Page 263 - Instrumentation Reference Book 3E

Measurement techniques: direct effects 247

used. In order to extend the range to 500"C, a Mercury is a good electrical conductor. By
pressure of about 20 bar is required. In spite of the introducing into the bore of a thermometer two
existence of this gas at high pressure. there is a platinum contact wires, one fixed at the lower end
tendency for mercury to vaporize from the top of of the scale and the other either fixed or adjust-
the column at high temperatures and to condense able from the top of the stem, it is possible to
on the cooler portions of the stem in the form of arrange for an electrical circuit to be completed
minute globules which will not join up again with when a predetermined temperature is reached.
the main bulk of the mercury. It is, therefore, The current through the circuit is limited to about
inadvisable to expose a thermometer to high tem- 25 mA. This current is used to operate an electro-
peratures for prolonged periods. nic control circuit. Contact thermometers find
At high temperatures the correction for the tem- applications in laboratories for the temperature
perature of the emergent stem becomes particu- control of water baths, fluidized beds and incu-
larly important. and the thermometer should, if bators. With careful design, temperature control
possible, be immersed to the top of the mercury to 0.1 "C can be attained.
column. Where this is not possible. the therm- Formerly, fixed temperature contact therm-
ometer should be immersed as far as conditions ometers were used for the temperature control
permit, and a correction made to the observed of quartz crystal oscillator ovens, but now this
reading, for the emergent column. To do this, duty is more usually performed by thermistors or
the average temperature of the emergent column semiconductor sensors which can achieve better
should be found by means of a short thermometer temperature control by an order of magnitude.
placed in several positions near to the stem. The
emergent column correction may then be found
from the formula: 14.3.2 Liquid-filled dial thermometers
correction = 0.0016(tl - t2)n on Celsius scale 14.3.2.1 Mercury-in-steel thermometer
where tl is the temperature of the thermometer Two distinct disadvantages restrict the usefulness
bulb, t2 is the average temperature of the emer- of liquid-in-glass thermometers in industry: glass
gent column, and n is the number of degrees is very fragile, and the position of the thermometer
exposed. The numerical constant is the coefficient for accurate temperature measurement is not
of apparent expansion of mercury in glass. always the best position for reading the scale of
the thermometer.
These difficulties are overcome in the mercury-
14.3.1.2 Use of liquids other than mercury in-steel thermometer shown in Figure 14.4. This
In certain industrial uses, particularly in industries type of thermometer works on exactly the same
where the escape of mercury from a broken bulb principle as the liquid-in-glass thermometer. The
might cause considerable damage to the products, glass bulb is, however, replaced by a steel bulb
other liquids areused to fill the thermometer. These and the glass capillary tube by one of stainless
liquids are also used where the temperature range of steel. As the liquid in the system is now no longer
the mercury-in-glass thermometer is not suitable. visible, a Bourdon tube is used to measure the
Table 14.8 lists some liquids together with their
range of usefulness.
14.3.1.3 Mevcuvy-in-glass electric contact
thermometer
A mercury-in-glass thermometer can form the
basis of a simple onloff temperature controller
which will control the temperature of an enclos-
ure at any value between 40 "C and 350 "C.

fin8 bom capiffq
Table 14.8 Liquids used in glass thermometers i
lubr tiffed wifb
mercury
Liquid Temperature range ("C)
Mercury -35 to +510
Alcohol -80 to f70
Toluene -80 to +loo Sled bulb fiffed
Pentane -200 to +30 wilh mercury
Creosote -5 to +200
Figure 14.4 Mercury-in-steel thermometer.