Page 268 - Instrumentation Reference Book 3E

252 Temperature measurement

Table 14.11 Comparison of three types of dial themometers
~
Liquid-in-metal Gus expansion Vapor pressure
(cons tan t volume)
~
Scale Evenly divided. Evenly divided. Not evenly divided. Divisions
increase in size as the
temperature increases. Filling
liquid chosen to given reason-
ably uniform scale in the
neighborhood of the operating
temperatures.
Range Wide range is possible Usually has a range Limited for a particular filling
with a single filling of at least liquid, but with the choice of a
liquid, particularly 50°C between -130°C suitable liquid almost any tem-
with mercury. By and 540°C. Can be perature between -50 "C and
choice of suitable used for a lower 320 "C may be measured.
filling liquid. temperature than Instrument is not usually
temperatures may mercury in steel. suitable for measuring
be measured temperatures near ambient
between -200 "C temperatures owing to the lag
and 570 "C; introduced when bulb
but not with a single temperature crosses
instrument. ambient temperature.
Power arriilable to Ample power is Power available is Power available is very much
operate the available so that the very much less than less than that from liquid
indicator Bourdan tube may be that from liquid expansion.
made robust and arranged expansion.
to give good pointer
control.
Effect of d$ference When the system is No head error, as the Head error is not negligible. as
in level of bulb and filled with a liquid pressure due to difference the pressure in the system is
Bourdon tube at high pressure, in level is negligible in not large. Error may be
errors due to comparison with corrected over a limited range
difference of level the total pressure in the of temperature if the ratio
between bulb and system. pressure to deflection of the
indicator will be pointer can be considered
small. If the difference constant over that range. In
in level is very large. this case the error is corrected
a correction may be by resetting the pointer.
made.
Effect of changes Negligible. May produce a large error. Error may be large, but may
in barometric Error due to using the be corrected by resetting the
pressure instrument at a different pointer as for head error. Day-
altitude from that at to-day errors due to variation
which it was calibrated in barometric pressure may be
may be corrected by corrected by zero adjustment.
adjusting the zero.
Day-to-day variations
in barometric pressure
may be corrected for
in the same way.
Crrpillary error Compensation for change in Difficult to eliminate. No capillary error.
ambient temperature
Changes in Compensation obtained by Compensation obtained by Errors due to changes in the
temperature at the means of a bimetallic strip. means of bimetallic strip. elasticity of the Bourdon tube
indicatov are compensated for by means
of a bimetallic strip.
Accuracy *&i range to 320 "C &l% of differential rage of fl% of differential range even
of
=t% range above 320°C. the instrument if the with wide temperature
of
temperature of the capillary variation of the capillary and
and Bourdon tube does not Bourdon tube.
vary too much.