Page 455 - The Mechatronics Handbook

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Thermometers That Rely Upon Phase Changes
Phase transitions of pure substances at specified pressures are used in the ITS 90 to define several of the
temperature points. This concept of a phase change being a function of temperature, as well as pressure
and the type of material, can be exploited in several forms as a means of determining the temperature
of a system by observing either the phase change itself or the conditions at which the two phases are in
equilibrium. Several useful applications of this are discussed below.

Liquid to Gas
A common remote thermometer consists of a bulb containing a liquid-gas two-phase fluid connected
via a capillary tube to a pressure gage. As long as both phases are present, the pressure read on the gage
yields the saturation pressure of the fluid. This arrangement overcomes many of the disadvantages of
the gas vs. solid thermometers with the same outward appearance described in section “Gas vs. Solid”
above. By monitoring the saturation pressure, the indicated temperature is independent of the temper-
ature of the rest of the system and is insensitive to the actual volume of the bulb and capillary. The fluid
is typically an organic solvent such as ethane selected for the particular temperature range desired. To
keep the two-phase fluid entirely in the bulb, the pressure can be transmitted through the capillary using
a single-phase fluid such as oil. Few fluids have a linear saturation curve. Therefore, most pressure gages
have a notably nonlinear scale when graduated into units of temperature. Special compensation springs
within the pressure gage can be used to allow for a nearly linear temperature scale, but the extra complication
is rarely warranted.
The temperature range of liquid to gas thermometers is limited by the two phases of the fluid and
they are typically useful from −40°C to 300°C, although a single instrument rarely will operate over more
than about a 150°C span. If the saturation pressure of the fluid is very much greater than 100 kPa, a
simple pressure gage referenced to atmospheric pressure can be used. Otherwise best accuracy is obtained
using an absolute pressure gage. The volume of the bulb must be large compared to the change in volume
of the capillary and bourdon tube in the pressure gage so that both phases are always present in the bulb.
The size of the bulb keeps these thermometers from being used for point measurements.

Reversible Phase Change Thermostats
Fixed-point thermostats may be constructed based upon the phase change of a particular sensing element.
An example is used in mechanical ice-point references where the sudden expansion of water as it freezes
is sensed to cycle the cooling system to maintain a two-phase bath. The actual melting and freezing of
the ice maintains the reference temperature. Waxes of various melting points can be used in a similar
manner.
Another example is a magnetic switch held closed by a permanent magnet until the Curie temperature
is reached at which point the magnet loses its magnetism, or more properly, changes from a ferromagnetic
material to a paramagnetic material, and the switch opens. When the material cools, it regains its
ferromagnetism, which closes the heater switch. The magnetic material can be selected to have the
appropriate Curie temperature.
Fixed Temperature Indicators
Any substance that changes phase at a fixed temperature can be used as a temperature indicator. Numerous
examples exist, the most common being a crayon made of a wax with a defined melting point. A mark
is made on the object whose temperature needs to be monitored, and if the wax melts, its temperature
is higher than the crayon point. These fixed temperature indicators are generally irreversible and can
take on many forms in addition to crayons.
A variation that can be either reversible or irreversible is a paint containing suspended solids of the
wax or similar material that melts at the desired indication temperature. As long as the particles are
solid and scatter light, the paint appears opaque, but when they melt and turn into a liquid with a
refractive index close to that of the base paint, it appears clear. These indicators can be made in a series
of spots with varying temperature points to help monitor actual temperature attained rather than just

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