Specifications

Section 3. CR3000 Measurement Details
When the thermocouple measurement junction is in electrical contact with the
object being measured (or has the possibility of making contact) a differential
measurement should be made.
Noise on Voltage Measurement
The typical input noise on the ±20 mV range for a differential measurement
with 250 µs integration and input reversal is 0.4 µV RMS. On a type T
thermocouple (approximately 40 µV/
o
C) this is 0.01
o
C. Note that this is an
RMS value, some individual readings will vary by greater than this. By using
the “60 Hz” integration for 16.67 µs the noise level is reduced to .19 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 CR3000's math and storage
capabilities, 4 separate 6th order polynomials are used to convert from volts to
temperature over the range covered by each thermocouple type. Table 3.4-3
gives error limits for the thermocouple polynomials.
TABLE 3.4-3. Limits of Error on CR3000 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 ± 0.08
-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 ± 0.4
-130 to 200 ± 0.005
200 to 1000 ± 0.02
K -50 to 1372
-50 to 950 ± 0.01
950 to 1372 ± 0.04
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 CR3000 environmental operating range, so there is
no problem when the CR3000 is used as the reference junction. External
reference junction boxes however, must also be within these temperature
3-14