General Parameters, Installation and Maintenance

General Selection Parameters

The conditions of measurement determine the type of thermocouple used. Temperature, atmosphere, protection, response, and service life should be considered. The following descriptions serve as a guide to selection.

Thermocouple Type

Select the thermocouple type that will be capable of operating in your application temperature range and be compatible with your instrumentation.

Protection Tube

Select a material that will withstand the temperature and possible corrosives of your application.

Tube Size

Use the tube size that will withstand the rigors of your application but with minimal effect on it.

Fitting or Mounting Type

To attach and/or seal the assembly in your application use a flange or fixed fitting.

Terminal and/or Extension Type

For connection to instruments various terminations and extensions are available.

General Installation Parameters

The thermocouple should “see”, as closely as possible, what the product in the process is experiencing in order to get meaningful measurements.

Location

Locate the thermocouple junction as close to the product as possible. A rule of thumb is to have at least 10 tube diameters immersion in the hot zone. Avoid direct flame impingement or stagnant areas.

Wire Extension

Select the wire insulation that environmental conditions dictate.

See Section on General Wire Insulation Characteristics

Use the correct thermocouple type throughout the circuit. “Red” color code is always negative in thermocouple circuits. Ideally, run thermocouple circuit wires in separate conduits at least one foot away from power lines. Twisted and shielded constructions may be required to avoid noise in the thermocouple circuit. The overall impedance of the thermocouple circuit must be compatible with your instrumentation.

General Maintenance Parameters

THERMOCOUPLE DO’s:

1. DO check in place.
2. DO replace at established, proper intervals.
3. DO have good connections throughout the circuit.

THERMOCOUPLE DO NOT’s:

1. DON’T reinsert at different immersions. (Avoid decreasing the immersion).
2. DON’T use for accurate measurements at lower temperatures after being exposed to higher temperatures.
3. DON’T use in defective protecting tubes.
4. DON’T insulate with used insulators.

If there is a reversal in the thermocouple circuit the indication will be down scale. A “double-reversal” in the circuit will give an upscale but erroneous reading. Keep the “Red” color coded leg negative throughout the circuit to avoid these reversals.