Temperature Sensors. The basic types of temperature sensors are thermometers,
Thermometers. Thermometers are located on equipment and piping to provide
local temperature indication.
Thermocouples. Thermocouples provide a reliable and accurate
temperature measurement for most remote temperature sensing applications. Thermocouples
can be used with pneumatic and electronic transmitters or they can be direct connected
to some instruments. Thermocouples are non-linear.
temperature range about the same as a copper-constantan thermocouple with detection of
temperature changes of 0.03 degrees F (0.02 C). The resistance detector does not have a
reference junction, as a thermocouple, since it operates on the measured change in the
resistance of a metal or semiconductor (thermistor) with temperature. Platinum, because
of its inherent stability and linearity is becoming the standard of the industry with
some copper and nickel still being used. Copper is quite linear but nickel is quite
nonlinear. Resistance detectors can be used with pneumatic and electronic transmitters
or can also be direct-connected to some instruments. See Table 23 for ranges and use.
Transmitters. Transmitters are primarily used to transmit an analog output
(pneumatic or electronic) proportional to its measured signal. The American Petroleum
Institute Recommended Practice 550 suggests for general service applications that the
pneumatic tubing length from transmitter to controller to control valve not exceed 400
feet. Neither run should exceed 250 feet. Standard output signal for pneumatic
transmitter is 3-15 psi with a 20 psi air supply and 4-20 mA DC for electronic
transmitters with a nominal 30 VDC power supply. There are options for other output
signals depending upon the manufacture.
Recent electronic enhancements have made most electronic transmitters much
more accurate, reliable, and smaller in size, weight, and less expensive than its
pneumatic counterpart. Electronic signals are easier to adapt to microprocessor and
computer based systems and should therefore be given prime consideration over
Materials of construction is of prime importance when selecting a
transmitter. Many options are available and should be selected on the basis of need or
life expectancy and cost. Most transmitters are field-mounted near point of
measurement. Most electronic transmitters are the two wire type. Sensing line lengths
longer than 50 feet are not usually recommended and should be used only when absolutely
necessary. Where transmission line length (pneumatic) poses a problem in a flow loop,
use a volume booster, or mount the controller near the valve.