VAPOUR PRESSURE - Wooh Scary! What is it?

[Simon B]

Went on a family hol to Paignton lovely - Land Rover now has a small fuel tank in it so I take a couple of jerrys to load up on cheap supermarket fuel usually without incident. However this time at the Sainsburys in Paignton the manageress comes scampering out and and tells me I can only put 10 litres in each jerry (20 litre cans) because of "vapour pressure!" I've a fair idea what it is but couldnt work out it was safer to have a container half full.

Interestingly she wasnt concerned how I was doing it and did not tell me the sfae procedure for grounding the tank etc etc which is what I normally do.

The last and best part of the vapour pressure explainaton was that it builds up and fuel needs tp breathe, this was from a mangeress of a fuel satation, (it beggared belief) that sells those minging green plastic cans. - BREATHE!! I was stunned at this, clearly I'll be unscrewing the caps on all my fuel containers now whilst in transport.

Codders, could you be a darling and see if the vapour pressure thing is better with the can full or half full please? The extra fuel wasnt needed in the end as the Landrover was in a fairly scary collision on the M5 northbound yesterday and now has its front quarter smashed up.

[Nos4r2]

Originally Posted by Simon B

Went on a family hol to Paignton lovely - Land Rover now has a small fuel tank in it so I take a couple of jerrys to load up on cheap supermarket fuel usually without incident. However this time at the Sainsburys in Paignton the manageress comes scampering out and and tells me I can only put 10 litres in each jerry (20 litre cans) because of "vapour pressure!" I've a fair idea what it is but couldnt work out it was safer to have a container half full.

Interestingly she wasnt concerned how I was doing it and did not tell me the sfae procedure for grounding the tank etc etc which is what I normally do.

The last and best part of the vapour pressure explainaton was that it builds up and fuel needs tp breathe, this was from a mangeress of a fuel satation, (it beggared belief) that sells those minging green plastic cans. - BREATHE!! I was stunned at this, clearly I'll be unscrewing the caps on all my fuel containers now whilst in transport.

Codders, could you be a darling and see if the vapour pressure thing is better with the can full or half full please? The extra fuel wasnt needed in the end as the Landrover was in a fairly scary collision on the M5 northbound yesterday and now has its front quarter smashed up.

This sounds like a case of what happens when you send a stupid person on a health and safety course.

Are you guys OK?

[Poly]

Originally Posted by Simon B

Went on a family hol to Paignton lovely - Land Rover now has a small fuel tank in it so I take a couple of jerrys to load up on cheap supermarket fuel usually without incident. However this time at the Sainsburys in Paignton the manageress comes scampering out and and tells me I can only put 10 litres in each jerry (20 litre cans) because of "vapour pressure!" I've a fair idea what it is but couldnt work out it was safer to have a container half full.

Interestingly she wasnt concerned how I was doing it and did not tell me the sfae procedure for grounding the tank etc etc which is what I normally do.

The last and best part of the vapour pressure explainaton was that it builds up and fuel needs tp breathe, this was from a mangeress of a fuel satation, (it beggared belief) that sells those minging green plastic cans. - BREATHE!! I was stunned at this, clearly I'll be unscrewing the caps on all my fuel containers now whilst in transport.

Codders, could you be a darling and see if the vapour pressure thing is better with the can full or half full please? The extra fuel wasnt needed in the end as the Landrover was in a fairly scary collision on the M5 northbound yesterday and now has its front quarter smashed up.

I will save codders the trouble (although I am sure he will chip in when he gets a moment!).

The vapour pressure is a property of the material (and the temperature) it is entirely independent of the volume of liquid you have. Assuming your cans are perfectly sealed then it is pretty irrelevant in terms of risk. If you assume that the seal is imperfect (or as she was suggesting they are left to breath) then your whole car becomes the sealed container (well if it was a normaly car it would be sealed - being a land rover it is presumably well ventilated!).

The vapour pressure of petrol is fairly high and so if left for long enough in your landy with an imperfectly sealed container it will be able to form a potentially explosive mixture. With 20 L split between two containers this is no less likely to happen, or no slower to happen than with 40 L in the same containers.

Her petroleum licence however probably limits her to selling you no more than 10L in a suitable metal can at any one visit! She has obviously misunderstood this.

[Bigmuz7]

Polwart... last time I picked up a 25 L drum of petrol, it felt considerably lighter than a 25 L drum of water, is it (petrol) so much less dense that it could be noticeable ?

[codprawn]

If you want to know about vapour pressure leave an almost empty can in the sun - even a steel one will swell up - it could even burst a seam. A full can is 100x safer - the less room there is for vapour the better.

It is scary just how stupid these women can be. Our local Tesco has really long hoses hanging from a high point. There is a sign saying use both sides. Nobody does(except me) and she reckons the sign means DONT use both sides because it's in a round circle..............

The good old mobile phone one is a great wind up as well. Apparently the radio signal will cause the whole place to explode - and yet radio keyfobs are fine along with dataloggers and police radios not to mention cars with 12 spark plugs and dodgy ignetion leads!!!

[codprawn]

And talking of petrol and supermarkets reminds me of cigarette lighters!!! The latest crazy stupid bit of nanny state nonsense is in the introduction of "child proof lighters". Basically the wheel is no longer round but has sharp edges. Quite how it will stop a child is beyond me. all it does is wreck your thumb. What is the answer? Everyone is buying matches instead - very safe!!!

[Nos4r2]

Originally Posted by codprawn

And talking of petrol and supermarkets reminds me of cigarette lighters!!! The latest crazy stupid bit of nanny state nonsense is in the introduction of "child proof lighters". Basically the wheel is no longer round but has sharp edges. Quite how it will stop a child is beyond me. all it does is wreck your thumb. What is the answer? Everyone is buying matches instead - very safe!!!

Personally I prefer them-it's easier to use with oily/wet hands. I can't work ouit why they are supposed to be childproof though.

[Poly]

Originally Posted by Bigmuz7

Polwart... last time I picked up a 25 L drum of petrol, it felt considerably lighter than a 25 L drum of water, is it (petrol) so much less dense that it could be noticeable ?

Yes.

• 25 L drum of water = 25 kg plus the weight of the drum.
• 25 L drum of water = roughly 20 kg plus the weight of the drum.

So roughly 5 kilos (11 pounds difference).

[Edit - I should point out I couldn't be bothered looking up the density of petrol so it has been done from a vague memory - but I am sure it will be roughly right]

[prairie tuber]

Originally Posted by Polwart

[*]25 L drum of water = 25 kg plus the weight of the drum.
[*]25 L drum of water = roughly 20 kg plus the weight of the drum.

Huh???

[codprawn]

Maybe it's heavy water? The Germans were always after that stuff during the war...........

[John Kennett]

Originally Posted by prairie tuber

Huh???

I think the second one was meant to say "petrol" not "water"!

Originally Posted by Polwart

[Edit - I should point out I couldn't be bothered looking up the density of petrol so it has been done from a vague memory - but I am sure it will be roughly right]

A quick search turned up a handy list of specific gravities

At 0.74 Kg/l the 25 litres of petrol is going to weigh 18.5 Kg so the difference is actually a bit more.

John

[Bigmuz7]

Originally Posted by John Kennett

I think the second one was meant to say "petrol" not "water"!
A quick search turned up a handy list of specific gravities

At 0.74 Kg/l the 25 litres of petrol is going to weigh 18.5 Kg so the difference is actually a bit more.

John

Thats a surprisingly large difference, no wonder it was noticeable .. ta

[Poly]

Originally Posted by John Kennett

I think the second one was meant to say "petrol" not "water"!
A quick search turned up a handy list of specific gravities

At 0.74 Kg/l the 25 litres of petrol is going to weigh 18.5 Kg so the difference is actually a bit more.

John

Ah yes sorry - thanks for spotting that . Must have been the extra vapour pressure from the full 25L fuel tank in the back garden !

[Bigmuz7]

Originally Posted by John Kennett

A quick search turned up a handy list of specific gravities

Interesting site that, I always thought uranium and tungsten were very heavy, (compared to something obvious like lead), but I see a cubic metre of gold is just as dense as both of them

[Simon B]

This sounds as I'd expected, I'll draft an illuminating letter to S-bury's elf and saiftee manager. I'm happy with sensible regs but enforcing daftness whilst not paying heed to what should be is nuts.

Petrol has been used as a non compressible source of bouyancy in early submersible craft, usually accompanied by a large weight to shed prior to leaving the sea bed.

[codprawn]

Originally Posted by Bigmuz7

Interesting site that, I always thought uranium and tungsten were very heavy, (compared to something obvious like lead), but I see a cubic metre of gold is just as dense as both of them

I see they don't have the heaviest of them all listed - Osmium - they have Iridium though which is very close - 22,154 compared to lead at just over 11,000.

Interesting to note some of the metal prices these days

US $ per troy oz

Gold 787
Platinum 1456
Osmium 380
Iridium 425
Rhodium 6225

It's amazing to think many people own some rhodium - in the catalyst on their cars!!!

I can't believe the price on gold - it was about 300$ an ounce when they sold off Britain's reserves..........

[Simon B]

Well that settles it then - I'm packing in the day job and I'm going to dig up all the speed bumps and junctions in Leicester with a mini excavator.

Later when I'm out on the town I'll clearly impress all the totty for miles around with my new job Rhodium miner!!

[Poly]

Originally Posted by Simon B

This sounds as I'd expected, I'll draft an illuminating letter to S-bury's elf and saiftee manager. I'm happy with sensible regs but enforcing daftness whilst not paying heed to what should be is nuts.

Petrol has been used as a non compressible source of bouyancy in early submersible craft, usually accompanied by a large weight to shed prior to leaving the sea bed.

Simon - the downside he will just tighten up on the not selling more than the set quantities in the petroleum spirit regulations. Her reasoning was wrong - but the reality is she is not supposed to sell you 40L of petrol in cans...

[codprawn]

Very true - the law doees say no more than 10L may be dispensed into a portable container in one go - so what you have to do is 1/2 fill it - then go and pay - then go back and stick in the other 10L - bonkers or what???

[Poly]

Originally Posted by codprawn

Very true - the law doees say no more than 10L may be dispensed into a portable container in one go - so what you have to do is 1/2 fill it - then go and pay - then go back and stick in the other 10L - bonkers or what???

Actually cod - I think the law says you can't fill a portable container which is larger than 10L (5 L for plastic). And that you can only put 2 of either type type of container in a road vehicle. Technically, as I understand it, 20L jerry cans are illegal to fill at the petrol station.