handbook of modern sensors

16
Temperature Sensors
When a scientist thinks of something, he asks,–‘Why’
When an engineer thinks of something, he asks,–‘Why not’
Since prehistoric times people were aware of heat and tried to assess its intensity
by measuring temperature. Perhaps the simplest and certainly the most widely used
phenomenon for temperature sensing is thermal expansion. This forms the basis of the
liquid-in-glass thermometers. For electrical transduction, different methods of sensing
are employed. Among them are the resistive, thermoelectric, semiconductive, optical,
acoustic, and piezoelectric detectors.
Taking a temperature essentially requires the transmission of a small portion of
the object’s thermal energy to the sensor, whose function is to convert that energy into
an electrical signal. When a contact sensor (probe) is placed inside or on the object,
heat conduction takes place through the interface between the object and the probe.
The sensing element in the probe warms up or cools down; that is, it exchanges heat
with the object. The same happens when heat is transferred by means of radiation:
thermal energy in the form of infrared light is either absorbed by the sensor or liberated
from it depending on the object’s temperature and the optical coupling. Any sensor,
no matter how small, will disturb the measurement site and thus cause some error
in temperature measurement. This applies to any method of sensing: conductive,
convective, and radiative. Thus, it is an engineering task to minimize the error by an
appropriate sensor design and a correct measurement technique.
When a temperature sensor responds, two basic methods of the signal processing
can be employed: equilibrium and predictive. In the equilibrium method, a temperature
measurement is complete when no significant thermal gradient exists between
the measured surface and the sensing element inside the probe. In other words, a
thermal equilibrium is reached between the sensor and the object of measurement. In
the predictive method, the equilibrium is not reached during the measurement time. It
is determined beforehand, through the rate of the sensor’s temperature change. After