Thread: Is this any good?

I used these propellor based anemometers for many years at work, but when I started testing DC ducting I found out quickly that they were not giving accurate results because the propeller and propellor housing interferes significantly with the air flow. This also means they cannot measure the airflow close enough to the wall of the ducting to measure the effect of wall friction. Propellor type anemometers are typically designed for open air/ outdoor use, or for door/window size openings or large ducts ie 12" and upwards, with low flows such as is found in air con or large ventilation systems where wall friction is less of an issue. To be truly accurate the duct needs to be about 10 times wider than the propellor/housing. In addition the flow must be measured in the middle of a test duct about 10 times longer than the duct diameter. This means cutting a hole in the side of the duct to poke the propeller thru but the hole MUST be air tight which makes using the propeller type sensor virtually impossible.

In 100 mm ducting moving 400 cfm the air speed is around 9000 fpm or 46 m/s which is beyond most anemometers to measure directly so connection to larger pipes is usually required to slow the air speed down to less than 30 m/s.

I have been in communication with a number of people with propellor type anemometers about DC air flow measurements. They typically measure air speeds between 10 and 25% too high and in one case about 35% too high. Some DC manufacturers probably use these and this contributes to the high readings they get.

If you want to get one a play around with it for relative testing then ebay has them for 1/2 the price of the Jaycar model. Ebay also has very cheap ones with the propellor built into the display housing but don't be folloed by the cheap prices as these are even more inaccurate that those with the prop connected by a wire to the display.

Here's one for $40 Digital Anemometer Wind Speed Meter Thermometer 0 45M S | eBay
It says it can measure 45 m/s but that might be 20% too high so it can really only measure 35 m/s. You can see by the fact that it measure temperature (most do that) and wind chill that it is designed for outdoor/open air use.