Page 314 - Instrumentation Reference Book 3E

298 Temperature measurement

tip is kept as small as possible, usually by using a cone is found which will just bend over. This cone
thermocouple or thermistor, to keep the thermal indicates the temperature of the kiln. This can be
mass of the probe to a minimum. Again, accuracy confirmed by the fact that the cone of next higher
can be improved somewhat by using thermally melting point does not melt.
conducting grease. Figure 14.40 shows a typical Since the material of the cone is not a very good
hand-held probe. conductor of heat, a definite time is required for
the cone to become fluid, so that the actual tem-
perature at which the cone will bend will depend to
14.7.3 Miscellaneous measurement techniques a certain extent upon the rate of heating. In order
Temperature measurement may be the primary to obtain the maximum accuracy, which is of the
measurement required for the control of a plant. order of &lO"C, the cones must, therefore, be
There are, however, many cases where tempera- heated at a controlled rate.
ture measurement is a tool to get an indication of
the conditions in a plant. For instance, in distilla- 14.7.3.2 Tenzyerntzire-seiisitive pigments
tion columns it is more convenient and quicker to
judge the compositions of the offtake by tempera- In many equipment applications it is necessary to
ture measurement than to install on-line analyzers, ensure that certain components do not exceed a
and as a further bonus the cost of temperature specified temperature range. A typical case is the
measurement is very significantly less than the electronics industry, where it is essential that semi-
cost of analyzers. conductor components remain within their rather
The reverse situation also exists where it is not limited operating range, typically -5 "C to 85 "C,
possible to gain access for a thermometer to the or for equipment to military specification -40 "C
region where the temperature requires to be to 125 "C. These components are too small to fix
known. In this instance some indirect measure- all but the finest thermocouples to them. To deal
ment technique must be resorted to. One case of with this situation temperature-sensitive paints can
indirect measurement that has already been dealt be used. These paints contain pigments which
with at some length is the case of radiation therm- change color at known temperatures with an
ometers. accuracy of *lT. The pigments are available
either having a reversible or a non-reversible
color change, the latter being the more usually
14.7.3.1 Pyrometric cones
used. In the case above, a semiconductor compo-
At certain definite conditions of purity and pres- nent in an electronic machine can have two spots
sure. substances change their state at fixed tem- of paint put on its case having color changes at,
peratures. This fact forms a useful basis for fixing say, 0°C and 110°C. On subsequent inspection,
temperatures, and is the basis of the scales of perhaps after equipment failure, it can be seen at
temperature. once whether that component has been beyond its
For example, the melting points of metals give temperature tolerance.
a useful method of determining the electroniotive As an alternative to paint these pigments are
force of a thermocouple at certain fixed points on available on small self-adhesive labels. In either case
the International Practical Temperature Scale as they are available for temperatures within the range
has been described. of 0 "C to about 350 "C in steps of about 5 degrees.
In a similar way, the melting points of mixtures
of certain minerals are used extensively in the 14.7.3.3 Liquid crystals
ceramic industry to determine the temperature of
kilns. These minerals, being similar in nature to A number of liquids, mainly organic, when not
the ceramic ware, behave in a manner which indi- flowing tend to form an ordered structure with,
cates what the behavior of the pottery under simi- for instance, all the molecules lying parallel to one
lar conditions is likely to be. The mixtures, which another. This structure is maintained against the
consist of silicate minerals such as kaolin or china thermal agitation by weak intermolecular bonding
clay (aluminum silicate). talc (magnesium sili- such as hydrogen bonding. These bonds hold
cate), felspar (sodium aluminum silicate), quartz the structure until the weak bonds between the
(silica), together with other minerals such as cal- molecules get broken, as will occur when the
cium carbonate, are made up in the form of cones liquid begins to flow. The structure can also be
known as Seger cones. By varying the composi- changed by electric fields, magnetic fields, or
tion of the cones, a range of temperature between temperature. Different compounds respond to dif-
600 "C and 2000 "C may be covered in convenient ferent stimuli. Most people will be familiar
steps. with the liquid crystal displays on digital watches
A series of cones is placed in the kiln. Those of and pocket calculators. These displays use com-
lower melting point will melt, but eventually a pounds sensitive to electric fields.