Does anyone know anything about measuring the CFM and pressure etc in a ducting system or at the business end of the fan etc? Can you buy or rent such instruments for a sensible price? What would you need?
Miles
Miles
tool to measure air flow and pressure in ducting?
- Thread starter miles_hot
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Mr Ed
Established Member
I think you want a manometer, which I notice Axminster have just started selling for this purpose, at about £50 IIRC.
Ed
Ed
RogerS
Established Member
Someone on the forum (woodbloke?) came up with an elegant Heath Robinson device for measuring the suck..if that helps.
All you need to get readings is a few meters of clear PVC fuel pipe to make a water U tube/manometer to take 'suck' readings at whatever points are needed.
Converting these to velocity and then CFM figures is easy, but i don't have the formula to hand. Chances are if you search you will find a calculator on one of the HVAC sites on the web, plus some advice on where to site the holes for the tube.
The latter can make a difference as you get turbulence and other peculiarities near bends/other kit, as does how far you poke the tube in - you don't get a uniform air velocity across the cross section of the duct. (the walls cause drag)
You could buy a packaged manometer/gauge with coloured alcohol or something in it, or a fancy Dwyer Magnahelix gauge, but unless my memory is screwy you actually don't need any more than the tube....
ian
Converting these to velocity and then CFM figures is easy, but i don't have the formula to hand. Chances are if you search you will find a calculator on one of the HVAC sites on the web, plus some advice on where to site the holes for the tube.
The latter can make a difference as you get turbulence and other peculiarities near bends/other kit, as does how far you poke the tube in - you don't get a uniform air velocity across the cross section of the duct. (the walls cause drag)
You could buy a packaged manometer/gauge with coloured alcohol or something in it, or a fancy Dwyer Magnahelix gauge, but unless my memory is screwy you actually don't need any more than the tube....
ian
Ondablade
Does the clear pipe have be a certain diameter? I assume that it should be inserted perpendicular so that it does not point into or away from the airflow?
Is it specifically the distance that the airflow pushes the liquid in the pipe which gives a direct pressure reading or is there some calculation based on the cross section of the pipe and hence the weight of the water etc which needs to be done?
Many thanks
Miles
Does the clear pipe have be a certain diameter? I assume that it should be inserted perpendicular so that it does not point into or away from the airflow?
Is it specifically the distance that the airflow pushes the liquid in the pipe which gives a direct pressure reading or is there some calculation based on the cross section of the pipe and hence the weight of the water etc which needs to be done?
Many thanks
Miles
Mr Ed
Established Member
lurker
Le dullard de la commune
Without wanting to climb too far up my own.........
Testing airflow for LEV requires quite some training - I did a two week course & would still need to check my textbooks before I did one with any accuracy.
As a crude example - if you stick a manometer tube into a extract duct exactly how far you shove it in will give wildly different results.
Personally I'd not worry, go for enough suck & be aware the system might deteriorate slowly & you not notice.
Easiest check is to have a flat surface somewhere in the shop at about nose level where you weekly check there is no dust build up (wipe your finger over it in other words)
If you run a commecial concern then you are required (COSHH regs) to have systems checked every 14 months by a competent person.
Testing airflow for LEV requires quite some training - I did a two week course & would still need to check my textbooks before I did one with any accuracy.
As a crude example - if you stick a manometer tube into a extract duct exactly how far you shove it in will give wildly different results.
Personally I'd not worry, go for enough suck & be aware the system might deteriorate slowly & you not notice.
Easiest check is to have a flat surface somewhere in the shop at about nose level where you weekly check there is no dust build up (wipe your finger over it in other words)
If you run a commecial concern then you are required (COSHH regs) to have systems checked every 14 months by a competent person.
woodbloke
Established Member
Not I Rog - RobRogerS":1ci0h6gw said:
lurker
Le dullard de la commune
This might help
http://books.hse.gov.uk/hse/public/sale ... 0717662982
Downloadable PDF
But more likely to send you to sleep
http://books.hse.gov.uk/hse/public/sale ... 0717662982
Downloadable PDF
But more likely to send you to sleep
The tube diameter does not in principle matter Miles, other than to make sure it's big enough to avoid capillary effects and no so large as to become problematical to handle. Something like 3/8 or 1/2 in bore should be fine.
A U tube manometer works as follows. Think of a a length of tube with water in it clipped to a board to form a 'U' shape. It's filled so the water comes about half way up the vertical leg, and when at rest and not connected to anything finds it's own level so that it's at exactly the same height in both legs. This is a reflection of the fact that the atmospheric pressure over each column of water is exactly the same since both legs of the tube are open to the air.
When air flows in a duct what's called the static pressure drops (i.e. the 'normal' pressure reading you would get if you stuck some sort of sensitive pressure gauge into the duct) The dynamic pressure (in effect the pressure you feel on your face in a wind) increases since the air is moving.
If you were to stick one end of your tube in the duct pointing upstream into the airflow then the dynamic pressure would act on the water column in that side of the U tube. Things like pitot tubes that measure airspeed another way do it this way, but we don't want that effect. Which means that to get a static pressure reading your tube should be placed at right angles to the flow of air.
If you do this the drop in static pressure in the duct will cause the column of water in that column of your U tube to rise by an amount proportional to the drop in pressure relative to the atmospheric pressure the other leg is exposed to. Which means the water level in the other leg will drop the same amount.
Some U tube info: http://www.engineeringtoolbox.com/u-tub ... d_611.html
You take the pressure reading by directly measuring the difference in vertical height between the water surfaces in both tubes - pressure in duct design is commonly expressed in 'inches of water'.
Do the same at another point a known distance of straight duct upstream or downstream from this and using the difference between the two readings you can (using the right formula) calculate the air speed. You'll have to Google or find a book for this.
The U tube has to be long enough so that the water column is not sucked high enough to overflow.
As Lurker says and as in my earlier post if you want to get accurate readings of airpseed the positioning of the tube in the duct is important.
On the other hand from my reading (i've only done this in practice on pre installed measuring ports, and it was almost 30 years ago) it's not that big a deal if you are only seeking to monitor the drop in air flow in your system as the filters start to blind. Make sure you position the tube the same every time and at the centre of the duct and you should get comparable readings. Bill Pentz discuses this issue somewhere on his dust collection sysytem design website: www.billpentz.com/woodworking/cyclone/index.htm
Water filled U tubes are not really suitable for permanent installation as the water grows all sorts of algae and stuff that bungs the tube up. Plus they can be bulky and inconvenient. Which is why you see mechanical gauges like Mr. Ed's link. Dwyer Magnahelic is pretty much the industry standard, but expensive: www.dwyer-inst.com
The U tube is dead cheap, but i guess needs some careful study to get it working right. Don't know what the H&S people may demand....
Pardon if i'm coming across as a bit lengthy.
ian
A U tube manometer works as follows. Think of a a length of tube with water in it clipped to a board to form a 'U' shape. It's filled so the water comes about half way up the vertical leg, and when at rest and not connected to anything finds it's own level so that it's at exactly the same height in both legs. This is a reflection of the fact that the atmospheric pressure over each column of water is exactly the same since both legs of the tube are open to the air.
When air flows in a duct what's called the static pressure drops (i.e. the 'normal' pressure reading you would get if you stuck some sort of sensitive pressure gauge into the duct) The dynamic pressure (in effect the pressure you feel on your face in a wind) increases since the air is moving.
If you were to stick one end of your tube in the duct pointing upstream into the airflow then the dynamic pressure would act on the water column in that side of the U tube. Things like pitot tubes that measure airspeed another way do it this way, but we don't want that effect. Which means that to get a static pressure reading your tube should be placed at right angles to the flow of air.
If you do this the drop in static pressure in the duct will cause the column of water in that column of your U tube to rise by an amount proportional to the drop in pressure relative to the atmospheric pressure the other leg is exposed to. Which means the water level in the other leg will drop the same amount.
Some U tube info: http://www.engineeringtoolbox.com/u-tub ... d_611.html
You take the pressure reading by directly measuring the difference in vertical height between the water surfaces in both tubes - pressure in duct design is commonly expressed in 'inches of water'.
Do the same at another point a known distance of straight duct upstream or downstream from this and using the difference between the two readings you can (using the right formula) calculate the air speed. You'll have to Google or find a book for this.
The U tube has to be long enough so that the water column is not sucked high enough to overflow.
As Lurker says and as in my earlier post if you want to get accurate readings of airpseed the positioning of the tube in the duct is important.
On the other hand from my reading (i've only done this in practice on pre installed measuring ports, and it was almost 30 years ago) it's not that big a deal if you are only seeking to monitor the drop in air flow in your system as the filters start to blind. Make sure you position the tube the same every time and at the centre of the duct and you should get comparable readings. Bill Pentz discuses this issue somewhere on his dust collection sysytem design website: www.billpentz.com/woodworking/cyclone/index.htm
Water filled U tubes are not really suitable for permanent installation as the water grows all sorts of algae and stuff that bungs the tube up. Plus they can be bulky and inconvenient. Which is why you see mechanical gauges like Mr. Ed's link. Dwyer Magnahelic is pretty much the industry standard, but expensive: www.dwyer-inst.com
The U tube is dead cheap, but i guess needs some careful study to get it working right. Don't know what the H&S people may demand....
Pardon if i'm coming across as a bit lengthy.
ian
Aled Dafis
Established Member
I used to to quite a bit of airflow monitoring on extracion ducts (albeit large ducts that you cloud crawl/walk through) during my Uni sandwich placement at British Steel/Corus (I was there during the merger/takeover). Please bear in mind that this was a few years ago, and some of the details are a little hazy in my mind, so please don't quote the details below, but they shouldn't be too far off.
Firstly, just sticking a pipe (U-Tube Manometer) against a hole in your ducting will only give an indication of the static pressure of your system - much like the square of plywood test. This is excatly what the Axminster doo daa does, and it would serve as a good indicator of how your system is performing, but does not show how much air (the flowrate) is flowing through the system
To work out the flowrate of your extraction system (cfm or m3/s etc.), you'll need some more apparatus, such as a Pitot Tube to take readings of differential pressure across the entire cross section of the duct and then apply a mathematical formula (which I can't remember) to work out the actual flowrate.
Sorry about the large pic, it was the first one I found on google :roll:
The process is very long winded, but relatively accurate if done correctly and in an appropriate section of duct - I seem to recall needing a straight run of 8 pipe diameters ahead of the point you take a reading, and 4 diameters behind.
The other option to measure flowrate would be to use an anemometer such as this one on the end of your duct and then multiply the air speed m/s by the cross sectional area of the duct. The problem with this system is that it won't take into account any losses further down the pipe from leaks/friction etc.
I hope that helped a little.
Cheers
Aled
Edit : We must have been typing at the same time, just as well we said pretty much the same thing eh? :wink:
Firstly, just sticking a pipe (U-Tube Manometer) against a hole in your ducting will only give an indication of the static pressure of your system - much like the square of plywood test. This is excatly what the Axminster doo daa does, and it would serve as a good indicator of how your system is performing, but does not show how much air (the flowrate) is flowing through the system
To work out the flowrate of your extraction system (cfm or m3/s etc.), you'll need some more apparatus, such as a Pitot Tube to take readings of differential pressure across the entire cross section of the duct and then apply a mathematical formula (which I can't remember) to work out the actual flowrate.
Sorry about the large pic, it was the first one I found on google :roll:
The process is very long winded, but relatively accurate if done correctly and in an appropriate section of duct - I seem to recall needing a straight run of 8 pipe diameters ahead of the point you take a reading, and 4 diameters behind.
The other option to measure flowrate would be to use an anemometer such as this one on the end of your duct and then multiply the air speed m/s by the cross sectional area of the duct. The problem with this system is that it won't take into account any losses further down the pipe from leaks/friction etc.
I hope that helped a little.
Cheers
Aled
Edit : We must have been typing at the same time, just as well we said pretty much the same thing eh? :wink:
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