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 50-200°C
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 1°C 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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Temperature Measurements
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