The Continuous Thermocouple

About the continuous thermocouple

The continuous thermocouple is one of a class of devices which detect and respond to heat. Other devices in this class include the line-type heat detector and heat-sensitive cable. The basic construction of all these devices consists of two or more strands of wire separated by insulation within a long, thin cable. Whilst they sense temperature, they do not in fact provide an output measurement of temperature. Their function is to respond to abnormal temperature rises and thus prevent fires, equipment damage, etc.


The advantages of continuous thermocouples become more apparent if the problems with other types of heat detector are considered. The insulation in the line-type heat detector and heat-sensitive cable consists of a plastic or ceramic material with a negative temperature coefficient (i.e. the resistance falls as the temperature rises). An alarm signal can be generated when the measured resistance falls below a certain level.

 

Alternatively, in some instruments, the insulation is allowed to break down completely, in which case the device acts as a switch. The major limitation of these devices is that the temperature change has to be relatively large, typically 50-200C above ambient temperature, before they respond. Also, it is not generally possible for such devices to give an output which indicates that an alarm condition is developing before it actually happens, and thus to allow preventative action. Furthermore, after a device has generated an alarm it usually has to be replaced. This is particularly irksome because there is a large variation in the characteristics of detectors coming from different batches and so replacement of the device requires extensive on-site recalibration of the system.


In contrast, the continuous thermocouple suffers from very few of these problems. It differs from other types of heat detector in that the two strands of wire inside it are a pair of thermocouple materials separated by a special, patented mineral insulation and contained within a stainless steel protective sheath. If any part of the cable is subjected to heat, the resistance of the insulation at that point is reduced and a hot junction is created between the two wires of dissimilar metals. An e. m. f. is generated at this hot junction according to normal thermoelectric principles.


The continuous thermocouple can detect temperature rises as small as 1C above normal. Unlike other types of heat detector, it can also monitor abnormal rates of temperature rise and provide a warning of alarm conditions developing before they actually happen. Replacement is only necessary if a great degree of insulation breakdown has been caused by a substantial hot spot at some point along the detector's length. Even then, the use of thermocouple materials of standard characteristics in the detector means that recalibration is not needed if it is replaced. Calibration is not affected either by cable length, and so a replacement cable may be of a different length to the one it is replacing. One further advantage of continuous thermocouples over earlier forms of heat detector is that no power supply is needed, thus significantly reducing installation costs.


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