Thread: Inexpensive hot wire anemometer..Any recommendations..?

Originally Posted by BobL

Propellor type air speed indicators are fine for AC ducts where the ducts are relatively large compare to the size of the device.
As soon as the measurement devoices starts to significantly impede the flow this does two things
A) it introduces even more turbulence
B) Blocking the flow ends up speeding up the flow through the remaining gaps.
It is not unusual to measure up to 40% greater air speeds than is really flowing though or into a duct.

The situation is just as bad or even worse at the opening to a duct when the air flow is already highly turbulent. Using a BMH improves things a bit, but the air flow across the BMH is so uneven anyway.

A basic rule of thumb is that the cross-sectional area of an air flow sensor should be significantly less than the cross sectional area of a duct. By significantly less I mean about 1/100th. Even some hot wire anemometers (HWA)are too fat to meet this criterion in small ducts. The Testo is about 9 mm wide and measuring in the middle of a 150 mm duct has an exposure cross section of ~9 x 75 = 675 mm^2. Inside a 154mm (74506 mm^2) diameter duct that's about 1/110th of the duct cross section. U use a 240 mm (180,995 mm^2) diameter test duct which is 1/268th

Those prop type anemometers are designed to be used at low speeds (ie much less or no turbulence) and large cross section area ducts ie doors, windows or large ducting eg 300 mm diameter ducts. I used them to measure flows through 1200 x 600 mm cross section HEPA filters at work - they are ideal for this sort of thing.

Thanks for the info bob.

Originally Posted by QC Inspector

havabeer69 take a look at this publication. It applies to the outlet end of the duct. I don't know how measuring at the inlet affects the readings but suspect it might be worse. https://www.cdc.gov/niosh/mining/use...dfs/ri9061.pdf

Pete

I posted that link in Dec 2018. In that thread there was also testing between a vane anemometer and a hot wire anemometer in ducting and in a chamber.

Anemometer on special

I still believe, although you may not get accurate results re the air flow you can see definite trends as you make changes to a ducting systems.
As the cdc article showed you could possibly come up with a correction factor between the vane and Hotwire if you had the time to do some experimentation.

OK the gen on measurement of airflow in ducts and flues from some-one who spent their working career diong stack sampling and other air pollution monitoring.

The go to document is Australian Standard AS4323.1

To minimise effects from turbulent flow etc the measuring position needs to be at least 7-8 duct diameters downstream and 2-3 duct diameters upstream from a flow disturbance. A disturbance can be a change in diameter, bend fan or blower etc.

The better instrument for measuring is either a "S" or "P" type pitot which is connected to a manometer.

The "S" type pitot is easily made and provided the nozzles are sharp and parallel a standard calibration factor can be applied to calculate the air flow. Temperature and barometric pressure need to be included in the calculation.

Thanks for the input.
Most of the test parameters and methods you have mentioned have been discussed in a number of threads. The problem is that most people are looking for a cheap method to give some idea of flow with some degree of accuracy. The problem with calibrated Hotwire anemometers, pitot tubes etc are that they aren’t cheap hence the discussion on cheaper units.

Lappa I knew I likely got that link from here somewhere and wasn't trying to steal your thunder.

Pete

No problems Pete - didn’t think you were. I just thought that the earlier thread content suited this thread content and, when you look back, it’s amazing how many times new threads are going over the same questions/content and old threads.