HR-33T Dew Point Microvoltmeter Instruction/Service Manual ...

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Change in sensitivity with temperature is found by dividing Equation (15) by R and differentiating with respectto temperature. This givesd(E/R) -P ab=(16)dT 8Since P and 8 increase with temperature at about the same rate, the change in sensitivity with temperature remainsapproximately constant at about 2.5 × 10 -3 µvolts bar -1 °C -1 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, temperaturecorrection for thermocouple sensitivity is required. However, A v does change with temperature. This temperaturedependence is discussed in Paragraph 3.5.24
SECTION 5 – ELECTRONICSThe electronic system of the <strong>HR</strong>-<strong>33T</strong>, when in the DEW POINT operational mode, is depicted in the blockdiagram, Figure 5. The voltage signal from the thermocouple is processed through the microvoltmeter section ofthe instrument, amplified by a gain of 10. This signal is fed to the panel meter through the range amplifier andinto 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 voltageat the – input terminal. The microvoltmeter signal is compared to the sawtooth ramp voltage by this circuit. Sincethe 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 thethermocouple will receive cooling current through the electronic switching circuits. When the ramp voltagebecomes higher than the signal voltage, the output of the comparator switches to “low,” and the cooling currentis discontinued. The microvoltmeter indicates the thermocouple voltage during this interval, and the sample holdcircuit maintains the signal level from one reading interval to the next. The negative pulses produced by thesawtooth/pulse generator circuit are added to the thermocouple signal at the + input of the comparator. Thesepulses represent a 0.05 duty cycle, and limit the cooling duty cycle to a maximum of 0.95, ensuring a minimumread 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 A v SET control such that for a given thermocouple, thecooling duty cycle, as dictated by the thermocouple temperature depression signal, will remove precisely thatamount of heat that is flowing into the junction from its surroundings. If the water on the junction then causesa movement of the junction temperature toward the dew point, the output signal will change, dictating acorresponding change in the cooling duty cycle, thus maintaining a balance of thermal energy being conductedinto and out of the junction.To further illustrate, Figure 6 depicts the temporal relationship between the various signal and control waveformsand 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 A v SET control to match the thermocouple coolingcoefficient, A v . For another junction having a different cooling coefficient, a different sawtooth height willaccordingly be set in the instrument.At output levels below approximately 0.75 µvolts (-1 bar), the cooling duty cycle becomes so small that thefrequency response characteristics of the processing circuitry begin to affect the linearity of the system, such thatautomatic dew point operation cannot be maintained in this range.25
Page 1 and 2: HR-33T Dew Point MicrovoltmeterInst
Page 3: CONTENTSSECTION 1 - INSTRUMENT SPEC
Page 7: SECTION 1 - INSTRUMENT SPECIFICATIO
Page 11 and 12: A v SET control. The numerical valu
Page 13 and 14: Output For Dew Point, Psychrometric
Page 15 and 16: 11Temperature Correction CurveFigur
Page 17 and 18: SECTION 3 - ADDITIONAL OPERATING IN
Page 19 and 20: 3.3 - Effect of Zero Drift on Readi
Page 21 and 22: SECTION 4 - THEORY OF OPERATION4.1
Page 23 and 24: 4.2 - Psychrometric MethodThermocou
Page 25 and 26: Heat transfer due to Peltier coolin
Page 27: 23Theoretical vs. Actual Instrument
Page 31 and 32: 27Typical Control and Signal Wavefo
Page 33 and 34: SECTION 6 - MAINTENANCE6.1 - Genera
Page 35 and 36: SECTION 7 - TROUBLESHOOTINGIf the i
Page 37 and 38: Output Characteristics for Contamin
Page 39 and 40: HR-33T Schematic DiagramFigure 835
Page 41 and 42: Test Point LocationsFigure 1037
Page 43 and 44: Front Panel AssemblyFigure 1239
Page 45 and 46: SECTION 8 - PARTS LIST5106 - 110 Vo
Page 47 and 48: Front PanelRef. No. Part No. Qty. U
Page 49: 5112 Battery PackRef. No. Part No.

HR-33T Dew Point Microvoltmeter Instruction/Service Manual ...

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