cooling the coefficients and II are related by the equation 3 where T is the absolute temperature Equation 2 may lead one to believe that the junction can be cooled to any desired temperature simply by providing a sufficient amount of cooling current At higher values of I however this simple relationship does not hold true inasmuch as the current passing through the junction not only cools by the Peltier Effect but heats the junction as a result of the resistance Equation 2 must therefore be modified to account for this effect such that dW ILI RP 4 dt 3Smith Jones and Chasmar The Detection and Measurement of Infra Red Radiation Oxford University Press 1960 p 62 17 where R is the electrical resistance of the junction and thermocouple lead wires in the immediate vicinity of the junction The second or heating term of the equation is negligible at small values of I but because the heating effect increases with the square of I it rapidly becomes the dominant factor in the equation Thus in thermocouple psychrometers there is an optimum value of cooling current that will produce maximum temperature depression beyond which a further increase in current
24. emperature of the thermocouple can be made using the relationship CORRECTED READING READING 0 325 0 027T where T is in degrees Celsius 2 3 Combined Dew Point and Psychrometric Mode Combined dew point and psychrometric measurementis sometimes useful This can be accomplished by switching the FUNCTION switch from DEW POINT to READ upon completion ofthe dew point measurement The plateau should be identical to the plateau obtained using the psychrometric mode for the same cooling time If the plateau is wider then water has been added to the thermocouple during the dew point measurement period If the plateau is narrower then water has evaporated during the dew point measurement period A strip chart recorder should be employed for this evaluation The combined mode permits II to be experimentally determined under wet thermocouple conditions The value of IL must be adjusted until the duration of the plateau is the same as in psychrometric measurement In re adjusting remember that increasing the II setting decreases the cooling of the thermocouple which will decrease the width of the plateau Figure 2 shows the output for each of the three modes of operation presented above The recordings were made with 0 5514 molal NaCl solution in a C 52 Sample Chamber The psychrometric output for all psychrometers should be between 0 34 uvolts bar and 0 55 uvolts bar at 25 C 30 25 o Output microvolts o
25. ermocouple will fall below the dew point temperature during COOL assuming the sample is within the measurement range When switched to DEW POINT the temperature will converge to the dew point The e m f produced by the temperature difference between the junction at the dew point temperature and the ambient temperature is a linear function of the water potential The proportionality constant is approximately 0 75 uvolts Within the normal range of temperatures encountered in water potential measurements little if any correction for ambient temperature is required However it is necessary to maintain the proper II setting as explained in Paragraph 3 5 2 2 Psychrometric Mode The psychrometric mode does not require the determination or setting of IL To use the HR 33T in the psychrometric mode follow the procedure outlined in Paragraph 2 1 through step 5 omitting step 2 After cooling the junction step 5 rotate the FUNCTION switch to READ The meter will indicate a plateau signal before falling to zero The proportionality constant for psychrometric determination of water potential is approximately 0 47 uvolts The microvolt output divided by this number can be used for rough approximations of water potential The temperature correction given below must be used if the temperature is different from 25 C More accurate measurements can be obtained by individually calibrating each psychrometer A correction for the t
26. ery Pack or the AC power supplies The HR 33T R will operate approximately 20 hours per charge Optional AC Power Supplies Model 5106 115V 50 60 Hz Model 5109 230V 50 60 Hz Controls ON OFF power supply module ZERO OFFSET COARSE ZERO OFFSET FINE RANGE FUNCTION II SET II Read C uV temperature microvolts Ranges 10 30 100 and 300 p volts full scale plus positive and negative battery voltage temperature 0 C to 30 C 0 100 C with accuracy as specified below Functions HEAT INPUT SHORT READ COOL DEW POINT Accuracy 1 of full scale recorder output 2 of full scale panel meter Temperature 0 5 C from 0 C to 40 C refer to Figure 3 Zero Drift Less than 0 5 uvolts per 24 hours Less than 0 1 p volts per 1 C Noise Less than 0 1 uvolts Input Impedance 100 ohms or less higher impedances will degrade accuracy Rise Times 2 seconds 10 90 Zero Suppression 75 uvolts Recorder Output 10 volts full scale 10mA Connectors SUREFAST bulkhead connector Binding Posts constantan binding post for reference junction External Dimensions 23 cm x 30 cm x 13 cm Weight 3 3 kg with batteries SECTION 2 BASIC OPERATING PROCEDURE The HR 33T permits water potential determination with a variety of sensors in either the dew point or the psychrometric mode Both methods of measurement after proper calibration
27. eter being linearly related to water potential within the range from 0 to approximately 70 bars With the psychrometric method the junction is cooled by the passage of an electrical current through it Peltier Effect to a temperature below the dew pont thus causing pure water to condense upon the junction Having accomplished this the cooling current is discontinued and the condensed water allowed to evaporate from the junction back into the surrounding atmosphere The evaporating water draws heat from the junction heat of vaporization depressing the temperature of the junction from that of the surrounding air temperature The magnitude of the temperature depression depends upon the relative humidity and the temperature of the surrounding air the drier and warmer the air the faster the evaporation rate and the greater the depression The temperature of the junction is measured by electrical means prior to cooling and during evaporation The differential temperature of the junction is an explicit function of the relative humidity and hence of the water potential in the media being measured Thermocouple psychrometers have a typical responsivity near 0 47 uvolts bar at 25 C Because the effects of temperature must be removed from the measurement a correction for ambient temperature if other than 25 C is made to obtain the true water potential refer to Paragraph 2 2 4 3 Dew Point Method In the psychrometric method the temperature
28. g the part numbers listed in Section 8 A schematic diagram of the instrument is shown in Figure 8 The power supply schematics are shown in Figure 9 Test point locations and waveforms are detailed in Figures 10 and 11 A thermocouple psychrometer can be simulated for checkout purposes by attaching a 10 ohm resistor to the instrument binding posts The ZERO OFFSET controls can be used to simulate a varying signal at the input terminals and the complete electronic system can be analyzed to isolate and identify any faulty component CAUTION Instrument damage resulting from repairs attempted by non factory authorized personnel may void the warranty 34 OF ANITWHTY LIOA 6 ALAVA VEGIG 0012 0 992 22 543 2 TANT COIG 4144106 60 HR 33T Power Supply Schematic Diagram Figure 9 36 7 R225 8 R292 5 R260 4 C204 6 R262 1 Test point 1 is the wiper of the II set potentiometer located on front panel not shown here Test Point Locations Figure 10 3 R215 2 CR207 37 TEST POINT 38 1 2 3 4 5 6 7 8 Wiper on II Set pot 10 x IL us p MAE 10 x 0 volts Frequency approximately 30 Hz to 60 Hz varies inversely with 10 volts Frequency and duty cycle same as 2 10 volts 10 x TI 0 volts 0 95 0 05 Frequency approximately 30Hz to 60Hz varies inversely with height INPUT SHORT READ 10 volts HEA
29. he thermocouple Most sample chambers can be easily taken apart so that the thermocouple is exposed The same method that was given for cleaning a disassembled soil psychrometer can then be used to clean the sample chamber thermocouple Sometimes with a sample chamber it is necessary to remove the o ring and clean around the thermocouple and in the o ring groove with a wet swab taking care to avoid touching the thermocouple wire In all cases it is important to avoid touching the thermocouple wire as the thermocouple is fragile and can easily be broken After cleaning contamination tests should again be performed If none of the above appears to solve the problem the instrument may be returned to the factory for repair Before returning an instrument for service please contact Wescor for return authorization and instructions See front pages of this manual for contact information Customers outside the USA must pay customs brokerage fees and freight charges both ways Freight to the factory should be prepaid Customs brokerage fees will be invoiced at the time instrument is returned to customer Ifreturn is impractical or it you prefer to make your own repairs a qualified electronics technician who has access to a standard VOM 20 000 ohm per volt and an oscilloscope may be able to identify and replace any defective part or component Some parts can be obtained from any radio electronic supply house or they can be ordered from the factory usin
30. hen necessary BATT measures internal battery voltage BATT full scale 30 volts 3 5 Versus Temperature It has been found both theoretically and empirically that II changes by a factor of 0 7 uvolts C with Wescor Psychrometers II for temperatures other than the temperature at which II was measured is given by the formula 0 7 T To where 7 is the value of II at the new temperature and is the value of II at the temperature of measurement An example of such a correction is as follows Given at 25 C 55 uvolts T 12 C IL at 12 C 0 7 12 25 55 45 9 uvolts For maximum accuracy I should be readjusted to the correct value whenever operating at temperatures different from 15 SECTION 4 THEORY OF OPERATION 4 1 Thermocouple Cooling Coefficient In a thermocouple consisting of substances a and b a temperature difference AT between the two junctions will cause a thermoelectric e m f E to be set up between the junctions The magnitude and direction of E is determined by the thermoelectric properties of the two substances and is given by E P AT 1 where P is the sensitivity of the thermocouple in volts degree When a current I is caused to flow through the junction in the direction of its thermoelectric e m f the junction is cooled by an amount proportional to the current This is known as the Peltier Effect after Peltier who
31. hould be connected to the blue binding post On Wescor hygrometer psychrometers the copper lead from the temperature measurement thermocouple is also connected to the constantan side of the hygrometer psychrometer thermocouple If a sensor is used which has these two leads separate both should be connected to the red post The temperature measurement thermocouple can be connected at the same time as the psychrometer Temperature measurements can be made by moving the switch to the position as explained in Paragraph 2 4 2 Determine and set IL as follows If II for the sensor to be used is known adjust the II SET knob while depressing the II switch until the meter is at the desired value a Allow the thermocouple to equilibrate in a dry isothermal surrounding dew point depression gt 1 C If the sensor is a C 52 Sample Chamber it is sufficient to empty and dry the sample chamber by wiping it with a kimwipe Room air is usually sufficiently dry Soil psychrometers or sample chambers which rely upon a waterbath or an insulated box for thermal stability must be air dried and thermally insulated while II is determined b With the FUNCTION switch in the READ position and the RANGE switch set to 30 uvolts adjust the ZERO OFFSET control to bring the meter reading to zero It is necessary to accurately zero the meter before determining IL c Rotate the FUNCTION switch to cool for a few seconds then to DEW POINT
32. ieq 4 T PT or tr 9v GENER o T LEE ERES rrr aE SS 23 Change in sensitivity with temperature is found by dividing Equation 15 by y and differentiating with respect to temperature This gives d E y a e a Since P and increase with temperature at about the same rate the change in sensitivity with temperature remains approximately constant at about 2 5 x 103 uvolts bar C or about 0 3 percent per degree from 10 to 50 C Thus over the normal range of temperatures encountered in water potential measurements little if any temperature correction for thermocouple sensitivity is required However II does change with temperature This temperature dependence is discussed in Paragraph 3 5 24 SECTION 5 ELECTRONICS The electronic system of the HR 33T when in the DEW POINT operational mode is depicted in the block diagram Figure 5 The voltage signal from the thermocouple is processed through the microvoltmeter section of the instrument amplified by a gain of 10 This signal is fed to the panel meter through the range amplifier and into the non inverting input of a level comparison circuit whose bistable logic output is either high positive or low negative depending upon whether the voltage at the input terminal is higher or lower than the volt
33. mocouple Cooling Coefficient 1 15 4 2 Psychrometric Method os eA a eu tie Belg elas Ne rede 17 43 Dew Point Method Sei eX AE e etel ve e EN e PUES 17 44 HE 33T D w Point System i esie 2 0 e se na 18 4 5 Effect of Ambient Temperature 1 2 4 20 SECTION 5 ELECTRONICS is sede eec I eee ee dA RR Wee d e EA 23 SECTION 6 MAINTENANCE u 280 8 eer GES pesca een ea peus ea et aen da p e 27 6 I General Maintenance voe Diner Rie RETRO AUR ey 27 6 2 Batteries ae Rb S EEN n eo BEES 27 SECTION 7 TROUBLESHOOTING 1 4 mm 42 29 SECTION 8 PARTS LIST Sri eese tu me uo m e ed ade eb e e eder ee 39 Modular Power Supply SET Power Switch Recorder Output Case Ground Input Input Read Const Input SUREFAST Receptacle C uV Switch Zero Offset Coarse Zero Offset Fine Range Switch Function Switch HR 33T Figure 1 SECTION 1 INSTRUMENT SPECIFICATIONS Standard Power Supplies HR 33T 5103A Modular Battery Pack included with instrument Batteries Four Eveready type 522 or equivalent Battery Life 10 hours nominal HR 33T R 5112 Rechargeable Battery Pack included with instrument Consists of 30 Nicad batteries in a case which is interchangeable with the 5103A Batt
34. plateau signal from a thermocouple using the wet bulb dry bulb temperature comparison or psychrometric method Nevertheless while the resulting output from the dew point meter is less subject to interpretive evaluation the electronic process by which this output is derived from the thermocouple is somewhat more involved than with the psychrometric method It is therefore recommended that the operator familiarize himself with the principles employed to obtain the dew point reading as reviewed here and in Section 4 Theory of Operation in order that he will have a better understanding of the overall operation of the instrument and the results obtained Inherent limitations in the electronic switching circuitry prevent the instrument from being used in the dew point mode for water potential readings between zero and 1 bar approximately 0 75 volts 3 1 U Set Adjustment The dew point method requires that the electronic gain ofthe duty cycle control circuitry be matched to the cooling coefficient IL of the thermocouple being used This operation is easily performed using the procedure in Paragraph 2 1 step 2 which allows the numerical value of IL to be read on the meter The alternative procedure described in Paragraph 2 3 may also be used This gain matching procedure is essential since it ensures the energy balance condition that will cause the junction temperature to converge to the dew point If during the matching procedure II is set
35. r If the junction is wet the needle will quickly rise to high readings and after several seconds drop to zero The time is dependent upon the quantity of water condensed on the junction The chamber is then closed and after a short vapor and equilibration time is ready for another reading If the junction is dry the reading drops to zero within one or two seconds after transient excursions associated with pressure changes on opening the chamber 2 In situ Soil Psychrometer If condensation is suspected turn the FUNCTION switch to HEAT to evaporate all water This may take 10 seconds or more Switch to READ and allow 5 minutes for thermal and vapor equilibration then make a reading One problem associated with use of the HEAT function is that water evaporated from the junction will condense on the nearest surface 1 the thermocouple mount rather than in the soil possibly leading to erroneous readings 31 32 Repeated or prolonged cooling of the junction can gradually build up moisture in the psychrometer even without use of the HEAT function and this water is only slowly dissipated throughout the soil Thermal gradients may also cause condensation within the chamber Excessive Temperature Gradients These can be detected in single junction psychrometers as a difference in readings where the FUNCTION switch is alternated between READ and INPUT SHORT Differences above 1 uvolt indicate undesirable temperature gradients and a
36. r potential and ambient temperature is easily obtained for the HR 33T Thermocouple output is simply the thermocouple sensitivity P multiplied by the dew point depression AT When the dew point depression is small as is the case for water potential measurement AT Ae S where Ae is the difference between saturation and chamber vapor pressure and S is the slope of the vapor pressure temperature function at ambient temperature The Clausius Clapeyron equation gives S as a function of temperature saturation vapor pressure ej and latent heat of vaporization A s 11 where R is the universal gas constant A relationship between Ae and water potential A is obtained using the expression y e amp exp TT 12 RT y Thus Ae amp e e amp 1 exp 7 13 RT P RT and AE P AT Ae S 1 exp y RT 14 Equation 14 is the desired relationship between thermocouple output and water potential At high water potentials where the exponential can be approximated by the first two terms of a series equation 14 becomes Pay Ty E 15 indicating a linear relationship between E and A with a sensitivity of about 0 75 uvolts Figure 4 shows the agreement between theoretical and actual calibration curves Values of constants used were T 208 K 4 60 bars 24330 bars and P 63 uvolts K 22 p 24181 enjoy SA eono s
37. reasonable care should be exercised to protect the instrument from severe mechanical shock or from extremes in temperature The instrument panel and case may be cleaned using a soft cloth and mild soap solution if necessary 6 2 Batteries The HR 33T with the 5103A battery pack uses four Eveready alkaline type 522 or equivalent storage batteries They are connected in series to provide 18 18 and zero volt connections to power the instrument The batteries are mounted in the power supply module which is removed from the instrument by rotating the fasteners and lifting straight up The battery voltage can be tested at any time by moving the RANGE switch to BATT or BATT The reading will be in volts with full scale 2 30 volts Two batteries are read at one time to give nominal readings of plus or minus 18 volts respectively Batteries should be replaced anytime the voltage reading falls below 16 volts A ON 22 22 Remove used batteries promptly to avoid the danger of instrument damage by corrosive leakage Inno event should the instrument be stored for extended periods without first removing the batteries The useful operating battery life is theoretically 10 hours but actual service may vary substantially from this depending upon operating condition and freshness of the batteries when placed in service The HR 33T R with the 5112 rechargeable battery pack uses 30 nickel cadmium batteries in a case which is interchangeable with the 5103
38. rent Since the temperature of the junction is directly related to its thermoelectric e m f the cooling coefficient will be expressed in microvolts this is in harmony with the Peltier Coefficient which as defined above also has units of Watts Ampere or Volts DEFINITION The cooling coefficient IL for a given thermocouple psychrometer is defined as the differential e m f in microvolts that results from the passage of a specified nominally optimum cooling current through the junction at a specified ambient temperature Stated mathematically II P AT 5 where is the temperature differential that results from the given current The measurement of IL must be performed in such a way that the differential e m f is measured immediately after cessation of cooling current flow and is completely independent of any consequential effects of the current other than the temperature depression of the junction These requirements are fulfilled in the HR 33T Statistical data on a large number of Wescor Thermocouple Psychrometers have shown that the optimum cooling current is typically 8 0 milliamperes This value of cooling current is standard in the HR 33T Dew Point Microvoltmeter RMS current 8 mA at 9596 duty cycle 18 4 2 Psychrometric Method Thermocouple psychrometers provide a measurement of water potential in situ in sample chambers through their ability to sense the relative humidity of their environment this param
39. s divided electronically by the value of II which is set into the instrument using the II SET control To illustrate assume II SET has been adjusted for a given thermocouple and the thermocouple has been placed in position for measurement The junction temperature is lowered below the dew point for a short time to condense water upon its surface when in COOL the cooling duty cycle is always at its maximum value or 0 95 When the FUNCTION switch is rotated to DEW POINT the cooling duty cycle immediately assumes a value dictated by the microvoltmeter output such that heat is removed form the junction at the same rate it flows in from the surroundings Since the junction temperature is below the dew point water continues to condense liberating the heat of condensation As the temperature of the junction rises AT becomes smaller as does the psychrometer output and the duty cycle L so that Peltier cooling continues to remove exactly the amount or heat flowing in from the surroundings This automatic process continues until the junction temperature reaches the dew point and condensation ceases The system will then maintain the dew point temperature However if a change in ambient temperature occurs the meter output will be affected proportionately since the initial ambient temperature is used as a reference point from which the dew point depression is measured 21 4 5 Effect of Ambient Temperature Thermocouple output as a function of wate
40. sample exchange technique in a Wescor Sample Chamber but this technique can only be employed in conjunction with the psychrometric method Wescor Soil Psychrometers are individually tested and labeled for cooling coefficient A strip chart recording showing the psychrometric response to a 0 55 molal NaCl calibration solution is included with each sensor 13 3 2 Thermal and Vapor Equilibration Thermal equilibration can be easily determined by comparing the reading on INPUT SHORT and on READ The difference between these two readings represents the difference between the junction temperature and the temperature at the head of the psychrometer chamber where the chromel and constantan wires fasten to the connecting pins The offset should be less than 3 uvolts for meaningful measurements Higher offsets are evidence of excessive thermal gradients Section 7 Vapor equilibration can occur very rapidly for a sample chamber containing a solution often less than one minute For a ceramic cup in a calibration solution equilibration may take one or two hours For ceramic cups buried in soil it is necessary that the disturbed soil come to equilibrium with the surrounding soil It is advisable to allow 24 hours after installing soil psychrometers before reading the water potential The equilibration time for in situ measurement of water potential of leaves with the L 51 or L 51A Leaf Psychrometers varies with the type of leaf being measured
41. sation will cease Therefore given the aforesaid independence from all heat transfer except that due to water the temperature of a wet junction will always converge upon the dew point 4 4 HR 33T Dew Point System In the real world it is not possible for a thermocouple junction to be independent of heat transfer mechanisms that nature calls into play Nevertheless considering the circumstances that will prevail whenever a measurement of water potential is to be made it is possible to simulate the above described hypothetical situation During the measurement the wet junction temperature will always be below the temperature of its surroundings Therefore heat will tend to flow from the surroundings to the junction Using Peltier cooling a counter flow of heat can be created whose magnitude is adjusted electrically to exactly balance the heat inflow for a net energy transfer of zero It this balanced condition is set up on a dry thermocouple to account for all heat transfer mechanisms other than condensing or evaporating water then when the junction is wet its temperature will be influenced only by the water just as in the hypothetical example To illustrate let the heat transfer from the surroundings to the dry thermocouple be represented by the relationship dW 6 dt where k is a proportionality constant representing effective thermal conductivity and AT is the temperature differential between the tempera
42. that the junction attains while water evaporates from its surface is always lower than the ambient but higher than the dew point temperature This fact becomes obvious when one recognizes that if the junction were somehow to be held at the dew point temperature no water would evaporate from it This simple observation is the fundamental principle upon which the dew point thermocouple method is based to wit If held at the dew point temperature a wet thermocouple junction will neither lose water through evaporation nor gain water through condensation Rigorous analyses of the principles of thermocouple psychrometry are available in the literature A simplified and generalized description of the process is used here to provide a basis for comparison with the dew point method 19 Consider a hypothetical thermocouple junction whose temperature is determined exclusively by the heat transferred to it or away from it by condensing or evaporating water Assume also that the junction has an initial temperature T and that it is covered with water If T is above the dew point water will evaporate from the junction carrying with it the heat of vaporization until the temperature of the junction falls to the dew point at which time evaporation will cease If T is below the dew point additional water will condense upon its surface and the heat of condensation will raise the temperature of the junction until it reaches the dew point at which time conden
43. ture of the surroundings and the thermocouple junction Although the general mathematical description for radiative and conductive transfer mechanisms to and from the junction are considerably more complex than the above relationship for small AT such as applies here the simple linear model of equation 6 is sufficiently accurate Smith et al op cit p 48 20 Heat transfer due to Peltier cooling and resistive heating is given by equation 4 Assuming optimum value of cooling current the maximum temperature depression AT will be obtained If the cooling current is pulsed at a regular periodic rate the actual cooling effect can be varied linearly between zero and maximum The average temperature depression of the thermocouple junction will be given by AT LAT 7 The cooling duty cycle L is a unitless number between 0 and 1 and is given by L 8 where t time of cooling current and t period of electronic impulses In the HR 33T L is restricted by design to the range from 0 to 0 95 By controlling the value of L we are able to adjust the magnitude of cooling to exactly balance heat inflow Then using equations 5 6 and 7 we have eM 9 dt dt D The psychrometer output is given by P AT If we use this output to control the value of L such that P AT 10 then the exact energy balance stated in 9 is satisfied The instrument output i
Download Pdf Manuals
Related Search