Back to Ganoksin | FAQ | Contact

Acetylene tank pressure drops below 60 psi


Hey Y’all

Just a maybe stupid question, but I’ve been told to not use my
acetylene tank when the contents drop below 60 psi. Is that right or
am I wasting acetylene?

Thanks in advance for all your great experienced advice!

Tim Dwornick
Hastings, NE


Having just gone through this, I had a couple of things happen when
the pressure dropped. as the tank got low on fuel. I don’t know the
exact level but it caused some problems.

  1. The low pressure flame was not as hot and I ended up heating
    pieces for what seemed forever to try to get solder to run.
    Particularly bad on larger pieces.

  2. The flame got dirty with a lot more smoke and black particles of
    something (soot?) that stuck to the piece and were a real pain. I
    got a new gas tank and everything went back to normal. I decided that
    as soon as I notice effects from low gas, I will get a new tank. Its
    not worth the hassle.

Be well JIm J


depends on your regulator and the minimum pressure it needs, but
even with that, it is never a good idea to allow any gas tank to go
to zero, you should always leave a residual pressure in the tank.



I take the advice of the experts at the welding supply houses where
I buy and refill my tanks.

I have to say, however, that I’ve been soldering and melting with
acetylene for over 30 years, and I typically run that tank down until
nothing but the last whisper of gas comes out of it. I’ve personally
never had any problems or other issues with doing that.

You can bet that I will ask the welding supply guys about that tank
pressure issue when next I’m in to refill one of my tanks.

Jay Whaley
Whaley Studios



My gauge has 1/4, 1/2 etc. no absolute pressures. If it indicates 0
it is probably empty. If the flame smells and looks different then
you are burning the acetone in a near empty tank. Being frugal (or
desperate) I will suck the tank down to almost 0. Funny flame and I
stop quickly.

I have not killed myself yet but use my advice with caution.

Demand Designs
Analog/Digital Modelling & Goldsmithing


CGA handbook, states that carry over begins at around 25PSIG. Gas
Pressure(acetylene in this case)will remain almost constant at a
given temperature. As you withdraw the acetylene from the solution
in the cylinder till very little remains. If you are taking the
gauge pressure down on the acetylene cylinder to below 25psig it can
cause acetone to be withdrawn with the gas.

Most books and courses, state is that is the suggested cut off of
use of the cylinder. 25 psig to 50 psig to minimize, acetone carry
over. Since the acetone in most tanks of an age over 5 years or more
isn’t what we think of acetone. They don’t change the acetone out in
the cylinder, just add fresh. So over time with the aging of the
fill material and changes dues to the loading and unloading of the
acetylene gas it gets thicker over time sort of approaching syrup.

At these lower pressures, the amount of solvent expelled is
dependent on the solvent vapor pressures, the condition of the
cylinder, and the conditions of withdrawal. The condition of the
cylinder is the inside and the number of charge discharge cycles, As
the solvent can be come thick with age,impurities and cycles etc.

Which has an effect on the impurities in the gas and then in the
liquid. There are two, acetone and Dimethylformamide DMF a polar
(hydrophilic) Aprotic solvent. Today, the gas is shipped and stored
in metal cylinders containing acetylene, dissolved in
dimethylformamide (DMF) or acetone.

A couple of other things that should be cleared up, are if you have
traveled with your B or MC cylinder laying down on its side. You
must place it in an upright position so the acetylene gas gas
reabsorb into the filler, and prevent carry over of the liquid into
the regulator,torch and flame. Which can cause contaminate the joint
you are working on. Not too mention the effect on the flame.

Something else that is often stated incorrectly is that the material
filling the inside of the cylinder is only part way that is
incorrect. As the material fills the cylinder, but it is extremely
porous. This along with the fill rate(dissolving rate into the
solvent 7 hours for a full charge, is were the 1/7 use rate comes

Agamassan (aga) is a porous substrate used to safely absorb
acetylene/solvent and thus allow the transport, storage and
commercial exploitation of an otherwise unstable gas. Asbestos was
only used by a limited number of cylinder makers for a few years,
and the Compressed gas association has very good records on them.

Since the rate of use is one seventh of the capacity per hour as the
acetylene comes out of solution. As it takes 7 hours to fill the
cylinder or put the acetylene in solution with the liquid in its

Acetylene cylinders are filled to 225 psig. That is a full cylinder.

The critical pressure is 15psig for acetylene in the atmosphere.
That is why it is pressurized and dissolved in acetone so the tanks
can hold a use able amount of acetylene. It becomes very unstable at
or above that pressure.

Ask for and carry in you vehicle, a MSDS at all times, in the glove
box. In case you are stopped, and they see it.

Just for fun did you know that a B cylinder refers to its old times
use as the size of cylinders used on buses, in the days before
electrical lighting on vehicles.

Am MC cylinder refers to Motorcycle as this size was used on the
cross member of the motorcycle hence the angled valve. To provide
lighting in the good old days.



Reference the 60 PSI recommendation, it has to do with the rate that
Acetylene can outgas from the acetone or similar solvent that it is
dissolved in at room temperature without the acetone being drawn
into the regulator and line.

If you are using a average cutting or melting torch, that number
makes sense and is a correct recommendation in the central US, or
inside a building. If the tank is stored outside in the winter used
with the same tip the needed pressure would be even higher. Or in
northern Canada, you actually have to fit a warming blanket (Some
mobile service vehicles wind 1 inch rubber hose around the outside
of the tank, cover it with a horse blanket and run their engine
coolant trough it to heat the cylinder). In some cases with really
large torches, multiple tanks need to be ganged together on a
manifold to provide the needed gas flow (rule of thumb is flow rate
should not exceed 1/7 of tank capacity when full to avoid solvent

If you are asking a counterman, first make sure he knows the
rationale behind the answer and then show him the tip, or even better
look up and tell him how much gas you are drawing from the tank (in
cubic feet per minute or the metric equivalent) and he should be able
to give you an informed answer (or call head office and get the
correct answer) If he is dogmatic about the 60 psi ( or another
number) ask him why, if he says he doesn’t know or God told him, ask
him to see the tablet that it came down from the mountain on.

More info than you wanted to know, but now you know the “why” behind
the number.



The only reason I can think of not to take a tank to 0 pressure is
that moisture laden air might seep inside and begin rust formation
inside the tank. If you are simply exchanging tanks this isn’t a
problem. If you own your tank and keep refilling the same one, it
might be a cause for concern when the tank comes up for periodic
internal inspection and pressure certification.

Mike DeBurgh


My acetylene tank pressure dropped to zero a few days ago and I
figured I was “out of gas.” Took it to my local welding supplier for
a full tank, and asked them how it is you know how much fuel is left
in the tank. I use Smith’s two dial gauge, by the way. The lady who
owns and runs the welding supply company (for years and years) told
me the gauge that measures the supply pressure in the tank also
indicates the level of fuel. In the “B” size tank I use, the supply
pressure in the tank is 200 PSI when full, so, as you use fuel, the
supply pressure drops. She didn’t say anything about when to stop
using it, and the only problem I had was I couldn’t get my torch to
light and just ran out of fuel.

That’s my two cents. I’ll be waiting to hear from more of the
experts here on the Orchid forum!

Au revoir!


Someone once told me that is that there is acetone in the bottom of
the tank, which is why you don’t want to run it all the way to empty,
and also that is why it is important to avoid laying a tank down. If
it does lay down horizontally, let it stand up for a while before

Cynthia Eid



Congrats on using a dual line regulator. I quote Wikipedia:

Acetylene is not especially toxic but when generated from
calcium carbide it can contain toxic impurities such as traces of
phosphine and arsine. It is also highly flammable (hence its use
in welding). Its singular hazard is associated with its intrinsic
instability; samples of concentrated or pure acetylene will
explosively decompose. Acetylene can explode with extreme
violence if the pressure of the gas exceeds about 200 kPa (39
psi) as a gas or when in liquid or solid form. It is therefore
shipped and stored dissolved in acetone or dimethylformamide
(DMF), contained in a metal cylinder with a porous filling
(Agamassan, which renders it safe to transport and use, given
proper handling.

As a rule, I usually swap mine at 15 psi or thereabouts. Make sure
when you do swap it out, do your leak test on site. Welding supply
outfits bash their tanks around and the square valve stem to open and
close the gas can get damaged. I turned back a few tanks that had
leakage problems until one passed MY quality assurance test.

Remember to release the t-bar valve after bleeding your line for
your line pressure. The diaphragm can become stressed and weakened if
you forget.

Karen Christians