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4.4 Measuring Temperatures
All vibrating wire piezometers are equipped with a thermistor that gives a varying resistance
output as the temperature changes. The white and green leads of the instrument cable are
normally connected to the internal thermistor.
The GK-403, GK-404, and GK-405 readout boxes will read the thermistor and display the
temperature in degrees C. (High temperature versions use a different thermistor, which must be
read using an 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 14.7 ohms per
one thousand feet. Multiply this factor by two to account for both directions.)
2)
Look up the temperature for the measured resistance in Appendix B, Table 7. For high
temperature models use Appendix C,
Table 8.
4.5 Checking the Calibration
The following procedure can be used to verify the calibration factor as supplied on the
calibration sheet:
1)
Saturate the filter stone and fill the space between it and the diaphragm with water (see
Section 2.1).
2)
Lower the piezometer to the bottom of a water-filled borehole using the cable to measure the
actual depth.
3)
Allow 15 to 20 minutes for the piezometer to come to thermal equilibrium.
4)
Using a readout box record the reading at that level (see Section 4 for readout instructions).
5)
Raise the piezometer a known amount and record the reading.
6)
Calculate the calibration factor using the change in pressure and the reading.
7)
Compare to the calibration sheet value. The two values should agree within
±
0.5%.
When doing this test please be aware that the actual water level inside the borehole might change
due to displacement of water by the different lengths of submerged cable. This is especially
critical where the cable length is long and the borehole diameter small. Allowing sufficient time
for the water level to equilibrate may solve this problem, or keep the borehole filled to the top.
