Readout Procedures; Initial Readings; Input Voltage; Converting To Pressures - Geokon 3400 Series Instruction Manual

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5. READOUT PROCEDURES

Connect the piezometer to the readout instrument using the appropriate wiring chart given in
Appendix C.

5.1 Initial Readings

Initial readings must be taken and carefully recorded along with the barometric pressure
and temperature at the time of installation. Follow the instructions of Section 4.1.

5.2 Input Voltage

The Model 3400 Piezometer uses a semiconductor strain gage type transducer with an output of
either 0-100mV (Model 3400-1), 0-5 volts (Model 3400-2), or 4-20 mA (Model 3400-3).
For the 100mV type, the output voltage is directly proportioned to both pressure and input
voltage, therefore it is very important that the input voltage be accurately controlled @ 10V DC.
If any other voltage is used, the gage factor G must be adjusted accordingly in the manner shown
on the calibration report. The 0-5 volt and 4-20mA sensors require an unregulated input of 7-35
VDC.

5.3 Converting to Pressures

Formulae for converting readout voltages to pressure are shown on the calibration reports. Both
linear and polynomial expressions are shown. For better accuracy, the polynomial expression
should be used with a proviso that the value for the C coefficient be derived in the field by taking
an initial reading when the sensor is subject to atmospheric pressures only as described in
Section 4.1. Then substituting this initial value into the formula and setting the value of P to zero
will yield the correct value for C.

5.4 Measuring Temperatures

Each piezometer is equipped with a thermistor for reading temperature. The thermistor gives a
varying resistance output as the temperature changes. Appendix C shows which cable conductors
are connected to the thermistor. These conductors should be connected to a digital ohmmeter.
To read temperatures using an ohmmeter:
1) Connect an ohmmeter to the green and white thermistor leads coming from the strain gage.
Since the resistance changes with temperature are large, the effect of cable resistance is
usually insignificant. For long cables a correction can be applied, equal to approximately
14.7 Ω per one thousand feet (48.5Ω per km). Multiply this factor by two to account for both
directions.
2) Look up the temperature for the measured resistance in Appendix B, Table 5.