Limits Of Error For Thermocouple Wire - Campbell Measurement and Control Module CR10 Operator's Manual

13.4.1 ERROR ANALYSIS
The error in the measurement of a
thermocouple temperature is the sum of the
errors in the reference junction temperature, the
thermocouple output (deviation from standards
published in NBS Monograph 125), the
thermocouple voltage measurement, and the
polynomial error (difference between NBS
standard and CR10 polynomial
approximations). The discussion of errors
which follows is limited to these errors in
calibration and measurement and does not
include errors in installation or matching the
sensor to the environment being measured.
REFERENCE JUNCTION TEMPERATURE
The junction which is created when a
thermocouple is wired to the wiring panel is
referred to as the reference junction. The
temperature of the reference junction must be
known in order to calculate the absolute
temperature of the measuring junction. The
CR10TCR Thermocouple Reference
Temperature (CR10TCR) is used to measure
the temperature of the reference junction
(terminal strips).
The CR10TCR uses a thermistor to measure the
relative temperature of the terminal strips. The
accuracy of this measurement is a combination of
the thermistor's interchangeability specification, the
precision of the bridge resistors, and the polynomial
error. In a "worst case" example, all errors add to
TABLE 13.4-1. Limits of Error for Thermocouple Wire (Reference Junction at 0°C)
Thermocouple
Type
T
J
E
K
yield a ±0.4°C error in the range of -24°C to +48°C
and ±0.9°C over the range of -38°C to +53°C. It is
emphasized that this is the worst case. Campbell
Scientific's experience shows that the overall
accuracy is typically better than ±0.2°C. The major
error component in the -0°C to +50°C range is the
±0.2°C thermistor interchangeability specification.
When a CR10 is outside of this temperature range,
the polynomial error becomes much worse (Figure
13.4-1), and may necessitate the use of an external
reference junction to improve the accuracy.
If the terminal that the thermocouple is wired into
is at a different temperature than the CR10TCR
thermistor, this difference in temperature
becomes an error in the thermocouple
temperature measurement. With the CR10 in
one of the enclosure options (Section 14) this
error will generally be less than 0.3°C.
THERMOCOUPLE LIMITS OF ERROR
The standard reference which lists
thermocouple output voltage as a function of
temperature (reference junction at 0°C) is the
National Bureau of Standards Monograph 125
(1974). The American National Standards
Institute has established limits of error on
thermocouple wire which is accepted as an
industry standard (ANSI MC 96.1, 1975). Table
13.4-1 gives the ANSI limits of error for
standard and special grade thermocouple wire
of the types accommodated by the CR10.
Temperature
Range °C
-200 to 0 ± 1.0°C or 1.5%
0 to 350 ± 1.0°C or 0.75%
0 to 750 ± 2.2°C or 0.75%
-200 to 0 ± 1.7°C or 1.0%
0 to 900 ± 1.7°C or 0.5%
-200 to 0 ± 2.2°C or 2.0%
0 to 1250 ± 2.2°C or 0.75%
SECTION 13. CR10 MEASUREMENTS
Limits of Error
(Whichever is greater)
Standard
± 0.5°C or 0.4%
± 1.1°C or 0.4%
± 1.0°C or 0.4%
± 1.1°C or 0.4%
Special
13-13