Page 172 - Mechanical Engineers' Handbook (Volume 2)

4 Thermistors 161

4 THERMISTORS
Thermistors are temperature-sensitive resistors whose resistance varies inversely with tem-
perature. The resistance of a 5000- thermistor temperature sensor may go down by 20
for each degree Celsius increase in temperature (in the vicinity of the initial temperature).
Driven by a 1.0-mA current source, this yields a signal of 200 mV/ C.
Thermistors are used frequently in systems where high sensitivity is required. It is not
uncommon to find thermistor data logged to the nearest 0.001 C. This does not mean the
data are accurate to 0.001 C, but the data are readable to that precision.
A thermistor probe is sensitive to the same environmental errors that afflict any im-
mersion sensor: Its accuracy depends on the care with which it was designed for that par-
ticular environment.

4.1 Types and Ranges
Thermistor probes can be used between 183 C (the oxygen point) and 327 C (the lead
40
point). 39 But most applications are between 80 and 150 C. The sensitivity of a therm-
istor (i.e., the percent change in resistance per degree Celsius change in thermistor temper-
ature) varies markedly with temperature, being highest at the lowest temperatures.
The long-time stability of thermistor probes is open to some question, although several
months of accurate usage between calibrations seem attainable. The evidence on drift and
its causes is not clear. It would be prudent, as with any temperature-measuring system, to
make provision for periodic recertification of thermistor probes on a time scale established
by experience within the system itself.
If accurate measurements are required, calibration facilities are needed, and this need
poses some problems. Few instruments are capable of providing a transfer calibration for
thermistors to their limit of readability, since few are that sensitive. Melting point baths and
precision-grade resistance thermometry are needed to capitalize on the available precision.
Thermistors have strongly nonlinear output. Linearizing bridges are available, but these
add to cost. Nonlinearity is not a significant issue when the data will be interpreted by
computer.

4.2 Physical Characteristics of Typical Probes
Thermistor probes range in size from 0.25 mm spherical beads (glass covered) to 6-mm-
diameter steel-jacketed cylinders. Lead wires are proportionately sized. Disks and pad-
mounted sensors are available in a wide range of shapes, usually representing a custom
design gone commercial. Aside from the unmounted spherical probes and the cylindrical
probes, there is nothing standard about the probe shapes.
Figure 22 shows some representative shapes of commercially available probes.
For medical applications, thermistor probes are often encapsulated in sterilizable, flexible
vinyl material. Such probes are frequently taped to a patient’s skin and used as the control
sensor for the temperature-regulating system.
The thermistor element itself is fabricated using the techniques of powder metallurgy.
A mixture of metallic oxides is compressed into a disk and sintered. The mixture’s com-
position, the sintering temperature, and the atmosphere in the furnace determine the resis-
tance and the resistance–temperature coefficient of the thermistor. The faces of the disk are
plated with silver, and the resistance of the thermistor is adjusted by removing material from
the edges until the desired value has been obtained. The lead wires are attached, and the
assembled thermistor is then potted in epoxy, rubber, or glass.