A conundrum: Gas cylinder pressure - how can this be?

[Spriddler]

I can't fathom this teaser out and I am of curious nature so need to know.

I use a 5kg Calor Patio gas cylinder for my BBQ but the flame had dwindled to the extent that this morning I decided to exchange it for a refilled one.
However, to my surprise I could feel that there was still a significant amount of liquid sloshing around inside it and the built-in gauge on the cylinder showed just over 1/3rd full. It's an internal float operated gauge which I know isn't particularly accurate so in order to assuage my curiosity I weighed the used one and later the new one with the following results in Kg......


Gross weight of used bottle was 9.10kg and the Gross weight of the new bottle was 12.50kg.

Tare weight of the used bottle was 7.26kg and the Tare weight of the new bottle was 7.31kg.

Therefore the weight of liquid gas in the used bottle was 1.84kg and the weight of liquid gas in the new bottle was 5.19kg.

This means that my part-used 5kg bottle still had 1.84kg of liquid gas remaining even though the BBQ flame was dying.
The new bottle was full with 5.19kg as it should be.

My ponderings are............
That the BBQ flame using the part used bottle shouldn't diminish all the while there is liquid remaining because as gas is drawn off and the pressure drops the remaining liquid will gasify until pressure is restored to the point where no more liquid will gasify until the pressure drops again. This being so the BBQ flame should remain of constant magnitude all the while there is liquid gas in the bottle, but my flame was dwindling even though 1.84kg (nearly 1/3rd of a full bottle) of liquid gas remained un-gasified. At £23 a pop the remaining (unusable) third of a bottle (1.84kg) is not to be sneezed at.

My only thought is that my regulator which attaches to the top of the bottle is faulty and yet with the new bottle the flame is back to its normal strength.

 

[Pausim]

I share your surprise that the pressure seems to have dropped off in the less full bottle, in theory it shouldn’t so long as the temperatures are the same. If it is propane gas then I find it very surprising but less so if it is butane, which needs a much higher boiling point.

If the gas is butane then it might perhaps be more vulnerable to temperature drop. When the liquid boils off into gas the latent heat is drawn out of the liquid reducing it’s temperature (latent heat of evaporation). If you have 5kg of liquid in the bottle, compared with 1.84kg, then there is more than twice as much bulk in the bottle to absorb the heat loss and the temperature will not fall so quickly. The theory is that the lost heat will be regained through the metal cylinder wall, the liquid in the full bottle may find this easier to do this having more area in contact with the metal wall than the part full one. It is said that aluminium bottles are slightly better than steel ones because of the greater conductivity, plastic ones are likely to have poor conductivity.

I am no expert so others may have better explanations.

 

[Richard n Ann]

Is the bottom of the gas bottle cold? Give the bottle a shake up, it exposes the liquid to the warmer sides of the bottle and the pressure goes up, usually the flame will recover for a few minutes.

 

[Spriddler]

Thanks. Food for (more) thought.
Both bottles have the same gas; either butane or propane but I didn't look at which gas it is. I can't check at the mo as the BBQ is now at my son's house.
Both were used and weighed within an hour of each other, so both were between 22 and 24 deg. ambient.

 

[John H.]

Red bottles are propane. If it is blue, then it is butane.

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[Richard n Ann]

Thanks. Food for (more) thought.
Both bottles have the same gas; either butane or propane but I didn't look at which gas it is. I can't check at the mo as the BBQ is now at my son's house.
Both were used and weighed within an hour of each other, so both were between 22 and 24 deg. ambient.

It will be butane. Does the bbq use quite a lot of gas? The faster it's evaporated the lower the temperature of the remaining liquid.
This is because the warmest butane molecules at the surface of the liquid break free (ie evaporate) , so the average temperature of the remaining liquid is lower. Pressure and temperature are relative and as this process goes on there will eventually be no pressure left.

 

[Lenny HB]

I think patio gas is a mix of propane & butane, I think the the propane tends to burn off first so you old bottle was mostly butane in it. In those temperatures butane should burn better than propane.
I would suspect the regulator is the problem.

 

[Sandancers]

With the used bottle, did you try opening the valve of the bottle (when it was disconnected) to see what sort of rush of gas came out ?

 

[Spriddler]

Red bottles are propane. If it is blue, then it is butane.

Yes, I've had/have both for my m/h but I didn't read what was on the bottles although they were blue.
Have just checked and no there aren't. They are green

 

[Lenny HB]

Yes, I've had/have both for my m/h but I didn't read what was on the bottles although they were blue.

Butane then, patio gas bottles are green.

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[cmcardle75]

Yes, I've had/have both for my m/h but I didn't read what was on the bottles although they were blue.

A bit confused as you called it Patio gas, which is green. It is also propane, just with a 27mm push fit regulator instead of POL and a £5 premium for marketing reasons. (BBQ owners are ripe for the picking, according to Calor).

There is something very strange if a bottle with > 1kg of liquid gas sloshing around gives a lower flame than a new bottle of the same type. I suspect the outlet valve on the bottle has seized.

 

[Spriddler]

It will be butane. Does the bbq use quite a lot of gas? The faster it's evaporated the lower the temperature of the remaining liquid.

Indeed. I developed and manufactured a range of inflatable marine safety equipment and not only did the CO2 cylinders frost over but in trying to get a more rapid inflation by increasing the flow the CO2 cylinder outlet froze stopping the CO2 gas flow.

 

[cmcardle75]

I think patio gas is a mix of propane & butane, I think the the propane tends to burn off first so you old bottle was mostly butane in it. In those temperatures butane should burn better than propane.
I would suspect the regulator is the problem.

Calor Patio gas is 100% propane, give or take. It's just a red bottle painted green with a 27mm push fit valve instead of POL.

 

[cmcardle75]

Indeed. I developed and manufactured a range of inflatable marine safety equipment and not only did the CO2 cylinders frost over but in trying to get a more rapid inflation by increasing the flow the CO2 cylinder outlet froze stopping the CO2 gas flow.

Yes, but in the current weather, even a mostly empty small butane bottle will gas off at a potential rate many times that required by a BBQ.

 

[Spriddler]

A bit confused as you called it Patio gas, which is green. It is also propane, just with a 27mm push fit regulator instead of POL and a £5 premium for marketing reasons. (BBQ owners are ripe for the picking, according to Calor).

There is something very strange if a bottle with > 1kg of liquid gas sloshing around gives a lower flame than a new bottle of the same type. I suspect the outlet valve on the bottle has seized.

Butane then, patio gas bottles are green.

I'm a plonker! Yes, they are green (not blue) and with a red plastic handle each side and a push fit regulator.

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[Spriddler]

There is something very strange if a bottle with > 1kg of liquid gas sloshing around gives a lower flame than a new bottle of the same type. I suspect the outlet valve on the bottle has seized.

Indeed, it was still about a third full.
I suspect a blockage or some such fault with the bottle or my regulator so I'll monitor the result with this new bottle. Just on principle I don't want to keep exchanging one that still has 8 quid's worth of gas in it. It might last a fair while as I have made a feed from a redundant 6kg bottle and regulator that I removed from my m/h when I installed a Gaslow system. In fact I've two redundant 6kg cylinders so they may see me out.

 

[cmcardle75]

Indeed, it was still about a third full.
I suspect a blockage or some such fault with the bottle or my regulator so I'll monitor the result with this new bottle. Just on principle I don't want to keep exchanging one that still has 8 quid's worth of gas in it. It might last a fair while as I have made a feed from a redundant 6kg bottle and regulator that I removed from my m/h when I installed a Gaslow system. In fact I've two redundant 6kg cylinders so they may see me out.

Presumably it's the same regulator, so that's (almost) eliminated. I guess it could still be a regulator fault if turning the handle doesn't poke out the "stick" enough to open the valve fully on one of the bottles. I won't suggest poking the bottle valve with a screwdriver to see if it works properly, as I don't want to responsible for the life-changing burns as you do so with a lit cigarette...

 

[Richard n Ann]

I took the top off a butane can once, it sat there like steaming water. When I stuck my finger in it, it made it boil sightly round my finger and it felt cold. Took it 5 minutes to evaporate completely.

I think you've just got cold butane, the last bit is always difficult to get out if your using it at any reasonable speed.

I used to use map gas (butane propane mix) for brazing with a double barrel turbo torch, then the bottle gets 3/4 empty you're forever shaking it up to get it out.

 

[Sandancers]

Indeed. I developed and manufactured a range of inflatable marine safety equipment and not only did the CO2 cylinders frost over but in trying to get a more rapid inflation by increasing the flow the CO2 cylinder outlet froze stopping the CO2 gas flow.

Boyles Law. If you reduce the pressure of a gas, as you do when it goes through a regulator, then you lower the temperature.

I have seen main-line gas reduction stations where 20" pipes are surrounded by a layer of ice 4" thick as there was such a pressure drop and no pre-heating involved.

 

[cmcardle75]

I took the top off a butane can once, it sat there like steaming water. When I stuck my finger in it, it made it boil sightly round my finger and it felt cold. Took it 5 minutes to evaporate completely.

I think you've just got cold butane, the last bit is always difficult to get out if your using it at any reasonable speed.

I used to use map gas (butane propane mix) for brazing with a double barrel turbo torch, then the bottle gets 3/4 empty you're forever shaking it up to get it out.

However, even a 25% full 3.9kg propane (not butane) bottle should be able to provide many times the gas off rate required by a BBQ, especially in the recent high temperatures we've been having. I think there is a fault with it. He isn't trying to run a gas combi boiler off it! If a propane bottle isn't showing signs of external water frost, and sloshes, it is gas, not vaccuum in there. Something else is limiting the gas output. Blocked pipes, faulty bottle valve, regulator, regulator valve actuator...

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[Richard n Ann]

Easiest way to find out is switch everything off for 24hrs and try again.

Can't be the regulator or the other new bottle wouldn't work. I have had faulty mapp gas connection valves occasionally, but they're a cheap screw on disposable bottle

 

[cmcardle75]

Easiest way to find out is switch everything off for 24hrs and try again.

Can't be the regulator or the other new bottle wouldn't work. I have had faulty mapp gas connection valves occasionally, but they're a cheap screw on disposable bottle

It can be the regulator if the "pokey stick" is worn. If it doesn't stick out enough, some bottles will work and others won't.

 

[Richard n Ann]

However, even a 25% full 3.9kg propane

Absolutely, propane will be no bother at all. I though we were talking about butane

 

[mitzimad]

you need to speak to our resident gas bottle specialis Jim

 

[Spriddler]

Thanks all.
We'll see how far the new bottle goes when it's getting low 'ish.
My son uses it far more than I do as I have to admit to being afflicted with chronic BBQ intolerance: Sitting on a wobbly chair with my chin at camping table level eating half-cooked and rapidly cooling chicken thighs off a cold plate whilst being pestered with wasps and flies is not my idea of fine dining. Not to mention the carting in and out of all the mealtime paraphernalia and then getting filthy cleaning the ruddy thing ready for the next social trauma.
I still can't drink a beer from a bottle since 16 years ago when I took a swig from a dumpy beer bottle that had been stood on a dwarf wall and had to spit out a slug.

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[Griff097]

It may be worth disconnecting the hose from the reg to the appliance and blowing it out with air as oils from the gas can get stuck in there, especially if it's long enough to sit in a loop the same oil can run back and make the reg stick.

 

[chaser]

Just an aside from this technical stuff, if you take a bottle back with gas in it, is it emptied, or do they just top it up and keep getting paid for the same gas over and over again?

 

[Minxy]

No-one has asked what actual BBQ you are using as some need a much hither supply of gas than others, so what have you got?

Also am I right to assume that you've used these type of bottles previously and not had this issue until they were virtually empty?

 

[Spriddler]

No-one has asked what actual BBQ you are using as some need a much hither supply of gas than others, so what have you got?

Also am I right to assume that you've used these type of bottles previously and not had this issue until they were virtually empty?

It's a Weber. Can't give more detail as it's at my son's at the mo.
Yes, we've had maybe half a doz bottles in the past but I've only just noticed/thought about why there is the remaining liquid sloshing about and yet the flame has dwindled to about half power as hitherto we've just replaced the bottle.

 

[DBK]

My guess, assuming there isn't a mechanical fault is the liquid sloshing around at the bottom of the bottle might not even be butane or propane but some sort of oil. All commercial gases contain impurities and over time these must accumulate in refillable bottles and each time they are refilled more impurities are added.

One way to check would be to turn the bottle upsidedown and open the valve and see what comes out.

(Insert fine-print disclaimers about liability here).

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