Science
How Does the Gas Pump Actually Know How Much You Owe?
It’s no secret that gas prices have been slowly creeping up in the US, and that means you are probably paying closer attention to how much money you’re pumping into your gas tank.
You see the prices on the sign out front, and on the pump itself — with despair, nonetheless, but how exactly does the display on the pump actually know how much money you owe for that transaction?
Have you ever wondered if it was ever off, in the gas station owner’s favor?
Or, maybe you’ve never given it a second thought until now…
Either way, let’s take a look at how you can be sure the display on the pump is not playing any tricks on you or your loved ones.
Measure.
Before we even talk about money, or the display itself, we have to first figure out how the display knows how much fuel has been dispensed into your tank since you started filling.
To do that, we have to first consider what is actually being measured at the pump! Obviously we are measuring gasoline, and the money leaving your bank account, but we are also measuring how much gasoline is flowing through the hose and into your car.
That is called the “flow rate”. We need to measure the flow rate of the gasoline as our first step.
But how exactly do you do that? I mean, think about it. If you have a bucket full of water, you could find a way to measure the volume of the liquid and tell someone how much water is in the bucket.
But, what if the water, instead of being in a bucket you can weigh or measure, was flowing through a hose? Or, what if it were flowing out of the faucet in your kitchen?
Well, you could easily capture the flowing water in a bucket, and then measure the volume that way, right? Sure, but you can’t exactly do that at the gas station. The pump has to know how much gasoline is flowing through the hose as it is happening.
To do that, we have to use an instrument called a “flowmeter”, or “flow meter”. The two spellings are used interchangeably between various manufacturers.
In fact, there is some pretty wild flowmeter technology out there, but we won’t go there in this article.
A typical gas pump will use a “positive displacement” flowmeter, or sometimes often referred to as an “oval gear flowmeter” due to the mechanical composition which includes two oval-shaped gears perpendicular to each other.
This type of meter is very exact due to the operating principle. The gears inside of the meter are simply held in by a pin in the middle of each gear which allows them to rotate freely. However, since they are gears, when one moves, the other must move as well.
As liquid comes in on the inlet side of the flowmeter, that side of the flowmeter starts building up pressure due to the gears restricting the flow of the liquid. When the pressure, and volume of liquid, reaches a certain threshold, it is able to force the gears to rotate, and let that same amount of liquid out on the outlet side.
Or, another way to say it is that the flowmeter is able to displace a certain volume of liquid to the outlet side.
So, why is that important?
It’s important because with a certain size positive displacement flowmeter, only a certain volume of liquid can be displaced for each revolution of the gears. That is why this type of flowmeter is very accurate, and precise. Yes, there is actually a difference between those two terms.
Now, there are ways to “size” the correct flowmeter for a particular application, but that won’t be covered here. It involves a lot of detail about line sizes, hose run lengths and a million other variables. You can trust that the flowmeter was sized and calibrated properly for the application we are discussing in this article.
Okay, now that you know how we measure the flow rate of gasoline, let’s take a look at how the flowmeter communicates that information to the display you look at while fueling up.
Well, every time the gears complete a revolution, a magnetic field is generated and collapsed. For simplicity’s sake, imagine there is a magnet of one charge on the tip of one of the gears, and there was a magnet of the opposite charge at the 12 O’clock position of the flowmeter housing.
When the magnet on the gear and the magnet on the housing are lined up, a magnetic field is created. However, as the two magnets are pulled away from each other, the magnetic field collapses, and that is an important concept here!
A change in magnetic field will induce a current of electrical charge. That change includes the magnetic field being created, and then “slowly” collapsing as the gear moves around the housing. Since that is technically changing the magnetic field, it will induce a small electrical current.
That said, since you already know that only a certain volume of liquid can cause the gears to complete a revolution, you can deduce that it will take a certain volume of liquid to generate an electrical current.
We call those small electrical currents “pulses” since they are only happening momentarily and repeatedly. It’s not a continuous flow of electrical current, it’s just short busts at a time.
The amount of volume that passes through the flow meter per pulse is what is called the “K-Factor”. The K-Factor can also be defined as, “pulses per unit of measure”.
The K-Factor will vary from flowmeter to flowmeter depending on all sorts of variables. For more precise applications, like pumping fuel (also known as “custody transfer”), the flowmeters are “proved” to ensure the K-Factor is as exact as possible before they are commissioned.
Now with that out of the way, we can finally take a look at the answer for which you’ve been patiently waiting.
The pulses generated by the flowmeter need to go somewhere! They typically go to something referred to as a “counter”, “totalizer”, or sometimes even a “register”. In this case, it’s just the display you look at while fueling up.
That display is purposely designed to count very fast pulses of electrical current. Obviously specifications range between different brands, but a lot of them can read pulses into the kHz (thousands of pulses per second) range.
What typically happens is the “K-Factor” from the flowmeter is programmed into the display so that it can count the pulses and know how much gasoline has flowed through the flowmeter.
The display on the gas pump is actually NOT showing you the flow rate. Rather, it is showing you the total amount of liquid that has been dispensed. Since gas prices are expressed as “dollars per gallon”, the display is also programmed to take that total flow reading and multiply it by the current price per gallon.
That, in not so many words, is how the gas pump knows how much money you owe when you finish filling your gas tank.
But, how can you trust it?
Well, you’ve probably seen those stickers on the pump that somebody has signed with a pen. It may even have your state seal on it, and some weird imprints and numbers.
That sticker is proof of calibration. Since the gas station is exchanging product for your money, and you have no way of easily validating how much volume was pumped into your tank, the gas station must adhere to strict calibration requirements to ensure they are not ripping people off by changing the K-Factor, or anything else, on the display module. Otherwise, they could have the display show something like 10 gallons when really only 8 gallons were pumped.
The calibration applies to both the flowmeter and the display, and now you can rest assured that what you are paying at the pump is fair. Well, that’s not to say that gas prices are fair, but you aren’t paying more than you were told you were for the amount of gas you pump!
The next time you fuel up, keep your ears open at the pump. It doesn’t happen all the time, but sometimes when the gas is pumping you can hear a bit of a high pitched humming sound. That’s actually the sound of the gears inside the flowmeter rotating really fast as the gas passes through it.
Pretty cool, huh?
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