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Old 28 October 2007, 10:03   #1
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VAPOUR PRESSURE - Wooh Scary! What is it?

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.
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Old 28 October 2007, 10:19   #2
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Originally Posted by Simon B View Post
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?
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Old 28 October 2007, 16:07   #3
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Quote:
Originally Posted by Simon B View Post
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.
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Old 28 October 2007, 16:14   #4
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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 ?
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Old 28 October 2007, 16:41   #5
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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!!!
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Old 28 October 2007, 16:50   #6
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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!!!
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Old 28 October 2007, 16:57   #7
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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.
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Old 28 October 2007, 17:13   #8
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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]
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Old 28 October 2007, 22:50   #9
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[*]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???
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Old 28 October 2007, 23:03   #10
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Maybe it's heavy water? The Germans were always after that stuff during the war...........
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