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Technical Information

There are several methods to identify and correct temperature control problems without special equipment or an electronics background. Usually visual indications give clues where the trouble may lie.

The first indication of trouble usually occurs when the process being controlled is either being overheated, not heating, or is alternately too hot, then too cold (wide temperature cycling).

In any of these, the problem may be due to the temperature sensing circuit, an indication problem, or may be a control problem.

In troubleshooting, the complete system must be considered. This includes the “thermo- couple”, which senses the temperature, the “indicator” which indicates the temperature and compares it with a controller, or may be in the “instrument chassis” that amplifies the error signal (difference between the setpoint and the temperature indicated) and the “final control” device. The final control device may be a contactor, SCR power controller or a electric actuator.

Since the instrument is in the control system between the temperature sensing device or thermocouple and the final control device, it’s a good place to begin troubleshooting. Further, the indicator and pilot lights give a visual indication of events happening in the control system, and may give a good clue of the exact location of the problem.

First, to eliminate the control instrument as the source of trouble, interchange it with another. If the trouble follows the instrument, the thermocouple or sensor and the final control device have been eliminated. If the trouble stays with the same zone or location, then the problem must exist outside the instrument.

The two most common sources of trouble in a typical control system are the thermocouple in the sensing portion and contactor in the control portion.

If the trouble did follow the instrument when it interchanged with another, the instrument should be replaced with a spare and the faulty unit returned to the manufacturer for repair.

If the problem stays at the location where first observed, the instrument should be observed for clues to the trouble. The indicator may be at room temperature, at control setpoint, above control point and, if observed for a time, the indicator may move up and down the scale erratically. This erratic motion may or may not be continuous. These observations can lead to a quick solution to the problem.

The following troubleshooting guide will assist in interpreting the clues furnished by the instrument, to locate problems in the temperature control system.

A treatise for corrective action procedures in the sensing portion of the circuit is considered here, assuming the sensor is a thermocouple.

A deteriorated thermocouple will always cause a low reading. With base metal couples, evidence of deterioration is often visible at the hot junction. The metal may have a scaly,

Barber-Colman Company, Loves Park, IL.


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