What Are the Characteristics and Specifications of a Thermocouple?

Thermocouples are sensors that can be used to measure temperature of many different items in industries such as manufacturing. Even small changes in temperature can affect machinery, so thermocouples are incredibly important.

Thermocouples work by using two different kinds of metal with different electrical charges. As the temperature changes in machinery, the metals themselves harden or soften depending on the polarity. Electricians and machinists use thermocouples to detect changes in temperature that could harm machinery.

Polarity

Each thermocouple has both positive and negative wires. The positive and negative wires are used together to read temperatures. The polarity - the charge between positive and negative wires - has to be maintained in order for a thermocouple to work. Because each manufacturer uses different colors to indicate positive or negative wires, you will need to check with your thermocouple manufacturer to make sure that you are handling it safely. For example, one manufacturer (ANSI/ASTM E230) uses red to signal the negative wire. Another manufacturer, IEC 60584 uses white for their negative wire. In addition to potential safety hazards, reversed polarity is one of the most common wiring errors.

Tolerance

Precision is the degree to which measurements vary from each other. Tolerance is the pre-determined, acceptable level of variance or maximum allowable error in precision as defined by the user of a product. Different thermocouples have varying levels of tolerance. Some manufacturers list their requirements for tolerance according to standard limits, special limits, and extension grade.

• Standard limits are the standard tolerance of a thermocouple across its range of temperature.
• If the thermocouple is measured using special limits of error, the precision is 1⁄2 of the precision under a standard limit of error.
• If your thermocouple's tolerance is extension grade, it is assumed the thermocouple will not be used in areas where there are significant temperature changes, but instead they can be used where temperatures do not occur often — if they happen at all.

Check your thermocouples to determine the grade. Sometimes, manufacturers classify thermocouples by different names but with the same meaning. For example, IEC 60584 has class 1, 2, and 3 thermocouples. Class 1 is special limit, class 2 is standard limit, and class 3 is extension grade.

Accuracy

Accuracy determines how close a measurement is to the actual value and is defined by the manufacturers of a product. Manufacturers of thermocouples demonstrate accuracy by measuring the output of the thermocouple against the standard output tables and checking for accuracy at many different temperatures to get an overall reading of accuracy. But as with everything else regarding thermocouples, you need to be cautious. Thermocouples are usually used when temperatures are at 32° F (or 0° Celsius) and above. If you need a thermocouple that works at a lower temperature, you will need to get a thermocouple specifically used with below zero temperatures. If you don't have a thermocouple for low temperatures, your low-temperature reading will not be accurate.

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Understanding and using thermocouples may mean the difference between a machine working as it is supposed to and a machine that is operating at a temperature that is either too high or too low. If you need more information, don't hesitate to contact us today.