Moisture Sensing Elements; Flow Sensors - Honeywell AUTOMATIC CONTROL Engineering Manual

CONTROL FUNDAMENTALS

MOISTURE SENSING ELEMENTS

Elements that sense relative humidity fall generally into two
classes: mechanical and electronic. Mechanical elements
expand and contract as the moisture level changes and are
called "hygroscopic" elements. Several hygroscopic elements
can be used to produce mechanical output, but nylon is the
most commonly used element (Fig. 52). As the moisture
content of the surrounding air changes, the nylon element
absorbs or releases moisture, expanding or contracting,
respectively. The movement of the element operates the
controller mechanism.
NYLON ELEMENT
RELATIVE HUMIDITY SCALE
Fig. 52. Typical Nylon Humidity Sensing Element.
Electronic sensing of relative humidity is fast and accurate.
An electronic relative humidity sensor responds to a change
in humidity by a change in either the resistance or capacitance
of the element.
If the moisture content of the air remains constant, the
relative humidity of the air increases as temperature decreases
and decreases as temperature increases. Humidity sensors also
respond to changes in temperature. If the relative humidity is
held constant, the sensor reading can be affected by temperature
changes. Because of this characteristic, humidity sensors
should not be used in atmospheres that experience wide
temperature variations unless temperature compensation is
provided. Temperature compensation is usually provided with
nylon elements and can be factored into electronic sensor
values, if required.
Dew point is the temperature at which vapor condenses. A
dew point sensor senses dew point directly. A typical sensor
uses a heated, permeable membrane to establish an equilibrium
condition in which the dry-bulb temperature of a cavity in the
sensor is proportional to the dew point temperature of the
ambient air. Another type of sensor senses condensation on a
cooled surface. If the ambient dry-bulb and dew point
temperature are known, the relative humidity, total heat, and
specific humidity can be calculated. Refer to the Psychrometric
Chart Fundamentals section of this manual.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
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32

FLOW SENSORS

Flow sensors sense the rate of liquid and gas flow in volume
per unit of time. Flow is difficult to sense accurately under all
conditions. Selecting the best flow-sensing technique for an
application requires considering many aspects, especially the
level of accuracy required, the medium being measured, and
the degree of variation in the measured flow.
A simple flow sensor is a vane or paddle inserted into the
medium (Fig. 53) and generally called a flow switch. The
paddle is deflected as the medium flows and indicates that the
medium is in motion and is flowing in a certain direction. Vane
or paddle flow sensors are used for flow indication and
interlock purposes (e.g., a system requires an indication that
water is flowing before the system starts the chiller).
ON/OFF SIGNAL
TO CONTROLLER
PIVOT
FLOW
PADDLE (PERPENDICULAR TO FLOW)
Fig. 53. Paddle Flow Sensor.
Flow meters measure the rate of fluid flow. Principle types
of flow meters use orifice plates or vortex nozzles which
generate pressure drops proportional to the square of fluid
velocity. Other types of flow meters sense both total and static
pressure, the difference of which is velocity pressure, thus
providing a differential pressure measurement. Paddle wheels
and turbines respond directly to fluid velocity and are useful
over wide ranges of velocity.
In a commercial building or industrial process, flow meters
can measure the flow of steam, water, air, or fuel to enable
calculation of energy usage needs.
Airflow pickups, such as a pitot tube or flow measuring
station (an array of pitot tubes), measure static and total
pressures in a duct. Subtracting static pressure from total
pressure yields velocity pressure, from which velocity can be
calculated. Multiplying the velocity by the duct area yields
flow. For additional information, refer to the Building Airflow
System Control Applications section of this manual.
Applying the fluid jet principle allows the measurement of
very small changes in air velocity that a differential pressure
sensor cannot detect. A jet of air is emitted from a small tube
perpendicular to the flow of the air stream to be measured.
SENSOR
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