Operator`s manual

SECTION 13. 21X MEASUREMENTS
13-11
polynomial approximation of the NBS TC
calibrations.
13.4.1 ERROR ANALYSIS
The error in the measurement of a
thermocouple temperature is the sum of the
errors in the reference junction temperature, the
thermocouple output (deviation from standards
published in NBS Monograph 125), the
thermocouple voltage measurement, and the
polynomial error (difference between NBS
standard and 21X polynomial approximations).
The discussion of errors which follows is limited
to these errors in calibration and measurement
and does not include errors in installation or
matching the sensor to the environment being
measured.
REFERENCE JUNCTION TEMPERATURE
WITH 21X PANEL
The 21X panel temperature is measured with a
Fenwal Electronics UUT51J1 thermistor
mounted in the center of the analog input
terminal strip. The accuracy of this
measurement is a combination of Fenwal's
interchangeability specification, the precision of
the bridge resistors, and the polynomial error.
In a "worst case" example, all errors add to yield
a ±0.3
o
C error in the range of -35
o
C to +50
o
C. It
is emphasized that this is the worst case. In
Campbell Scientific's experience, the overall
accuracy is typically better than ±0.2
o
C. The
major error component in the -35
o
C to +50
o
C
range is the ±0.2
o
C thermistor specification.
When a 21X is outside of this temperature
range, the polynomial error 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
channel to 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 terminal strip to
the other. In a test the 21X was subjected to
the severe thermal gradient caused by bringing
the 21X from equilibrium at -25
o
C into an
ambient temperature of approximately 20
o
C.
After 100 minutes the gradient between the
batteries and air temperature was still 15
o
C.
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 0.3
o
C, 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.
ACTUAL TEMPERATURE
ERROR COMPUTED-ACTUAL
LINEARIZATION ERROR OF 21X PANEL TEMP
-40-30-20-10 0 10 20 30 40 50 60
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
-1.2
-1.4
-1.6
FIGURE 13.4-1. Thermistor Polynomial Error