DAPL Operating System | Processing Command
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 Steinhart-Hart 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 Steinhart-Hart 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 ln3(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 ln3(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 well-selected temperature points representative of your
operating range, insert these values into the Steinhart-Hart
equation form, and solve for the coefficient values.
To support a very wide temperature range, the
THERMISTOR command supports a multiple-piece device model.
Each piece of the model is encoded into the VMODEL
parameter as a break-point temperature in degrees C,
followed by the three Steinhart-Hart 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 break-point term is ignored and can be set
to zero. Most applications will use a one-piece model.
Multiple-piece models only work for negative temperature
coefficient thermistors, which are the ones that most applications
use.
Example
VECTOR THERM44007 FLOAT = ( 0.0, 1.285E-3, 2.362E-4, 9.285E-8 ) ... THERMISTOR(PRESIST, THERM44007, TEMPR4)
Manufacturer-provided coefficients for a model 44007
thermistor are used, and specified in the vector
THERM44007. Because this is a one-piece model, the
temperature breakpoint between pieces is unused, and the
first term is set to 0. Measurements of the temperature-dependent
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
Steinhart-Hart 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