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Sensors, such as thermocouples, detect the temperature of processes. Often they need to be made long enough so that connections of extension or other cables are made in a cool zone. This is called a lagging distance. This connection needs to be made in an isothermal zone and a small enclosure is usually provided at the cold end of a thermocouple and is called a head. There is a range of head options which offer various connection space sizes, degrees of protection to dust and water, temperature resistance, and even pressure restraint to allow for a broken protection sheath.
Increasingly a transmitter is fitted in the head to convert the signal to a digital 4-20mA output. Transmitters constantly decrease in cost and offer advanced features such as sensor burnout signaling, sensor calibration offsets, on the run re-ranging, and signal multiplexing.
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