How is the correct amount of dry chemical confirmed when recharging?
- By filling the shell to the top of the neck
- By pressure, read on the extinguisher gauge
- By volume, using a marked measuring scoop
- By weight, matched to the nameplate charge
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D. By weight, matched to the nameplate charge
The charge is weighed to the nameplate amount. Overfilling leaves too little space for expellant gas, and the pressure gauge says nothing about how much powder is inside.
Reference: NFPA 10: recharging
Which gas is normally used to pressurize a stored-pressure dry chemical extinguisher?
- Shop compressed air
- Dry nitrogen
- Carbon dioxide
- Oxygen
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B. Dry nitrogen
Dry chemical needs a moisture-free expellant, normally standard industrial grade nitrogen. Ordinary shop air carries moisture and oil that cake the powder.
Reference: NFPA 10: recharging, pressurizing gas
Why is moisture in the expellant gas a problem for a dry chemical extinguisher?
- It makes the powder cake and corrodes the shell
- It raises the pressure above the gauge range
- It makes the powder conduct electricity
- It freezes in the nozzle during discharge
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A. It makes the powder cake and corrodes the shell
Damp powder lumps and will not flow through the siphon tube, and moisture attacks the shell from the inside. That is why dry nitrogen is used and why shells are dried fully after a hydrostatic test.
Reference: NFPA 10: recharging, pressurizing gas