The thing I don’t get is that no matter how much I shake the nozzle at the end of the pump cycle to make all the drips fall into the tank, when I take it out invariably there’s a dribble of gasoline.
This problem only gets worse as you get older.
…the problem with zombie threads is that I was going to debate the point made by Q.E.D., when I remembered that Q.E.D is no longer with us. 
Rest in peace dude.
Blimey. I never noticed this. I think this thread needs laying to rest (as it was resurrected by kevininthe661, who is either joking, wrong, unlikely to return, or some combination of the three)
I wish someone from a company that makes the pumps would respond to this question. There was the poster that stated something about wear and tear on the pump but I think he/she was just guessing.
Really, the “slow down” would not need to be more than just a few seconds if it is really truly important for whatever reason. However, this “slow down” is an incredibly long time at many stations.
I have heard that some people shake the hose or lift it up to get more gas out of it. Maybe this is what the slow down is for? Does it make it so that you get all/most of the gas you pay for instead of having to lift up the hose or whatever after it stops abruptly leaving the hose filled?
It is true that often some gas spills out of the nozzle as I take it out of the pump hook as I move it over to my car (before pulling the trigger). Maybe this is indeed because the person that just used the pump before me was someone that did not pre-pay and just stopped the pump abruptly because was using a credit card at the pump.
Still, though, the question remains as to why some stations have the “slow down” set for upwards to 15 seconds! Surely they are hoping that people leave instead of waiting! I actually did that just last week. I try to avoid going to that store because I hate the long slow down it has.
Closing this thread which has had little factual info added for a few years.
Reopened for info.
samclem
Thanks, samclem!
For what it’s worth, I’m a mechanical engineer, and I’ve spent a fair bit of time designing high-precision flowmeters (Coriolis meters, to be specific) and high-pressure pumps.
The water hammer effect described early in the thread is a real phenomenon, but that’s not why the pumps creep along to the pre-paid number. First of all, it’s easy to mitigate the pressure wave of a closing valve with a damping mechanism (sometimes called a water hammer arrestor) built into the system.
Also, the valve controlling gasoline flow shuts off quickly not only when you’re approaching a full tank but also when you release the handle. The water hammer effect is not the reason dispensing slows down so much for so long.
It’s also not to prevent overdispensing. It’s pretty trivial to calculate how long your valve takes to close once you send it the signal to do so. If you know the closing time and the flow rate, you can figure out when to close the valve in order to deliver the right amount of fluid.
This sort of problem is central to control theory, and has essentially been solved.
From an engineering perspective, there is no reason to let the pump creep along to a pre-paid limit.
Consider this: if the pump were slowing down to somehow meter more accurately, it would slow down when it approached a certain volume left to pump. But instead, according to the reports in this thread, it slows down at a specific dollar value remaining. A pump that slows for 15 seconds at $2/galon should slow down for 7 seconds at $4/gallon. But evidently the slowdown point is keyed to dollar values, not volume.
As far as I can tell, the creeping-towrards-the-prepaid-value behavior really is intended to encourage chilly drivers to hop in the car and ignore the rest of the gas they paid for. If someone with petroleum industry experience has a convincing reason for the slowdown that I haven’t considered, I’m all ears.
That was a tough crowd 10 years ago! The first two responses were variations of “Doctor, it hurts when I do [this].” Doctor says, “Don’t do [that].”
And. . .
It’s unfortunate that the OP was banned for whatever infraction. He was the one who solved the “Al Traina/Unknown Carson Guest” mystery.
EdelweissParate hit the nail on the head along with a few other members…In addition to them trying to get the customer to hang up the pump early I also believe that they do this as a marketing tactic, people believe that measurements are more accurate (the last 50 cents), screw the previous $40 you spent getting there…lol.
Now that we solved this problem can anyone explain why gas stations still advertise their prices in 9/10’s of a percent. Maybe when fuel was $.20 a gallon, a penny was a big deal but as the costs are much higher is this even necessary?
WAG: Nobody wants to be the first one to change. If your competitor across the street is charging $2.699, you wouldn’t attract more business by charging $2.69, and you’d lose business by charging $2.70.
My mom routinely prepays for gas, fills up her tank, and then goes back in to get her change. (Personally, I always use a card, but my mom is stuck in her cash-paying ways.) Do people really drive off without getting every drop of gas they paid for or the change they’re owed?
Further fueling the latest zombie revival …
As long as people will continue to think of $2.299 gasoline as costing $2.00 or $2.20 or even $2.29 the tenth of a cent pricing will continue.
There’s plenty of evidence that no matter how many of us loudly insist that “I’m smarter than that. I *always *round up to $2.30!” that isn’t what really happens in the real world with most people.
I always thought this was some way to reduce spillage.
No mystery:
**
When you want the exact amount of fuel dispensed based on the amount you choose, the pump must slow down to be precise to the penny.** This requires a slow down, to tune the flow and calibrate it to the exact goddam penny, you poor or cheap sonsabitches. PRECISION FLOW must kick in; it must hone the flow to a point so precise that the dispensed amount synchs up to the penny of what you spending.
IN CONTRAST… if your willing to pay for whatever stinkin’ amount of fuel comes out when it slams shut, then if it dispenses X sloppily, that converts in you paying for X. SO, when filling the tank, penny precision isn’t needed. You’re paying for X am’t of fuel, because they dispensed X am’t of fuel.
That’s why it has to slow down when you want a super precise amount of fuel dispensed. There is no room for error when you want 10 bucks worth.
The ‘sudden stop’ requires far less precision, because you’ve agreed to pay for whatever comes out.
The engineer above missed the mark… big time. It’s not about the hammer effect. It’s about being precise to the penny for people who are only willing to spend up to N pennies.
.
He’s saying – and I tend to believe him – that pump cutoff precision could be accomplished far quicker than the prolonged slowdown we actually see.
I still don’t think it’s about making impatient people leave behind pennies worth of gas they already bought though. I wonder if it’s just a matter of gas station owners having to configure these pumps somewhat, and they don’t know what they’re doing and configure them wrong or not at all.
For example, there’s a church near me that installed an LED sign out front, but it stays at daytime brightness all night and nearly blinds me every time I drive by after dark. I’m ~100% positive there is a setting to automatically change the brightness based on ambient light level or local time, but the owner just didn’t turn that setting on.
I suspect that gas pumps come with a default “slow cutoff” setting, and another setting to automatically tune the cutoff control optimally for the tank, pump and pipe specifications at that particular station, but gas station owners universally ignore the user manual and just leave the extremely slow default setting in place.
Not that anyone could ever underestimate the stinginess of gas station owners, but I can’t believe that intentionally slowing down your customers would be a good business decision. I imagine that faster pumps would actually result in increased turnover, and ultimately more paying customers in a given period of time. Don’t attribute to malice that which can be adequately explained by stupidity.
I’m a chemical engineer in the petroleum additives industry. I don’t deal with gas stations, but do deal with pumping and metering lots of petroleum and related products. (Like, the additives that go into your gas.)
Gas pumps are relatively simple; they’re not sophisticated flow devices like a Coriolis meter or something. It is to the gas company’s advantage to make sure they do not dispense one $0.001 more of gas than you purchased. If you know your flowrate, you can command a flow valve to begin closing in anticipation of when it should shut to get the exact charge, since the closing isn’t instantaneous. This early close is often called the “preact”. I’ve spent a lot of time adjusting preacts to improve charge accuracy. This method becomes more accurate if the flowrate is slower – particularly if your meter and flow valves aren’t particularly sophisticated.
So, in a big petroleum plant we’ve got a lot of flow controls that slow the flowrate as it begins to close the control valve to ensure we reach the exact charge. Because when you’re making huge volume charges, being a fractional % off adds up to big $ – enough to justify some tweaking of flowrates, even if it’s not enough to justify upgrading to fancier flowmeters.
It’s not cost-effective for gas stations to upgrade to fancy equipment that could achieve an accurate cutoff without slowing the flowrate. But it doesn’t cost them much at all if the customer has to wait a few extra seconds for the pump to finish how much they’ve prepaid.
I agree with your premise but not your conclusion.
A US gallon consists of 128 fluid ounces. At current prices hovering around USD2.50/gal, that says a penny’s worth of fuel is about 0.5 floz. AKA 1 tablespoon.
It’s not that hard to cut off a flow with that precision with only a second or two of advance slowdown. It’s also not that hard to program the overall pump software to, for prepaid purchases, stop the money on the money even if the volume dispensed runs very slightly over. Or under, as long as it’s within whatever the State’s weights & measures standards says is close enough.
I also agree with Dr Cube that stupidity is more likely than malice.
Finally I wonder if the 10-20 second slowdown was first implemented 20 years ago with much slower computers & cruder sensors, but that same code is still running on modern dispensers? Or how much it’s become a chicken and egg problem where the programmers believe the users or store operators expect it so they keep re-implementing that behavior.
I’m with this – except that current design is exagerating the problem, not reproducing it.
In the old days, when you had a full-service station with old pumps, and you’d roll in and ask for $10 gas, if the attendant didn’t turn off the pump /before/ he reached $10, there would still be gas flowing down the pipe and he’d put in more that $10.
So then he’d either have to ask you for more, or swallow the difference.
So you can undestand someone thinking it’s a good idea when they started building more complex pumps.
But now it must be just a marketing device: customers expect the pump to slow down at the end, they think it’s unsafe or broken or inaccurate if it doesn’t slow down, lets go real slow at the last 10 seconds (!) because that’s what you would do if you were using a bucket to fill the tank…
Yesterday I pumped $30 worth of gas at a self-serve, pre-paid pump and it didn’t slow down - just shut off at $30, bang! like that.
I hope this is the new normal. To think of all the hours spent waiting for those last few drops…