handbook of modern sensors

16.5 Acoustic Temperature Sensor 495
(A) (B) (C)
Fig. 16.27. A thermochromic solution sensor: (A) absorption spectra of the cobalt chloride
solution; (B) reflective fiber coupling; (C) transmissive coupling. (From Ref. [18].)
the sensor should consist of a light source, a detector, and a cobalt chloride solution,
which is thermally coupled with the object. Two possible designs are shown in
Figs. 16.27B and 16.27C, where transmitting and receiving optical fibers are coupled
through a cobalt chloride solution.
16.5 Acoustic Temperature Sensor
Under extreme conditions, temperature measurement may become a difficult task.
These conditions include a cryogenic temperature range, high radiation levels inside
nuclear reactors, and so forth. Another unusual condition is the temperature measurement
inside a sealed enclosure with a known medium, in which no contact sensors
can be inserted and the enclosure in not transmissive for the infrared radiation. Under
such unusual conditions, acoustic temperature sensors may come in quite handy. An
operating principle of such a sensor is based on a relationship between temperature
of the medium and speed of sound. For instance, in dry air at a normal atmospheric
pressure, the relationship is
√
ν ≈ 331.5
T
273.15
m
s , (16.52)
where ν is the speed of sound and T is the absolute temperature.
An acoustic temperature sensor (Fig. 16.28) is composed of three components: an
ultrasonic transmitter, an ultrasonic receiver, and a gas-filled hermetically sealed tube.
The transmitter and receiver are ceramic piezoelectric plates which are acoustically
decoupled from the tube to assure sound propagation primarily through the enclosed
gas, which, in most practical cases, is dry air. Alternatively, the transmitting and
receiving crystals may be incorporated into a sealed enclosure with a known content
whose temperature has to be measured; that is, an intermediate tube in not necessarily
required in cases where the internal medium, its volume, and mass are held constant.
When a tube is used, care should be taken to prevent its mechanical deformation and