Prompted by a comment in the MPG thread, I thought I'd ask if anyone here knows what the typical accuracy of a forecourt petrol pump is when dispensing less than 2 litres ? I've done a bit of googling but can't find what type of pump and measuring device are used in them. Weights and Measures regulations and Trading Standards procedures only seem to test continuous deliveries greater than two litres.
The reason for my question is that I always used to brim the tank when filling up to delay my return as long as possible. A few months ago though, while waiting for the tank to fill I was keeping myself occupied by reading all the print on the pump display.
Manufacturers name, Pump Serial Number, Minimum Delivery 2 Litres, Certificate of Conformity etc. etc.
I've read the Minimum Delivery thing hundreds of times and never really thought about it because I'm always putting loads in. This time however I realised that every extra squeeze of the pump to top off the tank was adding less than a litre or so and probably constituted a seperate 'delivery'.
Since then, I've stopped brimming the tank and just wait until the nozzle shuts itself off.
Unscientific I know, but the range I get from a tankfull doesn't appear to have changed very much but my refills are a few quid cheaper.
Could be a nice little earner for the garage if each pull of the handle counts as one delivery and the pump accuracy isn't too good.
Kevin...
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I think it's just that the overall accuracy is affected by the slight elasticity of the delivery hose (and the residue in the nozzle) and this will be a greater proportion of a small volume than a large one. There is no cumulative effect, though, so clicking on and off will make little difference. Petrol pumps use positive displacement metering that is pretty accurate - can't quote figures off the top of my head, but it's not worth worrying about. You probably lose more to evaporation...
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I was told by someone who should know that Esso pumps were set to deliver very slightly more than an indicated gallon ie to err on the generous side of the allowed tollerance. The reason being that it would appear to give greater mpg than other brands. Not sure if this is true but the Esso employee who told me had no reason to make it up.
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I remember reading an article a year or so ago about fuel pumps and there ability to deliver accurately. The conclusions to this article was that on average, over the year, the amount of fuel delivered to your vehicle was accurate to within 50p - £1.
I know for a fact that over the years, a lot of work (six sigma) has gone in to ensuring that the amount of fuel delivered to your pump matches what it says on the pump display. Coca cola did a similar exercise to ensure that in their factory, the amount of coca cola filled into a bottle was correct to 0.005%.
Also, in response to cabsmanuk, I believe that the esso employee was speaking the truth. If the amount of fuel actually delivered was below what was quoted on the pump display, they would leave themselves open to all sorts of claims. Better to be safe than sorry. Even if it is 0.5pence over on a fill up.
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was correct to 0.005%.
All relative, really.
Given the way my car pees petrol out of the overflow when I park it on a hot day, I'm sure expansion/contraction according to the weather makes a vast difference (can any chemists advise?)
I know local pumps are carefully checked very often - the pump attendants have all sorts of scams going to con the drivers and management! I also know that they have a tolerance of -0/+2% on the pumps.
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I've noticed a couple of people lift the hose up after filling - presumably to try and get more fuel in for their money. Does this actually work, and if so, does it mean that the next person to use that pump will lose out?
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I've noticed a couple of people lift the hose up after filling - presumably to try and get more fuel in for their money. Does this actually work.
No. Once the trigger of the nozzle is released, the fuel is cut off at the nozzle end of the hose, not at the pump end; thus leaving the hose full of fuel.
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Going off topic a little, I worked as a pump attendant many moons ago and the manager used to transfer a lot of two-star into the four-star tank late at night, to boost his profits.
A few customers complained about pinking, and probably many more swore never to buy Mobil again.
I wonder if this still goes on? (he doesn't, the garage is long gone)
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the manager used to transfer a lot of two-star into the four-star tank late at night, I wonder if this still goes on? (he doesn't, the garage is long gone)
I think the owner of one local garage forcourt near me - I now steer well clear of - used to work in a pub, as he has taken to adding water to his petrol, instead of his beer!! Last time I filled up there was a couple of years ago, and moments later my bike spluttered and coughed. It would only run on full choke. Limped it home and discovered I had a load of water in the tank.
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DD
I think that the water was probably a genuine error.
I used to audit a fuel distributors who picked up their supplies direct from the refinery and it wasn't unknown for the load to contain water which had come from the refinery tanks.
The other alternative is condensation in the garage's own tanks.
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Petrol pumps are very, very accurate but because they're allowed by Trading Standards to over-read by 0.5%, they're usually set deliberately to over-read by 0.49%.
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As others have said, there are more important things in life to worry about than this. Fuel dispensers are regularly calibrated by Trading Standards officers under the guidance of UK Weights & Measures at Teddington. The flow meters used are extremely accurate although one of the reasons that pre-pay is not big in the UK (i.e the ability to pay a specific monetary amount in advance of dispense as in the U.S) is as I understand it down to the high cost of the necessary shut-off valve. I'm sure we'd have pre-pay over here if drive-aways (pump and runs) were more prevalent.
With regard to water in the tanks, this certainly does happen but it sinks to the bottom of the tank. All service stations these days have intelligent tank gauges with the ability to detect an excess of water and to alert the operators to its presence.
RM
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"I'm sure we'd have pre-pay over here if drive- aways (pump and runs) were more prevalent."
Not worth trying, I assure you. I once did it by accident (mind on other things) and within an hour two policemen were on my doorstep!
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JBJ...That was because you were not using false number plates. I used to help out at a garage where 'Drive off's' happened at least once a week (generally to the tune of £20-£30 per time). Each time the offence was reported to the police but always the outcome was the same...false plates!
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Thanks, 007. I'm obviously not thinking sufficently like a criminal.. :-)
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Roger - if (as you say) pumps over-read by that amount, it means that they under-deliver by that amount, which is not what Cabsman is suggesting higher up. I must ask my daughter - she works for TS.
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SteveH - no it shouldn't, but at least I am happier that the tipped-up nozzle will not dribble over car or shoes while I put it back in the holster.
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I asked Weights and Measures about this but forgot to post the response so here goes....
/* Quote
Kevin
I am not sure I understand you when you say 'each pull of the pump handle',but assume you mean each time the nozzle lever is squeezed to allow flow.
All petrol and diesel dispensers have electric pumps and the dispense can occur when this mechanical lever on the nozzle is squeezed. Each delivery commences when the customer lifts the nozzle from its stowage point and has been authorised by the forecourt operator. The customer can then take fuel, including short stops as might occur if the customer wants to completely top up his tank, or get to exactly £20 say. The delivery ceases when the customer returns the nozzle to the dispenser stowage point. We require the dispenser to be accurate for any quantity greater than two litres. For type approval we carry out tests at the minimum delivery and various other quantities, and at different flow rates. We also carry out a flow interruptio >test to ensure that the dispenser is still accurate when the flow stops and starts within a delivery. The limitation of a two litre minimum delivery is mainly an acknowledgement that the dispenser is less accurate with smaller deliveries due to a number of random errors that become insignificant with larger quantities. However I would accept that the uncertainties of our test methods also tend to increase with smaller quantities.
I hope this helps
Endquote */
A seperate email also stated that deliveries must not be consistently over or under the required amount. That is to say that a pump that always delivers slightly more (or less) than the indicated amount would not pass approval.
Kevin...
PS. I must say that W&M were very helpful and PDQ in getting back to me.
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