Handbook of air conditioning and refrigeration / Shan K

Pressure Sensors
Flow Sensors
ENERGY MANAGEMENT AND CONTROL SYSTEMS 5.19
In an EMCS, an electronic humidity sensing element measures the relative humidity and sends
an electric signal to the DDC unit. A typical space air humidity sensor has the following specifications:
Power source 24 V ac
Humidity range (active) 10 to 80 percent
Nominal output range 0 to 5 V dc
Accuracy at 70°F (21.1°C) � 3 percent at 10 to 60 percent range
� 4 percent at 60 to 80 percent range
Speed of response 8 min (90 percent of response time)
Mounting Wall mount
A pressure sensor usually senses the difference in pressure between the controlled medium (air and
water) and a reference pressure; or the pressure differential across two points, such as the pressure
differential across a filter. The reference pressure may be an absolute vacuum, atmospheric pressure,
or the pressure at any adjacent point. Output signals from the pressure sensors may be electric
or pneumatic, and analog or binary.
Pressure sensors used for HVAC&R systems can be divided into high-pressure and low-pressure
sensors. High-pressure sensors measure in pound per square inch or feet WC (kilo pascals), and
low-pressure sensors measure in inches WC (pascals). The sensing elements for high-pressure sensors
are usually Bourdon tube, bellows, and sometimes diaphragms. For low-pressure air sensors,
large diaphragms or flexible metal bellows are usually used.
To measure the duct static pressure, if a long section of straight duct of a length greater than
10 ft (3 m) is available, a single-point, pitot-tube type of duct static pressure sensor can be used.
Otherwise, a multipoint pitot-tube array or flow-measuring station with airflow straighteners should
be used. The small hole used to measure static pressure should never be directly opposite to an
airstream with a velocity pressure that can affect the reading. A reference pressure should be picked
up at a point with low air velocity outside the duct, at a point served by the same air system, or in
the ceiling plenum.
In a DDC system, low air pressure is often sensed by measuring the capacitance of two
diaphragms; one is allowed to move toward or away from the fixed one, depending on the pressure
on two sides of the diaphragm. An electric circuit is used to convert the capacitance to a voltage or
milliampere signal. The other method used to sense the low air pressure in DDC systems works because
the air pressure differential compresses or stretches a diaphragm as well as a strain gauge.
The change of the resistance of the gauge is detected and amplified, and this electric signal in the
form of voltage or milliamperes is sent to the controller.
A space air pressure sensor having an operating range of � 0.1 to � 0.1 in WC (� 25 Pa to � 25
Pa) detects the resistance of a silicon diaphragm, and thus the space air pressure is then measured.
Stainless steel, rubber, etc., can also be used as the material of the diaphragms. A space air pressure
sensor should be located in an open area of the conditioned space where the air velocity is less than
40 fpm (0.2 m/s) and where its reading is not affected by the opening of the doors. The reference
pressure pickup is best located at the rooftop, at a level 10 ft (3 m) above the building to avoid the
influence of wind.
Flow sensors usually sense the rate of air flow and water flow in cfm (L/s) for air and gpm (L/s)
for water. For airflow sensors, the average velocity pressure p v, in. WC (Pa), is often sensed and