Input circuit monitor 277, Temperature compensation, Measured value correction – West Control Solutions KS98-1 User Manual

Page 277

Temperature compensation

Measurement of the cold-junction reference temperature is using a PTC resistor. The temperature error thus deter -
mined is converted into mV of the relevant thermocouple type, linearized and added to the measured value as correc -
tive value with correct polarity. The remaining error with varying cold-junction reference temperature is approx.
0,5K/10K, i.e. about one tenth of the error which would occur without compensation. Better results are possible

with controlled external TC, which is adjustable within 0...+140°C at the cold junction reference dependent of con -
trolled temperature. With cold-junction reference measurements for “reproducibility” assessment, however, utmost
care must be taken that constant environmental conditions are not exceeded when working with internal TC. An air
draft at the PTC resistor of the cold junction reference can be sufficient to falsify the measurement result.

Measured value correction

The measurement can be corrected in various ways using the measured value correction.
Pre-requisite: configuration

XKorr = ein. In most cases, the relative accuracy and reproducibility rather than the

absolute one are of interest, e.g.:

-the compensation of measurement errors in one working point ( control)
-minimization of linearity errors in a limited operating range (variable )
-correspondence with other measuring facilities (recorders, indicators, PLCs, ...)
-compensation of sample differences of sensors, transmitters, etc.

Measured value correction is designed for zero offset, gain matching and for both. It corresponds to scaling mx+b, with the
difference that the KS 98-1 firmware calculates gain m and zero offset b from the value pairs for process value (

x1in;

x2in) and (x1out; x2out) of two reference points.

Example 1:
Zero offset

x1in = 100°C

x1out = 100°C + 1,5°C

x2in = 300°C

x2out = 300°C + 1,5°C

The corrected values are shifted evenly with reference
to the input values over the complete range.

Example 2:
Gain change (rotation around the coordinate origin)

x1in =

0°C

x1out = 0°C

x2in = 300°C

x2out = 300°C + 1,5°C

The corrected values diverge despite equality with the
input values at

x1in

and

x1out

Inputs

9499-040-82711

III-277

AINP1 ( analog input 1 (No. 08))

X2out

X2in

X1out
X1in

korrigierte Kennlinie

Eingangs-Kennlinie

corrected characteristic

input characteristic

Fig. 98