Page 309 - Instrumentation Reference Book 3E

Temperature measurement considerations 293

ruled out. They may also not be usable if there is of analog readouts, since they are not limited to a
likely to be big ambient temperature variation at rotary dial.
the readout point or along the length of the capil-
lary.
The next question is: what is the height difference 14.7.1.3 Temperature traiismitters
between the thermometer bulb and the readout On large installations, or where a wide variety of
position? Long vertical runs using liquid-in-metal different measurements are being made with a
thermometers can cause measurement offsets due wide range of instrumentation, it is more usual
to the liquid head in the vertical capillary adding to transfer the signal from the measurement point
to (or the subtracting from) the pressure at the to the control area by means of temperature
instrument Bourdon tube. In the case of height transmitters. This has the great advantage of
differences greater than. say, 10 meters, liquid allowing standardization of the readout equip-
thermometers are liitely to be unsuitable. This ment. Also, in the case of electrical transmission,
then reduces the choice to gas-filled instruments. by, say, a 4-20 milliamp signal, the measurement
A further consideration when specifying instru- is much less liable to degradation from electrical
mentation on a new plant is that it is convenient interference. Also, the use of temperature trans-
from itinerary considerations to use as many mitters allows the choice of measurement tech-
instruments of the same type as possible. The nique to be unencumbered by considerations of
choice of instrument is then dictated by the most length of run to the readout location.
stringent requirement. The choice of electrical or pneumatic trans-
On large installations where many different mission is usually dictated by overall plant policy
types of instrument are being installed and espe- rather than the needs of the particular measure-
cialip where pneumatic instrumentation is used, ment, in this case temperature. However, where
capillary instruments can run into an unexpected the requirement is for electrical temperature mea-
psychological hazard. Not infrequently a hard- surement for accuracy or other considerations the
pressed instrument technician has, on finding he transmission will also need to be electrical. (See
has too long a capillary, been known to cut a Part 4, Chapter 29.)
length out of the capillary and rejoint the ends
with a compression coupling. The result is, of
course, disaster to the thermometer. Where on 14.7.1.4 Coniputer-coiyimtible measurements
installation the capillary tube is found to be sig-
nificantly too long it must be coiled neatly in With the increasing use of computer control of
some suitable place. The choice of that place plants there is a requirement for measurements to
may depend 011 the type of instrument. In gas- be compatible. The tendency here is to use thermo-
filled instruments the location of the spare coil is couples, resistance thermometers, or where the
irrelevant but especially with vapor pressure accuracy does not need to be so high, thermistors
instruments it wants to be in a position where it as the measuring techniques. The analog signal is
will receive the minimum of ambient temperature either transmitted to an interface unit at the con-
excursions to avoid introduction of measurement trol room or to interface units local to the mea-
errors. surement. The latter usually provides for less
FOP installations with long distances between degradation of the signal.
the point of measurement and the control room it As most industrial temperature measurements
1s almost essential to use an electrical measme- do not require an accuracy much in excess of 0.5
ment technique. For long runs resistance therm- percent it is usually adequate for the interface
ometers are to be preferred to thermocouples for unit to work at eight-bit precision. Higher preci-
two principal reasons. First. the copper cables sion would normally only be required in very
used for connecting resistance bulbs to their read- special circumstances.
out equipment are very much less expensive than
thermoco qle wire or compensating cable. Sec- 14.7.1.5 Temperature controllers
ond, the resistance thermometer signal is a higher
level and lower impedance than most thermo- While thermometers, in their widest sense of
couple signals and is therefore less liable to elec- temperature measurement equipment, are used
trical interference. for readout purposes, probably the majority of
An added advantage of electrical measure- temperature measurenients in industrial applica-
ments is that, whether the readout is iocal or tions are for control purposes. There are there-
remote. the control engineer is given wider fore many forms of dedicated temperature
options as to the kinds of readout available to controllers on the market. As briefly described
him. Not only does he have a choice of analog or in Section 14.3.5.1, the simplest of these is a
digital readout but he can also have a wider range thermostat.