Using Screw Terminal Accessories
4-5
This reference-junction voltage must be added to the thermocouple
reading to obtain an accurate temperature value.
An appropriate cold junction compensation algorithm follows:
1. Read the CJC circuit output voltage at pins 16 and 20 of the main I/O
connector of the DAS-TC/B.
2. Perform the following data manipulations in the host computer:
a.
Convert the CJC circuit voltage to the reference-junction
temperature.
b.
Convert the reference-junction temperature to a
reference-junction voltage, using the appropriate equation or
lookup table for your thermocouple type.
c.
Add the reference-junction voltage to the thermocouple reading.
d. Convert the corrected thermocouple reading to a temperature
value, using the appropriate equation or lookup table for your
thermocouple type.
Open-Thermocouple Detection Signals
The consequences of an open thermocouple circuit can be serious. For
example, when using a thermocouple for automatic control of an elevated
temperature, an open circuit in the thermocouple typically causes the
measured temperature to appear constantly below the control set point.
Consequently, the control algorithm could apply excessive, potentially
damaging heat in a futile attempt to restore the measured temperature to
the control set point.
However, some temperature control software includes a feature that
interprets a positive out-of-range input as an open-thermocouple event,
then takes protective action. Therefore, on both the STC-TC/B and
STA-TC/B accessories, open-thermocouple detection signals are available
to drive high-impedance (100M
Ω
minimum) DAS-TC/B inputs out of
range during open-thermocouple events. These signals are small enough
not to interfere with normal thermocouple temperature measurements. For
example, the open-thermocouple detection signal to each channel on an
STA-TC/B is roughly 10nA, causing only a tiny voltage drop across the
thermocouple circuit—roughly 1
µ
V across 100
Ω
.
