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:

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.

Multimeter Safety

Overvoltage Installation Categories
The most important single concept to understand about the new standards is the Overvoltage Installation Category. The new standard defines Categories I through IV, often abbreviated as CAT I, CAT II, etc. (See Figure 1). The division of a power distribution system into categories is based on the fact that a dangerous high-energy transient such as a lightning strike will be attenuated or dampened as it travels through the impedance (acresistance) of the system. A higher CAT number refers to an electrical environment with higher power available and higher-energy transients. Thus a multimeter designed to a CAT III standard is resistant to much higher-energy transients than one designed to CAT II standards. Within a category, a higher voltage rating denotes a higher transient withstand rating; e.g., a CAT III-1000 V meter has superior protection compared to a CAT III-600 V rated meter..

When is 600V more than 1000V?

Table 2 helps us understand an instrument’s true voltage withstand rating:

1. Within a category, a higher “working voltage” (steadystate voltage) is associated with a higher transient, as would be expected. For example, a CAT III-600V meter is tested with 6000V transients while a CAT III-1000V meter is tested with 8000V transients. So far, so good.
2. What is not as obvious is the difference between the 6000V transient for CAT III-600V and the 6000 V transient for CAT II-1000 V. They are not the same. This is where the source impedance comes in. Ohm’s Law (Amps = Volts/Ohms) tells us that the 2Ω test source for CAT III has six times the current of the 12Ω test source for CAT II.

The CAT III-600V meter clearly offers superior transient protection compared to the CAT II-1000 V meter, even though its so-called “voltage rating” could be perceived as being lower. It is the combination of the steady-state voltage (called the working voltage), and the category that determines the total voltage withstand rating of the test instrument, including the all-important transient voltage withstand rating. A note on CAT IV: Test values and design standards for Category IV voltage testing are addressed in IEC 1010 second edition.

Table 2. Transient test values for measurement categories. (50 V/150 V/300 V values not included.)

Electrical Safety

Overvoltage Installation Categories

The most important single concept to understand about the new standards is the Overvoltage Installation Category. The new standard defines Categories I through IV, often abbreviated as CAT I, CAT II, etc. (See Figure 1). The division of a power distribution system into categories is based on the fact that a dangerous high-energy transient such as a lightning strike will be attenuated or dampened as it travels through the impedance (ac resistance) of the system. A higher CAT number refers to an electrical environment with higher power available and higher-energy transients. Thus a multimeter designed to a CAT III standard is resistant to much higher-energy transients than one designed to CAT II standards.

Within a category, a higher voltage rating denotes a higher transient withstand rating; e.g., a CAT III-1000 V meter has superior protection compared to a CAT III-600 V rated meter.

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Phantom RCU compatible for portable,
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The Phantom Miro M120 is a 2 megapixel camera with 1.6 Gigapixels/second (Gpx/s) throughput.
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