Operator`s manual

SECTION 13. 21X MEASUREMENTS
13-13
error of about 0.6
o
C. 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
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 21X'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 21X
Thermocouple Polynomials (Relative to NBS
Standards)
TC
Type Range
o
C Limits of Error
o
C
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
polynomial range. The ranges covered by
these polynomials include the 21X
environmental operating range, so there is no
problem when the 21X 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.
Table 13.4-3 gives the reference temperature
ranges covered and the limits of error in the
polynomials 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
from the thermocouple output. For example,
suppose the reference temperature for a
measurement on a type T thermocouple is
300
o
C. The compensation voltage calculated
by the 21X corresponds to a temperature of
272.6
o
C, a -27.4
o
C error. The type T
thermocouple with the measuring junction at
290
o
C and reference at 300
o
C would output -
578.7µV; using the reference temperature of
272.6
o
C, the 21X calculates a temperature
difference of -10.2
o
C, a -0.2
o
C error. The
temperature calculated by the 21X would be
262.4
o
C, 27.6
o
C low.
TABLE 13.4-3. Reference Temperature
Compensation Range and Polynomial Error
Relative to NBS Standards
TC
Type Range
o
C Limits of Error
o
C
T -100 to 100 ± 0.001
J -150 to 296 ± 0.005
E -150 to 206 ± 0.005
K -50 to 100 ± 0.01
ERROR SUMMARY
The magnitude of the errors described in the
previous sections illustrate that the greatest
sources of error in a thermocouple temperature
measurement are likely to be due to the limits of
error on the thermocouple wire and in the
reference temperature determined with the
built-in thermistor. Errors in the thermocouple