Flash Arrestors are a tricky business. They are usually solid state devices that allow a fuel gas to flow through them, but do not allow a flashback to propagate back through. These usually contain some form of sintered filter, which acts as a diffuser and barrier. The sintering can be anything that is powdered, or porous. These materials have ranged from sand to fine bronze wool. Commercially made flash Arrestors also contain a sintered metal filter. Unfortunately, they also contain thermal shutoffs. If the Arrestor heats up, an internal mechanism shuts off flow. This poses a dangerous scenario for HHO enthusiasts, because HHO is produced on demand. This means if the individual doesn't pay attention, a dangerous level of pressure could build, increasing the risk of disaster.

The more a flash Arrestor diffuses HHO, the less likely an ignition will pass through it. Many have reduced the particle size to as low as 1 micron in an effort to further diffuse the HHO, but this poses huge flow restriction issues. The more you restrict flow, the more pressure will build behind the Arrestor; the more compressed the HHO, the more likely a flashback will pass through. So, a flash Arrestor design is a bit of give and take. Currently, there is no mass manufactured flash Arrestor for a mixed gas like HHO. Sure, manufacturers make them for Hydrogen or acetylene, but these are considered the "fuel gas". They are a gas that would be harmless unless introduced to a necessary oxidizer, like Oxygen. Of course Oxygen exists in our atmosphere, which makes both Hydrogen and Acetylene dangerous when introduced to atmosphere. But contained in a bottle, they are considered "inert until oxidized". The problem with HHO is that it contains both the fuel (Hydrogen) and its oxidizer (Oxygen) in the same hose or container. This poses a problem with flash Arrestor manufacturers, because having both the fuel and oxidizer in the same line can allow a flashback to travel explosively through most sintered materials.

Here is a guideline for Sintered Flash Arrestors
In our studies in flash Arrestor designs, we have come up with a general guideline for sintered filter flash Arrestors. Keep in mind, the larger the flash Arrestor, the more internal surface area. This means reduced flow restriction. As HHO production levels rise, the Arrestor must also be increased in size. If this does not occur, a dangerous level of pressure could build, inducing failure, due to compressed HHO.
25-30 Microns: 0-1 LPM HHO, 0-PSI
10-25 Microns: 1.1-6 LPM HHO, .1 to 1-PSI
5-9 Microns: 6.1-20 LPM HHO, 1.1 to 4 PSI
1-5 Microns: 20+ LPM HHO, 4.1 to 10 PSI

A WARNING about Bronze Wool
Be careful with commonly made "Stuffed Bronze Wool" flash Arrestors. In order for a flash Arrestor to work consistently, you need to keep the pass-through particle size below 10 microns. This is extremely difficult to do with bronze wool, because it is nearly impossible to pack them consistently. It is this reason why we do not recommend using them. These can work well with production levels below 1 LPM, but add a little pressure, or allow them to become saturated, and they fail.

A WARNING about Check Valve "Arrestors"
DO NOT purchase a check valve for the purpose of using it as a flash Arrestor. They simply do not work. We have seen many vendors selling check valves just for that purpose. We have thoroughly tested many check valves. In our tests, a check valve was only 80% effective, and ONLY IN LESS THAN 1-LPM production. We simply do not like those odds. If someone is advertising check valves as flash Arrestors, be wary of purchasing ANYTHING from them. This means they have not done any due diligence in researching the safety aspects of what they are selling, and do not care about your safety.

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