Campbell Scientific CR5000 Measurement and Control Module - Manual (Page 67)

Section 3. CR5000 Measurement Details
3-13
requiring the ±200 mV input range. At 1300 o C, a K thermocouple outputs
34.9 µV per o C. The possible slope error in the voltage measurement is
0.00075x52 mV = 39 µV or 1.12 o C (39/34.9). The basic resolution on the 200
mV range is 6.67 µV or 0.19 o C. Thus, the possible error due to the voltage
measurement is 1.56 o C on a non-reversing differential, or 1.31 o C with a
reversing differential measurement.
TABLE 3.4-2. Voltage Range for maximum Thermocouple resolution
TC Type and temp.
range
o C
Temp. range for
±20 mV range Temp. range for
±50 mV range Temp. range for
±200 mV range
T -270 to 400 -270 to 400 not used not used
E -270 to 1000 -270 to 280 -270 to 660 >660
K --270 to 1372 -270 to 610 -270 to 1230 >1230
J -210 to 1200 -210 to 360 -210 to 870 > 870
B 0 to 1820 0 to 1820 not used not used
R -50 to 1768 -50 to 1680 -50 to 1768 not used
S -50 to 1768 -50 to 1768 not used not used
N -270 to 1300 -270 to 580 -270 to 1300 not used
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 no integration and input reversal is 1.8 µV RMS. On a type T
thermocouple (approximately 40 µV/
o C) this is 0.05 o C. Note that this is an
RMS value, some individual readings will vary by greater than this. By
integrating for 250 µs the noise level is reduced to 0.6 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 CR5000'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.