WattMaster WM-WCC3-TGD-01B - Manual (Page 161)

WCC III Technical Guide 3-47
WCC3.EXE SCREEN DESCRIPTIONS
Thermistor+ Screen
The Thermistor+ Screen is to be used if you are using a thermistor
type sensor that WattMaster Controls has not provided. The
example provided here is for a BAPI™ compatible Alerton™
standard 3K @ 77 Deg F Thermistor Sensor. Other thermistor
type sensors can be connected to the Satellite controller’s analog
inputs with this screen. Please consult the WattMaster Controls
factory for all other thermistor type sensors that you wish to
connect to the Satellite Controller. WattMaster Controls can
provide the correct b0, b1, b3 values at our option. Also note that
the associated analog input jumper on the Satellite controller must
also be set to “THERM” in order for this screen to display the
proper temperature value.
A thermistor is a type of resistor whose resistance varies with
temperature. Thermistor sensors differ from resistance temperature
detectors (RTD) in that the material used in a thermistor sensor is
generally a ceramic or polymer type of material, while RTD type
sensors use pure metals.
Advantages of Using a Thermistor
Sensor
The typical interchangeability tolerance (Inherent Accuracy) for
any thermistor sensors are: ± 0.2 °C (0 to 70 °C)
Thermistor Stability (Drift)
Thermistor stability (drift) is the amount that the resistance
characteristics of a thermistor will change. WattMaster Controls
uses only the highest quality, “pre-aged” thermistor with very small
drift values. Over a ten-year span, WattMaster Controls thermistor
products will not change (drift) more than 0.1°C.
Operating ranges:
Typical maximum operating ranges for thermistor type sensors are
-55 to 150 °C (-67 to 302 °F)
Disadvantages of Using a Thermistor
Sensor
The only real disadvantage to using a thermistor type sensor is
that there are many different resistance types and corresponding
different temperature curves for different manufacturers of
thermistor temperature sensors. The temperature curve of a
thermistor is non-linear in nature.
To correctly calculate the temperature curve of a thermistor,
WattMaster Controls uses a formula based on the industry standard
Steinhart-Hart Equation. These formulas and resultant calculations
reside within the Satellite controller as the Satellite controller’s
CPU actually performs these resistance to temperature calculations
many times per second.
The Steinhart-Hart Equation
The three-term Steinhart-Hart equation (Equation 1) is the
most popular equation that is used for thermistor resistance to
temperature calculations. There are three coeffi cients values that
are needed for correct thermistor Resistance to Temperature sensor
calculations: b0, b1, and b3.
These three coeffi cients values should be provided by the thermistor
manufacturer for each possible type of thermistor sensor that is
used, but WattMaster Controls can also provide the correct b0, b1,
b3 values at our option.
= A + Bln (R) + C(ln(R)) 3
Where T is the temperature in Kelvins, R is the resistance at T in
ohms, and A, B, and C are the Steinhart-Hart coeffi cients which
vary depending on the type and model of thermistor and the
temperature range of interest. The temperature in Kelvins is then
converted to Fahrenheit or Celsius within the satellite controller.
1
T
Analog Input Thermistor+ Screen