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In a liquid process, addition of a stirrer will help to eliminate thermal
lag. Since the thermocouple is basically a point measuring device,
placing more than one thermocouple in parallel will provide an
average temperature readout and produce better results in most
air heated processes.
Proper sensor type is also a very important factor to obtain precise
measurements. The sensor must have the correct temperature
range to meet the process requirements. In special processes the
sensor might need to have different requirements such as leak-
proof, anti-vibration, antiseptic, etc.
Standard thermocouple sensor limits of error are +/-4degrees F
(+/- 2 degrees C ) or 0.75% of sensed temperature (half that for
special ) plus drift caused by improper protection or an over-
temperature occurrence. This error is far greater than controller
error and cannot be corrected at the sensor except by proper
selection and replacement.
Thermocouple input connections are shown in Figure 2-5. The
correct type of thermocouple extension lead-wire or
compensating cable must be used for the entire distance
between the unit and the thermocouple, ensuring that the correct
polarity is observed throughout. Joints in the cable should be
If the length of thermocouple plus the extension wire is too long, it
may affect the temperature measurement. A 400 ohms K type or
a 500 ohms J type thermocouple lead resistance will produce
approximately 1 degree C temperature error .
2-6 Thermocouple Input Wiring2-6 Thermocouple Input Wiring
UM L91-Rev 8
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