Surface temperature measurement is not as straightforward as it might
seem. Here, Paul Caston describes what to look out for and how to go
about selecting suitable sensors
The biggest technical difficulty with surface temperature measurement is
overcoming the effects that environmental conditions can have on the
results. Factors such as conformance with surface contours and thermal,
as well as electrical, paths between the sensor and the surface can
influence results. Other considerations include the thickness and
flexibility of the sensors, and the sensor/surface heat exchange
characteristics.
A surface sensor mounting should achieve maximum thermal contact with
minimal mechanical strain. It may need to be insulated or isolated to
ensure that its temperature is as close to that of the surface as
possible. A large mismatch in thermal expansion coefficients may cause
strain in a resistance temperature device, leading to a change in
resistance that could be misinterpreted as a temperature change. The
leads should be in contact with the sensing surface for some length, to
reduce the effects of thermal conduction from the sensing wire or
junction.
RTD or thermocouple?
Two kinds of sensors are common in surface applications, RTDs (resistance
temperature detectors) and thermocouples. RTDs offer high accuracy,
long-term stability and higher signal output. They are normally bonded to
the measured surface and, unlike thermocouples, do not require a
reference point, ice baths or temperature compensation circuits. RTDs
have low thermal mass, which is beneficial in surface temperature
measurement, a response time of around 50ms, and a signal output of
between 50 and 200 times that of a thermocouple, allowing the use of
standard instrumentation.
Take the example of a military prototype aircraft, which may require
surface temperature measurement at a number of locations during test
flights. For this application, the sensor needs a wide range because
temperatures can vary from +500oC near the engines to well below zero at
the wing tips. The sensor must have a very low mass and not affect the
normal function of the small parts in the aircraft's wings. Furthermore,
the signal it generates must be one that is unlikely to suffer from the
electrical interference generated by the rest of the aircraft. A
polyamide, insulated, flexible sensor is ideal for this type of
measurement. Polyamide is a thermally stable material and the sensor,
with Teflon insulated lead wires, weighs less than 6 grams. Both
materials also offer a high degree of chemical resistance.
Thermocouple sensors have a wider operating range, faster response and
are cost effective. They are also simple to use, rugged and available in
thin wire or foil types, and are unaffected by strain due to mounting
materials or methods. Thermocouples are characterized by their ability to
function at considerably higher temperatures than RTDs, but they do
generate a low voltage signal, which may require amplification, leading
to problems in high electrical noise environments.
Thermocouples have low thermal mass and are often employed on sensitive
applications in the laboratory. The author is aware of one such
application, which measures the temperature of a biological specimen as
it is cooled for frozen storage. Living tissue has a high water content
and water expands as it freezes, causing friction in the cells that can
destroy the specimen. For this reason, it is extremely important to have
an accurate indication of temperature so that the freezing rate can be
carefully controlled.
Paul Caston is with Rhopoint Components
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