TM 5-810-15
b. Field transmitters.
(4) Pressure transmitters. Measuring elements
(1) General. Electronic transmitters should
for pressure transmitters will be diaphragm, bel-
produce a 4-20 mA dc signal that is linear with the
lows, bourdon tube, or strain gage transducers.
measured variable. Electronic transmitters will be
Pressure transmitters will be accurate within 0.5
the two wire type except when unavailable for a
percent of span with ambient temperature effect not
particular application. Encapsulated electronics are
to exceed 1.0 percent of span per 100 degrees F
unacceptable in any transmitter. Transmitters will
variation. Measuring elements for pressure differ-
be selected such that the output signal represents a
ential transmitters will be diaphragm, bellows, or
calibrated scale range that is a standard scale range
sealed pressure capsule. Pressure differential
between 110 and 125 percent of the maximum
transmitters will be accurate within 0.25 percent of
value of the measured process variable.
span with ambient temperature effect not to exceed
Transmitters will be designed for the service
1.0 percent of span per 100 degrees F variation.
required and will be supplied with mounting
Output signals for pressure and pressure differential
brackets. Purge meters and differential regulators
transmitters will be linear with the sensed pressure
will be used on transmitters for boiler gas service or
or differential pressure.
(5) Temperature sensors. Several types of sen-
coal-air mixture service. A change in the load on a
transmitter within the transmitter load limits will
sors can be used for temperature measurement.
not disturb the transmitted signal. The load limits of
Thermocouples sense temperature by a thermoelec-
the transmitter will be a minimum of 600 ohms.
tric circuit which is created when two dissimilar
Transmitters can be supplied with local indicators,
metals are joined at one end. A wide variety of
either integral or field mounted, if desired.
thermocouples are available for temperature sens-
Transmitters used for distributed control systems
ing. Type J (iron-constantan) and type K (chromel-
should be the "smart" type which have duplex
alumel) are the most common types of thermocou-
digital communications ability transparent to the
ples for boiler plant applications. Type J
analog signal. Smart transmitters may be remotely
thermocouples can be used for temperatures from
calibrated via a hand held terminal. Data available
32 to 1382 degrees F. Type K thermocouples can
at the hand held terminal should include
be used for temperatures from -328 to 2282
programmed instrument number, instrument ID or
degrees F. The type of thermocouple to be used,
serial number, instrument location, date of last
Type J or Type K, will be selected based upon the
calibration, calibrated range and diagnostics.
temperatures to be sensed. All thermocouples in the
(2) Flow transmitters. Numerous types of flow
boiler plant will be of the same type. Resistance
transmitters are available. These include differential
Temperature Detectors (RTD) sense temperature
pressure with square root extractor, turbine
based upon the relationship between the resistivity
flowmeters, nutating disk type transmitter, ultra-
of a metal and its temperature. The most common
sonic flow transmitters, and magnetic flowmeters.
RTD used in boiler plant applications is a platinum
The most common method for measuring flow is to
RTD with a resistance of 100 ohms at 0 degrees C.
measure differential pressure across an orifice, flow
Sealed bulb and capillary sensors detect
nozzle, venturi, pitot tube, or piezometer ring.
temperature by sensing the change in volume due
Square root extraction is necessary to linearize the
to changes in temperature of a fluid in a sealed
output signal. Differential pressure measurement
system.
(6) Temperature transmitters. Measuring ele-
should be used for most steam plant flow applica-
tions. Nutating disk with pulse to 4-20 mA trans-
ments for temperature transmitters should be
mitters are normally used for fuel oil flow mea-
thermocouple, RTD, or sealed bulb and capillary.
surement. Flow transmitters will be accurate within
Temperature transmitters will be accurate within
0.5 percent of span from 20 to 100 percent span
0.5 percent of span with ambient temperature effect
with ambient temperature effect not to exceed 1.0
not to exceed 1.0 percent of span per 100 degrees
percent per 100 degrees F variation.
F degree variation. Output signals for temperature
(3) Level transmitters. Measuring elements for
transmitters will be linear with the sensed
level transmitters will be diaphragm, bellows,
temperature.
(7) Oxygen analyzers. Oxygen analyzers will
bourdon tube, strain gage transducer, caged float,
or sealed pressure capsule. Level transmitters will
be direct probe type utilizing an in situ zirconium
be accurate within 0.5 percent of span with ambient
sensing element. The element will be inserted
temperature effect not to exceed 1.0 percent of
directly into the gas stream and will directly contact
span per 100 degrees F variation. The output signal
the process gases. The sensing element will be
will be linear with the sensed level.
provided with a protective shield to prevent direct
9-22
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