Indoor return laser fume handling?

As I actually get close to firing up the #monocle, I’m getting less and less excited about cutting a hole in the wall just to blow laser exhaust out under the porch that my family like to sit on. If I treat the exhaust well enough to blow at my family, I don’t think I actually need to cut a hole in the wall either.

I’ve been looking at indoor-capable fume extractors. Of course I need ones that can handle a larger unit than a K40 because the #monocle is a bit oversized.

The BOFA AD350 gets high marks and I’ve seen remarks that it works for filtering indoors. Sadly, it’s too tall to fit underneath the #monocle next to the chiller but the folks who have done DIY versions (even using the BOFA filters) often mention that they get too much leakage.

Are there any indoor fume handling units other than the BOFA AD350 that folks have good experience with?

I’m going to be cutting wood, cardboard, and acrylic, nothing nasty. I’ve already gone through the “I’m willing to pay to not cut a hole in my wall” cycle of grief and also (rare for me!) the “I want to DIY” phase is past.

There is a lot of info on DIY systems. The biggest issue with any fume extractor is that depending on what you are cutting, they can clog up quickly and can be expensive to replace the filters. I am sure there are as many recommendations as there are people!

Yeah, a lot of the DIY systems say that getting them fume-tight is hard.

The AD350 filter sets are about $500, and at least one of the better DIY configurations I saw was built around the AD350 filter set. While I have a table saw I could theoretically use for making a DIY filter box, the saw is old junk. And a lot of the DIY systems mention not filtering well enough for indoor circulation; they are just making them less noxious for the neighbors.

I’m wondering whether there’s a non-DIY option folks would recommend that I choose over the BOFA AD350. I’ve looked at several, but I could be missing whole swathes of options, since I only recently came to this conclusion.

You might want to search for ‘indoor growing air filters’, though you might also want incognito mode enabled first…

People growing certain pungent indoor crops have been quite creative in this aspect, and as legislation has changed recently the commercial market has started to respond with some high-capacity carbon based systems that might fit cnc and workshop needs too. And it is less of a premium market, so prices are sharp :wink:

Heh. To keep bears from noticing me while I am camping, I looked for scent-proof locking plastic bags to store food and pack waste out of the wilderness. I got a lot of targeted advertisement for products associated with “trees” as a result. It turns out that although I was attempting to evade the attention of Ursus Americanus, the American Black Bear, others interested in the same product wanted to evade the attention of an entirely different kind of “bear.” (I think this slang for police was current in CB radio culture for decades, though I have no idea of its original provenance.)

I would assume that those filters for “indoor crops” only have to deal with odor, not with loads of particulate; they are concerned with living plants, not burning plants. What I’m looking for is a multi-stage filter, not just odor control. If I bought something that did only odor control, I’d be back doing DIY for the prefilter, at which point I might as well just DIY the whole thing, because I already have a fan and would have to buy the filters in any case. If I DIY, I can design it to fit under the #monocle so I guess I could reconsider that part.

I’m concerned with total air volume; with the fairly large enclosure on the #monocle the AD350 is on the low end of what I’d want. The ~1000CFM units are around $10K which is definitely out of range; the AD350 is expensive enough. Since they are rated in CFM, I’ll convert to archaic units: my laser chamber is about 22 CF, so ~220CFM from the AD350 would be 10 air changes per minute or one air change every 6 seconds. Obviously that would depend on impedance, but I can be intentional about ventilation to improve airflow. I could put adjustable air valves in the front to concentrate airflow where I need it. Easy enough.

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I will need a filter for Mobeam and was considering these two approaches.

I have an air conditioner fan saved for the evacuation.
My needs may be less stringent as it will operate in a shop.

The second one below is lower cost no idea how well it works.


Man, y’all are talking me back into thinking about DIY after all.

Yeah, I had definitely seen at least one of those DIY options. I’m amused by the amount of work they put into making them look good. The idea of putting a plexiglass window on a fume handling box amuses me but seems a bit silly.

The #monocle will be running in my shop (not part of the recirculating air handling for the rest of the house), but the shop doesn’t even have a window. I do cut wood in there (though mostly behind a vinyl curtain to contain dust), so sometimes it smells a bit anyway.

I want to avoid smells strong enough that the family objects, and I want to manage particulates that might affect health, reducing them enough to not worry much.

I might end up talking myself into trying to DIY a plenum that fits underneath the #monocle after all, just so I don’t need to find more floor space. Hmm. 20" square is a normal filter size, and 20x20 plenum is a thing, e.g.:

I could stack 20x20 filters and have plenty of room for an activated charcoal cage if I wanted to add one. I could mount it front to back underneath the bed of the #monocle and the fact that the monocle is on wheels would mean I could open both the front and the back easily to service it; I wouldn’t even have to make it slide on rails. I can duck / duct tape filters in place to prevent leaking and account for difference from nominal sizes without having to design mounting hardware. This might be easy.

First stage, fiberglass MERV4 to remove the worst junk with least cost in $$ and least back pressure to airflow, $3 each, replace most often, e.g.:

(Alternatively use a humidifier wet filter which has also been reported to work well.)

Second stage, deep-pleated MERV8 electrostatic $10 each, replace somewhat frequently, e.g.:

(Potentially add more stages…)

Last stage, 4" extra deep carbon MERV16 for fine particulate and odor $100 each, replace when the Mark I Nose detects need, e.g.:

Fan on the output side so that leaks matter less.


I just realized that another reason to consider this DIY fume handling is that the fan I bought is at least rated for twice the volume of the AD350. It’s a 440 CFM fan. Using this to pull fumes through those filters should clear up to one full air change every 3 seconds (ignoring impedance). If that doesn’t turn out to be enough, I could mount an 8" fan with 740CFM for $100 instead. (And yes @easytarget this also is aimed at :deciduous_tree: aficionados.)

I have now realized that mounting filters in a duct immediately under the #monocle can solve a duct plumbing problem. I designed this thing to have a 6" duct tube coming out behind it, which is inelegant when it could otherwise sit back against the wall.

I also have unused space under the laser tube.

I also found that the 6" hole in the back is partially obscured by the 60mm tall bed frame. That’s no good.

So now I’ve realized that if I just do some sheet metal work, I can make a 160mm x 270mm hole in the back, and use some of that empty space to make a plenum that leads directly down into the top of the back of the 20x20 ductwork as the intake. which would avoid anything sticking out the back.

Now I’m glad for FreeCAD’s Sheet Metal Workbench.

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I have now realized that using metal plenum and hooking up the fan pulling vacuum will have a tendency to implode the plenum. I should consider making the plenum out of MDF instead to be stronger.

Yes, I recognize that this is looking more and more like what everyone else has already done. :grin:

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That’s the maximum throughput at zero pressure. The blower in an AD 350 draws up to 1100 W and goes up to 9600 Pa. It’s way more powerful. It can pull a lot more air through that filter stack.

The theoretical maximum throughput at zero pressure by itself doesn’t really mean much.

Sometimes they will also tell you the maximum pressure at zero throughput.

If you want the full picture, you need a pressure/throughput curve like this one:

NRG 137 performance curve (220 + 250 W)

Here is a rough comparison between my old bouncy castle blower, my new Papst blower, and an AC Infinity S6 mixed-flow inline fan:

This is a bit simplified and only for illustration purposes. The actual curves are of course slightly bent upwards and not straight lines.

Anyhow, if the goal is to move at least 70 m³/h (which is what the stock fan of my K40 can move under ideal conditions), the quieter Papst blower would actually outperform the much beefier bouncy castle blower.

And there is another factor to consider: The pressure drop across filters drastically increases if you pull more air through them.

E.g. if the pressure drop across everything is 1000 Pa at 110 m³/h with the Papst blower, then the bouncy castle blower would not be able to move 200 m³/h as that graph suggests since the pressure drop would be significantly higher at that higher flow rate.

If you need more throughput, the best option are larger filters with more surface area. If you got twice the area, you can move twice as much air at a similar pressure level.

If you want to build a filtration unit for a specific existing machine, I recommend to measure the actual throughput (a sub $10 anemometer is fine for this) to figure out what kind of minimum and desired throughput you’re actually looking for. Then you’ll know which range of a pressure/throughput graph is relevant to you.

Well, it’s a pretty complicated topic and it gets more complicated if you add differential pressure sensors, VOC sensors, flow rate sensors, automatic flow control, a basic remote interface, and whatever. There are more caveats with all of those things.

Anyhow, recirculating the air a couple of times is already a bit hairy with better commercial units. Filtration isn’t perfect. Most VOC sensors are pretty crude and not super reliable. Most units do not monitor the CO levels in the room. You definitely need at least some ventilation.


Yeah, that’s why I wrote “(ignoring impedance)”… I had no idea the rating of the blower in the AD350, thank you! I bought a radial fan instead of an axial fan because of the general understanding that axial fans have higher pressure at zero throughput.

I’m building a custom unit, so there’s nothing to match.

This was why I was looking at deep-pleat filters for the finer filtration levels, for additional surface area.

I hadn’t thought about CO levels. That’s not something that the Mark I Nose is able to detect. And CO poisoning leads insidiously to being stupid. If I’m working in my shop, I might be working metal as well, including lathe work. Being stupid while using the lathe is a really bad idea. (“Warning: This machine will kill you, and it will hurt the whole time you are dying.”) I never touch the lathe if it would be inappropriate to fly an airplane (e.g. not within 8 hours of consuming any alcohol, not if I’m feeling ill, etc.) and a fingertip O2 sat sensor won’t even catch CO poisoning. And there’s no trivial way that I know of to scrub CO.

My overall goal is to be able to occasionally make things that are hard to make now. Mostly cutting rather than engraving; my imagination is mechanical not artistic. Things like acrylic, masonite, and baltic birch boxes. I don’t expect to be using it frequently; i just want to have it for when I do want it. I go weeks without using my CNC router but I’m glad I have it. I’d expect the same here.

I do have wheels on it. While my shop doesn’t have a window, it does have double doors, and those doors lead under our porch, so I have a roughly 3 meter by 5 meter outdoor space that has a roof over it. Maybe I should make my rule that to use the laser cutter, I wheel it outside and use it there, and the nice spot I found for it in the shop is merely a parking space. The problem is that the chiller isn’t on wheels.

Alternatively, I could just always run with the door open and run an exhaust pipe out the door for some distance. I could rearrange the shop a bit to make that easy. I’d still want to filter it at least a bit so I’m not just sucking the raw fumes back in through the open door; I don’t have another door to open to have air cross-flow. That would let me leave the chiller in one place.

Then if I end up using it so much that doing that becomes an inconvenience, I reconsider cutting a hole in the wall. Maybe I could have a window installed and install a vent in the window when I’m cutting!

Thanks for the reality check!


I used to use a Rainbow vacuum that used water, for drywall sanding some years ago. It would captivate all of the dust into the water for awhile. You might look into a used one