handbook of modern sensors

16.2 Thermoelectric Contact Sensors 485
avoid dry contact, thermally conductive grease or epoxy should be applied for better
thermal tracking. A reference temperature detector in this example is a semiconductor
sensor LM35DZ manufactured by National Semiconductor, Inc. The circuit has two
outputs: one for the signal representing the Seebeck voltage V p and the other for the
reference signal V r . The schematic illustrates that connections to the circuit board
input terminals and then to the amplifier’s noninverting input and to the ground bus
are made by the same type of wire (Cu). Both board terminals should be at the same
temperature T c ; however, they do not necessarily have to be at the “cold” junction
temperature. This is especially important for the remote measurements, where the
circuit board temperature may be different from the reference “cold” junction temperature
T r .
For computing the temperature from a thermocouple sensor, two signals are essentially
required. The first is a thermocouple voltage V p and the other is the reference
sensor output voltage V r . These two signals come from different types of sensor and
therefore are characterized by different transfer functions. A thermopile in most cases
may be considered linear with normalized sensitivity α p (V/K), whereas the reference
sensor sensitivity is expressed according to its nature. For example, a thermistor’s
sensitivity α r at the operating temperature T is governed by Eq. (16.21) and has
dimension /K. There are several practical ways of processing the output signals.
The most precise method is to measure these signals separately, then compute the
reference temperature T r according to the reference sensor’s equation, and compute
the gradient temperature from a thermocouple voltage V p as
= T x − T r = V p
α p
. (16.41)
Finally, add the two temperatures and T p to arrive at the measured absolute temperature
T x . A value of sensitivity (α p ) can be found from Table A.10.
For a relatively narrow reference temperature range, instead of adding up the
temperatures, voltages from the reference sensor and the thermocouple can be combined
instead. Because α r and α p are very much different, a scaling circuit must be
employed. Figure 16.17 illustrates a concept of adding up voltages from a thermocouple
and a thermistor (reference sensor) to obtain a combined output signal V c .
When adding up the voltages, the thermocouple amplifier gain should be selected to
Fig. 16.17. Combining thermopile and thermistor signals.