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Using an anemometer for setting up dust collection

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Forum topic by MikeGraw posted 04-10-2016 08:35 PM 880 views 1 time favorited 12 replies Add to Favorites Watch
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MikeGraw

21 posts in 2383 days


04-10-2016 08:35 PM

I purchased an inexpensive digital anemometer as I am planning on updating my dust collection system. It has a setting for feet per minute and it also has a cfm setting.

I purchased this model as it did have the cfm setting and I thought I could use the device without having to make any other calculations.

When I put the unit up to the hose by where it goes into my separator it shows something like 4242 in cfm mode. Behind the number it shows for the cfm it shows x10 in very small type. The dust collector is a delta 1 1/2 hp unit rated for 1250 cfm, so I don’t think 4242 cfm is correct.

When I use the ft/m it shows 111. The hose is 4 inches so doing the calculation of ft/m X area shows cfm of about 1394. This sounds high but more reasonable.

I was wondering, are there other calculations I need to do when using the unit in cfm mode or should it be showing the proper cfm on the unit or am I not using the unit correctly?

Any help is appreciated as I want to be sure I am having enough cfm for the tools as I redesign the dust collection system.

-- Mike's having fun in Central Wisconsin


12 replies so far

View RobS888's profile

RobS888

1984 posts in 1306 days


#1 posted 04-10-2016 10:54 PM

I have to input the size of the pipe, around 12.7 square inches for a 4 inch pipe. Is there a way to set it on yours?

-- I always suspected many gun nuts were afraid of something, just never thought popcorn was on the list.

View Redoak49's profile

Redoak49

1940 posts in 1449 days


#2 posted 04-10-2016 11:13 PM

You can use an anemometer for rough estimates. There are issues with how close to hold it to the end of the hose or pipe as you can get very different numbers if it is too close or too far from the end.

Most mfg over rate their dust collectors for actual conditions.

Based upon some articles that tested 1-1/2 hp dust collectors, you probably will get at most 600-800 cfm from a 4” pipe.

Unfortunately, this is not an easy topic. A good start would be to do a search for Bill Pentz and read what he has and the spreadsheet for learning about and calculating the effects of various set ups.

Good Luck

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JeffP

573 posts in 853 days


#3 posted 04-10-2016 11:29 PM

I haven’t seen the system you’re using, so this is somewhat theoretical…all possible disclaimers apply.

An anemometer can measure wind speed, but in order to measure “CFM”, all of the air in question would have to be flowing through the small space occupied by the anemometer’s vanes. Just having it positioned in the airstream is not sufficient to measure cfm…ALL of the air would have to be flowing through just the space occupied by the vanes.

OTOH, if the anemometer is just taking up part of that stream, one would have to know what the cross sectional area of the whole pipe is in order to calculate the cfm.

Of course, if you set up a configuration where ALL of the air IS flowing only through the area encompassed by the vanes, then by it’s very nature, your setup will limit the cfm and therefore affect your measurement accuracy.

I think your best bet here is to get a (hopefully) accurate measure of the air speed, and then multiply it by the cross sectional area of the pipe to get cfm.

To describe that more completely, you would need to get “feet per minute” (which would be the air speed) using your anemometer, and then multiply that by “square feet” of cross sectional area to get “cubic feet per minute”.

The cross sectional area of a pipe is Pi times the radius (in feet) squared.

example:
111 feet per minute air speed in a 4 inch pipe:
111 times 3.14 times 0.333^2 = 38 cubic feet per minute (which doesn’t sound very likely)

conversely, 1000cfm would require a wind speed of 2866 feet per minute with a 4” pipe. (if my calculations are correct)

me thinks that something is off here.

I would propose that you use your anemometer just for relative measurements, to see when something makes it better or worse.

-- Last week I finally got my $*i# together. Unfortunately, it was in my shop, so I will probably never find it again.

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MikeGraw

21 posts in 2383 days


#4 posted 04-11-2016 01:01 AM

JeffP

That is the formula I was using. A 4 inch pipe has a 4adius of 2 inches. 2 squared is 4. So 4 times pi, 3.14, equals 12.56. 111 of from times 12 56 comes out to 1396 for cfm.

Rob, there is no way to set for size of pipe.

Redoak, I thought too that I would just have to use it as a guideline. The hood of the meter is almost exactly the same diameter of the hose so the is little loss to the side of it. It is a very cheap unit though so I don’t know about the accuracy of it.

Thank you all for you tips.

-- Mike's having fun in Central Wisconsin

View JBrow's profile

JBrow

817 posts in 381 days


#5 posted 04-11-2016 01:52 AM

MikeGraw,

If you have the instructions that came with the meter, it is best to check them. But since you are asking I assume you do not have them. I have encountered instruments which use the “x10” nomenclature. Sometime it means 1) “the reading from the instrument” X 10 = the measured reading while in other instances it means 2) “the measured reading” = actual value X 10. Confusing to you I hope, because this always confused me. Your question suggestions you cannot believe the meter may have the meaning of 1), 4242×10 = 42,420 cfm, which would be pretty incredible. It probably means 2) that is 4242 = 424.2×10. If 2) is correct, then the actual reading is 424 cfm.

The formula relating pipe Cross Sectional Area of the Pipe in Square Feet (SF) to Cubic Feet per Minute (CFM) and Velocity in Feet per minute (FPM) is:

Cross Sectional Area of the Pipe (square feet) = CFM/FPM; where the Cross Sectional Area must be in Square Feet, not square inches.

I did some calculations of my own using the formula above and could not reconcile the readings you provided. Even the 424 CFM value (if correct) sounds low and 111 FPM is inconceivable. There are probably some caveats associated with the Delta Dust Collector associated with their 1250 cfm claim. A departure from the conditions Delta used could result in different readings, which were probably originally derived with a new and clean filter taken at a 6” inlet (if the collector is so equipped) with no pipe.

An alternative to measuring differences in air flow is making a pitot tube or as some call it, a Manometer. Stumpy Nubs shows how to make one. The end of the tube should be placed in the center of the air stream and the end of the tube must be perpendicular to the direction of air flow. Placement of the end of the tube in the air stream must be consistent to have comparable readings. I am pretty sure that inches of water rise measured with the pitot tube can be converted to CFM and FPM, but I do not know how.

https://www.youtube.com/watch?v=f92mDnLpWFU

View RobS888's profile

RobS888

1984 posts in 1306 days


#6 posted 04-11-2016 03:24 AM

I fist bought this anemometer, but could get it to give me anything appropriate…

http://www.amazon.com/Lerway%C2%AE-Anemometer-Velocity-Temperature-Humidity/dp/B00LGK6XXQ?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_search_detailpage

Sent it back and got this..
http://www.amazon.com/Pyle-PMA90-Anemometer-Thermometer-Temperature/dp/B009TQ6ILQ?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_search_detailpage

Entered in the square inches and got realistic results.

I believe it is programmed with the volume of its vane and multiplies the result by the square inches you enter. It has 10 memory positions, so I was able to test 1 five inch outlet compared to 2 4 inch outlets.

The manual describes how to hold it in relation to the opening of the pipe or hose.

-- I always suspected many gun nuts were afraid of something, just never thought popcorn was on the list.

View MadMark's profile

MadMark

977 posts in 914 days


#7 posted 04-11-2016 04:04 AM

Hook it up. If it works great, if it doesn’t, keep tinkering.

You haven’t given much data and there are many variables. Determine the best configuration empirically.

M

-- Madmark - Madmark2150@yahoo.com Wiretreefarm.com

View MikeGraw's profile

MikeGraw

21 posts in 2383 days


#8 posted 04-11-2016 05:48 AM

Rob, the first one you bought looks very similar to mine.

Madmark, that’s what I am going to do. Didn’t want to spend much on this unit as it will have limited use. Was hoping for a little more accuracy though.

-- Mike's having fun in Central Wisconsin

View OSU55's profile

OSU55

1056 posts in 1450 days


#9 posted 04-11-2016 11:20 AM

MikeGraw

A quick look at the manual from Pyle’s website shows it very similar to one I have used, and the duct area can be input, units of measure, and several other aspects of using the unit. You should be seeing 5,000 to 8,000 fpm velocity type numbers.

View JeffP's profile

JeffP

573 posts in 853 days


#10 posted 04-11-2016 11:58 AM

removed…see next post

-- Last week I finally got my $*i# together. Unfortunately, it was in my shop, so I will probably never find it again.

View JeffP's profile

JeffP

573 posts in 853 days


#11 posted 04-11-2016 12:01 PM


JeffP

That is the formula I was using. A 4 inch pipe has a 4adius of 2 inches. 2 squared is 4. So 4 times pi, 3.14, equals 12.56. 111 of from times 12 56 comes out to 1396 for cfm.

Rob, there is no way to set for size of pipe.

Redoak, I thought too that I would just have to use it as a guideline. The hood of the meter is almost exactly the same diameter of the hose so the is little loss to the side of it. It is a very cheap unit though so I don t know about the accuracy of it.

Thank you all for you tips.

- MikeGraw

oops, I never use power tools after my evening glass of scotch. I guess I should include the calculator after this egregious error :) I did my calculations all using 4” as the radius…should have been 2. Important to note though, the calculations need to be done in FEET, not inches. The 2” radius of a 4” pipe is 1/6th of a foot. You either need to convert the radius to feet BEFORE you square it, or else take into account there are 144 sq. inches in a square foot.

- JeffP

-- Last week I finally got my $*i# together. Unfortunately, it was in my shop, so I will probably never find it again.

View RobS888's profile

RobS888

1984 posts in 1306 days


#12 posted 04-11-2016 05:50 PM



Rob, the first one you bought looks very similar to mine.

Madmark, that s what I am going to do. Didn t want to spend much on this unit as it will have limited use. Was hoping for a little more accuracy though.

- MikeGraw


See if you could return it, the Pyle is not that much and it is easier to use.

-- I always suspected many gun nuts were afraid of something, just never thought popcorn was on the list.

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