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Old 20-04-2020, 12:21   #1
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Water Mist Fire Extinguishers

If you respond with "but they're not rated B" or "it isn't done that way" I will rate your response boring. These are obviously true. But they are used in boats and they are rated, both USCG and abroad. Really.

The USCG says these are the minimums, although you can add more. If you've ever had a fire on a boat, you'll want more.

Vessel Length
No Fixed System
With approved Fixed Systems
Less than 26'
1 B-1

26' to less than 40'
2 B-1 or 1 B-II

40' to 65'
3 B-1 or 1 B-II and 1 B-1
2 B-1 or 1 B-II

B is the UL rating for oil and gasoline fires. (Note if you Google: US and Euro fire categories are different, although they both used letters. The numbers relate to different tests as well.) There is no requirement for type C (electrical) or type A (wood, cloth, fiberglass etc.). The B numbers are related to standard tests. The C rating only means it is not conductive. In the case of the A rating, it is known to be equivalent to 125% of the number in gallons of water. Fore example, A 2A fire extinguisher contains 2.5 gallons of water. If you look at the ratings of dry chemical extinguishers, 1A10B:C is typical. If you have a 1B extinguisher, that means it is equivalent to 2 ounces of water. Small wonder they aren't much good for puting out fires including paper and fiberglass. It's right in the rating. They don't even rate them for ordinary combustibles because they stink at it.

In the UK and Europe, water mist (not water spray--a fine fog) has some popularity.

The USCG also approves mist systems for shipboard use. I understand they are quite common in engine rooms.

* Not as good on oil. But they do not spread it like a water stream and they are somewhat effective if used intelligently.
* Not B rated. You still need a B extinguisher.

* Much better on wood, cloth and fiberglass. 10 times better. It cools the embers, other types do not.
* No hazardous fumes... other than the fire itself.
* Reduced visibility problems. You have to use a dry chemical in a tight space to understand how bad this is.
* Very little damamge, since very little water is used. DI water is not corrosive.
* Safe around electrical equipment (C rated). DI water is no conductive and they use very little.

Thoughts, as a secondary extinguisher? I have no opinion at this time, I'm just posting this as a conversation starter.
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Old 20-04-2020, 12:52   #2
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Join Date: Apr 2020
Boat: Amel 53, Super Maramu
Posts: 223
Re: Water Mist Fire Extinguishers

I was trained to fight chemical plant fires with water fog, and other techniques. That it works is certainly true. But, and it is a huge but, it is far more technique dependent than dry chemical, or foam based systems.

My opinion: Leave firefighting with water fog for the professionals, unless you are going to get the real hands on training to use it correctly. Note that professional ship crew get just such training.

Just as an aside, if your local fire department offers hands on fire training, jump on it. No matter how smart you are, actually putting out a fire will teach you something that might save your boat or your life.
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Old 20-04-2020, 13:28   #3
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Re: Water Mist Fire Extinguishers

My Amerex Model B402 3A.40BC (Ammonium Phosphate) in the cabin weighs 8 lb.

An Amerex 90-B270NM 2A:C Water Mist Extinguisher has a shipping weight of 8 lb and holds 1.75 gal (14.5 lb) of water, so I guess its full weight is about 20lb.

So, 2.5X the weight and 2/3 the effectiveness.

A deionized water fire extinguisher would have to be protected from freezing.

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