Flow Hood Calibration

Is my flow hood in calibration?
It was calibrated with the last year!

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

The following information is based on my experience with Shortridge Flow Hoods.

Flow hoods have been around since the mid 60’s in various forms. Flow hoods are a device that reads airflows from various air outlets with the hood being placed over the device. It channels airflow down through a cone of some type, square, rectangle or round to a flow sensing device. There are two typical devices that are used to sense the velocity pressure / velocity, a highly sensitive flow sensing manifold (Velgrid) or a thermal anemometer device that reads a temperature difference across a heated wire. The velgrid manifold simultaneously senses the total pressure and the static pressure at sixteen precision orifices spaced at the correct representative measurement points for a known cross-sectional area of the manifold. Most velgrids will have a minimum of sixteen precision orifices which meets industry standards for the number of measurement points for accuracy. The velgrid is actually two devices glued together with one exit point in the center which averages all of the sixteen test locations. These velgrids are reading velocity pressure, as you well know is total pressure minus static pressure. The exit points for the total pressure and static pressure are connected by rubber hoses to the meter. The meter uses the combined total pressure reading as well as the combined static pressure reading and converts it to velocity, and using the area (AK) displays its reading in CFM.

Now to clear up some important points, FLOW HOODS do not read Cubic Feet Per Minute (CFM). No meter known to man or women reads CFM directly, it must be calculated. Back when I had way too much time and was very curious as to how this flow hood device could calculate the CFM from the velocity obtained by the meter. Using a Shortridge flow hood (analog), removed the velgrid and measured its actual area and then measured the inside area of the flow hood where the velgrid is located. Subtracting the area of the velgrid from the area where it is attached demonstrated the actual area was one square foot or 1.0.

For the record, all meters that read airflow, especially flow hoods are actually reading velocity and if the area is 1.0, then that’s also the CFM. Just a reminder, CFM = A x V (Area x Velocity). Remember, it’s just a calculation and the flow hood read area does not need to be one square foot for a CFM reading with today’s electronic meters.

Now, the big question and the purpose of the article, is my flow hood accurate and how do I find out? Here’s a bulletin: just because it was calibrated within the last year and for others in the last few years, does not mean its calibrated now and is reading correctly.

Here is another issue to contemplate: just because the meter is calibrated does not mean that the flow hood assembly is in calibration.

In recent years, NEBB has allowed that the flow hood meter only needs to be calibrated, questionable in my opinion and as a past President of NEEB, I can say that. Just because the meter is in calibration does not mean that the airflow displayed is correct. Here are some reasons why:

  • Crack in the flow hood base, typical for the fiberglass hood bases that Shortirdge has.
  • Crack in the velgrid assemble.
  • Crack in the rubber hose connecting the velgrid and meter.
  • Velgrid orifices plugged with dirt.
  • One or both pressure devices in the meter malfunctioning.

In the past, the whole assemble was checked by the calibration labs and those issues were easily found and corrected or reported. Now it’s up to the individual/organization to make that determination.

Here are some ways to verify that your flow hood is reading accurately:

  • If the velgrid appears to be compromised, take it out and put tape over all of the orifices, top and bottom and cap off one side of the common exit point with a plug on the one hose of the side not being tested. Then take a squeeze bulb and connect it to the side you want to test and put pressure on it and see if it holds pressure. If it does, then that side is not compromised and then test the other side.
  • To field test the meter only, take meter out of the flow hood, select differential pressure, take a reading with no movement across the connection ports. The meter should read all zeros. You can also just set the hood down with no air movement through the hood and take a CFM reading which should also read all zeros. If in either method you get a reading, then your meter is not reading the velocity pressure accurately and needs to be sent back to factory for repair. Typically, the pressure conversion units are bad in the meter.
  • I had a Pharmaceutical customer that required that I get my flow hood calibrated for each day of use. After a short panic attack, I came up with a solution. A board, approximately four foot long, twelve inches wide with a U-Tube manometer mounted on it. With a hose connection between the U-Tube and my meter and a squeeze bulb, I would pressurize one side of the U-Tube down approximately 4” which would have the other side up 4”, I would set meter to read differential pressure and then would take a reading. The reading should be 8” as shown on the U-Tube manometer. Lucky for me I convinced the customer that if the meter was accurate, then the hood was accurate as well. Each reading uses approximately an eighth of an inch on the U-Tube.
  • Back in the good old days on TAB projects, we located one grille, typically a supply air grille with a perforated plate diffuser with a decent run of duct to that one grille. After checking for any air leakage and sealing any areas found, the duct was pitot tube traversed and a CFM was obtained and then the same grille was read with the flow hood. Hopefully they are the same or within 2-3%.
  • Another simple way to confirm accuracy of the flow hood is to read the same air outlet with other flow hoods. Take a company with many flow hoods, pick a day for all employees assigned with a flow hood to come to the office and everybody reads the same grille.
  • Another method of confirming accuracy is to build your own test device. Obtain a direct drive forward curved fan with a speed controller, a heavy gage metal base to connect the fan to, rectangle ductwork approximately 6 feet long, two long radius ninety’s and a 24 x 24 perforated grille. Connect this together, traverse airflow at various speeds, the read airflow at the grille with the hood.

Routinely maintenance of the flow hood assembly, including the velgrid, rubber connection hoses and the flow hood base is required. This involves cleaning the skirts, cleaning the velgrid with alcohol and cleaning the base. I have never seen many construction sites that are clean.

All NEBB Certified firms should have something inhouse to check the calibration of the flow hoods being used. Finding out that the flow hood used on a project was not accurate is not in the best interest of your company.