Page 454 - The Mechatronics Handbook

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The ratio of the volume of the bulb to the bore of the capillary determines the resolution of the
thermometer. The amount of liquid initially in the thermometer determines its range. The accuracy of the
bore and graduation markings determines its precision. The temperature read from a liquid-glass ther-
mometer is only valid if the liquid in the bore is continuous from the bulb to the point of reading.
Separations can be eliminated by either contracting the fluid entirely into the bulb or, in some cases, by
expanding it into a reservoir at the top of the thermometer. Mercury is useful from near its freezing
temperature (about −40°C) to around 500°C. The boiling point of mercury is only 357°C at standard
pressure, so mercury thermometers designed for very high temperatures must be pressurized with an
inert cover gas when sealed. Alcohol or other liquids can be used in place of mercury, but the accuracy
is generally not as great. The temperature range of alcohol extends from about −200°C to +250°C.
An alternative means of using the difference in expansion coefficients of liquids and solids to measure
temperature is to use a system filled completely with a liquid and to monitor the change in the volume
of the liquid by the position of a bourdon tube or bellows. If the volume-measuring element has a high
spring constant, the compressibility of the liquid might have to be considered (Doebelin, 1990). This
scheme has the same disadvantages as the gas bulb thermometer discussed above, as well as the same
compensation means for overcoming these disadvantages.
Gas, Liquid, and Solid
Some early thermometers consisted of a gas bulb connected to a sealed U-tube containing mercury or
another liquid. This, in effect, is really a gas pressure thermometer using a mercury manometer to indicate
pressure. To be accurate, the various coefficients of expansion of all three phases must be considered.
These instruments are rarely used where accuracy is important. The only example still widely used is a
style of minimum-maximum thermometer for ambient air measurements where a small metal fiber is
displaced in either leg of the manometer by the mercury. The fiber has enough friction in the tube and
is not wetted by the mercury so that it remains free in the glass tube after the mercury shifts. The indicator
on the gas bulb side stays at the minimum temperature while the indicator on the other leg stays at the
maximum temperature as the mercury recedes. Once the minimum and maximum temperatures are
observed, the fibers can be repositioned to the top of the two mercury columns by either centrifugal
force (slinging the whole thermometer) or with a magnet if iron fibers are used.
Solid vs. Solid
Bimetallic thermometers consist of two metals with differing temperature expansion coefficients bonded
together. As the temperature varies from the temperature at which the metals were initially bonded, the
metals expand by differing amounts and the composite experiences a shearing force. The most common
means of monitoring the shearing is to allow the metal composite to bend in response to temperature
changes. The form of the composite can take on many configurations varying in complexity from a
simple leaf fixed at one end with a pointer on the other to a small helix fixed at one end and a turning
shaft at the other that is linked to a pointer, possibly through a gear train. The shaft is supported on fine
bearings with the pointer as much as a meter away from the bimetallic helix. Stick thermometers with
a dial at one end are an example of the latter.
Since the temperature variations produce a force, there must always be some gradated restoring force
applied to the bimetallic strip. The most common application is to use the bimetallic strip itself as a
restoring spring. The final position of the strip is a balance between the shear imposed by the differing
temperature coefficients and the spring constant of the strip. There are instances when bimetallic ther-
mometers are required to actuate a switch. In these cases the load imposed by the switch must be overcome
by the shear forces in the strip and the designer must consider it as an external load. The temperature
range of bimetallic thermometers is limited by the annealing temperature or phase transformation of
the metals. Bimetallics are thus mainly used well below 700°C, and they can be permanently damaged
if the metals change their properties or the bonding between the different metals fails. A common pair
of metals is a nickel steel, such as Invar with a very low thermal expansion coefficient, bonded to a brass
alloy with a high thermal expansion coefficient.

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