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Weight and charge of 5kg CO2 extinguisher

Guest Robert H

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Guest Robert H

Hi all, I run a small workshop and operate a MIG welder but ran out of shield gas half way through a job over Christmas.

It was suggested that I could use a CO2 fire extinguisher as a source of gas to get the job finished, so obtained an out of date 5kg extinguisher and completed the job.

Out of interest I weighed the cylinder to compare it against the markings on the cylinder and commissioning label.

This has left me a bit puzzled and am seeking clarification of the markings on the cylinder and weighing procedure.

Cylinder markings;

V7.5L (Volume 7.5 Litres?)

PT250BAR (Test Pressure 250 Bar?)

5KG CO2 (Net weight of charge)

17.44KG G (Gross?)

12.44KG (Tare?)

Commissioning label (2005);

Full Mass 13.018 kg

My measurements;

Weight 12.290 kg with hose / horn removed

Unfortunately I disposed of the horn, but kept the hose so don't have an exact weight, but estimate 0.500 kg.

My questions are;

Are CO2 fire extinguishers generally weighed with the horn/hose attached?

By my calculation, from the full mass above it only contained just over 0.5 kg charge new. Why are these referred to as 5kg when the charge is only a tenth of that?

I'm trying to do a cost comparison vs rental and refill charge of a new cylinder.

Many thanks.



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Sounds like it's been serviced by a rag and tag unqualified firm based on the information on the service label. Your true weights are stamped in the neck of the cylinder - 17.44kg full, 12.55kg empty, CO2 charge 5kg. 

The manufacture date is also stamped in the neck, usually mm/yy or mm/yyyy. If it's 2005 as on the service label it's overdue it's 10 year statutory hydrostatic pressure test and can't be refilled (but can be exchanged for a tested unit and sent off to be tested & revalved).

You should weigh a CO2 with the horn assembly removed.

A 5kg CO2 is 5kg (unless the cowboys commissioned a faulty unit that had leaked down to nearly empty!)

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Guest Robert H

Ok Anthony, thanks.

Out of interest I measured the pressure of the contents to be 38 Bar (at 10*c), I understand they are charged to 55 Bar so am I right in thinking approx 2/3 of the charge remains?

I'll see if I can get a different set of scales in case my measurements are wrong, but that still doesn't explain the weight on the sticker.

I don't intend to get it refilled, I'll just look out for another one when the time comes.

Thanks again.

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The pressure is constant in a CO2 until virtually empty, that's why they don't have pressure gauges, as  the liquid CO2 level drops more CO2 vapourises in the cylinder to keep the pressure constant. Only when there is no liquid CO2 left does the pressure drop and the nature of the discharge change (higher pitch, almost clear discharge) as only CO2 in it's gaseous state is being discharged.

It shouldn't be 38 bar unless it's virtually empty.

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