Campbell Scientific CR9000X Measurement and Control System - Manual (Page 100)

Section 3. CR9000X Measurement Details
speed is more important than the reduced accuracy, the temperature of a single
CR9050 module can be used for thermocouples connected to other modules.
A foam block that fits under the terminal cover is sent with the CR9000X.
When installed, this block insulates and limits air circulation around the
terminals. This helps to limit temperature gradients on the analog input
modules, particularly when the CR9000X is subjected to rapid temperature
changes and/or convective air currents.
Figure 3.1.4-1 shows the thermocouple temperature errors experienced on
different channels of the CR9051E analog module when a CR9000X, in a lab
enclosure with the foam block inserted under the lid, was subjected to an
abrupt change in temperature. The logger was enclosed in 1 mil plastic, to keep
convective air currents from directly impinging on the logger surfaces, and
placed inside a test chamber. Throughout the test, channels 1, 7, and 14 of the
CR9051E module were used to measure the temperature of an ice bath. The
Logger was soaked until it reached -40 ˚C and then the chamber was cycled
from -40 ˚C to 60 ˚C in 12 minutes. The measured temperature of the ice bath
was compared with the actual temperature, which was measured using an
independent, calibrated device. The measurement errors on Channels 1, 7, and
14 are plotted against the left axis. The reference temperature (PRT_Ref) of the
CR9051E and the ambient chamber temperature are plotted against the right
axis.
‐4
‐3
‐2
‐1
0
1
0 0.
2
5
0.
5
0.
75
1 1.
25
1.
5
1.
75
2 2.
2
5
2.
5
2.
75
3 3.
25
3.
5
3.
75
4 4.
2
5
4.
5
4.
75
Time (hrs)
Ch
a
n
ne
ls
(?
C
)
Te
m
p
e
rat
u
re
Di
ff
e
re
n
ce
‐60
‐40
‐20
0
20
40
60
80
Ch
a
m
b
er
/
PR
T
_
Re
f
(?
C
)
Channel 1
Channel 7
Channel 14
Chamber_Ambient
PRT_Ref
FIGURE 3.1.4-1. Thermocouple temperature errors during rapid
temperature change
Thermocouple Limits of Error
The standard reference which lists thermocouple output voltage as a function
of temperature (reference junction at 0
o C) is the National Institute of Standards
and Technology Monograph 175 (1993). 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 3.1.4-1 gives the ANSI
limits of error for standard and special grade thermocouple wire of the types
accommodated by the CR9000X.
3-12