Gadget to stop wasting camping gas near empty cans

The limits that are in place for a saturated liquid are there not just because of thermal expansion they also have to consider the rise in pressure caused by the increased mass of gas in the head spaced. The fact that this liquid is saturated is the fundamental flaw in your analogy.

Unless I’m misunderstanding stuff from ancient Science lessons if any of the contents are in gas form at all the cannister will be within it’s design constraints because the gas pressure is related only to temperature, it doesn’t matter how full it is *until* it’s full of fluid. At the point it’s fluid it’s no different to milk/meths.

The headspace is purely to allow for potential changes in temperature.

It’s fairly easy to liquify butane at RTP/STP. It’s why they work in fridges as a coolant. The pressure keeps it liquid but as you increase temp more will become gaseous and increase pressure. There will be an equilibrium/vapour pressure/temp that can be worked out. But I’d say you need that volume for expansion. There will be a maximum storage temp for full cans so they don’t go pop.

As a reminder to those who did high school chemistry. 1 mole of water in a liquid state has a volume of 18ml and mass of 18g if we could have that mole (18g) as a gas at RTP (room temp pressure) it would occupy a volume of 24,000ml (molar gas volume, this volume increases with temp). So yes expanding gases very dangerous.

They’re not intend to be bidirectional valves are they?

How do you think they get the gas in? 😀

Less stuff = less pressure?

We’re talking about a gas. 😀

Anyway, two specific questions above. Can one/all of the experts answer:

I *have* noticed that emptier cannisters have lower pressure and since pressure is relative only to temperature I can’t explain that unless the remains at the end of the can have a higher butane ratio and lower propane.
Maybe a forum chemist/physicist can explain what’s actually happening.

What circumstances should trigger the testing you’re suggesting? If you need to test every time you screw something onto the cannister then you need to test every time you connect a burner. If you need to test every time the pressure changes you need to test all your cannisters every time the temperature changes.

OOB – add to that what about refilling refillable lighters

How do you think they get the gas in? 😀

Leave it in a room full of pressurised butane for a few 000 years until it permeates through obviously.

But seriously, just because it’s good for 1 fill doesn’t mean it’s refillable. Try putting the juice back in a carton then carrying it round for instance.

the design of the cans makes it impossible not to leave an expansion gap as the filled protrudes into the can below the top. there will always be a expansion space even if overfilled.

Are you sure about that? Does that give the 20% capacity needed for LPG? Basically, it can’t.

Its actually very difficult to overfill to the point you get liquid gas out of the burner

Again, not the point – the risk in overfilling is in overpressurising the container to the point that it bursts. Disposable cannisters don’t have a separate pressure relief valve. They’re designed to survive exposure to whatever temperature range with the factory fill. You go over that and they will not survive.

A valve is a valve. ON there cans when you screw in either the burner of the adapter there is a hollow tube that pushes into the valve and opens it by pushing the moving part of the valve inwards. I cannot see how the direction of the gas alters how it works at all.

What circumstances should trigger the testing

Disposable canisters don’t get tested (other than during manufacture) – they’re meant to be filled, used, and thrown away.

Refillable cylinders are inspected and/or tested on an age basis when returned to the filling depot.

Pressure isn’t relative only to temperature, in the scenario where you use the gas you are removing the gas, so less gas, this gas the effect of vapourising some of the liquid, as it vaporises this reduces temperature.

So you are doing two things that will have the effect of reducing pressure. Removing mass and cooling.

tilly dog –

The design of the cans means even if overfilled there is always expansion room. Yes I am certain of that.

How on earth can an overfilled can increase pressure if there is always expansion room. Pressure is a function of temperature not fill level. Basic physics!

Please – just have a look at a can and a wee think about it

But seriously, just because it’s good for 1 fill doesn’t mean it’s refillable.

I think it *is* refillable because the valve is good for a large number of connections and nobody has yet been able to offer a reason why these cans are not refillable. Maybe someone will in which case I’ll change my opinion.

How on earth can an overfilled can increase pressure if there is always expansion room. Pressure is a function of temperature not fill level. Basic physics!

Because over filling reduces the expansion room (it’s got liquid in it, not gas). Pressure is also a function of volume and the quantity of stuff in that volume – same basic physics.

Anyway, I think you’re trolling now, so I’m oot! 🙂

so tilly dog – how about refillable lighters?

In use, pressure drops/the cannister cools. At low ambient temperatures I’ve had to warm the cannister with my hands to get any decent flame out of it. Winter mixes suffer less but partially filled containers can be next to useless.

Pressure isn’t relative only to temperature,

Pressure *is* relative only to temperature.

No tillydog – the expansion room contains gas! The design of the can is such that you cannot fill it completely with liquid. You are not using the pressure from the top can to pressurize gas into liquid. You are using a tiny differential in gas pressure to push liquid gas from one to the other

Its clear you really do not understand this at all. go and look at a can, take it apart to see the design. I have!

the valve part sticks down into the main chamber. any space above that cannot fill with liquid gas. It can only remain full of gaseous gas

How about refillable lighters?

So 1mole of a gas in a fixed volume has the same pressure as two moles in same volume at same temp?

No

I cannot see how the direction of the gas alters how it works at all.

I’m not saying it *does* but it might, it depends on how the valves are constructed as to the risks of damage – the nozzle is designed to engage and open the valve to a specific depth to allow outward flow, then close off under internal gas (or mechanical) pressure. The depth of compression is dictated solely by the mechanical interface and the nozzle on your stove will be produced to a specific design and tolerance.
Applying pressure beyond the mechanical engagement by filling could, theoretically, over depress and damage the valve.

Anyone who has ever refilled a cheap lighter will know the valves occasionally stick, and they definitely are designed to allow inward flow (note those same lighters have a different fill point to exit point)

~Scotroutes – have you tried the primus winter mix? It has a “wick” to help evaporate the gas at low temperatures especially when part empty and the mix is even more volatile – it comes in a brown can and not the 100g ones. It really does work.

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Nope dangerous brain – the valve is opened mechanically in exactly the same way when attached to a burner and when attached to the refill gadget. Some stoves use these cans inverted so liquid gas flows past the valve and then its evaporated in the burner.

In use, pressure drops/the cannister cools. At low ambient temperatures I’ve had to warm the cannister with my hands to get any decent flame out of it. Winter mixes suffer less but partially filled containers can be next to useless.

So you reckon the emptier cans just cool faster as the gas leaves? That sounds quite likely thanks.

So 1mole of a gas in a fixed volume has the same pressure as two moles in same volume at same temp?
No

Gas pressure is relative to temperature and nothing else. More volume = more fluid, not more pressure. The gas (and therefore the can) remains at exactly the same pressure.

Google is your friend here.

the valve is opened mechanically in exactly the same way when attached to a burner and when attached to the refill gadget.

Fair do.

In use, pressure drops/the cannister cools

Gas is removed which is the replaced by the liquid evaporating. The liquid evaporating needs energy from somewhere so the temperature of the liquid drops.

At low ambient temperatures I’ve had to warm the cannister with my hands to get any decent flame out of it

If the differential temperature between the atmosphere and the can isn’t high enough then the energy flow into the can won’t be high enough to evaporate the liquid quickly enough to maintain the pressure. Your hands are much warmer so heat transfer has a higher driving force and will be more efficient as the air is a rubbish at heat transfer. The mix of fuel is also a potential issue here.

Winter mixes suffer less but partially filled containers can be next to useless.

Likely a higher propane content.

I may be able to shed a little light on the gas pressure / volume thing. more gas in less volume does equal more pressure when it is all in its gaseous state. However once you have reached a pressure high enough that some of the gas is liquid then the pressure will stabilize at whatever pressure is needed to liquefy that gas. thus a container that is part liquid gas and part gaseous gas the volume factor no longer matters – temperature is the only variable that alters pressure of a container that is part full of liquid gas and aprt ful of gasueous gas.

anyway – anyone interested in this its simple, perfectly safe and easy to do. I have been doing it for years and so have many other folk I know – including one chap who works for an organisation were they do hundreds of cans a year refilling them

the only possible is is if you overfil you get liquid gas out of the burner 0- sam as if you knocjk it on its side while lit.

It is impossible to fil these cans to the point that the whole can would be full of liquid with no expansion room. \just as well as that would be dangerous but it cannot happen

I may be able to shed a little light on the gas pressure / volume thing. more gas in less volume does equal more pressure when it is all in its gaseous state. However once you have reached a pressure high enough that some of the gas is liquid then the pressure will stabilize at whatever pressure is needed to liquefy that gas. thus a container that is part liquid gas and part gaseous gas the volume factor no longer matters – temperature is the only variable that alters pressure of a container that is part full of liquid gas and aprt ful of gasueous gas.

TJ I for one don’t need an explanation from you on how the behaviour of saturated hydrocarbon mixtures.

anyway – anyone interested in this its simple, perfectly safe and easy to do. I have been doing it for years and so have many other folk I know – including one chap who works for an organisation were they do hundreds of cans a year refilling them

Messing about with hydrocarbons is never “perfectly safe” and you don’t think that maybe, just maybe, a commercial enterprise might be able to achieve the task with far better control on the suitability of tools, location to manage the hazard associated with the transfer of hydrocarbons? And that this is very different to someone “doing it in their kitchen/garage/shed” (if you do want to do this, please do it outside)

It is impossible to fil these cans to the point that the whole can would be full of liquid with no expansion room. \just as well as that would be dangerous but it cannot happen

I’ve heard that statement too many times for it to not ring alarm bells in my head.

Messing about with hydrocarbons is never “perfectly safe” and you don’t think that maybe, just maybe, a commercial enterprise might be able to achieve the task with far better control on the suitability of tools, location to manage the hazard associated with the transfer of hydrocarbons? And that this is very different to someone “doing it in their kitchen/garage/shed” (if you do want to do this, please do it outside)

In terms of the balance of risks. If I’m refilling a gas canister at home I can think of very few ways it could possibly go wrong. Compare that to meths. If someone kicks over my Meths stove there’s gonna be burning meths all over me.

I think gas (including the refilling process) is safer than meths. Indeed I wouldn’t run a meths stove with my 3yo running about, I happily use gas.

I’d be interested in the safety margins and where this 20pc figure comes from. I’m guessing they bottle at a set temperature but lets assume they don’t. 20pc must be enough margin that they can safely bottle somewhere very cold and ship to somewhere very hot. Maybe that’s more margin than required for a weekend camping where the bottle is filled at roughly the same temperature it will be used at.

I always leave the same gap the canister came with but I suspect that’s OTT.

You are using a tiny differential in gas pressure to push liquid gas from one to the other

Or gravity. That’s generally how I do it with both cans at the same temperature, and therefore pressure.

In a pinch you can do it with a ‘straw’ from a plastic shafted cotton bud and a slightly shorter piece of washer hose. Now that will get the chemical engineers frothing.

In a pinch you can do it with a ‘straw’ from a plastic shafted cotton bud and a slightly shorter piece of washer hose. Now that will get the chemical engineers frothing.

You can also fill a Coleman Canister with lighter fluid from a standard lighter fluid can. I’ve done it once and it works.

Unfortunately I did it in September which meant I had a cannister full of butane for most of the winter and in the cold after a few minutes it literally wouldn’t push butane out at all. 🙁

Or gravity.

That’s how I thought it worked. Liquid denser than gas, innit. Pressure differential will be minimal (as others have explained) and presumably will equalise between the two cans when you open the valve anyway?

TJ I for one don’t need an explanation from you on how the behaviour of saturated hydrocarbon mixtures.

I do. I thought that was useful.

I do. I thought that was useful.

…then you should probably be aware that there are errors albeit ones that won’t generally apply here.

TJ I for one don’t need an explanation from you on how the behaviour of saturated hydrocarbon mixtures.

Justy trying to shed a little light – tillydog was obviously confused over this.

I don’t think you are not understanding gonefishin – just condemning something you have not seen.

No it is not pushing thru from gravity – it is the pressure differential. Yes the pressure does equalise between the cvans – its that equalising of pressure that pushes the liquid thru

gonefishing – I do actually understand that if you work in this field then it would appear dangerous as it would set all your alarm bells ringing. But in practice it is not.

I note no one has answered the point about refillable lighters. Similar gas, much more dodgy process

Much much smaller volumes (therefore the hazard is reduced) and a sight glass so the level can be seen, oh and as was actually said earlier a system that is designed to work that way.

gonfishin. Not all lighters can y see the gas level – look at clippers. The donor can is not significantly smaller. the valves are much cruder.

The donor can is not significantly smaller. the valves are much cruder.

You aren’t refilling the donor can.

Why I have no doubt that this can easily be done, I still think it’s bizarre that the people on here that claim to be so safety conscious and careful in some areas of their lives, think this is a sensible approach.

I think doing it 2 or 3 times to a canister that is good condition is probably fine, anymore than that is pretty stupid.

You are using a potentially hazardous piece of equipment, way out of it’s original parameters and specification, with no way of knowing it’s in the same condition it was when you first bought it. Basically “it’ll be reet”. All to save a couple of quid. Fair enough, crack on, but if you use these on campsites near other people, then I don’t see the difference from the people who drink 2 pints then drive their car.

I wonder how much the rest of your camping kit cost?

gobuchal – and its clear from your posts yo simply do noit understand the process 🙂

I do understand yor concerns – its the same as I get to fringe medicine. Its a result of your training

None of the folk who think this unsafe have actually been able to give any concrete reasons why

I think doing it 2 or 3 times to a canister that is good condition is probably fine, anymore than that is pretty stupid.

Once again – as you attach and detach the can from burners many many times there clearly is no limit to how often the valve can be opened and closed mechanically. Some systems use the can upside down so the valve passes liquid gas – so that is clearly no issue. It is impossible to overfil the cans to the piont there is no expansion room. You cannot get pressures in the cans higher than in normal use.

so what is the issue? Where is the danger? What can go wrong?

educate us please.