How does the petrol pump know when your tank is full?

The filling station’s pumping system is based on the Venturi effect, which describes the pressure decrease in a liquid as it flows from a big pipe to a smaller one. The Venturi effect is one of those common phenomena that you experience every day, but don’t know the name or even recognize the phenomena until pointed out by a physics professor. Take any rubber (or any other elastic material) pipe and run a fluid (water, in this example) through it. Now, gently press the pipe right in the middle. What happens? The speed with which water flows through the pressed region (or the area where you have applied pressure) increases. The other change that you may not experience here is the change in the fluid pressure in that region (fluid pressure decreases in the compressed region). That’s basically what the Venturi effect is all about.

Back to the car, the pressure reduction causes suction, which the petrol pump uses as a safety measure. By diverting the petrol through the narrow passage of the gun, a suction effect results, sucking in air through a smaller pipe that begins at the end of the petrol gun which is inserted into the car.

Next time you go to a gas pump, take a closer look at the pump nozzle (i.e., the thing that goes inside the tank of the vehicle to fill it). Upon inspection, you will find a small, half-inch hole at the end of it. If you don’t know already, then I should tell you, this hole does not pass gasoline to the tank. From the looks of it, it may seem like it just sits there idly, not doing anything at all. But is that true? What purpose does the hole serve?

Well, that inconspicuous hole performs a very important function that ensures that not a single drop of the precious (and expensive!) oil is spilled.

This hole is connected to a small pipe that runs into the handle. At a point near the end of its length, let’s call it Point A, the pipes veers off in a different direction to a diaphragm of air. At Point A, there is a Venturi tube through which gasoline flows. This is how the nozzle appears from the inside.

How the nozzle shuts off automatically with a click may seem like magic to toddlers and adults alike, but it actually involves a very simple and innovative process.

When the tank is not full, air is drawn from the tiny hole (of the nozzle) to the diaphragm due to suction. This goes on until the level of gasoline reaches the level of the hole (or passes the hole). As soon as the hole dips beneath the level of gasoline, it starts to suck up gasoline. Since the density of oil is greater than air, the Venturi tube experiences a change in fluid pressure. As a result, the diaphragm collapses slightly and an automatic shut-off of the flow of gasoline through point A occurs.

What happens next is what we’re all familiar with – the crisp click announcing a freshly full tank.

It’s incredible how simple stuff from our daily lives can help inspire such path-breaking techniques and make our lives easier; in this case, the employees in charge of filling up tanks at a gas station are the real winners! And PS, shaking your car, does not allow you to pump more petrol so take note and be kind to your cars!

Check out my related post: What is the history of the public bus service in Singapore?

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7 thoughts on “How does the petrol pump know when your tank is full?

    1. You are right. As things progress, who knows, we might not even need it and robots pump gas for us. But whether they use this age old theory of ensuring that the tank is full or some new way is still out for verdict.

      Liked by 1 person

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