Thread: Anemometer on special

Propellor type anemometers are not appropriate for measurement of air speeds in or around wood dust extraction ducts.
In general they significantly over estimate the air speeds and as their measurements are highly position sensitive cannot be used reliably even as a rough indicator..
Most changes that are made to DC systems produce small (a few % changes) so basing changes on propeller type measurements can end up making things worse rather than better.

Measurement of air speeds for dust extraction requires the use of an anemometer with a much smaller sensor.
see DRAFT: FAQ - Dust Extraction (Practical Aspects)
This shows what to use and how to do it. It's long winded and painful.

Oh ok...

Originally Posted by elanjacobs

Oh ok...

There, I hope you have learnt your lesson. (1) Made to look and feel like a fool by the best of them and (2) don't bother contributing to the "Dust Extraction" forum unless your a qualified and trained specialist scientist (who is into hot air and pain).

I personally like the anemometer because you can watch the propeller go round and it makes that really great wizzing sound.

TinyTool would you prefer money is spent on measuring tools that aren't right for the application? If you really want to get one there are lots direct from China for as low as $6US. Upside is you can watch the spinny bits and have money leftover to waste elsewhere.

Perhaps the content of a message should be the focus and not the delivery.

elanjacobs the intent of your post is appreciated. There are hot wire anemometers available for about $100 - $120 that are better for checking dust collectors. More bucks for sure but they are coming down in price all the time.

Pete

While not suitable for dust extraction, ideal for other uses such as sailing, operating elevated work platforms, and cranes.

Everything is relative; what works for one woodie is of no use to another and vice-versa. Besides, measuring DC flow is far from the only use for an anemometer.

I use a vane type when setting up the bell-hood for the DC on my lathe. While it's inaccurate inside the duct-work it's more than good enough for checking that I have decent flow across the work-piece outside of the duct-work.



At $30- I might have to see about grabbng myself a spare. I've a habit of misplacing these things in "safe" spots that are quite patently too safe. (What's the betting that when I find my micrometer, I'll find the other two or three simultaneously?)

Tiny Tool, not sure if your comments were tongue-in-cheek or not, so I'll give you the benefit of the doubt. If, however, they were not, feel free to insert an appropriately smarmy response here

I thought it might be useful for measuring at the source, rather than in-line, as Skew mentioned.

We're all here to learn, sometimes it's by making mistakes.

Originally Posted by elanjacobs

We're all here to learn, sometimes it's by making mistakes.

well said.

I know I have made my share

Originally Posted by TinyTool

I personally like the anemometer because ... it makes that really great wizzing sound.

you sound like Douglas Adams (The Hitchhiker's Guide to the Galaxy) who also like whizzing sounds, especially the ones made by missed deadlines

I'm sorry if my response came across the the manner suggested - I really am just trying to save you guys time and dust exposure.

I'm not sure how big Skewie's hood is, but the same criteria for the size of the sensor relative to the size of the cross section of the still applies measuring either inside or outside ducts and especially around relatively smaller duct openings at DC flow rates. This criteria is the total cross section of the sensor in the path of the air flow should be at least ~10x smaller than the the cross section of the air way.

This means prop type sensors are suitable for accurate measurements measuring air speeds across larger openings like a door or window frame.
Although less accurate they will also give reasonably indicative measurements across large openings to AC ducting ie 300 x 300 mm
The other reason it works for these openings is that there is a good chance the air flow though these openings is non-turbulent and slow enough for the air flow to be reasonably uniform across the opening so a single measurement or two will suffice.

Now contrast that with the air flow into any sort of small hood at DC air flow rates where, except for a true bell mouth hood (BTW the ones I make are not true bell mouth hoods - a true bell mouth hood is trumpet like in shape) the air flow will be fast and highly turbulent. If the hood opening is anywhere near 10x the size of the sensor, locating a prop type sensor in the air path only makes things worse. The net effect is counter intuitive. One would thinks the sensor is going to partially block the air path so the sensor will measure a lower air speed. However in most cases the air speed measured is higher because more air no cannot flow through th non measuring part of the sensor (and any hand or apparatus holding the sensor) thus increasing its speed. Even the prop blades themselves contribute to this problem. I haven't performed exhaustive tests but the tests I have done shows the prop type sensor over estimates the air speed by up to 40% whether it's inside a duct or outside a small duct opening. So, if you are hoping to assess flow and think you have enough you might be 40% short so you might start by discounting your air flow by 40% and be closer to the true flow. The higher the flow the more this applies.

Air flow through small openings is classically performed by placing the air speed sensor in the middle of the air pathway because this is where most the highest air speed is. BUT the sub-crossectional air way in the middle with the highest speed is actually relatively small so the total flow though this sub-cross-section is relatively low. In contrast the air speeds near the edges of the opening are slower BUT the total relative sub cross section of this area is much larger so more air actually ends up flowing through the sides of openings than through the middle. This means the measurements need to be taken systematically across the air way and the air flow computed using their relative sub-cross sectional areas. If the sensor is not small enough it never really can get close enough to the edges of the opening to measure the flow in these positions.

All this is why measuring air flow through into small duct openings is not performed by placing any air flow sensors around the outside of an opening but measurements are made inside a test duct attached to the opening. I have had many length email exchanges with people over the use of prop type sensors and those that have then gone out and purchased hot wire anemometers had to reluctantly agree with my estimate of their worth.

Hot wire anemometers can also run into the same problems,. The telescopic wand of standard hot wire anemometers can be anywhere from 5 to 10mm thick but even this is enough to interfere with the high speed flow inside a 100 mm test duct at DC air flows, and even inside a 150 mm test duct. They, like any air flow sensor, are more accurate at slower air speeds which is why I use a 240mm diameter test duct for higher (DC type) flows and 150mm diam test dust for the much lower flows such as that produced by vacs and power tools.

Now here is a situation where the prop type type sensor could be reasonably accurately used in a shed.
It assumes a still day and that you have a DC outside and a shed that is reasonably well sealed (it does not have to be air tight, ie even 1mm gaps are are OK). You can also always temporarily block of gaps with pieces of wood or foam rubber.
The idea is to get all the air flow into the shed though a single large opening like a door way or large window which means the air flow will low speed, relatively uniform across the doorway, and non turbulent.
Use your prop sensor to systematically measure the flow at various positions across the opening. ie divide the doorway up into an imaginary ~250 x 250 mm grid and measure in the centre of each grid.
You cannot stand in the doorway and do this or you will run into the same problems as above but will need to stand to the side and rig up some sort of a boom arm to locate the sensor into positions for the reading.
Then average the results and computer a total air flow.
Just for fun measure it three times and see if you get the same results

As I've said previously - its a relatively laborious process but you could use it to eg, to check the flow though specific machines.

Could you use it to make improvements to DC systems? Unless the improvements were significant then it's unlikely you could see most differences without repeated measurements and statistical testing. If this is not done in many cases you may end up making as many improvements as not.

No problem Bob, your initial response was polite and succinct, which is more than can be said of some others. Can't really ask for more than that.

Pretty much everyone here knows that you know what you're talking about and I think we're all pretty comfortable accepting your opinions without the need for an essay to justify them.

Originally Posted by elanjacobs

No problem Bob, your initial response was polite and succinct, which is more than can be said of some others. Can't really ask for more than that.
Pretty much everyone here knows that you know what you're talking about and I think we're all pretty comfortable accepting your opinions without the need for an essay to justify them.

Thanks. Sorry about the essay .

Originally Posted by BobL

I'm not sure how big Skewie's hood is, but the same criteria for the size of the sensor relative to the size of the cross section of the still applies measuring either inside or outside ducts and especially around relatively smaller duct openings at DC flow rates. This criteria is the total cross section of the sensor in the path of the air flow should be at least ~10x smaller than the the cross section of the air way.

I don't measure flow into the hood; just across & around the work-piece.

The blank is mounted, the tool-rest positioned and then the hood adjusted to get best possible air-flow across the blank/tool-rest in both the area I'll be working and in the direction the shavings/dust will tend to want to fly. The hood is not up-close 'n personal to the workpiece... I need elbow room for the chisels.

Precision of measurement isn't necessary; I'm not aiming for set data points. It's more a "blowing cigarette smoke" or "waving a length of ticker-tape" type of thing.

Heh. It sounds awkward, but takes less time to do than to type this sentence and it saves me from the "try a cut, adjust the hood, try a cut, adjust the hood" routine.

I understand what BobL is saying and agree he is correct.

However, there is a real downside to this problem. Many people are working on the dust collection and want to improve it. But how do you know if a change improved cfm or not. Given BobL's comments and requirements, none of us are really capable or want to spend the big money to have the correct equipment. It is not a good feeling to know we are not capable of doing any meaningful measurements according to BobL. What we need is a way of estimating the flows in a simple and cost effective manner. It may not have anywhere near the accuracy of BobL's measurements but something is better than nothing.