Thermistor is device that changes its electrical resistance in response
to changes in ambient temperature. Since the resistance is easily measured the thermistors
are commonly used as temperature transducers. They can be manufactured in very small sizes
thus reducing their thermal capacity and in consequence reducing response time. Relatively
high temperature coefficients can be achieved making the thermistors suitable for high
resolution measurements.
The relationship between temperature and resistance of the thermistor
is highly non-linear. However, using the Steinhart-Hart formula programmed into the
PMA2160 interface, the PMA2100 linearizes this relationship and calculates temperature to
the accuracy of 0.35°C. The Steinhart-Hart formula has wide industry acceptance for
computation of temperature.
The particular thermistor used in PMA probes has a negative temperature
coefficient and a very good aging characteristic. Self heating is controlled by limiting
the current to less than 100 µA, which in turn limits the power dissipated the thermistor
to approx. 100 µW.
The PMA2160 family of temperature probes measure temperature using the
combination of thermistor probes and the PMA2160 interface to the PMA2100. The interface
and probes can be purchased as a kit or separately, and one interface can accept any of
the thermistor probes offered. Measurements of surface temperature, liquids, or gases can
be easily interchanged by connecting the proper thermistor probe to the interface. The
interface contains a memory capsule which holds a program that the PMA2100 can read and
execute. This program contains all of the necessary information to convert the thermistors
resistance into a temperature as well as unit conversions and all setup information for
the PMA2100.
The Surface temperature probe (PMA2161) can be used in situations where
the temperature of a solid mass is needed, as in a semiconductors heat-sink or to monitor
skin temperature. Measurements of soil temperature can be achieved using the stainless
steel immersion probe (PMA2163), Air or gas temperatures can be obtained using the
stainless steel air/gas probe (PMA2164), and for general purpose temperature measurements
the PMA2162 or PMA 2163 probes can be used.
The measurements can be displayed in units of degrees Celsius (°C),
degrees Fahrenheit (°F), or in Kelvin (K). Unit conversions are as follows:
T[°C] =T[K] - 273.16
T[°F] = 1.8 * T[°C] + 32
