Campbell Scientific CR10X Measurement and Control System - Manual (Page 205)

SECTION 13. CR10X MEASUREMENTS
13-13
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 CR10X 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 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
yield a
± 0.4
°C error in the range of -24°C to +48°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 CR10X 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 CR10X 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
NIST Monograph 175. 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 CR10X.
TABLE 13.4-1. Limits of Error for Thermocouple Wire (Reference Junction at 0
°°°°C)
Limits of Error
Thermocouple Temperature (Whichever is greater)
Type Range oC Standard Special
T -200 to 0 ± 1.0
o
C or 1.5%
0 to 350 ± 1.0
o
C or 0.75% ± 0.5
o
C or 0.4%
J 0 to 750 ± 2.2
o
C or 0.75% ± 1.1
o
C or 0.4%
E -200 to 0 ± 1.7
o
C or 1.0%
0 to 900 ± 1.7
o
C or 0.5% ± 1.0
o
C or 0.4%
K -200 to 0 ± 2.2
o
C or 2.0%
0 to 1250 ± 2.2
o
C or 0.75% ± 1.1
o
C or 0.4%
R or S 0 to 1450 ± 1.5
o
C or 0.25% ± 0.6
o
C or 0.1%
B 800 to 1700 ± 0.5% Not established
N <0 Not established
0 to 1250 ± 2.2
o
C or 0.75% ± 1.1
o
C or 0.4%