Page 262 - Instrumentation Reference Book 3E

246 Temperature measurement
to the emergent column of mercury and the glass Glass thermometers used in industry are
stem being at a different temperature than the bulb. usually protected by metal sheaths. These sheaths
Errors introduced this way should be allowed for if may conduct heat to or from the neighborhood of
accurate readings are required. Some therm- the thermometer bulb and cause the thermometer
ometers, however, are calibrated for partial immer- to read either high or low according to the actual
sion and should be used immersed to the specified conditions prevailing. A thermometer should,
depth. therefore, be calibrated, whenever possible, under
When reading a thermometer an observer the conditions in which it will be used, if accurate
should keep his eye on the same level as the top temperature readings are required. If, however,
of the mercury column. In this way errors due to the main requirement is that the temperature
parallax will be avoided. indication be consistent for the same plant tem-
Figure 14.3 shows the effect of observing the perature, then an error introduced is not so
thermometer reading from the wrong position. important, so long as the conditions remain the
When viewed from (a) the reading is too high. same, and the error is constant.
Taken from (b) the reading is correct, but from
(c) it is too low.
A mercury-in-glass thermometer has a fairly Errors due to aging It is often assumed that pro-
large thermal capacity (i.e., it requires quite an vided a mercury-in-glass thermometer is in good
appreciable amount of heat to change its tempera- condition it will always give an accurate reading.
ture by one degree), and glass is not a very good This is not always so, particularly with cheap
conductor of heat. This type of thermometer will, thermometers. A large error may be introduced
therefore, have a definite thermal lag. In other by changes in the size of the bulb due to aging.
words, it will require a definite time to reach the When glass is heated to a high temperature, as it
temperature of its surroundings. This time should is when a thermometer is made, it does not, on
be allowed for before any reading is taken. If there cooling, contract to its original volume immedi-
is any doubt as to whether the thermometer has ately. Thus, for a long time after it has been made
reached equilibrium with a bath of liquid having a the bulb continues to contract very slowly so that
constant temperature, then readings should be the original zero mark is too low on the stem. and
taken at short intervals of time. When the reading the thermometer reads high. This error continues
remains constant the thermometer must be in equi- to increase over a long period, and depends upon
librium with the bath. If the temperature is varying the type of glass used in the manufacture of the
rapidly the thermometer may never indicate the thermometer. In order to reduce to a minimum
temperature accurately, particularly if the tested the error due to this cause, during manufacture
medium is a gas. thermometers are annealed by baking for several
days at a temperature above that which they will
be required to measure, and then cooled slowly
over a period of several days.
Another error due to the same cause is the
- Capillary depression of the zero when a thermometer is
tube cooled rapidly from a high temperature. When
cooled, the glass of the thermometer bulb does
not contract immediately to its original size so that
the reading on the theimometer at low temperature
is too low, but returns to normal after a period of
time. This period depends upon the nature of the
glass from which the bulb is made.

High teinperature tlzerniometers Mercury nor-
I Mercury mally boils at 357 "C at atmospheric pressure. In
order to extend the range of a mercury-in-glass
thermometer beyond this temperature, the top
end of the thermometer bore is enlarged into a
bulb having a capacity of about 20 times that of
the bore of the stem. This bulb, together with the
bore above the mercury, is then filled with nitro-
gen or carbon dioxide at a sufficiently high pres-
sure to prevent the mercury boiling at the highest
Figure 14.3 Parallaxerrorswhen reading glass
thermometerer. temperature at which the thermometer will be