Page 250 - Fluid mechanics, heat transfer, and mass transfer

DIMENSIONLESS NUMBERS, TEMPERATURE MEASUREMENT, AND CONDUCTION HEAT TRANSFER
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& Disadvantages: Nonlinear, limited temperature & Bimetallic strips are usually configured as a spiral or
range, fragile, current source required, and self-heat- helix for compactness and can then be used with a
ing. Being semiconductors, thermistors are more pointer to make a cheap compact rugged thermom-
susceptible to permanent decalibration than thermo- eter as shown in Figure 8.5b.
couples or RTDs. & These are portable and do not require a power supply,
. What is the principle of operation of infrared temper- but are usually not as accurate as thermocouples or
ature sensors? RTDs. Temperature recording is not possible.

& They obtain temperature by measuring the thermal & Their operating range is from 180 to 430 C and can
radiation emitted by a material. Infrared sensors be used in applications from oven thermometers to
are noncontacting devices. These are also known home and industrial control thermostats.
as pyrometers. Used for furnace temperature & The bimetallic strip is extensively used in ON/OFF
measurements. applications, and is not requiring high accuracy as it
& Based on Stefan–Boltzmann equation, is rugged and cost-effective.
& Bimetallic thermometers are relatively inaccurate,
q ¼ sT 4 ð8:1Þ slow to respond, not normally used in analog appli-
cations to give remote indication and have hysteresis.
for a black body.
. What are the characteristics of fluid expansion
. What are the advantages and disadvantages of
thermometers?
pyrometers?
& Fluid expansion devices are liquid-in-glass thermo-
& Advantages: Pyrometers can measure high tem-
meters. The liquid may be mercury or organic liquid
peratures without melting or oxidation. They can
(typically alcohol). Versions employing gas instead
also be used for measuring low temperatures.
of liquid are also available.
& Disadvantages: Pyrometers are not as accurate as
& Mercury is considered an environmental hazard, so
other methods and readings are based on black body
there are regulations governing the shipment of
radiation. Many bodies are not black in practice.
devices that contain it.
. How do bimetallic thermometers work?
& Fluid expansion sensors do not require electric
& Bimetallic devices take advantage of the difference
power, do not pose explosion hazards, and are stable
in rate of thermal expansion between different even after repeated cycling.
metals. Strips of two metals are bonded together.
& Useful for calibrating other temperature measuring
When heated or cooled, one side will expand or
devices. For this purpose, standard short-range ther-
contract more than the other and the resulting bend-
mometer sets, called Anschutz thermometers, are
ing is translated into a temperature reading by
available.
mechanical linkage to a pointer.
& On the other hand, they do not generate data that are
& If two strips of dissimilar metals such as brass and
easily recorded or transmitted, and cannot make spot
invar (copper–nickel alloy) are joined together along
or point measurements.
their length, they will flex to form an arc as the
& Glass being breakable, the liquid may pose problems
temperature changes, as shown in Figure 8.5a.
on spillage.
& Need to make sure liquid is continuous in the cap-
illary column.
& Readings are subject to human error in reading.
. What are change-of-state temperature measuring
devices?
& Change-of-state temperature sensors consist of
labels, pellets, crayons, lacquers, or liquid crystals
whose appearance changes once a certain tempera-
ture is reached. They are used, for instance, with
steam traps—when a trap exceeds a certain temper-
ature, a white dot on a sensor label attached to the trap
will turn black. Response time typically takes min-
(a) Effect of temperature change on a bime- utes, so these devices often do not respond to tran-
FIGURE 8.5
tallic strip. (b) Bimetallic strip thermometer. sient temperature changes. Accuracy is lower than