How do I measure air flow inside a duct?

[digitrex]

Friends, can you suggest what portable instrument is suitable for measuring air-flow rate in a duct?

Somebody tells me that a single point measurement in the centre of the duct is not a correct figure because the air speed is not uniform in a duct.

For an old duct, I can foresee problem because of few constraint,eg:
1. If I put my hand(which carries the measuring instrument) into the air duct, I cannot close the opening anymore.
2. If I have a small probe that can insert through a small hole, I can't make multiple measurements in order to get average value(unless I make many holes on the duct wall).

 

[MintJulep]

I didn't read it in detail, but this looks pretty comprehensive.

http://www.flowmeterdirectory.com/flowmeter_artc/flowmeter_artc_02111201.html

 

[digitrex]

Thank you MintJulep,
I will try to read and see if it can help me.

 

[MarauderX]

Is there any reason you can't get a balancing contractor/building engineer to do it for you? They will typically measure the air flow at each outlet then calculate a total - and they can do it in a very short time. If you have a large system this is the way to go, unless you need the exact measurements inside the duct instead of estimating them.

 

[ChrisConley]

What you need to do is called a 'pitot tube traverse'. One penetration in the duct, multiple readings within the duct, averaged to get an idea of airflow.

I know that ASHRAE textbooks have a proceedure for taking the measurement, but I don't have them on me right now to verify which one.

 

[billygoat22]

Depending on the duct size, you could measure at the outlet of duct with an air flow hood (balometer?)
Or use an anenometer at the outlet
If this is the only duct off a heater, use temp rise calculation to get airflow.

 

[kevinf]

Hello Digitrex,

There are lots of portable air flow/velocity tools out there. A company called Dwyer Instruments carries a few models (

http://www.dwyer-inst.com)

of anemometer type devices that are easy to use and digital.

KevinF

 

[friartuck]

Pitot static tubes are the most accurate but a hot wire anemometer could also be used. Kane and May make them as do a lot of other companies. Several holes ned to be drilled and a full traverse undertaken for optimum accuracy.

Drapes