Page 249 - Fluid mechanics, heat transfer, and mass transfer

TEMPERATURE MEASUREMENT 229

. What are the advantages and disadvantages of resis-
tance temperature devices?
Advantages
& The response time compared to thermocouples is
very fast, in the order of fractions of a second.
& Within its range, it is more accurate, more linear, and
has higher sensitivity than a thermocouple.
& Unlike thermocouples, radioactive radiation (beta,
gamma, and neutrons) has minimal effect on RTDs
since the parameter measured is resistance, not
voltage.
& An RTD will not experience drift problems because it
is not self-powered.
& In an installation where long leads are required, the
RTD does not require special extension cable.
Disadvantages
Resistance—temperature curve of a thermistor.
& Because the metal used for an RTD must be in its FIGURE 8.4
purest form, they are much more expensive than
thermocouples.
temperature elements available, but with very non-
& In general, an RTD is not capable of measuring as
linear characteristics.
wide a temperature range as that of a thermocouple.
& These are low-cost devices and manufactured in a
& Current source is required. A power supply failure
wide range of shapes, sizes, and values.
can cause erroneous readings.
& When in use, care has to be taken to minimize the
& Small changes in resistance are being measured, thus
effects of internal heating.
all connections must be tight and free of corrosion,
& Not suitable for wide span temperature measurement
which will create errors.
as resistance changes are too large to be conveniently
& Among the many uses in a nuclear station, RTDs can
measured by a single instrument. Maximum spans
be found in the reactor area temperature measure-

are about 85 C.
ment and fuel channel coolant temperature.
& Particularly suitable for narrow temperature spans
. What is a thermistor?
due to large resistance changes (i.e., good accuracy)
& Thermistor is a thermally sensitive resistor, whose
involving high sensitivity, which can be up to 10%
primary function is to exhibit a change in electrical change per C, making them the most sensitive

resistance with a change in temperature. temperature measuring devices available.
& It is made from compressed and sintered metal oxides
& The typical response times are 0.5–5 s with an
(semiconductor materials). operating range from 50 C to typically 300 C.

& Metals used are Ni, Co, Mn, Fe, Cu, Mg, and Ti. Devices are available with the temperature range

& By changing the oxide proportions, basic resistance extended to 500 C.
of the thermistor can be varied. & When in use, care has to be taken to minimize the
& Thermistors are a class of metal oxide (semiconduc- effects of internal heating.
tor material), which typically have a high negative & The nonlinear characteristics make the device diffi-
temperature coefficient of resistance, but can also be cult to use as an accurate measuring device without
positive. compensation, but its sensitivity and low cost makes
& Thermistors are based on resistance change in a it useful in many applications.
ceramic semiconductor. The resistance drops non- & The nonlinear characteristics are illustrated in
linearly with temperature rise as shown in Figure 8.4. Figure 8.4.
& Temperature ranges for thermistors are from 50 to . What are the advantages and the disadvantages of

300 C. Devices are available with the temperature thermistors?
range extended to 500 C. & Advantages: Fast and high output. Being low in heat

& Thermistors have high sensitivity that can be up to capacity, more accurate than liquid thermometer for
10% change per C, making them the most sensitive measurements involving small volumes.