The Primary Sensing Elements are aerodynamic
head devices which generate a differential (velocity) pressure output
signal. The Primary Sensing Elements output is similar to that of an
orifice plate, venturi, flow nozzle, or other head-producing primary
elements, in that the differential pressure produced by these devices is
related to the air quantity passing through the measurement plane.
The basic design of the FE Series is a
"cylindrical tube within a cylindrical tube" that permits the simultaneous
measurement of both impact (total) pressure and static pressure. The
total and static sensing port design eliminates the need for air
straightening based on a known pressure distribution over a cylindrical
surface. The incorporation of multiple total and static pressure
sensing ports strategically located along its length provide comprehensive
averaging of the velocity variations within the duct.
The inner tube is an isolated chamber serving as
a manifold for the total pressure sensing ports. The pressure within
this manifold is influenced by the internal movement of air between the
multiple ports, which occurs because of their pressure differences.
Similarly, the outer tube is an isolated chamber
serving as a manifold for the static pressure sensing ports. The
offset static sensing ports are located at the critical angle of the
cylinder's surface where there is no influence of dynamic (velocity)
pressure when the element is inserted perpendicular to flow. The static
port design of the FE Series permits flow-angle variations of up to ±20
degrees to occur without affecting the output value due to the linear
characteristic of this portion of the pressure distribution curve. As
the flow-angle changes from nominal, one static port is exposed to a higher
pressure and the other static port is exposed to a lower pressure of equal
value.
The pressure within each manifold is sent
through signal ports to become the output source of the FE. Total and
static pressure outputs are transmitted through interconnected tubing as
primary input signals to a differential pressure measurement device (gauge,
transmitter, etc.) for flow indication, alarming, data logging and/or
feedback control purposes. The duct-mounted Model PE-5000s are similar
to the FE Series, but are furnished with static pressure ports only.
The cross-sectional views shown in Figures 1 and
2 identify the distribution of pressure around the
cylindrical surface of the Primary Sensing Element in a moving stream of air
or gas. These figures illustrate the innovative "double-hollow"
design, the implementation of existing pressure measurement fundamentals,
and the ability of the Primary Sensing Element to measure true air velocity
in moderately turbulent flow conditions.
Multiple Model FE-1000s are required for most
applications. Application and configuration tables are found in the
Technical Data Sheet for the Model FE-1000 series products, and were
developed to comply with the guidelines for pitot-tube traversing of air
ducts as published in the American Society of Heating, Refrigerating, and
Air-Conditioning Engineers (ASHRAE) Fundamentals Handbook.
PCI
Primary Sensing Elements are designed for use in HVAC airside systems or
industrial process control loops. The Primary Sensing Elements provide
output signals readily convertible to air velocity, air volume, and
differential pressures by devices such as indicating meters, transducers,
and controllers. PCI has designed and manufactured Primary Sensing
Elements to meet the needs of all commercial and most industrial
applications since 1984. PCI Primary Sensing Elements are installed
worldwide as part of systems requiring continuous signals for HVAC or
industrial automatic control, alarming, and process indication.
