CEMP-E
TI 811-12
18 August 1998
CHAPTER 5
INSTRUMENTATION AND CONTROLS
1.
SENSING DEVICES.
a. General. The following sensor descriptions include the majority of instrument types used to
interface the UMCS to utility systems for monitoring of system conditions and operation. Transmitters
providing a DC signal proportional to the required analog measurement will be included as a part of each
instrument to provide a linear conditioned signal for input to field equipment panels. The designer will
refer to TM 5-815-3, HVAC Control Systems, for additional information regarding the application of
sensing devices to HVAC systems.
b. Temperature instruments.
(1) Temperature instruments include various configurations of platinum resistance temperature
detectors (RTDs) and require proper housing for temperature measurement in rooms, ducts, piping, and
outside air (OA). The selection of a platinum RTD for the specific application depends upon the required
range and accuracy. Thermistors will not be used in UMCS applications. Thermocouples will not be used
in UMCS applications except in the specific case when the measured temperature might exceed the
maximum recommended temperature for platinum RTDs (about 1,800 degrees F) or when the
thermocouples are provided by the manufacturer of rotating machinery for bearing or coil temperature
measurement. Conditioning circuitry is required, and will be integral to the sensor.
(2) Continuous averaging RTDs have long, flexible sensing elements. They are installed in a
serpentine fashion in the cross-section of a duct to reduce measurement errors due to air stratification.
The continuous averaging RTD transmitter output signal represents the average temperature along the
sensing element.
(3) Temperature switches are bimetallic or filled elements affected by temperature changes
that cause contacts to open (or close) at a selected temperature setting. Temperature switches must be
adjustable over the operating temperature range.
(4) Temperature instruments on pipes or boiler stacks will be installed in thermowells. The
thermowell material will be selected based on the piping material and properties of the fluid in which the
thermowell is immersed.
(5) Outside air temperature instruments will be installed in instrument shelters to prevent the
sun from directly striking the sensors, and will be located and mounted to minimize direct solar radiation
and conductive heat transfer to the building.
c. Relative humidity instruments are used to measure percent relative humidity in spaces, ducts,
and OA. Where OA measurements are required, shielding will be provided to neutralize the effects of
solar heating and rain.
d. Pressure instruments.
(1) Pressure transducers are pressure measurement devices which use the deformation of an
elastic membrane as the primary measuring device. The various pressure transducers include bellows,
diaphragm, bourdon tube, and strain gage types. Pressure transducers are subdivided into a number of
categories including those for measuring gauge pressure, absolute pressure, and differential pressure.
5-1
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