- So, I bought a petrol pump
nozzle and cut it in half. You might call it a gas pump nozzle. The reason I cut it in half is because I want to answer one question. How do these things know
when to turn themselves off? You hear that click sound? That's the nozzle turning itself off. Like, when you go to fill
up your car with petrol, you don't have to worry
about it overflowing. That's because these nozzles
switch off automatically when the tank is full, but how? My first thought was that
there must be some kind of electronic sensor in there, but the way these really work
is much smarter than that. There are actually two very
clever mechanisms in here. The first one relates to this tube you can see that runs to
the end of the nozzle, and the second relates to
all these connected levers. The whole thing's quite compact, so I built a few different things to illustrate each part of the mechanism, to make it clearer. Let's start with this hole that's usually located here or here. It's called the Venturi sensor, which sounds like it's electronic, but actually this is
entirely fluid mechanical. It's called the Venturi sensor because it works on the Venturi effect. The Venturi effect happens
in this part of the nozzle, but to be able to see what's going on, I got this made; it's
called a Venturi tube, because it demonstrates
the Venturi effect. It's a wide tube that
narrows in the middle, and then you have this
narrow U-bend shaped tube coming off the restriction in the middle. There are two other U-bend
shaped tubes either side, but we're ignoring those for now. Look what happens when I
partially fill this U-bend with water, and then
blow through the tube. You might expect air to
be forced into the U-bend, causing the level of
water here to go down. In fact, the water level here goes up. That means there must be a
reduction in pressure here in the constriction, and
that's sucking the water up. By the way, the water in the U-bend is there just so that you can
see the change in pressure. There's no equivalent to that
water in the nozzle itself. But why does the pressure go down? Well, it's Bernoulli's principle, but actually, we can
explain it quite easily from first principles. The air traveling through the
constricted part of the pipe must be traveling faster than the air in the
wider part of the pipe. That makes intuitive sense. To get the same amount of
mass through a narrower pipe, the mass must have to travel more quickly, but you can also think about it in terms of conservation of energy. The gas here has kinetic energy, but it also has potential
energy stored as pressure, a bit like how you can store energy in a spring by compressing it. But that total energy, kinetic
energy plus potential energy stored as pressure, needs to be conserved. That means that when the
kinetic energy goes up in the fast moving fluid
in the constricted part of the tube, the potential
energy must go down. The pressure must go down. By the way, those two
other U-bend shaped tubes either side of the middle
one are there to illustrate the fact that the pressure goes down in the wider parts of the tube, as well, when there's air flowing, but
just not to the same degree as it does in the
constricted part of the tube. The two outer U-shaped
pipes have nothing to do with the discussion we're having
here about petrol nozzles; I just thought I should
explain what they were. But thinking about that
central U-shaped tube, what does that tell us about
how the petrol nozzle works? Well, this pipe in the model
is this pipe in the nozzle. The constriction in the pipe
that happens here in the model actually happens here in the nozzle. This spring-loaded stopper
here opens slightly under the pressure of the petrol to reveal a really narrow ring for the liquid to pass through. And you can just about
see, inside that ring, there's a tiny hole there. Actually, there's a number of holes. The one I just illustrated
with the paperclip. There's also this one,
and there's probably one on the other side as well,
but they all lead up to here, and then into this hole,
which comes down through here, which feeds into this long tube. This tube that runs to the
end of the petrol pump nozzle is equivalent to this tube in the model. So, this constriction here
creates low pressure in the tube. There is low pressure here
at the end of the nozzle. That means air is actually
drawn in through this tube. That air simply mixes with the petrol in this part of the nozzle. So, how is this used to
detect when your petrol tank or your gas tank is full? This bit's really clever. The tube that comes away
from the constriction in the flow of the main
pipe is actually forked. One tube goes off to the end
of the nozzle, as we've seen, but there's actually a second
tube that goes off up here. Because of the way this
thing has been cut in half, it's not that easy to see, but there is a tube coming off here. Most of it's been cut
away, but it was there. And that tube leads to this cavity here. That cavity is sealed by a membrane. You can see part of the
membrane just there, before it was cut in half. In other words, before I cut it in half, this whole chamber was
sealed off with a membrane. So, schematically, it would look like this with two tubes forking
off the restriction. This now represents the tube that goes to the end of the nozzle, and this represents the tube that goes to the sealed chamber. So, when I blow through this pipe, it will reduce the pressure
in this tube and this tube. But look what happens when I
put my finger over this tube. It turns out that this tube was relieving some of
the negative pressure. And when I put my finger over it, look, you see a sudden jump in the
water level in this tube. The same thing happens
with the petrol nozzle. This opening is allowing some
of that negative pressure to be relieved by allowing
air to flow into the system. But then, what happens when the
level of petrol in your tank reaches the end of the nozzle? Well, it covers up that hole. The tube is now sucking
on petrol instead of air. Petrol is heavier than air, so
the tube can't suck as much. It can't relieve as much
of that suction force. And just like when I put my finger over one pipe in the model, the other pipe experiences
an increase in suction force. This is my attempt to
put all that together. So, you've got the main tube. It has a constriction here. Here's the tube coming
off from the constriction. Here is where it forks. One tube goes to the end of the nozzle, the other goes to this chamber that's sealed off with a membrane here. And look, when the liquid
in the tank reaches the end of the Venturi tube, you see that membrane gets
sucked into the chamber. The membrane moves up
only slightly in my model. That's because I don't know
much about fluid dynamics. I'm sure there's a lot of things
I could tune in this model, like how much is the pipe constricted? How wide are the Venturi tubes? Where does the fork happen? All that sort of stuff. But for me, it was incredibly satisfying to see that membrane move at all. So, when the petrol in your tank reaches the end of the
nozzle, this membrane moves. And you'll notice that this membrane is attached to something. It's attached to this rod here. And that's really important. That's how the nozzle actually turns off. It's quite hard to see
what's going on here, so I built another model. So, imagine this is a valve
that lets petrol through. I need to push this thing
up to open the valve. In the actual nozzle, this
valve is spring loaded. In this model, I'm representing
that fact with a mass. The mass is pushing back down on the valve like the spring in the real thing. And this here is the handle of the pump. So, hopefully, if I pull this handle up, it will open the valve. But look, it doesn't actually work because we've got a lever happening here. Pulling the handle up just
makes this thing move down. What I need to do is
hold this thing in place so that this becomes the
fulcrum of the lever. Now, when I pull the handle
up, it opens the valve. Great. Now, one way I can hold
this thing in place is to put a couple of circles in here. And then, I put this piece
in to hold these circles in place, to stop them
falling into the middle. And there you go. That works perfectly. Now, here's the clever part. This funny wedge thing is
attached to the membrane. And remember, when petrol
reaches that Venturi tube, it causes the membrane to pop up. And when it pops up, it
pulls this part with it. And when it does that,
those circles are now free to fall into the middle. They're no longer jamming
that shaft in place, and this point stops being the fulcrum. Everything collapses, and the valve shuts. In three dimensions this is
achieved three ball bearings in these two positions, and one round the back that you can't see. So, let's put all those
mechanisms together inside the nozzle itself, and recap. So, petrol comes in here under pressure, and it meets this closed valve. So, you pull on this handle here, you'll notice it doesn't open the valve. Instead, this thing moves. But look, you get to this point here, and these ball bearings, they get jammed against the constriction
here in the housing. Of course, because I've
cut this thing in half, that doesn't work, the
constriction doesn't work, and the thing can move when it shouldn't. So, I'm just gonna use brute
force to hold that in place to illustrate the point. And look, now... oh, I've
lost the ball bearing. Doesn't matter. Now, when I put on this handle, this point acts as a fulcrum,
and the lever mechanism opens the valve, and
petrol can flow through. So, let's imagine the valve is still open; petrol flows through here. It looks as if this housing
is in the way of the petrol, but actually there's a gap underneath, and there's a gap on the top as well. So, petrol flows around
this part of the mechanism, and then the pressure of the petrol pushes against this
spring-loaded thing here, which creates a thin circular channel for the petrol to continue
to flow through the nozzle. Now, because it's a thin channel, and due to the Venturi effect, the pressure is lower in that part. And look, there's a hole
here and a hole here that links to that low pressure region. And that hole leads over
here, down into this tube, but it also goes up into this cavity here that is bounded by a
membrane at the bottom here that's cut in half. Now, because this hole is open to the air, it sucks air in, and that acts to relieve the negative pressure. But when your petrol tank is full and it comes up to that hole, it's harder to suck on petrol
than it is to suck on air, so this tube here is less good at relieving that negative pressure, which means that this chamber here feels a stronger negative pressure, which pulls up on the membrane. The membrane is attached to this thing, so this thing gets pulled up as well. So, let's put all that
together at the very end, and see what happens when the
petrol nozzle in your hand makes that clicking sound. So, the ball bearings are
holding this thing in place. And look, you've opened the valve. But now, look, this thing...
I'm gonna try and reach over and do it. This thing lifts up, you
see the ball bearings fall into the middle of that thing, which means it can now slide down, which causes this thing to move down, which causes this thing to move down, which closes the valve. And of course, that prevents
petrol from spilling out of your full tank onto the full court, which would be a pretty big fire hazard. This video is sponsored by 80,000 Hours, a nonprofit that helps
people to find couriers that solve the world's biggest problems. Like, I remember careers advice at school where they'd ask questions like, "Do you like being outside?" "Do you like people?" And at the end, they'd say something like you should be a flight
attendant, or whatever. And you're left thinking,
"Did they even ask the right questions?" You know what I mean? The thing is, there is
actually good advice out there for how to find a fulfilling career. It's just that careers advice at school doesn't tend to focus on that. Most importantly, it's hard
to find good careers advice for people who want to make a difference. The advice tends to be follow one of these well known
career paths, be a doctor, be a teacher, be a charity worker. But those aren't the only options, and they might not be the
best option for you either. 80,000 Hours aims to
help you find a career that's fulfilling and
makes a big difference. The name comes from the
average length of a career: 40 hours a week, 50 weeks
a year, for 40 years. Their insights come from
10 years of research alongside academics at Oxford University. And it turns out, the best way
to make a positive difference might be pretty different
to what you'd expect. And you won't find any
unsupported generalizations at 80,000 Hours either. Like, their recommendations
are based on careful research, but if they aren't sure about something, they'll say so. And to me, that says a lot. If you care about what the evidence says about having a fulfilling
and impactful career, and you want advice that goes
beyond, "Follow your dreams," then 80,000 Hours can help. And by the way, everything
they provide is free forever, because they're a nonprofit. Their only aim is to help
solve global problems by helping people like you find the most impactful careers they can. If you sign up for their newsletter now, you'll get a free copy of
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you'll enjoy this video next. (upbeat groovy music)
I nervously tried to figure out when to manually stop the pump for like a year before someone told me they automatically shut off when the tank is full
Not in any way is this cutting edge technology. Which, to me, makes it even cooler. I was using this tech 30 years ago in the Navy, and it was old and dusty then. But itβs comforting to me, at least, to know physics and mechanical parts are keeping things flowing (so to speak).
The Venturi effect is the same that powered carburetors on older cars built before the 80s and 90s. Higher intake volume from the pistons moving faster caused more fuel to be sucked up into the intake.
Man, Steve Mould is a top tier science educator. Iβve been curious about this for a long time and it wasnβt until this video that it finally clicked for me, no pun intended.
Very clear explaining and use of working models to further show illustrate the concepts. I love this stuff.
Some time ago a German childrens education TV show (Sendung mit der Maus) also made a video about this: https://www.youtube.com/watch?v=7bPoVtybOu0
Its interesting to compare the different approach of essentially conveying the same information
Also make sure you dont keep trying to put more gas into your tank after its full. Good way to mess up your evap system.
This is so cool. My wife asked me how the pump knew when to turn off a few months ago. My explanation was along these lines, but this video is MUCH better. Can't wait to show it to her
Something that I never thought I would care about but this is a pretty good video. I watched it to the end.