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of 0.05%
error of
appropriate
the voltage
of a degree.
is
25pV
which
is a
temperature
O.6oC.
In
the environmental
with voltage measured on an
error
in
temperature due to
rements
is a
few hundredths
SECTION
13.
21X MEASUREMEI{TS
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
300oC. The compensation voltage calculated
by
the 21X corresponds to
a
temperature of
272.6oC, a
-27.4oC
error.
The type
T
thermocouple with
the measuring junction
at
29OoC
and reference at
3O0oC
would output
-
578.71tY;
using
the reference temperature of
272.6oC,
the 21X
calculates
a
temperature
difference of
-10.2oC,
a
-O.2oC
error. The
temperature
calculated
by
the 21X
would
be
262.4oC,27.6oC
low.
TABLE 13.4-3. Reference Temperature
Compensation Range and Polynomial Error
Relative
to
NBS
Standards
THERMOCQUPLE
POLYNOMIALS: Voltage
to Temperafure
NBS
Monogfaph 125 gives high order
polynomials for computing the output voltage of
a given
therinocouple type over a broad range
of
temperatqres.
In
order to speed processing
and accomnhodate
the 21X's math and storage
capabilities,
4 separate 6th order polynomials
TC
Type
T
are used to
from volts to
temperature
over
the
covered by each thermocouple
type.
Table 13.4-2 gives error limits for the
polynomials.
TABLE 13.4-2. Limits of Error on
21X
Thermocorlple Polynomials (Relative to NBS
Standards)
Ra4ge
oC
Limits of Error
oC
-27q
-27q
-2AA
-10q
100
-150
-100
760
300
1000
-130
200
1000
*0.008
*0.002
r0.4
*0.005
t0.02
to
400
to
-200
+18@
-270
to
-100
+0.08
to 100
+0.001
to 400
+0.015
E
.24Q
-24Q
_130
20Q
TC
Type
T
J
E
K
to
to
to
to
to
to
to
to
to
1372
950
+0.01
1372
t0.04
JUNCTION COMPENSATION:
to Voltage
The polynoinials used for reference junction
compensatipn (converting reference
temperaturE to equivalent TC output voltage) do
not
cover
tl'p
entire
thermocouple range.
Substantial
lerrors
will result
if
the reference
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 ternperature determined with
the
Range
oC
-100
to
100
-150 to 296
-150 to
206
-50
to
100
Limits of Error
oC
r
0.001
r
0.005
t
0.005
+ 0.01
REFERE
13-13
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