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Episode #59: Make a manometer to measure DC filters

We’ve been talking a lot about dust collection and filters lately, and we’re not done yet. By the time we’re finished our new dust collection section at will be full of great information and resources to keep your shop cleaner and safer. But today I’m going to show you how to tell when your filters need cleaning. So let’s get started!




So, you’ve upgraded your dust collection filters and now you have all kinds of airflow. Everything is running at peak performance and the air in your shop has never been cleaner. Time to sit back and enjoy it, for a while. But eventually those filters will start to plug up. And as they do, they become restricted, the airflow through the system is reduced, and suddenly your dust collection isn’t collecting as much dust. Now, it’s easy to say “when that happens I’ll know it’s time to clean the filters, right?” It’s not that simple. Since the filters accumulate dust slowly, the changes are subtle. It’s like the story about a frog in a pot of water. Boil him slowly and he won’t even notice until he’s dead! Dust collection may not be as dramatic, or cruel, but you still need a way to keep an eye on your filter condition so everything is running at peak efficiency. The answer is a manometer, a simple water gauge that measures pressure. These work on just about any type of dust collector, from a single stage unit to a cyclone, even ambient air filtration units if you’ve upgraded them to a canister filter like I showed you in the last video. And the best part is, you can make one yourself.


All you need is a piece of plywood, about 4” wide and 10” long; three to six feet of 3/8” clear tubing; and a rubber grommet. These are made for sinks with garbage disposals and you can get them in most home centers.


Now comes your first big decision. Where do you want to cut a hole in your filter? We’ll talk about ambient filters in a bit, but for now let’s concentrate on dust collectors. If you have a single stage dust collector like a Harbor Freight, Jet, Grizzly or something similar, you’ll want to cut your hole right in the top of the metal top of the filter. Now, this is assuming you have a canister filter on your collector. If you have a bag filter, I suggest you stop here and go to to watch our video about why you shouldn’t be using a bag filter. If you have a cyclone, you’ll want the hole in the bottom of your filter stack. A lot of cyclones have wooden cleanout boxes down there. That’s a perfect place to cut a hole to attach your meter. On mine I replaced the cleanout box with a removable filter pan from which attaches to the bottom of the main filter with some clips. This sucker gives me even more filter area, which makes my cyclone run even better. Anyway, that means I am going to cut my hole right in the filter pan.


By now you’re probably wondering why we’re cutting holes to begin with. Well, we have to attach one end of our tubing to our filter somehow, now don’t we? So we’ll trace around the outside of our rubber grommet, and use whatever tool we have on hand to cut it out. Remember, you’ll be doing this on the top of your filter with a single stage collector, and in the side of your wooden cleanout box if you have one of those on your cyclone. I actually tried mounting it above the filter on my cyclone, and that just didn’t work well at all for all sorts of complex scientific reasons.


After you get your big hole cut, you’re going to drill a ½” hole through the center of your grommet to fit your tubing. These things don’t drill well, so take it easy. You want a snug fit, not a torn up mess. You’ll probably have to cut the last of it out by hand. Now, use some good, goopy adhesive to attach your grommet to the hole in your filter or on your cleanout box if you went that way.


Time to make the meter itself. Get out your chunk of plywood and lay your tubing along one edge and mark a point on either side. We’re going to drill holes at these points to attach some zip ties around the tubing to secure it to the wood. Mark three pairs down each edge. Keep them about an inch and a half away from the bottom end of the plywood so there’s room for your hose to bend in a nice U shape without kinking. An eighth on an inch drill bit should be fine.


After you finish boring your holes, measure about 4” from the bottom end and mark that point with a pencil. From there, mark off every 1/8” up for about 6”. Use a square to draw a line across your board at each of those marks so you have a nice even scale. It doesn’t have to be perfect, but you know, try. Now get yourself some plastic zip ties, or some wire would work too, and use them to attach your tubing to the board. You’ll want one end to start in the upper right corner then run down the edge, make a nice U turn at the bottom, and then run up the other edge. You should have lots of extra tubing. Fight the urge to trim it for now, that would be a big mistake.


Now where are you going to mount your meter? I mounted mine on the wall next to my filter at eye level. Just make sure wherever you mount it is close enough to the filter that you can slip the end of the tubing into the hole in rubber grommet without kinking it. That’s why I said you needed three to six feet of tubing. It all depends on where you mount your meter. Speaking of meters, if you’ve plugged the tube into the grommet, you’re almost finished. Time to calibrate.


Get yourself some nice liquid like scotch, or water, or scotch and water. If you’re a teetotaler you can use water with some food coloring. Carefully fill your tube until the level reaches the first line. Don’t go over, because if you do, you’ll have to go through a complicated recalibration process that includes sliding the tube down. In fact, anytime your water level changes due to evaporation or refilling, you can adjust it by sliding the tube up or down in its ties so you always start with your water level at that bottom line. That’s the zero point zero.


Now turn on your system. We’re only concerned with the right hand side of the gauge, where the water level rises when you turn the dust collector on.  If your filters are new, or recently cleaned, you’ll see the water rise just a little bit, maybe one or two lines. Of course this is all relative to the size of your blower and the size of your filters, but it shouldn’t rise much. It’s a good idea to make a mark on your gauge at that point so you know where your level should be with a clean filter. Now, we need something to compare it too. So get a couple of large trash bags and some duct tape. You want to cover up as much of your filter as you can. A perfect seal will be impossible, but use lots of duct tape and do your best. Now either unhook your dust collector from the ductwork, or if that’s not convenient, open up all of the blast gates. You want maximum airflow coming into the system. Of course, since we covered the filters, we should have minimal airflow coming out of the system, which will build up pressure in the filters. Don’t worry, the shop is fairly unlikely to explode, so turn the system on and watch what happens with your meter- on my cyclone it shot up several inches. That represents a really, really, really plugged filter. Way more plugged than you will ever let it get. If I had completely mummified this thing so it was sealed right up perfectly, it would have shot water out the top because this is a powerful cyclone. A single stage collector won’t move the water as much. Wherever your level is with your filter wrapped, mark it on the meter. We’ll consider that maximum pressure. So, we have our zero, we have our new filter level, and we have our max level. At what point should we clean our filters? I would suggest cleaning them at about 25% of maximum. On my cyclone that’s at about an inch and a half above zero. On a smaller system it will be lower. Just make a mark on your meter about a quarter of the way from zero to your max line. When your water level reaches that point, you know it’s time to clean the filters. Don’t forget to take the trash bags off.


So, what if you want to mount a manometer on one of those ambient air filtration units we modified in our last video to accept a canister filter? Well, if you remember we attached a disc to the top of the filter so we could hook up our duct. I made mine out of plastic, but you might have used plywood. Get yourself a nylon hose barb. Drill a hole right in that disc and glue the hose barb into it. If you didn’t duct to your filter and instead attached it right to the system, you’ll want to put your hose barb in the bottom of the filter. Now, attach your manometer and follow the same process as before to make your guide marks: zero, new filter, max and then about 25% for cleaning. Remember, these units generate far less pressure than a dust collector. So your water level changes will be more subtle. But if you do the same wrap test to find your max level, you can then mark 25% of that as your clean-it point and you’ll be golden.


So there you have a homemade manometer. Sure beats buying one. And you’ll always know that your system is running at peak performance. Check out for more dust collection resources and we’ll see you next time at the Stumpy Nubs Workshop!










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