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polynomial approximation
of
the NBS TC
calibrations.
l
13.4.1
ERROR
ANALYSIS
The
error
in
lhe
measurement of a
thermocouple
temperature
is
the sum of the
errors
in
the feference junction temperature, the
thermocouplb
output (deviation from standards
published
in
Monograph 125), the
voltage measurement, and the
polynomial
(difference between NBS
standard
an{
21X polynomial approximations).
The
of errors which
follows is limited
to
these errdrs
in
calibration and measurement
and
does not include errors in installation or
matching
thq sensor to the environment being
measured.
REFERENCE
JUNCTION TEMPERATURE
WITH
21X PANEL
The
21X parfeltemperature is measured with a
Fenwal Electronics
UUT51
J1
thermistor
mounted
in
fre
center of the
analog input
terminal
stri$. The
accuracy of this
measuremerirt
is
a
combination of Fenwal's
interchange{bility specification, the precision of
the bridge
rQsistors,
and
the polynomial error.
ln
a "worst
c!se"
example,
allerrors
add to yield
a
*0.3oC
err[r
in
the range of
-35oC
to
+sooO. lt
o.8
o-6
o-4
o -2
o
-o-2
-o-4
-o -
6
-o-a
-1
-L-2
-L-4
-L-6
SECTION
13.
21X MEASUREMET{TS
is
emphasized that
this
is
the worst
case.
In
Campbell Scientific's experience,
the overall
accuracy
is
typically better than
t0.2oC. The
major error
component
in
the
-35oC
to
+50oC
range
is
the
+0.2oC
thermistor specification.
When
a21X
is
outside
of
this temperature
range,
the polynomialerror becomes much
worse (Figure 13.4-1), and may necessitate the
use of an
external reference junction to improve
the accuracy.
Any difference
in
temperature between the
thermistor and the actual reference junction (the
channelto
which the
thermocouple
is
attached)
will become an error in the measurement. The
terminal strip cover is designed to reduce the
gradient
from one end
of
the
terminalstrip to
the
other.
ln
a
test the 21X
was subjected to
the severe thermal gradient caused by bringing
the 21X
from equilibrium at
-25oC
into
an
ambient
temperature of approximately
20oC.
After.100 minutes the gradient between the
batteries and air
temperature was still
15oC.
The
time rate of change
of
temperature in
a
natural environment would
seldom cause even
this great a
gradient. The
maximum error at
this
time, approximately
O.3oC,
is used as
an
estimate
of
the error which could occur in
a
field
situation
with proper radiation shielding. The
error is lowest on channel 4 because it
is
directly over the thermistor.
LINEARIZATION
ERROR
OF
2]-X
PNEL
TEMP
I
,1
n
h
U
)
I
q
tI]
tr
lr
H
E
0
L)
0
lrl
/.-.
-._-:-
.--.--'-\..
,/--_--\
-4c
-30-20-10
0
ao
20
30
40
50
ACTUAL
TEMPE%TURE
FIGURE
13.4-1. Thermistor Polynomial Error
60
13-1
1
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