measuring static pressure.

A

AlanBienlein

New member
I want to measure the static pressure ( water column) of my system. Do I install the test tube in the 8" ductwork to test all of my duct runs? Also do I leave it in this location to measure my max static pressure?
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Can I plug this into a formula to give me cfm at my various tools?
I read your supposed to test the blower seperate from the cyclone and then test with no filters but I don't care about big numbers. I just want to know what my system does installed as I would use it. To me those are the numbers that mean anyhting. Any assistance would be appreciated.

Alan
 
Measuring ...

Measuring ...

Alan,
Measuring static pressure with various airflows is useful when designing the system. I think it buys little once the system is built. You can use a pitot tube along with a U-tube manometer to measure velocity pressure in the pipe. From the velocity pressure you calculate air velocity or look it up in a table. Then with the area of the pipe you get airflow. You can do this at the one location you point out for all your runs. Just close off all runs except the one you're measuring. Make sure you don't have leaks as that can mess up this approach.
I bought a Dwyer 167-6 pitot tube (google Dwyer) for about 56$ with shipping. I built my own manometer but they aren't very expensive from Dwyer. You make the measurement thru a small hole in the pipe and plug it later. You should have at least 8.5 pipe diameters upstream and 1.5 diameters downstream (see Dwyer on-line literature) of smooth pipe from the measurement point to minimize the effects of turbulence. It looks like you may be a little short on that but you can search around in the pipe with the pitot tube to see if the variation is smooth. Hope this helps.
bababrown
 
airflow

airflow

Alan,
I measured the air flow in my system by using a Dwyer Magnehelic 0-5 in WC gauge that I bought on eBay, the Dwyer 166-6 pitot tube that I had to buy new, and a homemade manometer. A gauge to measure 0 to 3 in. for up to about 7000 fpm would be better for accuracy. I don’t think you can get the cfm from just measuring the static pressure. The static air pressure will be the same throughout the whole system so the location you have shown should work for that.
The Dwyer pitot tube comes with a calculator to convert the velocity pressure, air flow through the duct directly into the pitot, to fpm. You can disconnect the tube for the velocity pressure and the gauge will give you the static pressure which also agreed with my manometer. The fpm is then calculated by taking the fpm*duct area(sqft)=cfm. You could measure the velocity pressure in the 8 in. line and calculate the 6 in. lines by using the ratio of the duct areas.
I like to see what air flow I am getting and it shows what happens after any hood and machine modifications.
 
It sort of helps. I'm just curious as to what my system pressures and cfm flow is at my various machines. I can't totaly seal up my blast gates but am pretty sure everything else is sealed up.

Just messing around last night with a piece of 3/16" copper tube I formed and installed in the ductwork I measured almost 14" w.c. with everything closed up. My ductrun to the tablesaw read about 7" w.c.and if I opened a second blast gate it dropped to about 3" w.c.

Probably not the most accurate but it did match what I put into Bill's static pressure calculator for the duct run for the tablesaw. I just haven't been able to find the tables or formulas to figure cfm or velocity.
 
airflow

airflow

Alan,
I did a search for 'how to calculate cfm with static pressure' and found the www.physicsforum.com that says this;

Static pressure is pressure without motion (that's what the word "static" means). So there is no positive relationship between fan static pressure and flow rate, with two exceptions:

1. For a specific fan, each possible static pressure corresponds to a specific airflow. Ie, if you close a valve and increase the backpressure the fan has to overcome, the new cfm can be read from a fan curve by matching the new pressure to the curve.

2. In a situation where all of the static pressure is turned into velocity pressure - such as in a pressurized tank with an open valve, you can use Bernoulli's equation equating the two to find velocity from static pressure.

If you have the blower data, may be you colud get some estimates using the first part. I don't think the second part is going to help.
 
airflow ..

airflow ..

Alan,
The problem using static pressure alone is that none of the other factors are well known. If we knew the cyclone curve well and then measured static pressures at the filter and at the input of the cyclone, we could get airflow because the point has to fall on the cyclone curve. I've used the cyclone data in the spec section but I doubt it applies since the stick was put in the blowers. If we knew the head losses for all our duct runs we could use the static pressure to derive airflow. But again we don't know them well. I decided to measure airflow with a pitot tube so I know where I'm at and then I monitor pressure on the filters to see when the system degrades. That just uses a static pressure tap in relatively clean air.
bababrown
 
So will the few measurements I got be able to tell me anything? I don't want to tear my cyclone apart just to measure the blower to get a fan curve. Will leaving the setup in place at least let me know if the filters are getting clogged up?
 
measuring static pressure

measuring static pressure

Alan,
I installed a permanent homemade U-tube manometer to monitor filter pressure alone. I have the standard CV-1800 with the filters stacked. I drilled and tapped a 1/4 male pipe to barb fitting straight into the side of the transition as the pressure tap. You want to get the face flush with the inside of the transition. I connected the barb fitting to my manometer. I made it using marvel mystery oil so it doesn't evaporate. it's been there for months now and hasn't evaporated any oil that I can see. It shows about 0.35" of positive pressure and has stayed that way for months. I think the filter pressure gauge might prove useful although I'm getting very little dust in the filters. I sand quite a few bowls so I'm pleasantly surprised at the cyclone separation.
I've run the numbers on my system (they're published in the gallery) and I guess they probably have helped me some. I know which runs are strong and which are weak. It cost me about $100 and some time to find the numbers. On the other hand my laser particle counter tells me how much dust I'm missing at each machine. That's a lot more useful.
bababrown
 
bababrown,
I like your idea of monitoring the pressure at the filters. What's the name on your gallery?
I attempted to measure the air flow in a line that has a damper that opens with all the blast gates closed to monitor the filters but with the damper weighted heavily, there isn't enough air flow to make accurate measurements with my 0-5 in. gauge.
 
measuring static pressure

measuring static pressure

Dan,
It's Richard L Brown under the CV-1800 installs. Good luck.
bababrown
 
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