Operator`s manual

SECTION 13. 21X MEASUREMENTS
13-12
THERMOCOUPLE LIMITS OF ERROR
The standard reference which lists
thermocouple output voltage as a function of
temperature (reference junction at 0
o
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 21X.
TABLE 13.4-1. Limits of Error for Thermocouple Wire (Reference Junction at 0
o
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%
When both junctions of a thermocouple are at
the same temperature, there is no voltage
produced (law of intermediate metals). A
consequence of this is that a thermocouple
cannot have an offset error; any deviation from
a standard (assuming the wires are each
homogeneous and no secondary junctions
exist) is due to a deviation in slope. In light of
this, the fixed temperature limits of error (e.g.,
±1.0
o
C for type T as opposed to the slope error
of 0.75% of the temperature) in the table above
are probably greater than one would experience
when considering temperatures in the
environmental range. In other words, the
reference junction, at 0
o
C, is relatively close to
the temperature being measured, so the
absolute error (the product of the temperature
difference and the slope error) should be closer
to the percentage error than the fixed error.
Likewise, because thermocouple calibration
error is a slope error, accuracy can be
increased when the reference junction
temperature is close to the measurement
temperature. For the same reason, differential
temperature measurements, over a small
temperature gradient, can be extremely
accurate.
In order to quantitatively evaluate thermocouple
error when the reference junction is not fixed at
0
o
C, one needs limits of error for the Seebeck
coefficient (slope of thermocouple voltage vs.
temperature curve) for the various
thermocouples. Lacking this information, a
reasonable approach is to apply the percentage
errors, with perhaps 0.25% added on, to the
difference in temperature being measured by
the thermocouple.
ACCURACY OF THE THERMOCOUPLE
VOLTAGE MEASUREMENT
The accuracy of a 21X voltage measurement is
specified as 0.1% (0.05% 0 to 40
o
C) of the full
scale range being used to make the
measurement. The actual accuracy may be
better than this as it involves a slope error (the
error is proportional to the measurement being
made though limited by the resolution). The
error in the temperature due to inaccuracy in the
measurement of the thermocouple voltage is
worst at temperature extremes, where a
relatively large scale is necessary to read the
thermocouple output. For example, assume
type K (chromel-alumel) thermocouples are
used to measure temperatures at 1000
o
C. The
TC output is on the order of 40mV, requiring the
±50mV input range. The accuracy specification
of 0.05% FSR is 25µV which is a temperature