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There are many different types of thermocouples. Each has its advantages and disadvantages over other types of thermocouples that you may find in the market. Some of the factors to guide your selection of thermocouple for any given applications are discussed below.
COST
There are cost differences among the various types of thermocouples. These differences are depend largely on the type of materials used in making the particular thermocouple. Low cost thermocouples are those made from common metals or so called “base metals.” The base metal thermocouples include types: E, K, J, N,T. Type K and J are the most popular base metal thermocouples. They are inexpensive and accurate and therefore mainly used in most industrial applications that require a thermocouple.
There are also thermocouples made from the noble metals. These thermocouples are very expensive. The noble metal thermocouples include types B, R and S. These thermocouples are based on the platinum-rhodium alloy and are many times more expensive than base metal thermocouples. This is because Platinum and Rhodium are very rare and expensive metals. The obvious advantage of using noble metal thermocouples in any industrial application is that they can be used at a higher temperature and they have better accuracy than the base metal thermocouples.
There are also thermocouples made from the noble metals. These thermocouples are very expensive. The noble metal thermocouples include types B, R and S. These thermocouples are based on the platinum-rhodium alloy and are many times more expensive than base metal thermocouples. This is because Platinum and Rhodium are very rare and expensive metals. The obvious advantage of using noble metal thermocouples in any industrial application is that they can be used at a higher temperature and they have better accuracy than the base metal thermocouples.
TEMPERATURE RANGE
The maximum temperature anticipated in the application where the thermocouple is to be used will determine the type of thermocouple required. The higher this temperature the more costly the thermocouple required might be.
ACCURACY
Every measuring instrument is expected give a certain band of accuracy. The thermocouple is not an exception. Accuracy here may be defined as the error which exists in a given temperature measurement. It indicates how close measured values are to the actual temperature value. This is also referred to as tolerance or error. In thermocouple applications, initial calibration tolerance tables tell us what tolerance or accuracy we can get from a given thermocouple. The more accuracy is required for a given application, the more expensive the thermocouple required for this particular application could be. Often there is a compromise between accuracy and cost.
LIFE EXPECTANCY
How long can a thermocouple last before it fails? It depends on a lot of factors which are many and varied. For thermocouple applications, life denotes accuracy of the device over time. How long can it continue to guarantee its initial calibration tolerance? Some factors which affect thermocouple life include:- Operating temperature
- Thermocouple wire size
- Thermocouple protection
- Operating environment
- Accuracy required etc
As time goes on, a thermocouple gets more inaccurate. At some point in its life, the accuracy of the thermocouple becomes so unreliable that we consider the thermocouple to have failed. This is often manifested in the form of drifting milivolts signal of the thermocouple. For a given thermocouple application, the more drift in milivolts signal that can be tolerated, the longer the thermocouple can last.
As a maintenance strategy, in most plants, the life of thermocouples can be prolonged by recalibrating it at periodic intervals. Calibrating a thermocouple entails measuring the accuracy of the milivolt signal and adjusting the controller to compensate for any errors. In most plants calibration is usually done at determined intervals say every 3 months or even 6 months.
Other factors to consider when selecting a thermocouple include:
