How do gas pumps know when the tank is full?
I have literally wondered this for 20 years. Surprise, it's not a sensor or a camera; it's a work of mechanical genius.
As I demystify this decades-old question, let’s be clear about something. Oil is not a renewable resource. The oil and gas industry is both a signatory and a sponsor of our world’s current climate crisis and ecological collapse. Consumers bear a responsibility to demand less oil, but the responsibility falls upon the industry to develop sustainable alternatives.
And while we’re talking, a few other thoughts:
Encourage your members of parliament and provincial representatives to support public divestment from oil and to push for public investment in renewable energy! Demand the protection of Mi’kmaw fishing rights! And those who rationalize, ignore, or dismiss domestic terrorism against Indigenous peoples are each a partner in state-sponsored genocide.
Off we go now.
You know that feeling of pumping gas and the handle nozzle thingy clicks the gas off by itself, and then you look over at the pump and you’re like, $1.58 away from a flat number? Of course you do. So naturally you squeeze the handle once, twice, (three times a lady) until, sweet relief, the price arrives exactly at $50.00. Or $40.00 for those of you in Alberta.
But something that’s been weighing on me for decades is how that nozzle, in all of its analog, perpetually grimy glory, always knows when to stop pumping gas.
Every gas pump nozzle is built to control the flow of petroleum from the hose to the tank. In the nozzle’s construction, the the primary flow pipe narrows at the end in what’s called a Venturi tube. This establishes a pressure differential between the tank and the hose. Stay with me.
Thanks to some laws of physics (I am laughing so hard at my humanities-educated self right now), fluids will typically (but not always) flow from a high-pressure area to a low-pressure one in a closed system. Since there’s a difference between the reservoir (higher pressure) and the empty tank (lower pressure), the differential helps to draw gas through the hose and into the tank like a straw.
That’s how the fluid dynamics work here to keep gas flowing into the tank, but there’s a little detail in the construction of these nozzles that act as a failsafe against overflow. It’s the same concept, but kind of used in reverse.
Since fluid flows faster and at lower pressure through the narrow portion (thanks, Bernoulli!), the mechanical measuring of the pressure differential is how the shutoff mechanism works. Venturi tubes contain air that expands and contracts based on the hydrostatic pressure (p1 and p2 in the diagram) in the pipe in relation to the other fluids that might come into contact with one of the ends. That air (the measurement of h) is the key here.
In a fuel nozzle, there’s a small hole near the end; that’s the Venturi tube, which is bookended by an airbag in the handle that’s attached to a shutoff valve. Gas flows out of the main part of the pipe and into the tank. It fills and fills while air flows freely through through the hole and keeps a little air pocket inflated in the handle. As long as the airbag stays inflated with the maintained air pressure, the shutoff valve isn’t triggered.
As the gas reaches the tip of the nozzle, the air pressure changes. All of a sudden, the air has to work extra hard to fill the balloon. Think about how it feels to drink a milkshake through a straw; the thin and melty part doesn’t take much suction to drink, but encountering the bottom of the glass or some residual ice cream means your cheeks get pulled toward your teeth and you’re forced to exert more force to fill the straw.
It’s sort of the same thing with the gas pump. When the gas fills the hole leading to the Venturi tube, the airbag acts like your cheeks. It deflates after the change in static pressure and releases the mechanical shutoff valve that stops the flow of gas.
Just like that! Next time you’re at the pump, spare a thought for the simple brilliance of Venturi tubes. The world is safe from overflows thanks to ol’ hydro physics and a wee airbag. No sensor, no microchip, no technology at all.
See you next time!