Page 310 - Instrumentation Reference Book 3E

294 Temperature measurement

Thermostats A thermostat is a device in which 14.7.2 Sensor location considerations
the control function, usually electrical contacts To obtain accurate temperature measurement
but sometimes some other control function such careful consideration must be given to the siting
as a valve, is directly controlled by the measure- of temperature sensing probes. Frequently in
ment action. The instrument described in Section industrial applications temperature measuring
14.3.5.1 uses solid expansion to operate electrical equipment does not live up to the expectations
contacts, but any of the other expansion tech- of the plant design engineer. The measurement
niques may be used. In automotive applications error is not infrequently ten or even twenty times
the thermostat in an engine cooling system is a the error tolerance quoted by the instrument
simple valve directly operated either by vapor manufacturer.
pressure or change of state; e.g., the change of Large measurement errors in service may be
volume of wax when it melts. due to the wrong choice of instrument but more
Thermostats, however. are very imprecise
controllers. In the first place their switching frequently the error is due to incorrect location
of the measurement points. Unfortunately the
differential (the difference in temperature location of temperature sensors is dictated by
between switch-off and switch-on) is usually the mechanical design of the plant rather than
several Kelvin. Second, the only adjustment is by measurement criteria.
setpoint.

Contact dial tlzemoineters A first improvement 14.7.2. I Immersion probes
on a thermostat is the use of a contact dial therm- To minimize errors in the measurement of the
ometer. The dial of this instrument carries a sec- temperature of process fluids, whether liquid or
ond pointer, the position of which can be set by gas, it is preferable to insert the sensor so that it is
the operator. When the indicating pointer reaches directly immersed in the fluid. The probe may be
the setpoint pointer they make electrical contact directly dipped into liquid in an open vessel,
with one another. The current that then flows inserted through the wall of the vessel. or inserted
between the pointers operates an electrical relay into a pipe.
which controls the load. In this case the switching
differential can be very small, typically a fraction Measurement of liquid in vessels Temperature
of a Kelvin.
measurement of liquid in a plant vessel may illus-
trate the dilemma of the control engineer when
Proportional temperature controllers Dedicated faced with mechanical problems. Consider Figure
one-, two- or three-term temperature control- 14.62, which represents a vessel filled with liquid
lers are available in either pneumatic or elec- and stirred by a double anchor agitator. The ideal
tronic options. The use of such controllers is place to measure the temperature would be some-
mainly confined to small plants where there is where near the center of the mass at, say; T1. The
a cost advantage in avoiding the use of trans- best arrangement would seem to be a dip probe
mitters. T2. But even though the design level of the liquid
In the case of pneumatic controllers the input is at A in operation the liquid level may fall as low
measurement will be liquid, vapor pressure, or as B leaving probe T2 dry. The only remaining
gas expansion. The Bourdon tube or bellows possibility is T3. This is not a very good approach
used to measure the pressure in the capillary to T1 and is subject to error due to conduction of
system operates directly on the controller heat from or to the vessel wall.
mechanism. An approach that can be used if the tempera-
However, in recent years there has been an ture measurement is critical is to mount a com-
enormous increase in the number of electronic plete temperature measuring package onto the
temperature controllers. The input to these shaft of the agitator. Wires are then brought up
instruments is from either a thermocouple or a the shaft out of the vessel from where the tem-
resistance thermometer. The functions available perature signal can be taken off with slip rings,
in these controllers vary from on/off control to inductively coupled, or radio telemetered to a
full three-term proportional, integral, and deriva- suitable receiver. This is, of course. only possible
tive operation. Some of the more sophisticated where the temperature of the process is within
electronic controllers use an internal micropro- the operating range of the electronics in the mea-
cessor to provide the control functions. Some surement package. The use of slip rings is not
units are available with the facility to control very satisfactory as they add unreliability, but
several temperature control loops. Of course the in the absence of slip rings the package must
use of an internal microprocessor can make direct also carry its own power supply in the form of
computer compatibility a simple matter. batteries.