Page 266 - Instrumentation Reference Book 3E

250 Temperature measurement

6Q Inert Steel 7 &$ Outercasing X I
'ap I I I a r! Tubing -
i

EFFECT OF INCREASE TEMPERATURE
OF
AS SHOWN IN SECTION 'XY'
A Remains a constant diameter
Low Temp High Temp B Increases in diameter
SECTION XV C Volume of space increase? more
than the volume of mercury therein
Figure 14.9 Ambient temperature compensation chamber.

to compensate for the relative increase in volume tubes are made from materials other than
of the mercury within the chamber and in the stainless steel, it may be necessary to protect
intervening length of capillary tube. them from corrosion or mechanical damage.
This may be done by covering the tube with
thermal insulation material-formerly asbestos
14.3.2.2 Other filling liquids
was used-and winding the whole in a heavy
Admirable though mercury may be for therm- spiral of bronze. In cases where a bronze outer
ometers, in certain circumstances it has its limita- casing is likely to be damaged either by acid
tions, particularly at the lower end of the fumes or mechanically, it may be replaced by a
temperature scale. It is also very expensive to stainless steel spiral which results in a much
weld mercury systems in stainless steel. For these stronger but slightly less flexible construction.
and other reasons, other liquids are used in place For use in damp places, or where the tube is
of mercury. Details of the liquids used in liquid- liable to be attacked by acid fumes, the capil-
in-metal thermometers, with their usual tempera- lary and bronze spiral may be protected by a
ture ranges, are given in Table 14.9. Comparison covering of molded rubber, polyvinyl chloride,
with Table 14.8 shows that liquids are used for or rubber-covered woven-fabric hose. For use
different temperature ranges in glass and metal on chemical plants, such as sulfuric acid plants,
thermometers. In general, in metal thermometers, both the capillary tube and the bulb are pro-
liquids can be used up to higher temperatures tected by a covering of lead.
than in glass thermometer as they can be filled The construction of the liquid-in-metal therm-
to higher pressures. ometer is the same as that of the mercury-in-steel
When liquids other than mercury are used, the thermometer, and compensation for changes in
bulb and capillary tube need no longer be made ambient temperature may be achieved in the same
of steel. The material of the bulb may, therefore, ways.
be chosen from a wide range of metals and alloys, Further facts about liquid-in-metal therm-
and is selected to give the maximum resistance to ometers will be found in Table 14.1 1, which gives
any corrosive action which may be present where a comparison of the various forms of non-elec-
the bulb is to be used. trical dial thermometers.
The capillary tube, too, may be made from a In installations where liquid-filled instruments
variety of materials, although copper and with very long capillaries are used, care must be
bronze are the most common. When capillary taken to see that there is not a significant height
difference between the bulb location and that of
the instrument. If there is a large height differ-
Table 14.9 Liquids used in metal thermometers ence, the pressure due to the column of liquid in
(expansion type)
the capillary will be added to (or subtracted from)
Liquid Temperature range ("C) the pressure due to the expansion of the liquid in
the bulb, resulting in a standing error in the tem-
Mercury -39 to +650 perature reading. This problem is at its worst with
Xylene -40 to +400 mercury-filled instruments. Instruments with
Alcohol -46 to +150 double capillary ambient temperature compensa-
Ether +20 to +90 tion, Figure 14.8, are, of course, also compen-
Other organic liquids -87 to +260
sated for static head errors.