View Full Version : 3M Fresh Air Hoods
05-06-2008, 10:17 PM
Does anyone use one of the 3M L-series positive pressure hoods? I am looking at the L 500 and L 900 ones and am wondering if there are any opinions? Are there any other brands? I need to start breathing better air while welding and fitting but want to talk with someone who's using these before dropping $1200 for the setup! Thanks!:)
05-07-2008, 05:20 AM
Will you be using your supplied-air system at a fixed location or all around the shop? That is, do you need lots of mobility while wearing your respirator? If you can operate with a long umbilical cord, consider making your own rig.
Proper supplied-air respirators have to meet specific standards for air flow, but since you will not be working in a toxic or oxygen-deficient environment or other conditions requiring strict performance, you might be able to hack together something like I did a few years ago when I had to do some extensive concrete restoration with stinky epoxy in a small underground chamber.
Basically it was simply a full plastic hood with clear faceplate having a perforated plastic tube around the crown. I adapted a Bullard sandblasting hood, but think you could make do with a welding helmet by taping a fire-resistant fabric to the helmet and allowing it to drape down over your shoulders.
My recollection is that the air distribution tube I used inside the hood was just thin-wall vinyl tubing about 3/4" O.D. with a bunch of approx 1/8" holes punched in the wall of the tube to allow the air to blow down over my face.
I chose to use new, uncontaminated corrugated black sump pump output hose for the run between my air pump and my hood. About $6 for 50 feet, I think. Lightweight, flexible, and cheap. Since I was working in a relatively confined space, that hose gave me adequate mobility.
You have to be careful in your choice of air pump, since you don't want to breathe contaminated air from a lubricated pump. As I recall the commercial supplied-air pumps use special unlubricated vane pumps with output filters, but I used a small, surplus "regenerative blower," that has no sliding parts that could produce wear particles or require lubrication. They are somewhat expensive new (see the Grainger catalog), but I used a new surplus unit that I picked up for about $50. Beware of possible contamination from prior use. I used a foam inlet filter on the pump to avoid breathing insects.
A regenerative blower requires a certain minimum flow for cooling, so instead of using a valve to throttle air flow into my helmet tube I used a valve on my belt that dumped excess air to the atmosphere. I also provided an air dump orifice near the pump that would allow the safe minimum air flow through the pump even if the long hose got kinked. I think the regenerative blower was about as small as they come and provided plenty of air.
The whole rig couldn't have cost more than $100 and worked great. Since I was working in a pit under the public sidewalk, I had the pump on the flat roof of the single-story building I was restoring, away from curious passers-by. If you are interested I could dig out the hardware and give you more exact info on pump, hose and orifice sizes.
05-09-2008, 03:27 AM
Awright thanks! I frequently cobble stuff together or even over build my own whatevers but this time I am thinking of just purchasing a positive pressure system. I appreciate you sharing your experience. I really am hoping someone has experience with something off the shelf (again in particular the 3m L -series hoods)
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