Shopsmith Forums

After some recent testing I got to wondering just what the cfm (cubic feet per minute) numbers are for several items I have, you know, dust collector and shop vacs.

I own a cheap anemometer which will read wind speeds but should also get in the ball park for air flow on other things. It has feet per minute setting so we can get close to the results in terms of time and then just have to adjust the vacuum hose to a unit of feet.

So as an example a 2-1/2" hose would have a radius of 1-1/4. Area is pi*r squared so converting that to feet it would be like 0.0340711805555556 sqft (approx for pi 3.14).

Now if the meter read say 5000 feet per minute then that would mean about 170 cfm.

Anyone see any problem with this?

I know that a lot of factors come in when you go from the numbers they claim to what you see at the machine and while this doesn't tell the whole store it might at least tell part of the story.

I've got other thing planned for tomorrow but maybe Wed I can see what numbers come up..........

Ed

C'mon Ed!!! You know better than to use two two place decimals and get a 16 place result.

.034 in your case.

Using 3.141592654 for Pi, (1.250000000 x 1.250000000 x 3.141592654) / 144.000000000 = 0.034088461.

Now go measure that 2 1.2" hose.

JPG wrote:C'mon Ed!!! You know better than to use two two place decimals and get a 16 place result.

.034 in your case.

Using 3.141592654 for Pi, (1.250000000 x 1.250000000 x 3.141592654) / 144.000000000 = 0.034088461.

Now go measure that 2 1.2" hose.

Oh not the 2-1/2" hose isn't 2-1/2"???? Actually I have no idea on vacuum hoses, some say 2-1/2 some say 2-1/4 and finding things that actually fit together is quite the job. I have a whole box of fittings because there are "hard" fittings and all sorts of terms that never get explained.

I don't think it matters much when I compare the shop vacs because I can use the same hose and get comparative results and for the dust collector I can do a step down to the same hose, and also look at the splitter effects with the same hoses. I don't expect to get results that actually tell me real numbers but will I see twice the air flow off the dust collector or three times or four times????? I also expect the numbers to be a lot smaller then the claims. 10 feet of "corrugated" hose has got to be downer.

Ed

Ed you started out with

After some recent testing I got to wondering just what the cfm (cubic feet per meter) numbers are for several items I have, you know, dust collector and shop vacs.

Isn't CFM actually Cubic Feet per Minute a measure of volume, in this case air and not a conversion of cubic feet to meters? Much like measuring the flow of water through a hose that I did as a firefighter, you should be able to find a formula for air speed & hose diameter to determine CFM. Use the hoses inside diameter. As an example, my shop vac's 2-1/2" hose is actually 2" ID hose. I didn't measure my dust collectors hose. Yes, corrugated hose will cause some turbulence but that's the difference between theoretical and actual flow. You will be measuring actual flow. Most of all, have fun with it.

The test results are all interesting but I find that the best way for me to determine what works and what does not work is to get all setup and then MAKE SAW DUST. If there is none (or very little) on the floor and/or on the carriage then "the setup works". Any thing different this means there is room for improvement.

I have been trying to make improvements for years now. I am about to just settle with what I have. I have a Shopsmith DC3300 and a Shopvac.

I use the Shopvac to pick up what the DC3300 fails to collect (no matter where in gathers).

chapmanruss wrote:Ed you started out with

After some recent testing I got to wondering just what the cfm (cubic feet per meter) numbers are for several items I have, you know, dust collector and shop vacs.

Isn't CFM actually Cubic Feet per Minute a measure of volume, in this case air and not a conversion of cubic feet to meters? Much like measuring the flow of water through a hose that I did as a firefighter, you should be able to find a formula for air speed & hose diameter to determine CFM. Use the hoses inside diameter. As an example, my shop vac's 2-1/2" hose is actually 2" ID hose. I didn't measure my dust collectors hose. Yes, corrugated hose will cause some turbulence but that's the difference between theoretical and actual flow. You will be measuring actual flow. Most of all, have fun with it.

The conversion was to get like units, the area of the hose was done with inches and needed to be in feet to match the other units of feet. The CFM is cubic feet per minute like wise the air speed is in feet per minute so all units need to be in feet for this to work right.

The original post has been corrected, not sure why I wrote that because I know better.

This is the formula, abet in simple terms. No need for anything else. And as I said I don't care so much about how close my numbers are to real numbers but more about how they related to each other.

I always have fun with projects like this, it is in my nature.

Ed

dusty wrote:The test results are all interesting but I find that the best way for me to determine what works and what does not work is to get all setup and then MAKE SAW DUST. If there is none (or very little) on the floor and/or on the carriage then "the setup works". Any thing different this means there is room for improvement.

I have been trying to make improvements for years now. I am about to just settle with what I have. I have a Shopsmith DC3300 and a Shopvac.

I use the Shopvac to pick up what the DC3300 fails to collect (no matter where in gathers).

I totally agree that the best way is to experiment with the equipment you have to get the best dust collection you can. This is a slightly different tack, figuring out what the equipment I have yields as far as they compare to each other. It is mostly a learning experience for me.

Ed

chapmanruss wrote:Ed you started out with

After some recent testing I got to wondering just what the cfm (cubic feet per meter) numbers are for several items I have, you know, dust collector and shop vacs.

Isn't CFM actually Cubic Feet per Minute a measure of volume, in this case air and not a conversion of cubic feet to meters? Much like measuring the flow of water through a hose that I did as a firefighter, you should be able to find a formula for air speed & hose diameter to determine CFM. Use the hoses inside diameter. As an example, my shop vac's 2-1/2" hose is actually 2" ID hose. I didn't measure my dust collectors hose. Yes, corrugated hose will cause some turbulence but that's the difference between theoretical and actual flow. You will be measuring actual flow. Most of all, have fun with it.

Cubic feet is volume.

Cubic feet per minute is total volume over time.

Yes area x velocity yields volume over time.