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b1 Airflow Sensing Elements
b1 Pressure Sensing Elements
b1 Flow Transducers
b1 Differential Pressure Transducers
b1 Indicating Meters
b1 Calibration Tools
 
 
 
Theory of Operation
 
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 below 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.
 
Figure 1
Pressure distribution on the surface of a Primary Sensing Element when inserted nominal to the direction of flow. The total pressure sensing port is located in the center of the positive pressure portion of the cylinders surface. The static pressure sensing ports are located in the same plane and are separated by about twice the critical angle. One of the static pressure sensing ports is centered in each of the two neutral pressure portions of the cylinder's surface.
 
Figure 2
Pressure distribution on the surface of a Primary Sensing Element when inserted at a 20 degree flow angle. The total pressure port remains within the positive pressure portion of the cylinders surface. The effect of one static sensing port entering into the positive portion of the cylinders surface is offset by the other static sensing port entering into an equally negative portion of the cylinders surface.
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.
 
 
 
 
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