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 Post subject: Filling up with fuel
PostPosted: Tue Jul 18, 2006 16:29 
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Hi all.
I was filling my car up the other day (as one does when the little yellow light on the dash starts throwing a wobbler) and, during a slight episode of paranoia, was wondering how accurate the reading was on the petrol pump. After all, you can't SEE how much is going in, so unless you run your tank bone dry and fill it from empty (not recommended) you'll never know. I guess it would be relatively easy for the pump to under-deliver by a few mls per litre and consequently have us all paying that little bit more.

Any petrol pump fans care to put my mind at rest?

Or am I being a little too cynical? :roll:


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PostPosted: Tue Jul 18, 2006 16:40 
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Pumps are subject to checking by the local Trading Standards office. I don't know how often they have to be calibrated or what level of accuracy is typically achieved.

An error of 2ml per litre dispensed is only 0.2% which seems quite a high level of accuracy for a device of this type.

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PostPosted: Wed Jul 19, 2006 11:16 
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Thanks MalcolmW. I suspected that in reality they would probably be subject to checks. I just had my cynical head on yesterday :wink:


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PostPosted: Wed Jul 19, 2006 17:58 
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I had a bigger episode of paranoia when after putting 70L of Diesel in the fuel gauge still said empty, the light was on the dash display said low fuel and the computer estimated 21 miles left in the tank.

Thankfully a couple of taps on the fuel gauge fixed it.

Whick means all these electronic gismo's are reliant on one little mechanichal needle prone to getting stuck. :?


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PostPosted: Thu Jul 20, 2006 15:28 
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Could I just mention that in very hot weather it is best to avoid filling the fuel tank (especially with a petrol engined car) and then leaving the car parked in the hot sun. This can cause the tank to overflow, as we just found out with Eileen's Fiesta.

Best wishes all,
Dave.


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PostPosted: Thu Jul 20, 2006 17:58 
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TripleS wrote:
Could I just mention that in very hot weather it is best to avoid filling the fuel tank (especially with a petrol engined car) and then leaving the car parked in the hot sun. This can cause the tank to overflow, as we just found out with Eileen's Fiesta.

Best wishes all,
Dave.


I found that out about 18 years ago with a Fiat 128, fille dup, drove home and it pi$$ed on the floor.

Modern cars shouldn't do it, perhaps a breather pipe is rotted?


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PostPosted: Thu Jul 20, 2006 21:50 
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No car should EVER have done it!

They should all be designed with sufficient space for the fuel to expand as it warms. It's not like manufacturers don't know this could happen!

Modern cars have their tank breathers routed through a rollover valve (to stop fuel leaking out of the breather if they roll) and then on into a carbon can. This is just a cannister of activated charcoal which absorbs fumes evaporating from the tank breather. Every time the engine is switched on, a "purge valve" opens and sucks air backwards through the carbon can for a few seconds to purge the charcoal ready for the next time the engine is switched off.


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 Post subject:
PostPosted: Wed Jul 26, 2006 14:50 
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Do pumps have thermal compensation for the metering or are we all paying more in summer than winter?


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PostPosted: Wed Jul 26, 2006 16:25 
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A litre or gallon is a measure of capacity and is independent of temperature. A metering pump does not, therefore, require compensation.

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PostPosted: Wed Jul 26, 2006 16:38 
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A 500 gallon pot on a road tanker, loaded during the day, would drop half an inch on the dipstick if the temperature dropped a lot overnight! :(


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 Post subject:
PostPosted: Wed Jul 26, 2006 16:53 
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malcolmw wrote:
A litre or gallon is a measure of capacity and is independent of temperature. A metering pump does not, therefore, require compensation.


I went round that loop. Then I started to discover some complexities. If I buy a litre of fuel and then take it to a place with a standard temperature, I will have LESS in summer and MORE in winter. So I thought, hang on - what is the definition of a litre of Petrol? And it seems to me that any useful definition must specify a temperature, otherwise we simply don't have knowledge of what we have purchased.

So maybe fuel pumps should have temperature compensation and we should be buying petrol in "20 degree litres".

Well, this is brainstorming.

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 Post subject:
PostPosted: Wed Jul 26, 2006 17:05 
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Fuel should really be sold in Lbs/Kilos, Its just that volume is easier to measure "On the fly" as it were

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 Post subject:
PostPosted: Wed Jul 26, 2006 17:26 
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Well look what I found - second google hit:

[url=http://www.tuvnel.com/upload/pdfs/warm-fuel.pdf]Temperature Compensation of Liquid Fuels
A Study for National Weights and Measures Laboratory
Report No: 184/99
Date: 21 July 1999[/url]

Chapter and verse... :)

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 Post subject:
PostPosted: Wed Jul 26, 2006 22:53 
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Location: Treacletown ( just north of M6 J3),A MILE OR TWO PAST BEDROCK
Seem to remember somat about some sort of law---

about pressure/volume / tempeture being linked

P1*V1/T1 = P2*V2/T2


Sorry - wqs about 40 years ago .

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 Post subject:
PostPosted: Wed Jul 26, 2006 22:58 
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botach wrote:
Seem to remember somat about some sort of law---

about pressure/volume / tempeture being linked

P1*V1/T1 = P2*V2/T2


Sorry - wqs about 40 years ago .


When they said 'gas' they weren't talking american.

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 Post subject:
PostPosted: Wed Jul 26, 2006 23:00 
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SafeSpeed wrote:
botach wrote:
Seem to remember somat about some sort of law---

about pressure/volume / tempeture being linked

P1*V1/T1 = P2*V2/T2


Sorry - wqs about 40 years ago .


When they said 'gas' they weren't talking american.


aS I SAID -- WAS A VERY LING TIME AGO :roll:

BUT - dont forget that fuel vapour in a tank is a gas ( and not American Gas) :twisted:

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 Post subject:
PostPosted: Thu Jul 27, 2006 01:58 
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When it is stored in underground tanks, I dont suppose the outside temperature has much influence - from what I have read recently on heat pumps as a means of energy storage, below 1 metre the ground never freezes, and temperature change is minimal and slow.

It's once it gets in your car that it really heats up, and if you have filled to the brim, it CAN over flow via the breather pipe which exits alongside the filler - common occurence when you work in a "gas" station!
Many pumps are belt driven - and when it rains hard after a dry spell, they slip, resulting in a slow fill - but the guage always reads right AFAICT - AND are checked periodically by the Gilbarco service van.
The console has an option to allow the measured fuel to be cashed off without payment! :idea:

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 Post subject:
PostPosted: Sat Jul 29, 2006 14:33 
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Dusty wrote:
Fuel should really be sold in Lbs/Kilos, Its just that volume is easier to measure "On the fly" as it were


Martin Brundle, commentating from Hockenheim an hour ago, said something like this concerning F1 fuel amounts being by weight due to temperature effects.

If you're lurking, Martin, I'd love to hear from you. :yesyes:

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 Post subject:
PostPosted: Sat Jul 29, 2006 15:15 
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SafeSpeed wrote:
Martin Brundle, commentating from Hockenheim an hour ago, said something like this concerning F1 fuel amounts being by weight due to temperature effects.


Aircraft generally uplift and measure fuel on their gauges by mass; a C130K with Nos 1 - 4 tanks full carries 34,000 lbs (or 34k for short) of fuel.
The aircraft itself uses capacitance gauging units connected to an electric box which, from the quantity in the tanks and the capacitance of the fuel, determine how much weight there is.
The bowser also delivers in lbs (or kilogrammes), the drivers I think sets the delivery system up with the appropriate SG of the fuel for that day so that the meter will read in mass not volume; AVTUR (Jet A1) has an SG of approximatly 0.81 on an ISA standard day (15 C).
At the end of a refuel the groundcrew usually carry out a quick 'reconciliation' check to ensure that what the gauges say has been uplifted and what the bowser says it dispensed are more or less the same.
Quite a lot off faff just to put some juice in :o


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PostPosted: Sat Jul 29, 2006 21:01 
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In both the above, the fuel is stored/delivered above ground.
I dont know of any roadside filling station which does not have underground storage tanks - although they dont stay full for long.
Most have a facility for the tanks to "breath" - usually an arrangement of tall standing pipes.
Deliveries TO the station are in litres.

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