2640A/2645A NetDAQ
Users Manual
A-14
2640A RTD’s 2-Wire per ITS-1990 Measurement Specifications A-18.
The 2640A specifications for the 2-wire Resistance-Temperature Detector (RTD)
measurement function is based on the 4-wire RTD measurement specification
(above) except you add a nominal 5-Ohm (roughly 13ºC ) positive offset. This
value varies for each channel and temperature gradient (nominal +0.4%/ºC). Also
note that the resistance of the RTD wiring adds directly to the error. After 100
million operations of a measurement channel, the offset will increase at an
indeterminate rate.
2640A Thermocouple per ITS-1990 Measurement Specifications A-19.
Tables A-20 to A-21 provide 2640A specifications for the thermocouple
measurement function per ITS-1990.
Table A-20. 2640A Thermocouple General Specifications
Specification Characteristic
Input Impedance 100 MΩ minimum in parallel with 300 pF
Open Thermocouple Detect Operates by injecting a small ac signal into the
input after each measurement. A thermocouple
resistance greater than 1kΩ to 10kΩ is
detected as an open input.
Temperature Coefficient To calculate thermocouple accuracy for
temperatures between 28ºC and 60ºC, or -10ºC
and 18ºC, use a linear interpolation between
the two applicable points. e.g., if the applicable
spec at 28ºC is .6 and the spec at 60ºC is 1.1,
then the spec at 40ºC = (1.1-.6)*(40-28)/(60-
28)+.6= .5* (12/32) + .6 = .7875.
Accuracy at -20°C Multiply the -10°C to + 60°C accuracy
specification by 2. After 1 hour warm-up. For
accuracy between -10°C and -20°C, interpolate
linearly.
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