c) DELTA
d) BETA (only calculated if criteria explained above is met)
6.
Residuals are calculated for each set-point used in calculating the coeffi-
cients. Residuals are always in the same scale as the Reference Probe
readings.
3.7.4
Polynomial Coefficients
The following requirements must be met when Polynomial is selected as the
type of coefficients to calculate for Platinum or Thermistor probes. For more
information concerning the method used to calculate the Polynomial coeffi-
cients, see Section 3.8, Methods Used for Calculating Coefficients.
1.
The Reference Probe readings must be in temperature (°C, °F or K).
Note
: The accepted thermistor model is based upon the logarithmic resis-
tance-temperature characteristic in terms of absolute temperature (Kel-
vin). The accuracy of a model based upon °F or °C has not been tested.
Therefore, TableWare always calculates polynomial coefficients for therm-
istor probes using Kelvin. If Reference readings are entered in °F or °C,
TableWare converts the readings to their Kelvin equivalent values before
the fit is computed.
2.
The UUT (test probe) readings must be in resistance (Ohms or KOhms).
3.
The order of the polynomial must be selected. Allowable choices are
from 3rd order to 9th order for platinum probes, and from 3rd to 6th or -
der for thermistor probes.
4.
For thermistor probes, the Steinhart-Hart method may be selected. Stud-
ies have indicated no significant loss in accuracy when using the
Steinhart-Hart method (eliminating the squared term) over the range of
–2°C to 70°C (28°F to 158°F). The formula used when the
Steinhart-Hart method is selected is:
1
T
a
bln R
dln R
T
T
= +
+
(
)
(
)
For more information on this topic, refer to “Worldwide Capability in Thermis-
tors”, Thermometrics, Inc., 1993, pages 14–15.
5.
For normal calculations, at least 4 set-points must be used but no more
than 10 set-points can be used.
6.
For Steinhart-Hart calculations, only 3 set-points should be used.
7.
The minimum number of set-points used must exceed the order of the
polynomial by at least 1 (i.e. for a 5th order polynomial, at least 6
set-points must be used).
8.
The following Polynomial coefficients are calculated:
41
3 Coefficients menu
Requirements for calculating coefficients
