Campbell Scientific CR23X Micrologger - Manual (Page 217)

SECTION 13. CR23X MEASUREMENTS
13-17
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 Institute of Standards and Technology
Monograph 175 (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 CR23X.
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°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°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°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 CR23X voltage
measurement is specified as
±
0.05% (
±
0.025%
0 to 40°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°C. The TC output is on
the order of 40 mV, requiring the ±50 mV input
range. The accuracy specification of 0.025%
FSR is 25 µV which is a temperature error of
about 0.6°C (
±
(.00025)(100 mV)=25 µV). In the
environmental temperature range with voltage
measured on an appropriate scale, error in
temperature due to the voltage measurements
is a few hundredths of a degree.
THERMOCOUPLE POLYNOMIALS: Voltage
to Temperature
NIST Monograph 175 gives high order
polynomials for computing the output voltage of
a given thermocouple type over a broad range
of temperatures. In order to speed processing
and accommodate the CR23X's math and
storage capabilities, 4 separate 6
th
order
polynomials are used to convert from volts to
temperature over the range covered by each
thermocouple type. Table 13.4-2 gives error
limits for these thermocouple polynomials.