Campbell Scientific CR7 Measurement and Control System - Manual (Page 136)

SECTION 13. CR7 MEASUREMENTS
13-12
thermocouples attached to it and to one 723
Analog Input card to either side of it (i.e. Analog
Input cards 1,2, and 3, where card 2 contains
RTD). If more than these three cards are used,
it is necessary to measure a new reference
temperature to stay within the desired 0.05oC
limit. This can be done by using one of the
thermocouples from the first set of
measurements to measure the reference
temperature for the next set. The second
reference temperature could provide the
reference for another bank of three cards. The
measuring junction for this reference should be
clamped (along with the lead from the
thermocouple being measured) into one of the
inputs in the center of the second card in this
bank. If more severe temperature gradients within the I/O Module are anticipated or
suspected this technique can be used to
quantify these gradients and supply additional
reference temperatures if necessary.
THERMOCOUPLE LIMITS OF ERROR
The standard reference which lists
thermocouple output voltage as a function of
temperature (reference junction at 0oC) is the
National Bureau of Standards Monograph 125
(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 CR7.
TABLE 13.4-1. Limits of Error for Thermocouple Wire (Reference Junction at 0oC)
Limits of Error
Thermocouple Temperature (Whichever is greater)
Type Range oC Standard Special
T -200 to 0 ± 1.0
o
C or 1.5%
0 to 350 ± 1.0
o
C or 0.75% ± 0.5
o
C or 0.4%
J 0 to 750 ± 2.2
o
C or 0.75% ± 1.1
o
C or 0.4%
E -200 to 0 ± 1.7
o
C or 1.0%
0 to 900 ± 1.7
o
C or 0.5% ± 1.0
o
C or 0.4%
K -200 to 0 ± 2.2
o
C or 2.0%
0 to 1250 ± 2.2
o
C or 0.75% ± 1.1
o
C or 0.4%
R or S 0 to 1450 ± 1.5
o
C or 0.25% ± 0.6
o
C or 0.1%
B 800 to 1700 ± 0.5% Not Estab.
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 can
not 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.g.,
±1.0oC for type T as opposed to the slope error
of 0.75% of the temperature) in the table above
are probably greater than one would experience
when considering temperatures in the
environmental range (i.e., the reference
junction, at 0oC, 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.