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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 A4degrees F (A
2degrees 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.
2-6 Thermocouple Input Wiring
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 avoided, if possible.
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 .
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UM0L911C
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