One Test Worth a Thousand Opinions

[Floyd Cantrell]

Well I received the needed equipment to test my dust collection system. Many thanks to Allan Johanson's information on the cheep test equipment, see previous post: Measuring CFM/Efficiency of DC/Cyclone
http://www.sawmillcreek.org/showthread.php?t=23857
I Bought the Pitot tube from Dwyer and the Dwyer Differential Gauge from Ebay. I bought all that was needed for less than a good saw blade. I set up and tested the flow rate about 2 feet down stream from the connection to my jointer that I was having a problem with collecting all the chips. The reading was only 230 cfm, way below the required flow of 650 cfm stated by Mini-Max. I moved up-stream in my system and looked at the point were it exists the shop. The reading was 264 cfm with the same jointer gate open. I then stared opening up other gates to see what would happen. The readings increased as I opened up the gates to a max of 350 cfm. I then whet out and knocked some of the caked dust from the top filter bag, reading went up to 350 cfm. This is all with 4" PVC ducting and about 4 to 6 feet of flex line to each machine.

With these tests I now see I need to make some changes, not sure which way I will go. I think to do it right may cost between $400 and $2500 depending on whether I but a new DC system.

[Richard Wolf]

Thanks for sharing the info.

Richard

[Dennis Peacock]

Floyd,

When Terry Hatfield and I installed my cyclone setup and tested it, we tested at the end of a 75 foot run with 5 turns in the pipe. It's all piped at 6" piping and got 1,057CFM at the TS (end of the 75' run). Reduced it down to 4" right behind the TS and we neasure 735CFM. So, this proved to me in real life, that going from 6" to 4" will in fact reduce your CFM flow drastically. Go as big as you can for all your DC piping run. You'll be glad you did.

Just my 2¢ worth on this subject.

[Jim Becker]

Since you have 4" duct work, those numbers are not all that surprising...you can only fit so much air in there!! Even a "kick-butt" system will not pull great numbers through a 4" port. I think I can speak with experience that the big J/P needs a lot more air to move chips and dust. (The hoods are pretty demanding from an air flow standpoint) Aside from making my shop much larger, it was the second contributing factor to putting in a larger system with a 7"/6" main. No clogs since...

[Allan Johanson]

Hi Floyd,

I'm glad you got the testing gear running. It's really an eye opener when you can see for yourself just how much different things affect the airflow.

With my cyclone I can have over 1400cfm in a 7" main duct. But when I hook up my Performax sander with the factory 4" port, the airflow will drop down to around 700cfm. With a better port I can do a much better job at that nasty dust-producing tool. Having the testing gear shows me how much better it could be so I really appreciate having it around.

If you have any questions, drop me a line.

Have fun!

Allan

[Steve Aiken]

Ok, I don't mean to be a party-pooper, but how reliable is the test equipment? How repeatable are the tests? Is there room for introducing error by the operator?

I know we don't need absolute scientific accuracy here, but I wouldn't want to spend the money unless the readings were fairly accurate and reliable.

Steve

[Mike Cutler]

Ok, I don't mean to be a party-pooper, but how reliable is the test equipment? How repeatable are the tests? Is there room for introducing error by the operator?

I know we don't need absolute scientific accuracy here, but I wouldn't want to spend the money unless the readings were fairly accurate and reliable.

Steve

Steve.
I have 26 years of experience in the instrumentation and testing field. I routinely perform calibrations on large and small ventilation systems with the result being traceable to NIST standards.
The Dwyer products being used in this thread and others, are commercial grade instruments. There accuracy is, OK not great, but OK.
The testing methodology however can contibute enormous inaccuracies if not performed corectly, and if the physical instrumentation is not set up properly.
Flow, in this case CFM, is a measurement of the differential pressure(DP) across an orifice in this case the pitot tube. A pitot tube can heve one or two orifices depending on the application. If you have two tubes coming off off the pitot tube you will be looking at the DP of the process stream at the pitot tube. If it has only one tube it is looking at the DP as referenced to ambient pressure, and needs to be sized accordingly.
Flow is equal to the square root of the DP across the orifice. What does this mean? It means that the curve for flow is very steep in the first 25% of measured range. The lower the DP of the entire range the greater the error can be by a function of square.
Flows in DC systems are typically low inches of water("H2O)measurements, static, and DP. This causes problems in other areas. The size of the test tubing, and length can have dramatic effects, the air temperature can effect the measurement. The density, amount of moisture in the process stream, can also have a dramatic result. If the pitot tube is not positioned in the process stream properly it can have a huge effect, additionally the location of the pitot tube on the process system(DC Piping) can also effect readings. You want a minumum number of piping diameters ahead of the pitot tube and behind it for an accurate reading. The lower the static inches of water the greater the effect on measurement if the minimum distances are not followed.
One large contributor to accuracy is the range of the measurement instrument. You would like your instrumentation to fall in the 50% of expected reading and range application, in other words, if you are expecting to see Dp's on the order of 0-10" h2o for example you wouldn't want to use an instrument that had a range of 0-200" h20. The resolution and the accuracy will drop off.
After having stated all this, what does it mean? It means that I can refute the actual numbers being posted by folks using home testing methods, with untraceable standards as being accurate on an academic level, but this would be a gross error on my point, and a disservice to assume this position.
If the testing being performed in the home shops is performed in an repeatable manner, and the values recorded I can establish a fairly usable baseline. If I change the input variables, ie piping size, piping run, or placement of tee's and drops, and I repeat the mesurement in the same manner. I can look at the difference in the baseline values versus the new value and get a very accurate percentage of change value that can be quantified. I may not be able to tell you that I am getting a specific CFM accurately, but I can accurately derive the incresed performance in CFM with baseline data versus improved system responses.
In a nutshell The larger the piping that you can use the better, as long as the DC can handle it, additionall the straighter the runs, and the smoother the inside walls of the piping the better the flow rate will be. Use "Y"s instead of Tee's if you can, and make your blast gates as large as possible.
Sorry for being so long winded.

[Ken Fitzgerald]

Even if the test equipment/tools are not totally accurate, the measurements should be relative. In other words, it might not be exactly 750 cfm but if you double the cfm by increasing the size of your pipe, you have increased the cfm.

[Lee Schierer]

You need to look at a couple more things before you run out and buy a new dust collector.

First, what is the rating for your existing impellar or dust collector? If it can't produce the amount of CFM you want it should be replaced.

Second, try giving it larger pipe to suck through. Your 4" pipe is only 0.087 square feet in area. To get 350 CFM through that pipe it will be traveling at 4,022 feet per minute. To get 650 CFM through the same pipe it needs to travel at 7,471 feet per minute. The same amount of air through a 5" duct is only traveling at 2,573 feet per minute for 350 cfm since the area of the duct is almost twice that of a 4" pipe. A 6" pipe would be even better since its area is more than three times the area of the 4" pipe. Also any reduction in the pipe anywhere will be the limiting factor for how much air you can move. The longer that diameter reduction is the more effect it has. Corrugated flex duct and Elbows also have a similar effect. One elbow is equivalent to 10' feet of straight pipe.

Try sucking coffee through a coffee stirrer. It takes a lot of effort to get much coffee. Suck the same coffee through a straw and you get lots more coffee for much less effort.

[Allan Johanson]

Steve.
I have 26 years of experience in the instrumentation and testing field. I routinely perform calibrations on large and small ventilation systems with the result being traceable to NIST standards.
The Dwyer products being used in this thread and others, are commercial grade instruments. There accuracy is, OK not great, but OK.
The testing methodology however can contibute enormous inaccuracies if not performed corectly, and if the physical instrumentation is not set up properly.

[Lots of great info snipped]

Mike,

Thank you so much for your excellent post. I certainly don't have your training, but have seen for myself how inconsistent a person's results can be if they are not careful.

The way I see it with this type of home testing gear is that when I record airflow through my planer of say 750cfm, that it probably isn't 750cfm "exactly", but somewhere close to that. I focus on the change in airflow when I make ducting or hood changes. If I can get repeatable results (higher or lower by swapping in and out the changes), then I'm happy with the results. I always test things a couple times.

When it comes to reading the gauges, what I do to minimize human error is to clamp the gauges to a board so they can't move since altering their vertical position will move the needle. I also have my pitot tube fastened securely to a piece of wood that holds it directly into the airstream. Not at some arbitrary angle which will affect the results.

When I got my Dwyer pitot tube, it said where to place the tube for accurate results. They specified inserting the pitot tube in a piece of straight ducting that is at least "10 diameters long". So 4" ducting must be minimum 40" long, 5" should be 50" long, 6" should be 60" long. Then you insert the tube no closer than "8.5 diameters" downstream from the tool end of the straight pipe and "1.5 diameters upstream" from the blower side of the pipe. Placement here will ensure the airflow is stable enough for reasonably accurate measurements.

The Dwyer gauges I use are from this line:

http://www.dwyer-inst.com/htdocs/pre...s2000Price.cfm

These are accurate to within 2% if you use them correctly. So right off the bat you can't get any better than +/- 2%. Not a huge deal, but since we're trying to be accurate with our inaccuracies, I figured I'd mention it.

I have 0-10" and 0-2" gauges. When I use the 0-10" and read a low reading, say 1.2", I don't expect it to be that accurate. When I swap out gauges and use the 0-2" instead, that 1.2" reading might change to 1.4". I'm just making numbers up, but you can see what I mean. This just reinforces one of Mike's points.

By securely fastening the pitot tube directly into the airstream, fastening the gauges to a block of wood so they don't move, use Dwyer specs for pitot tube placement, using the correct "size" of gauge so I'm more often than not reading values in the middle range of the gauge, and running the tests a couple times (swapping changes in and out)....I figure what I do makes my testing be "accurate enough" for hobbyist use and I'm not sure what else I could do to make things more accurate. Do you have any suggestions, Mike?

Thanks,

Allan

[Mike Cutler]

Allan.

The Magnehelic that you specified is a perfectly fine instrument. It's a basic mechanical device that works by applying a preesure to an internal diaphagm assembly and converting that pressure to a rotational movement. It' rugged, easy to read, and it's analog. The only adjustment is a zero adjust, and it either works or it doesn't. The only failures I have seen with these is that the gasket around the face gets brittle in high temps, and the zero screw goes thru a little buna o-ring that can crack over time. This device requires an airtight seal on the face.
If possible I would use 3/8" hard pipe tubing, ie. refrigerant tubing and the appropiate adapter fitting( 3/8" tube to 1/4" npt I think). Flexible tubing, and that "surgical" type tubing that dwyer sometimes provides has some limitations. Flexible tubing is more susceptible to physical changes based on ambient temperature that can affect your readings and response times. If you want to see an example, put your magnahelic in service and grab just one of the tubes with your hand, and warm it up. You will see the reading change just by the differential temperature of the tubing.The tubing should also be as close to the same lengths as possible or response time, and reading errors can be induced in a dynamic process (flow stream) due to lead/lag response times due to tubing lengths, the lower the input pressure the greater the effect.
A 0-2.5" H2O may need a volume chamber to settle out the reponse times, and process oscillations. At the low CFM rates of DC's and Cyclones it's propbably not an issue. (I have a 0-2.5" transmitter that monitors a 0-100,000 cfm flow range in a 6'x6' vertical duct that requires a lot of finesse to cal properly. I need tuned tubes and volume chambers to be able to do it.)
Mostly though, I wouldn't get too hung up in the quantifying of results, unless you find it interesting that is. There is a lot of posted info about Dust Collection, and it seems that folks are coming to the proper conclusions, by discussions such as these. I think this is because there is a very eclectic cross section of people that do woodworking as a hobby, and a job. I know that we have professional engineers, from multiple disciplines, and some seriously educated folks on this forum. I think that either Lee Schierer, or Ken Garlock could have written exactly what I did, they seem to have a suspicous background in instrumentation . Bill Pentz's site is a wealth of information, I'm not sure that I agree with every model presented, but the end conclusions I agree with. He's done some great research. I find his "newer" design of the neutral vane to be interesting. I showed it to some of the HVAC Engineers and they were fairly interested.

[Steve Aiken]

Thanks Mike and Allan, it sounds like the testing equipment would give good indications of relative changes in airflow, as long as you pay close attention to carefull methodology.

It sounds like Floyd will be able to make modifications to his existing system and get good feedback about the effectiveness of those modifications, instead of relying solely on Oneida engineers, books, and forums like this--as valuable as each source may be.

Do keep us up-to-date on your journey towards DC "perfection."

Steve

[Floyd Cantrell]

Thanks for the encouragement. The intent of this testing was to assure myself that I wanted to make changes. I do realize the measured CFM may not be exact but will give me a frame of reference. I do have some experience in this are, my first 25 years in the working world was in the calibration lab of a major aerospace manufacture. I have spent a lot of time calibrating pressure gauges and as was stated the Dwyer Magnehelic gauge is a very rugged gauge. I did do a comparison between two of the gages I have and they compared acceptable. I mounted the gauge to a small platform that holds the gauge vertical and stable while taking the readings. The tubing is also the same length for sides of the differential gauge. I also bought the pitot tube with a compression fitting to set my depth into the air flow. The pitot tube was also rotated in both plans slowly to achieve the highest reading. Several of the tests were repeatedly made going back and forth between the two test points. I feel very comfortable in using the measurements to make decisions on changing the DC system. As was stated I plan to check my DC with a 8-10 foot section of 6 inch pipe and see what the maximum I can expect from the DC before all the added length and bends. I have a Jet DC1200 with the oversize top bag from AF.

[Douglas Robinson]

Thanks

Everyone.

[Allan Johanson]

As was stated I plan to check my DC with a 8-10 foot section of 6 inch pipe and see what the maximum I can expect from the DC before all the added length and bends. I have a Jet DC1200 with the oversize top bag from AF.

Very cool, Floyd. Please post some results after you play around with it. I've been doing lots of tests and have learned a lot doing it. One is that one of my brands of flex hose is really poor for airflow, but it flexes great! When you use one of those on-line ducting airflow estimators like the Beacon Engineering Estimator or a simpler one like Bill Pentz', you may discover that these estimates can be off a fair amount compared to the materials you have in your shop. I'm not dissing these estimators (I find them very useful), but how can they know what the friction losses are for your materials?

I actually set up a repeatable test where a 10' length of 6" flex hose with a 6" port on a tool flowed LESS air than a 10' length of 5" flex hose with a 5" port. A big part of that had to do with how the ultra-soft 6" flex hose laid on the floor when compared to the almost frozen-stiff 5" flex hose - but each shop is different and your materials may be different too.

Anyhow, you will discover what's going on in your shop with your materials and that's what is important to you.

Have fun!

Allan