Thermistors provide accurate and reliable long-term temperature measurements and are used widely in the extremely harsh environments found within Geotechnical monitoring.
They are available in two types:
- Probe – A single point sensor mounted within a PVC or stainless steel housing which is attached to a cable length.
- String – A series of sensors mounted along a multi-core cable which provide a temperature profile and is manufactured to customer requirements in terms of the number and spacing of each sensor.
The NTC (negative temperature coefficient ) thermistor sensor has a resistance that decreases with increasing temperature and with a coefficient >4%/ºC allows it to detect very small changes in temperature. They have a non-linear output that is represented by the Steinhart–Hart equation.
T= (1 / (A + B (LnR) + C(LnR) 3)) –273.2
Where: T = Temperature in degrees Centigrade
LnR= Natural log of Thermistor resistance in ohms
Readings can be made with a wide range of readout units including the VW2106 and the MP12 which display the reading directly in engineering units (degrees Celsius) or by an ohmmeter in combination with look-up tables. Readings can also be automated using an automatic data acquisition unit.