Campbell Scientific CR7 Measurement and Control System - Manual (Page 137)

SECTION 13. CR7 MEASUREMENTS
13-13
In order to quantitatively evaluate thermocouple
error when the reference junction is not fixed at
0 oC, 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 CR7 voltage measurement is
specified as 0.02% (0.01% 0 to 40 oC) 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 oC.
The TC output is on the order of 40mV,
requiring the ±50mV input range. The accuracy
specification of 0.01% FSR is 10µV which is a
measurement error of about 0.2 oC. 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 POLYNOMIAL: Voltage to
Temperature
NBS Monograph 125 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 CR7's math and storage
capabilities, 4 separate 6th 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 the
thermocouple polynomials.
TABLE 13.4-2. Limits of Error on CR7
Thermocouple Polynomials (Relative to NBS
Standards)
TC Limits of
Type Range oC Error oC
T -270 to 400
-270 to -200 +18 @ -270
-200 to -100 ±0.08
-100 to 100 ±0.001
100 to 400 ±0.015
J -150 to 760 ±0.008
-100 to 300 ±0.002
E -240 to 1000
-240 to -130
±0.4
-130 to 200 ±0.005
200 to 1000 ±0.02
K -50 to 1372
-50 to
950 ±0.01
950 to 1372 ±0.04
REFERENCE JUNCTION COMPENSATION:
Temperature to Voltage
The polynomials used for reference junction
compensation (converting reference
temperature to equivalent TC output voltage) do
not cover the entire thermocouple range.
Substantial errors will result if the reference
junction temperature is outside of the
linearization range. The ranges covered by
these linearizations include the CR7
environmental operating range, so there is no
problem when the CR7 is used as the reference
junction. External reference junction boxes
however, must also be within these temperature
ranges. Temperature difference measurements
made outside of the reference temperature
range should be made by obtaining the actual
temperatures referenced to a junction within the
reference temperature range and subtracting as
described in Section 7.5. Table 13.4-3 gives
the reference temperature ranges covered and
the limits of error in the linearizations within
these ranges.
Two sources of error arise when the reference
temperature is out of range. The most
significant error is in the calculated
compensation voltage, however error is also
created in the temperature difference calculated