Connection heads are used to connect temperature sensors and extension wires. They not only protect the connection, but also are easy to install, allowing access to the terminal block. Connection heads are also referred to as thermoheads or thermocouple heads. They are available in a number of styles, most notably thermocouples, resistance temperature detectors (RTDs), thermistors and solid-state style sensors.
Thermocouple connection heads are used in situations where there is a junction of two dissimilar metals could cause a difference in voltage output proportion. This occurs where there is a voltage output (seeback potential) proportional to the difference in temperature between the hot junction and the lead wires (cold junction).
Resistive temperature detectors, both standard and precision, operate on the principle that the electrical resistance of a metal conductor changes as a function of temperature. RTDs provide an accurate, stable and repeatable means of absolute temperature measurement.
Thermistor element connection heads are very sensitive temperature sensors. The base element is composed of metal oxide ceramics, and has a relatively low cost. Thermistor elements are also the most nonlinear temperature sensors, and have a negative temperature coefficient. Often, two thermistors are combined to provide a more linear output.
Thermoheads connect to and protect the connections with extension wires via a number of connection heads. Some of the more common head types include screw cap and chain, hinged or latching closures, bolted or screwed on covers, and open head type.
Screw cap and chain connection heads have threaded caps that screw onto the body of the thermohead, protecting the connection within. Often, a chain is used to secure the head on more firmly after it has been screwed on. This type offers the advantage of a very secure fitting, even in industrial areas where vibration could be an issue. The drawbacks include a somewhat cumbersome disassembly process and the possibility of the threads being stripped if opened and closed often.
Hinged or latching closures are easy to use and convenient. The cap snaps or latches onto the body, protecting the connection within. They can be opened and closed easily, and are very useful in situations where the connections need to be inspected or changed regularly.
Bolted or screwed on connections are very secure, and provide excellent protection in hazardous environments. The only drawback is that tools are required to access the connection point.
Open head style connection heads do not provide protection for the connection. In this style, the wiring is left exposed. This style is useful in situations where access to the connections is often, but they are not recommended for hazardous environments, for obvious reasons.