Air Velocity Measurement

Calculating Air Velocity from Velocity Pressure

Manometers for use with a Pitot tube are offered in a choice of two scale types. Some are made specifically for air velocity measurement and are calibrated directly in feet per minute. They are correct for standard air conditions: i.e. air density of .075 lbs. Per cubic foot which corresponds to dry air at 70°F, barometric pressure of 29.92 inches Hg. To correct the velocity reading for other than standard air conditions, the actual air density must be known. It may be calculated if relative humidity, temperature and barometric pressure are known. Most manometer scales are calibrated in inches of water. Using readings from such an instrument, the air velocity may be calculated using the basic formula: 

With dry air at 29.9 inches mercury, air velocity can be read directly from curves on the following page. For partially for fully saturated air a further correction is required. To save time when converting velocity pressure into air velocity, the Dwyer® Air Velocity Calculator may be used. A simple slide rule, it provides for all the factors needed to calculate air velocity quickly and accurately. It is included as an accessory with each Dwyer® Pitot tube. 


To use the Dwyer® Calculator: 

1. Set relative humidity on scale provided. On scale opposite known dry bulb temperature, read correction factor. 
2. Set temperature under barometric pressure scale. Read density of air over correction factor established in #1. 
3. On the other side of calculator, set air density reading just obtained on the scale provided. 
4. Under Pitot tube reading (velocity pressure, inches of water) read air velocity, feet per minute. 

Determining Volume Flow
Once the average air velocity is know, the air flow rate in cubic feet per minute is easily computed using the formula:


Q = AV
Where: Q = Quantity of flow in cubic feet per minute.
            A = Cross sectional area of duct in square feet.
            V = Average velocity in feet per minute.


Determining Air Volume by Calibrated Resistance
Manufacturers of air filters, cooling ad condenser coils and similar equipment often publish data from which approximate air flow can be determined. It is characteristic of such equipment to cause a pressure drop which varies proportionately to the square of the flow rate. Fig. 5 shows a typical filter and a curve for air flow versus resistance. Since it is plotted on logarithmic paper, it appears as a straight line. On this curve, a clean filter which causes a pressure drop of .50” w.c. would indicate a flow of 2,000 CFM.

Other Devices for Measuring Air Velocity
Electronic manometers. The latest advance in the field of pitot tubes and manometers is the electronic manometer. Like a liquid manometer, this instrument is a true differential pressure measuring device. Therefore, it can be used with any standard pitot tube to measure pressure and/or velocity. The primary differences between the electronic and the liquid manometers stem from the fact that the electronic manometer is a digital, sometimes microprocessor-based, instrument. The electronic manometer needs to be zeroed only once each day it is used and does not require leveling.


Its hand-held operation can be carried out in any orientation. It reads directly in in. H2O or fpm (or in pascals or meters/second). On a duct traverse, the readings along with their average can be directly printed on a microprinter. Up to 125 readings may be entered (or printed) and averaged. The electronic manometer can be operated with or without the printer.