Campbell Scientific CR7 Measurement and Control System - Manual (Page 138)

SECTION 13. CR7 MEASUREMENTS
13-14
from the thermocouple output. For example,
suppose the reference temperature for a
measurement on a type T thermocouple is 300
oC. The compensation voltage calculated by
the CR7 corresponds to a temperature of 272.6
oC, a -27.4 oC error. The type T thermocouple
with the measuring junction at 290 oC and
reference at 300 oC would output -578.7 µV;
using the reference temperature of 272.6 oC,
the CR7 calculates a temperature difference of -
10.2 oC, a -0.2 oC error. The temperature
calculated by the CR7 would be 262.4 oC, 27.6
oC low.
TABLE 13.4-3. Reference Temperature
Compensation Range and Polynomial Error
Relative to NBS Standards
TC
Type
Range oC Limits of Error oC
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 with the CR7 are likely to be due
to the limits of error on the thermocouple wire
and in the reference temperature determined
with the 723-T RTD. Errors in the thermocouple
and reference temperature linearizations are
extremely small, and error in the voltage
measurement is negligible.
To illustrate the relative magnitude of these
errors in the environmental range, we will take a
worst case situation where all errors are
maximum and additive. A temperature of 45 oC
is measured with a type T (copper-constantan)
thermocouple, using the ±5 mV range. The
nominal accuracy on this range is 1µV(0.01% of
10mV) which at 45 oC changes the temperature
by 0.012 oC. The RTD is 25 oC but is
indicating 25.1 oC, and the terminal that the
thermocouple is connected to is 0.05 oC cooler
than the RTD.
TABLE 13.4-4. Example of Errors in
Thermocouple Temperature
Source Error oC % of Total Error
1oC 1% Slope
Error Error
Reference Temp. 0.15 12.8 39.9
TC Output
ANSI 1.0 85.0
0.01 x 20
o
C 0.2 53.2
Voltage
Measurement
0.024 2.0 6.3
Reference
Linearization
0.001 0.1 0.3
Output
Linearization
0.001 0.1 0.3
Total Error
With ANSI error 1.176 100
Assuming 1% 0.376 100
slope error
13.4.2 USE OF EXTERNAL REFERENCE
JUNCTION OR JUNCTION BOX
An external junction box is often used to
facilitate connections and to reduce the
expense of thermocouple wire when the
temperature measurements are to be made at a
distance from the CR7. In most situations it is
preferable to make the box the reference
junction in which case its temperature is
measured and used as the reference for the
thermocouples and copper wires are run from
the box to the CR7 (Section 7.4). Alternatively,
the junction box can be used to couple
extension grade thermocouple wire to the
thermocouples being used for measurement,
and the CR7 I/O Module used as the reference
junction. Extension grade thermocouple wire
has a smaller temperature range than standard
thermocouple wire, but meets the same limits of
error within that range. The only situation where
it would be necessary to use extension grade
wire instead of a external measuring junction is
where the junction box temperature is outside
the range of reference junction compensation
provided by the CR7. This is only a factor when
using type K thermocouples, where the upper
limit of the reference compensation linearization