Does an Anemometer measure airflow in CFM at return/exhuast Grille / Register / Diffuser 2

[katwalatapan]

Hello,

I have a query concerning whether an Anemometer either Vane type or Thermal type, would be able to measure volumetric air flow in CFM at the exhaust or return grille / register / diffuser?

I've mostly observed Anemometers being used to measure CFM airflow at the supply grille / register / diffuser, but not at the return or exhaust. I suppose a reversible vane head may be required to measure volumetric air flow at the return or exhaust grille / register / diffuser.

I would appreciate if I could be directed towards an Anemometer manufacturer that has a product that would be able to measure volumetric air flow at the supply as well as return grille / register / diffuser.

Thank you.

 

[rb1957]

i see no reason why it can't. but, i think you'll have a problem with the static pressure within the diffusor.

maybe you need a pitot tube, like aircraft use, to measure static and dynamic pressure ?

another day in paradise, or is paradise one day closer ?

 

[TheTick]

An anemometor measures rotational speed of it's vane, which is roughly proportional to air velocity. It's the engineer's job to correlate that to volumetric air flow.

 

[berkshire]

He also mentioned thermal type, which measures temp drop by thermistor.
B.E.

You are judged not by what you know, but by what you can do.

 

[IRstuff]

"which measures temp drop by thermistor."

which vaguely proportional to square root of air velocity.

The issue, as mentioned above, is to then tie velocity to air volume.

TTFN
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[MintJulep]

Anemometers measure velocity.

Vane types, and most thermal types are directional. That is, they measure most accurately when the air stream you are measuring is "collimated" (for lack of a better term).

That is typically not the case at a return grill.

If you need to measure flow at a return grill your best chance at a reasonably accurate reading is with a flow hood.

 

[3DDave]

I've wanted to see this test done with a big plastic bag. Expect 300CFM? 300 cu-ft bag and a stopwatch. On outflow it is easy to smash the bag flat and time how long it takes to inflate it. For intake, make a bag that is rectangular with a frame at intervals to maintain the initial section and volume before it wads up like an accordion. Some means needs to be in place to prevent problems when it finally goes flat and cuts the air supply, but other than that it should be OK.

Velocity measurements to get volume require assuming the flow field is uniform and picking a representative spot or trying to make a number of measurements without affecting the flow field, which is hard to do.

One thing is that mass flow will remain constant if the air system is free of gross leaks and such. Measuring anywhere in the duct work is better than trying to capture it outside. Hence mass-flow measurement for engines is done in the throttle-body, not outside the air filter. If a de-turbulating stage is added to straighten the flow it is pretty easy to do a velocity sweep across a duct section and get results with good accuracy. Tedious, but easy. Some makers supply venturi adapters for intakes to do just that.

 

[TheTick]

Procedures for measuring air flow in HVAC systems are well-established. Most are common sense. There is some specialized equipment to make the job easier. Check out web sites for NEBB and AABC.

 

[dvd]

Testing and balancing companies are more likely to use an airflow hood to determine flow -

http://www.dwyer-inst.com/Product/TestEquipment/AirFlowHood/ModelAFH2

 

[drawoh]

I've wanted to see this test done with a big plastic bag. Expect 300CFM? 300 cu-ft bag and a stopwatch. On outflow it is easy to smash the bag flat and time how long it takes to inflate it. For intake, make a bag that is rectangular with a frame at intervals to maintain the initial section and volume before it wads up like an accordion. Some means needs to be in place to prevent problems when it finally goes flat and cuts the air supply, but other than that it should be OK.
...

Wouldn't the bag affect airflow?

I would stick with a set of anemometer or Pitot tube readings across the conduit.

--
JHG

 

[3DDave]

In a sense, yes - it prevents air that is not in the control volume from interfering with the measurement. It's a practical application of the textbook control volume. It's not as convenient as purpose built tools for building airflow measurement, but for a one-off or a classroom exercise, it clearly demonstrates the control volume principle and should be sufficiently accurate for most applications.

The reason the bag works best on the suction side is that the air that surrounds it is the same temperature as the air inside, so the volume it represents at the outset is not going to vary with time. There is a delta-p contribution to the density from deforming the bag as it shrinks, but even .1 psi = more than 1 lbf/square foot, which should be enough to get the bag to move, while representing less than 1% error. There may also be some wrinkles in the bag at the end, but a large enough initial volume should drop them to a few percent as well.

If it is on the output side of the system, there's a chance the air is warmer or colder so one has to wait for the air temp in the bag to stabilize to get a decent measurement; and that makes it harder to measure the volume of the bag.

 

[katwalatapan]

Thank you very much for all your responses. It helps me better understand our requirements and appropriate tools that I should look into.

Cheers!