What effect does cable length have on the thermistor reading?
The AWG 22 copper wire in signal cable for piezometers has a nominal resistance of 16 ohms per 1000 feet. The round trip of the wire-pair to and from the thermistor effectively doubles this value to 32 ohms. The thermistor used in our sensors, as well as in those of most other instrument manufacturers, provides a resistance of 3,000 ohms at 25 degrees C. The thermistor is non-linear, however. (See manufacturer’s chart). At 25 degrees C, 32 ohms represents about 0.24 degree C, and at 10 degrees C, 32 ohms represents about 0.11 degree C. In other words, the effect of cable length has less effect as the temperature decreases. In either case, though, correction for cable length is probably not needed.
