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SECTION
13.
21X MEASUREMENTS
THERMOCOUPLE LIMITS OF ERROR
The
standard reference which lists
thermocouple output voltage as
a
function
of
temperature (reference junction at
OoC)
is
the
National Bureau of Standards Monograph
125
Thermocouple
Type
T
Temperature
Range oC
-200 to
0
0
to
350
0
to
750
-200 to
0
0
to
900
-200 to
0
0
to
1250
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.9.,
*1.OoC
for type T as opposed to the slope
error
of
O.75oh
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
OoC,
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.
(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
OoC)
Limits of Error
(Whichever is greater)
E
K
Standard
+
1.OoC
or
1.5o/"
r
1.OoC
or
O.75"/o
+,2.2oC
or
0.75"/"
x.1.7oC
or
1.Ooh
t
1.7oC
or 0.5%
*2.2oC
or
2.Oo/o
x.2.2oC
or
0.75"/"
Special
+ 0.5oC
or
O.4o/o
+
1.1oC
orO.4o/o
i
1.OoC
or
O.4o/o
+
1.1oC
or 0.47o
In
order to quantitatively evaluate thermocouple
error when
the reference junction is
not
fixed at
OoC,
one needs limits of error for the Seebeck
coefficient (slope
of
thermocouple voltage
vs.
temperature curue) for the various
thermocouples. Lacking this information,
a
reasonable approach
is
to apply
the percentage
errors,
with perhaps
O.25o/o
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
40oC) 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
proportionalto the measurement being
made
though limited by the resolution). The
error
in
the temperature due
to inaccuracy
in
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 1000oC. The
TC output
is
on the order of
40mV, requiring the
*50mV
input
range.
The accuracy specification
13-12
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