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1560 Thermometer Readout
User's Guide
12.2.1
148
Internal CJC temperature coefficient
Sample time
Number of channels
Recommended operating temperature range
Absolute operating temperature range
Weight
1
The accuracy specifications apply within the recommended operating temperature range. Accuracy limits
are increased by a factor of the temperature coefficient outside this range.
Short-term accuracy includes nonlinearity and noise uncertainties. It does not include long-term drift or
2
calibration uncertainties.
Temperature accuracies do not include sensor uncertainties. Accuracies using external CJC do not in-
3
clude cold-junction reference uncertainty.
Calculating Accuracy
The temperature accuracy specifications listed are derived from the voltage ac-
curacy and CJC accuracy. The accuracy of a thermocouple readout device in
terms of temperature depends on the slope of the voltage-temperature relation-
ship (Seebeck coefficient) at the temperature of interest (which depends on the
type of thermocouple and its temperature), the accuracy of the voltage mea-
surement, the slope of the voltage-temperature relationship at the cold-junction
temperature, and the accuracy of the cold-junction temperature according to the
following equation:
u
S t
(
)
=
+
v
CJ
u
u
t
CJ
S t
( )
S t
( )
For the 2565/2566 modules the voltage accuracy is specified. If internal CJC is
used, its accuracy is also specified. If external CJC is used, it is up to the user
to determine the accuracy of the cold-junction temperature. The temperature
accuracies listed above for external CJC are assuming negligible cold-junction
temperature error.
Here is an example of how temperature accuracy is calculated. Suppose we are
measuring with a type T thermocouple at –200°C and using internal CJC. At
–200°C a T thermocouple has a Seebeck coefficient of about 0.016 mV/°C, and
at about 25°C (the cold-junction temperature) the Seebeck coefficient is about
0.041 mV/°C. From the specifications, the voltage accuracy of the 2565 is
0.002 mV (at the voltage we are measuring) and the CJC accuracy is 0.05°C.
Applying the above equation for this situation gives a temperature accuracy of
0.253°C:
0 041
.
0 002
.
mV
=
+
u
t
mV
0 016
.
0 016
.
°
C
1
1
mV
°
°
=
° +
C
0 05
.
C
0125
.
C
0128
mV
°
C
0.005°C/°C (0.005°F/°F)
2 seconds
2
18 to 28°C
(64 to 82°F)
5 to 35°C
(40 to 95°F)
2.5 lbs.
°
=
°
.
C
0 253
.
C
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