Coefficients a, b, c are common for all three
measurements, therefore their value can be calculated by solving three equations
simultaneously. The a, b, c coefficients are important for an instrumentation
setup. Resistance versus temperature Tables are published for thermistors
with nominal values. Most temperature controllers have the ability to define
the a, b, c setpoints when temperature and corresponding resistance are
entered.
NTC thermistors are the most sensitive of all the temperature sensing elements. Small
dimensions of wafer, bead, disc and chip thermistors result in a rapid response time. This
is especially useful for control system feedback.
Interchangeability is another important feature. NTC thermistor interchangeability
guarantees close tolerances (0.1 to 0.2 °C) in a certain temperature range, usually
70°C. Instruments and control systems do not have to be recalibrated when a thermistor of
the same nominal value is replaced. The ceramic manufacturing process of NTC thermistors
produces extremely hard and rugged sensors. NTC thermistors are able to handle mechanical
and thermal shocks better than any other temperature measuring device. Table 2 is a
comparison chart for thermocouples and thermistors.
An RTD is a resistance temperature detector. The principle of RTD function is similar to a
thermistor. Unlike thermocouples, RTDs are not self-powered. A current must be passed
through the RTD, the same as with thermistors, and the change of voltage with temperature
is measured. Materials for RTDs can be gold, silver, copper or platinum. Platinum,
however, has become the most-used metal for RTDs. A thin film of platinum or a thin
platinum wire is deposited on a flat ceramic material and sealed.
Platinum has a nearly linear temperature versus resistance relationship. The Callendar-Van
Dusen equation approximates the RTD curve: 
|
|
where:
The lowest practically
used resistance of RTDs is 100W at 0°C. The operating temperature range is from
–220°C to 850°C. RTDs have a self-heating error that depends on the electrical
energy input. The formula for the self-heating error is:
dt = P/EK
where:
Thermocouples, NTC thermistors and RTDs are advantageous devices for remote
temperature measuring and sensing. Thermocouples are useful in applications
where a wide temperature operating range is anticipated. Thermistors are
recommended for applications with a specified temperature range and where
interchangeability without recalibration is required. NTC thermistors
are the most sensitive temperature sensors with an extremely fast response
time. Microcircuit technologies enhance applications of NTC thermistors.
RTDs offer almost linear electrical output but can be fragile. RTDs are
recommended for applications where accuracy over a wide temperature range
is the most important factor.
 |
| Alena Svab is a sales
engineer at Sensor Scientific. She has been working in the sensing industry since 1984,
and her current responsibilities include sales, export and custom applications of NTC
thermistors and assemblies. She also participates in development and marketing of new
industrial products. Alena graduated from Industrial Engineering School and attended
Charles University, Mechanical Engineering, Prague, Czech Republic. She holds a BSME. For
more information, please contact Sensor Scientific, Inc., 6 Kings Bridge Rd., Fairfield,
NJ 07004, 800/524-1610, FAX 973/227-8063. |
|
