Campbell Scientific CR9000X Measurement and Control System - Manual (Page 103)

Section 3. CR9000X Measurement Details
Noise on Voltage Measurement
The input noise on the ±50 mV range for a measurement with no integration is
4 µV RMS. On a type T thermocouple (approximately 40 µV/
o C) this is 0.1
o C. Note that this is an RMS value, some individual readings will vary by
greater than this. By integrating for 500 µs (50 samples) the noise level is
reduced to 0.6 µV RMS (4/
√50=0.6). If a 500 µs integration is combined with
reversing the differential input, there are 100 samples in the measurement and
the noise level is reduced to 0.4 µV RMS.
Thermocouple Polynomial: Voltage to Temperature
NIST Monograph 175 gives high order polynomials for computing the output
voltage of a given thermocouple type over a broad range of temperatures. In
order to speed processing and accommodate the CR9000X's math and storage
capabilities, four separate 6th order polynomials are used to convert from volts
to temperature over the range covered by each thermocouple type. Table
3.1.4-3 gives error limits for the thermocouple polynomials.
TABLE 3.1.4-3. Limits of Error on
CR9000X Thermocouple Polynomials
(Relative to NIST Standards)
TC
Type Range o C Limits of Error o C
T -270 to 400
-270 to - 200 +18@ -270
-200 to -100
±
0.080
-100 to 100
±
0.001
100 to 400
±
0.015
J -150 to 760
±
0.008
-100 to 300
±
0.002
E -240 to 1000
-240 to - 130
±
0.400
-130 to 200
±
0.005
200 to 1000
±
0.020
K - 50 to 1372
- 50 to 950
±
0.010
950 to 1372
±
0.040
Reference Junction Compensation: Temperature to Voltage
The polynomials used for reference junction compensation (converting
reference temperature to equivalent TC output voltage) do not cover the entire
thermocouple range. Substantial errors will result if the reference junction
temperature is outside of the linearization range. The ranges covered by these
linearizations include the CR9000X environmental operating range, so there is
no problem when the CR9000X is used as the reference junction. External
reference junction boxes however, must also be within these temperature
ranges. Temperature difference measurements made outside of the reference
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