Temperature is measured with precision interchangeable
thermistors that provide highly accurate and stable temperature readings.
The thermistors use couple glass hermetic encapsulation with 100-percent
resistance shift screening. The components have tight interchangeability,
providing precise measurements without calibration of circuitry to match
individual components. The thermistors are designed to eliminate the typical
problem of sensor drift, a common problem in maintaining sensor accuracy
A thermistor is a thermally sensitive resistor composed
of mixed metal
oxide semiconductors. The temperature ranges are defined using Negative
Temperature Coefficient of resistance (NTC). NTC thermistors exhibit a
steep drop in resistance as temperature increases. Four-hundred ohms per
change in degree C on average after calibration was observed on the 10,000
ohm thermistors. This difference provides high sensitivity to temperature
The manufacturer-rated accuracy of the thermistors is
0.09 degrees Fahrenheit;
however, they have been calibrated to 0.008 deg F, for the full range of
values, with high-end calibration equipment and a NIST traceable procedure.
The calibration procedure includes triple point of water and gallium melting
Thermistors for the cooling tower and outside air wet
temperatures are enclosed in an enthalpy wet bulb (EWB) unit, a self-contained
aspirated enclosure, shown below. The EWB enclosure contains wet
and dry bulb wells, a pressure-compensated float valve assembly, and a
fan, which draws air over the thermistors. The wick is bathed continuously
by distilled water (stored in a reservoir above the sensor) as moisture
evaporates. The unit is designed to be reliable and low-maintenance.
Go to the Weather Station for more about
the EWB unit.