Module SENSORM :: THERMISTOR
Convert resistance measurements to the corresponding
temperature for a thermistor device.
Syntax
THERMISTOR( RIN, VMODEL, TEMPOUT )
Parameters
 RIN
 Input resistance values in ohms
 FLOAT PIPE
 VMODEL
 Coefficients for Steinhart and Hart thermistor model
 FLOAT VECTOR
 TEMPOUT
 Output data pipe, temperatures in degrees Centigrade
 FLOAT PIPE
Description
The THERMISTOR command converts resistance
values received from the RIN pipe, using the
device model specified by the VMODEL vector,
and delivering the corresponding output temperatures in
degrees C to the TEMPOUT pipe.
The device model used is the SteinhartHart equation.
This equation is an empirical formula capable of conversion
accuracy within a small fraction of a degree. There are no
standardized conversion curves, but most thermistor
manufacturers will provide SteinhartHart coefficient values
that work over a suitable range with their devices.
The form of the Steinhart and Hart equation is
T = 1.0 / ( a + b ln(R) + c ln^{3}(R) )
where R is the measured device resistance in
ohms, and the a, b, and c terms
are the model coefficients. The THERMISTOR command
uses a slight variation of this
T = 1.0 / ( a + b ln(R) + c ln^{3}(R) )  273.16
where the final 273.16 term converts the
temperature units from Kelvins to degrees Centigrade. For
maximum conversion accuracy, you can calibrate the curve for the
individual device you are using. Measure the actual resistance
at three wellselected temperature points representative of your
operating range, insert these values into the SteinhartHart
equation form, and solve for the coefficient values.
To support a very wide temperature range, the
THERMISTOR command supports a multiplepiece device model.
Each piece of the model is encoded into the VMODEL
parameter as a breakpoint temperature in degrees C,
followed by the three SteinhartHart coefficients to use at that
temperature and beyond. The model pieces are encoded in order
from lowest temperature range to highest temperature range. The
first piece is the default that provides conversions at low
temperatures, so its breakpoint term is ignored and can be set
to zero. Most applications will use a onepiece model.
Multiplepiece models only work for negative temperature
coefficient thermistors, which are the ones that most applications
use.
Example
VECTOR THERM44007 FLOAT = ( 0.0, 1.285E3, 2.362E4, 9.285E8 )
...
THERMISTOR(PRESIST, THERM44007, TEMPR4)
Manufacturerprovided coefficients for a model 44007
thermistor are used, and specified in the vector
THERM44007. Because this is a onepiece model, the
temperature breakpoint between pieces is unused, and the
first term is set to 0. Measurements of the temperaturedependent
thermistor device resistance in ohms are obtained from pipe
PRESIST. Each resistance value is converted to the
corresponding operating temperature of the thermistor using the
SteinhartHart equation with the coefficients from the
THERM44007 vector, producing the corresponding
temperature results in degrees C. The temperature results are
placed into pipe TEMPR4.
See also:
DIVIDER, BRIDGE
