5.1.5 Thermoelectric potentials
Whenever metals of two different types are connected together, a
thermoelectric poten[1]tial (sometimes
called a thermal e.m.f.) is generated according to the temperature of the
joint. This is known as the thermoelectric effect and is the physical principle
on which temperature-measuring thermocouples operate (see Chapter 14). Such
thermoelectric potentials are only a few millivolts in magnitude and so the
effect is only significant when typical voltage output signals of a measurement
system are of a similar low magnitude.
One such situation is where one e.m.f.-measuring instrument is used to
monitor the output of several thermocouples measuring the temperatures at
different points in a process control system. This requires a means of
automatically switching the output of each thermocouple to the measuring
instrument in turn. Nickel–iron reed-relays with copper connecting leads are
commonly used to provide this switching function. This introduces a thermocouple
effect of magnitude 40 µV/°C between the reed-relay and the copper connecting
leads. There is no problem if both ends of the reed relay are at the same
temperature because then the thermoelectric potentials will be equal and
opposite and so cancel out. However, there are several recorded instances
where, because of lack of awareness of the problem, poor design has resulted in
the two ends of a reed-relay being at different temperatures and causing a net
thermoelectric potential. The serious error that this introduces is clear. For
a temperature difference between the two ends of only 2°C, the thermoelectric
potential is 80 µV, which is very large compared with a typical thermocouple
output level of 400 µV.
Another example of the difficulties that thermoelectric potentials can
create becomes apparent in considering the following problem that was reported
in a current-measuring system. This system had been designed such that the
current flowing in a particular part of a circuit was calculated by applying it
to an accurately calibrated wire-wound resistance of value 100 and
measuring the voltage drop across the resistance. In calibration of the system,
a known current of 20 µA was applied to the resistance and a voltage of 2.20 mV
was measured by an accurate high-impedance instrument. Simple application of
Ohm’s law reveals that such a voltage reading indicates a current value of 22
µA. What then was the explanation for this discrepancy? The answer once again
is a thermoelectric potential. Because the designer was not aware of
thermoelectric potentials, the circuit had been constructed such that one side
of the standard resistance was close to a power transistor, creating a
difference in temperature between the two ends of the resistor of 2°C. The
thermoelectric potential associated with this was sufficient to account for the
10% measurement error found.
5.1.6 Shot noise
Shot noise occurs in transistors, integrated circuits and other
semiconductor devices. It consists of random fluctuations in the rate of
transfer of carriers across junctions within such devices.
5.1.7 Electrochemical potentials
These are potentials that arise within measurement systems due to electrochemical action. Poorly soldered joints are a common source.
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