This may be extremely important depending on what the volumetric flow is being used for, such as reporting emissions information. If we consider volumetric flow rate in a 3 foot diameter duct, this would cause a difference of 175.58 CFM. These yield a difference of 24.84 ft/min. Next, we’ll solve for the same differential pressure, but corrected for air density. Now let’s bring it all together into one example:įirst, we will calculate the velocity without correcting for air density. Our Pitot Tube, Series 160, includes a simple equation in its instruction manual, which is similar to the standard equation above, but takes air density into account in pounds per cubic foot (PCF). In order to ensure accurate velocity measurements, corrections must be applied to the standard equation. What does all of this mean? The environment in which flow measurements are monitored will rarely be taken under standard conditions. In this example, the standard temperature used for our 4004.4 constant in Fahrenheit is 70☏. Absolute Rankine temperature is derived as follows: The 4004.4 constant is derived from standard conditions set at 29.92 in Hg atmospheric pressure and absolute standard temperature 530°R. The equation above is based on standard temperature and density. The units of pressure for this example are in inches of water column. The standard equation for calculating velocity in feet per minute (FPM) is as follows: By subtracting the static pressure of a system from the total pressure, a differential pressure is generated. Pitot tubes are used to measure static pressure and total pressure. When using a mechanical flow sensor, such as a pitot tube, differential pressure can also be used to determine flow velocity and volumetric flow. As the filter becomes dirty, this differential pressure increases. A clean filter will typically have a baseline differential pressure generated as air flows through it. Filter health can be established by monitoring the pressure on the primary and secondary sides of a filter. One of these applications involves determining if a filter in your system needs to be cleaned or replaced. Differential pressure measurement can be beneficial in a wide variety of building automation system (BAS) applications.
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