I keep cylinders of various gases here (the big, full-size ones). They each have pressure regulators attached appropriate for the composition of the gases therein. The question is, does the pressure indicated on the regulator give any reasonably accurate idea of how much actual
*quantity* of gas is in there? I mean the gauges are showing like
3000psi and more. Is it safe to assume they're full? Or can gases behave a bit like electricity where you can get huge pressures but hardly any actual 'current' behind them, as in high impedance voltage sources?
I certainly found that the pressure gauges on my late partner's portable oxygen cylinders, used when we were out in the car, were a very reliable indication of time to run before I need to change to the backup.
Depends on the gas and its temperature. If it's just a compressed gas such as oxygen, then the pressure will fall as the cylinder empties. If it's a liquified gas such as butane, the pressure will remain constant until there is no more liquid, or in some cases of very high demand the pressure may fall before then as the liquid gas cools significantly and the outside of the cylinder frosts up.
Don't know about acetylene, as that's a solution of gas in a liquid, usually acetone, I believe.
Depends entirely on what state the gas is in inside the cylinder. If a gas liquifies under pressure at ambient temperature then the pressure above the gas gives little, if any indication of content. It it remains gaseous, the pressure will directly correlate with content.
a pressure guage will only show the pressure in the top vapour bit of the tank which will stay the same throughout and will only drop just before the tank runs out...uselesss
Provided the gas is notr liqified and behaves as an ideal gas:
The universal gas law applies:
PV = nRT
P = pressure V = Volume n number of moles or amount of substance R = Universal gas constant T = temperature
See
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You also have to allow for the fact that n decreases as you discharge the gas form the cylinder. I suppose one could calculate the value of n for a full cylinder and then somehow work out how n decreease with gas use.
V would be the internal volume of the gas cylinder.
P would be the pressure reading on the gauge
T will be influenced by the expansion of the gas leading to a temperature drop so teh cylinder will absorb heat from its surroundings meaning the pressure will come back up a bit until thermal equilibrium
The R value depends on whether the gas is monoatomic or diatomic or polyatomic.
The easiest thing to do really is to put the cylinder on a set of scales and substract the mass of an empty cylioner and gauge from that assumign all gas cylinders and gauges weigh the same!
Reminds me of an incident while I was working for Racal. They had all sorts of gasses delivered and one was supposed to have them laid down with the valve protected or in a vertical rack. Of course the delivery body was interested in was getting somebody to accept it and get a signature. So things got sloppy inside the Goods in Dept, and one day a cylinder of gas got knocked over and the valve end hit a pile of solid metal billets. Whooshe it went out across the car park, and straight in through a glass window in the building opposite which mercifully only was a store room, abut it took several minutes until anyone went in to look at the mayhem, which of course there was. If anybody or their vehicle had been in the way I hate to imagine what might have happened. Brian
If the gas is liquefied the header pressure will be pretty constant. If you have just drawn some gas there will be a temperature difference between the liquid and gas parts which (if you have one) can be picked up with a thermal camera and thus determine how much liquid is left.
No doubt. They'll punch their way straight through brick walls when that happens. Nothing stands in their way. If you haven't seen it happen, you wouldn't believe it were possible.
If so, there are a few ways to determine the amount of gas.
The cylinders should have a ‘tare’ weight either stamped on them or on a tag. This is the empty weight, sometimes in pounds and ounces ( heaven knows why when the contents are here are almost invariably measured in kg etc).
Weigh the cylinder, without the regulator etc, subtract the tare weight after converting to kg, and that gives the gas left in kg.
That is ( more or less) the method I use for my cylinders in the motorhome
- I use a luggage scale. The only difference is, I weigh each new cylinder and work backwards - I don’t need to find the tare weight, I know when I’ve used 5.5kg on a 6kg cylinder, it is all but empty.
You can get various gizmos to determine where the top of the liquid gas is in the cylinders. Some only work on a ‘common’ / standard cylinders.
One, made by Truma ( and not cheap) fixes via magnets to the bottom of the cylinder and uses ‘sonar’ to determine the depth of liquid. It conveys the data to an app on you phone via Bluetooth and you tell the app the cylinder size. I understand these are very good but they are expensive - from memory over £100 each.
Truma do another unit which also determines the liquid level - I suspect by temperature but I’m not sure. It looks like a large USB stick. There are other makes. The Truma one is about £40. I’ve heard varying reports as to how good these are. I’ve never used one.
There are stickers which you fix, I think magnetically, to the cylinders and they indicate the liquid level. Again, I’ve never used them and reports vary.
There is a technique where you pour hot or cold water ( I forget which) and watch for the a different ‘pattern’ as it flows down the side of the cylinder. I’ve not tried this but it is on YouTube.
I’ve never heard good reports of ‘inline’ gauges based on pressure. I assume due to the theory of how the systems work. What you need to know is the level of liquid- not the pressure of the gas above it or post the regulator or even before it.
Personally, in over 10 years of using LPG, I’ve always used weight and found it reliable.
They can be quite robust though. I was once standing close to the place where a scuba diving cylinder, freshly filled to over 200 bar, landed after falling at least
15m down a cliff onto the rocks below. I did throw myself onto the ground very quickly, but nothing spectacular happened. The pillar valve bent, there was lots of white noise for a few minutes and the cylinder ended up with a thick crust of ice all over.
It was put back into service after a new pillar valve had been fitted and the cylinder retested.
I think CO2 will be liquid if the pressure is over 80PSI as long as it is not too warm but the others will be true gases still at ambient temperatures.
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