Optimizing thermocouple performance. (Process Control & Instrumentation).

Ward (Bud) Kinney

Proper selection and maintenance of a sensing device for capturing and continuously measuring a process variable is essential for successful process control. Thermocouples are simple in form and function, but you must take great care in selection and use to get the optimum performance and accuracy.

The most common sensor in a thermal system is a thermo-couple (TO), which is sensitive to change in process conditions reasonably accurate and often the required precision at modest cost. Thermocouples are rugged and can be used over a wide range of process temperatures--from subzero to over 4000[degrees]F (2205[degrees]C) However, a combination of many factors influence the successful application of a thermocouple, including thermocouple material purity and manufacturing quality, temperature exposure, thermal cycling, chemical exposure, the protection applied and physical abuse. This article provides a perspective on the selection and application of thermocouples.

All thermocouples are not equal

One would think that the process of fastening a couple of wires together to form a thermocouple would result in the same useful device every time. However there are several important factors to consider when selecting a thermocouple, from the type of material to quality M the supplier. For example, manufacturing derived material inconsistencies are a source of loss in thermocouple accuracy, which, in this case, is defined as the amount of error that exists in a temperature measurement. Material inconsistencies can be introduced during the melting of the metals (either when the alloys are produced or when the TC tips are welded), during cold forming such as swaging, drawing and bending and during heat treatment and cooling. Most thermocouple manufacturers provide information on the initial calibration tolerances for their thermocouples (Table 1). For example, standard tolerance for a Type K thermocouple at a temperature of 2000[degrees]F (1095[degrees]C) is about 15[degrees]F (8[degrees]C). In critical high-t emperature applications, special tolerances (e.g., 8[degrees]F, or 4[degrees]C) may be required to ensure a favorable process outcome.

Selecting the right thermocouple

Selecting the proper thermocouple requires that needs are defined and matched with the appropriate kind and type of thermocouple. For example:

* What are the maximum and minimum temperatures the thermocouple will see?

* What is the furnace atmosphere?

* What are the process specifications and allowable tolerances for error?

* How long is the measurement process expected to run?

* What is the expected thermocouple life?

* Are there any cost limits?

* Are the users experienced in proper thermocouple management?

For example, …