Reading Air Velocity

By Eric T Jenison, NEBB past President & FEBB Past President

Having proctored over 100 practical exams one observation seen constantly, is a lack of understanding of how to properly read air velocity to obtain the CFM present. It is a straight forward process that is not performed correctly very often.

Here is a little history lesson: when starting in the trade, an Alnor Deflecting Vane Anemometer (DVA) was used to read air velocities on all types of air distribution devices. That device came with three different probes, a lo-flow probe, a diffuser probe and a pitot tube. The diffuser probe was used all the time and it had wings on it to hold the probe in the correct and consistent location on the air distribution device so the only thing you had to concentrate on was to hold the probe straight. Manufacturer’s catalogs were available, with AK factors for all types of grilles based on the Alnor Deflecting Vane Anemometer, making it relatively easy to calculate the CFM from the air device.

Velocity is a vector quantity which denotes, at once, the time rate and the direction of a linear motion. The air velocity may be at filter banks, coils, air outlet grille/diffusers or even an open duct.

Everything measured concerning airflow is read in velocity (FPM). Remember this formula?

CFM = Area (A) x Velocity (V).

For example: Flow hoods read velocity (V) calculated from velocity pressure (VP) and times the AK of the air flow sensing area of the hood, displays that value in CFM.

There are many reasons why reading air velocity is important. Determining possible coil air stratification, reading velocity patterns from HEPA grilles to verify lamenter flow requirements; reading air velocities at kitchen hood filters; reading air velocity from all types of air distribution devices when a flow hood reading is not possible; and readings air velocities above the seating for drafts in movie theaters and many more applications.

At the Total Dynamic Balance, Education and Training Center (ETC) current office, there are many possibilities for air velocity readings which were done on purpose. Mostly for training technicians on the proper way to read air velocities with various instruments to obtain accurate CFM readings. As part of the NEBB practical examination at the ETC there are three opportunities to read air velocities as part of the exam.

The one that provides the most challenge is the reading of a side wall grille that requires the use of a velocity reading instrument to obtain the grille CFM. That part of the exam has been a struggle for all but a few candidates taking the exam. The main reason is the predominate use of flow hoods which can be modified by various means to read almost anything leaving very little use for a velocity reading meter. NEBB currently does not require a RVA for required instrumentation, which is highly questionable by this author.

As a young technician, it was desirous to know how to use all of the available instruments. At that time for determining air velocity readings, both the Alnor Deflecting Vane Anemometer (DVA) and a Rotating Vane Anemometer (RVA) were used. Quickly it was learned that these instruments were difficult to use to obtain an accurate CFM’s. A solution to fine tune the use and application of these instruments was to find one air distribution device that was connected by a single duct, verifying that no air leakage was present and then traversed the duct serving that single device by a pitot tube traverse to obtain the CFM of that air device. Next came practice reading the air velocity out of that one grille to figurate out how to read it accurately. After fine tuning the procedure on reading grille velocities, it was identified that two items were important, application of the instrument and most important the correct AK for the air device.

Here is the procedure developed for reading air velocities for air devices using an RVA:

  • Determine the free area of the grille. That would be the inside area of the grille. For example, if you have a 10 by 6 side wall grille, the inside free area is typically 9.25 by 5.25. The calculation to obtain the free area or AK of the grille would be 9.25 x 5.25 = 48.56 divided by 144 = .34.
  • For use of the RVA directly on the grille, which is not considered the correct application of the RVA, you would measure all of the deflection blades width and length and subtract that number from the free area to get an AK for this application of the meter.
  • If the grille had deflection blades which are used to set the air velocity pattern, typically up and down as well as right and left, set them to the neutral position. That would be a straight-out air velocity pattern.
  • If the grille has an opposed blade damper (OBD) behind the deflection blades, set it wide open.
  • Check the grille for velocity patterns, such as high and low velocities. Very seldom does a grille have laminar velocity coming out of it. This will determine where you start the RVA readings. If you start on the high side and then move to the low side, the RVA will coast down to the lower velocities causing an inaccurate reading. You always start on the lower velocities and proceed to the higher velocities.
  • After much research, it has been determined that the correct distance off the grille for the use of an RVA is 1” with the free area calculated as previously stated. There is a known phenomenon about air velocity moving through the deflection blades of a grille, it speeds up and then slows down after about 1” off the grille face. This is known as the vena contracta effect. Davis Instruments used to sell flow rings that can be connected to the RVA holding the distance off the grille at exactly 1”.
  • The typical measurement time is one minute, desiring the reading in FPM (Feet Per Minute). You can divide the grille being read for a thirty second reading and multiply that number by two or for a more accurate readings, take a two-minute reading and divide that reading by 2 for the one-minute reading. It can be done in two different styles, one with specific locations at a timed amount or moving the RVA slowly from specific location to specific location, low to high velocities.

For the above referenced example, 10 x 6 grille with a free area of .34. Take the readings at 1” off the grille starting at the lower velocities to the higher velocities and take a time measured reading. Let’s say the velocity reading for one minute was 864 fpm. It would be (CFM = A x V) or 864 x .34 = 294 CFM.

There is now a wide array of devices that you can use to read air velocities instead of an RVA. There are velgrids which can be obtained in all different sizes, an Air Foil Probe by Shortridge and Thermal Anemometers (Hot Wire).

Here is the procedure developed for reading air velocities for air devices using a Velgrid:

  • With the belief that 1” is the correct distance off the air device being measured, we have replaced the 1.5” stand off’s that all velgrids come with as a stand distance from Shortridge Instruments to 1” stand offs. Shortridge sells stand offs at various distances depending on what device you are measuring, especially for Cleanroom HEPA grilles.
  • With the use of the ETC, I have developed AK factors for devices such as kitchen hood filters for use of the velgrid which is very handy for measuring Kitchen hood filters instead of an RVA.
  • For other air devices such as side wall grilles, coil and filter banks, we use the standard free area calculation.

It is my personal considered opinion that all technicians should experiment with their velocity reading instruments to verify their proper use. By using procedures previously stated with a duct traverse of airflow to obtain a known and trusted CFM, test velocity readings with a velocity instruments to obtain the same CFM readings within +/- 5%. Also, always refer to the manufacture’s recommendation use of the instrument as a start.