Operation Manual

Model 2750 Multimeter/Switch System User’s Manual Temperature Equations F-7
Selecting a thermistor — The thermistor’s manufacturer’s specified curve fitting values
(A, B, and C) may not be exactly the same as the ones used by the Model 2750. If they are
not exactly the same, perform the following steps to select a thermistor to use with the
Model 2750:
NOTE The specified thermistor temperature measurement accuracy of the Model 2750
(see Appendix A) is based on the curve fitting constants listed in Table F-9. If the
thermistor manufacturers curve fitting constants are not exactly the same as the
ones listed in Table F-9, accuracy will be affected.
1. Choose the thermistor type to be used; 2252Ω, 5kΩ, or 10kΩ.
2. Compare the A, B, and C constants from the thermistor manufacturer with those
used by the Model 2750 (see Table F-9).
3. Select a thermistor that closely matches the A, B, and C constants in Table F-9.
4. Analyze the differences between the two sets of curve fitting constants to deter-
mine the affect on measurement accuracy.
Converting K to °C — The temperature in Kelvin can be converted to °C as follows:
T
°C
= T
K
- 273.15
where: T
°C
is the temperature in °C.
T
K
is the calculated Kelvin temperature.
Example — Calculate the temperature for a Series 44007 thermistor that measures
5kΩ (R):
lnR = ln(5000) = 8.5172
A = 0.001288
B = 0.0002356
C = 9.557e-8
T
K
= 1 / {A + (BlnR) + [(C)(lnR)
3
]}
= 1 / {0.001288 + [(0.0002356)(8.5172)] + [(9.557e-8)(8.5172
3
)]}
= 1 / (0.001288 + 0.002007 + 0.000059)
= 1 / 0.003354
= 298.15
T
°C
= T
K
- 273.15
= 298.15 - 273.15
= 25°C
2750-900-01.book Page 7 Wednesday, August 3, 2011 7:56 AM