"Bouncy" sulfur hexafluoride gas in tennis balls?

today on Applied Science we're going to take a look at the myth that there's sulfur hexafluoride in tennis balls this is also known as the doop forged gosh because it's very dense but what's weird is that people claim they put it in tennis balls because it makes the ball bounce here which seems a little strange it also seems to show up in shoes and even car tires people suggest that it's in because it has this bouncy property but then even stranger some people claim that it's a shock absorbing gas which would make it not a good choice for something bouncy like a tennis ball so in today's video we're gonna take a look of which of these claims if any are true if there's any sulfur hexafluoride in tennis balls and then take some careful measurements which I actually found to be really surprising so this whole investigation got started when I was searching for commercial uses of sulfur hexafluoride I was just looking for something neat to demo and I thought I found it everything from anonymous forum posts all the way up to like government sponsored reports on industry say that tennis balls have sulfur hexafluoride and I thought that was a pretty cool story and I think the reason that everyone says it is because there are actually a few patents that go into detail about why you would put sulfur hexafluoride in a tennis ball and it has to do with the longevity of the ball not the bounciness so as you know if you have like a latex balloon filled up with especially with helium but even with air it will deflate over time and that's not because it has a leak it's just because the air molecules are diffusing through the rubber and just leaving the balloon because it's higher pressure inside and lower pressure outside so I didn't know this when I started this but tennis balls are in fact pressurized in fact they even come in this pressurized can to keep them fresh right so the tennis ball is basically like a balloon it just has a thicker skin so if there's pressure inside their air for example over time the air leaks out the ball loses pressure and it won't be as bouncy so the patent introduced this new idea of putting sulfur hexafluoride inside the ball with the idea that the sulfur hexafluoride molecule is bigger and it won't diffuse through the rubber as fast and so it'll stay pressurized longer in fact there's an interesting twist to this if you filled the ball completely with sf6 100% would actually be a diffusion of air molecules into the ball there'd actually be osmotic pressure trying to inflate the ball over time and the patent goes into this and says well the ideal ratio is about 60 or 70 percent sf6 the remainder air inside the ball with the idea that osmotic pressure will try to be filling the ball and very slowly sf6 will be diffusing out because it eventually makes it through the rubber - it's just much slower than the air so over time the ball will we get to be filled more and more with air the ratio will change but the pressure will be mostly constant very cool the only problem is I can't find a single tennis ball that has any sf6 in it and I this was an extensive search I wouldn't say it was exhaustive but I went to some pretty extreme measures here so I started off just buying all kinds of different brands of tennis ball at a local sports store and was testing it couldn't find any sf6 and then I moved up to buying like less common brands on Amazon these Green Dot balls and orange dot balls actually have different pressure levels in them for beginner players and I tested those still nothing and then finally I went to ebay and bought sealed never opened before vintage tennis ball cans so this was kind of fun these these are these are old enough such that the ball or the can is metal I mean it's all these plastic ones are modern but this is vintage metal and this is a pretty weird-looking tennis ball I guess they used to be white if you go far enough back or something even the diameter is a little bit different compared to modern ones anyway I went through all this and he and also even went as far as testing the gas that was in the can in addition to testing the gas that was in the ball so I thought well maybe you know since these have been sitting on the shelf for 20 or 30 years or 40 even maybe even more maybe the sf6 is diffused out into the can so I even took one of these little vampire taps meant for like a refrigeration line and modified it so I could put it on the can and then twist in and Pierce the can while collecting the gas that was coming out and so I measured the pressure and density of the gases in these cans and in the balls for everything that you see here and didn't find any trace of sf6 so it's possible that someone weird brand was doing it way back in the day but I captured basically all the major brands Dunlop Spaulding and Wilson from the vintage era and then also from the modern era and there's no trace so if you're cynical like me you can see why this tennis ball sf6 thing didn't work out right like imagine that the company you're on the engineering team and you said hey we came up with this cool patent that allows the balls to self inflate and actually stay pressurized for years and then the folks on the commercial team say well ok it's gonna cost more to make the ball and we're gonna sell fewer of them because they stay inflated longer so that means the price it's gonna have to be much higher and the whole ball only costs about a dollar anyways there's really not much room we can we're not gonna be able to recover the cost that we put into it so I have a feeling that's probably why sf6 was never put into tennis balls but there's another product that actually has a different set of economics around it where it does make sense and that is these Nike Air Products right so these these absolutely were filled with sf6 and I actually measured this one myself and found it to have a trace of sf6 left what's interesting is Nike stopped using sf6 not because it didn't work it actually worked great but sf6 is a very powerful greenhouse gas in fact it's just repeating figures that I've heard on the internet 20,000 times more potent than co2 so they had to stop doing it for that reason but this she was falling apart this is a vintage shoe that I got off of ebay because the modern ones don't have sf6 but I found this old one that was falling apart in fact it's new never worn just old new old stock and I pierced one of these and pulled a gas sample out and measured it and did find some sf6 left not a lot there's not much pressure left in these pads after I think this shoe is 20 or so 20 something years old that never worn and the reason that Nike used sf6 in the first place was primarily because it didn't leak out same with the tennis balls right this is a rubber you know bladder here and if you put air in here after the you know years of use they would so the sf6 keeps it inflated but again these claims of the squishiness or the softness or the springiness of the gas kept coming up so when I was searching on the internet people would say oh yeah they put it in shoes because it makes them springier so that's a claim that we're gonna look at in the second part of this video but for now let me show you the method that I was using to detect if there was sf6 in here in the first place a mass spectrometer would be the ideal instrument but since I haven't built one of those yet just using the density of the gas is a really good way of doing this sulfur hexafluoride is about five times as dense as air and there's very few other gases that are anywhere near is dense and so if you have a scale that reads out in milligrams or even tenths of milligrams like this one you have a pretty good chance of measuring the density accurately and I was actually surprised that this works as well as it does but basically just using a syringe piercing the ball and what's cool is that there's actually enough pressure in the ball to cause the syringe to fill all by itself you don't have to pull on the plunger and then cap this off and measure the weight but something funny happens here right so let's let's show you how this works so here's just the empty syringe with the valve on the end and we'll just take a baseline reading and I have the scale cranked up to maximum sensitivity as they call it so it may take a little bit longer to stabilize it looks like we're okay so we'll tear it and then if I take this out and close the valve and then pull a vacuum in here so I'm pulling on it and there's now 60 ml of vacuum it should be the same right because obviously it's the same molecules physically there's the same thing here and I have to admit I got this wrong the first time I thought about it but you'll be very surprised if you think that it was going to be the same it's actually about 70 milligrams lighter than it was okay this is pretty funny right how did the syringe become lighter by me just pulling this thing out it's literally the same molecules they're same atoms are here which is true but what we have to remember is that we are in fact measuring it in air so some things are buoyant like if I measured a helium balloon what would you say the weight of a helium balloon is well it's negative because it's floating and the same thing that's happening here this is actually vacuum inside here so it's even more buoyant than helium because this way is you know it's in theory nothing whereas helium just weighs less than air so basically we were measuring a vacuum balloon and it's it's literally floating or it's trying to float and that's what we measured was the buoyant force and it came out to be about 70 milligrams which is about right since 60 ml of a vacuum but well basically what we measured was the air like the negative whatever the air displaced and air is about 1.1 or 2 milligrams per milliliter so it's pretty close I mentioned that these Green Dot and orange dot balls are purposefully less inflated from the factory so Green is only like half pressure and orange is like zero pressure but I still wanted to test the gas sample that was in here so what I did was I pierced them with the needle and then squeezed it in the vise to sort of squeeze out all the gas that was in there and again found basically just air or nitrogen inside there so anyway let's talk about this claim of bounciness or squishiness my first thought was well if there are no tennis balls with sf6 in them why don't I make one and then try it out so that's what I did I came up with this filling rig and I used luer lock parts to basically put all this together and you can get bags of these things from McMaster for just a couple bucks and you can get like valves and tees and everything it's actually a very cheap and easy way to put together little tubing systems like this and then of course since I'm using sharp hypodermic needles these have lure locks on them to start with so it's very convenient if your if your piercing something and filling it with a gas for example so what I would do is take this and pierce it with the needle and then use the vacuum pump to pull out everything that was inside there and then fill it up from my tank of sf6 and I'd fill it up to about 10 psi or about 3/4 of an atmosphere which is about the average of what I measured from the factory balls and then I did the same thing but filled one up with argon the first thing that's apparent is that they do sound different so if you had a tennis ball that was sf6 and one that was argh out and you bounced them even with the same pressure in each ball they would sound different you can hear its back and forth here so then I wanted to measure the bounciness let's actually see if the fill gas will make the ball bouncier or not even at the same pressure and I thought of some ways to do this it's very hard to control the experiment because what I wanted to do was sort of measure the ball under real conditions in other words being hit by a racket pretty hard and I don't really have that kind of a setup going so what I do is I just threw it at the ground and used a high-speed camera to accurately measure the speed of impact going in and then coming out and I expected there to be some difference but couldn't I spend a while like trying different combinations and different drop heights and throw higher speeds and everything and it always seemed to be about the same so I had to sort of step up my efforts meanwhile I was doing a lot of background reading and realized there actually is quite a lot of interesting stuff with this claim of squishy gases it's just too difficult to test in a tennis ball so I came up with this setup which will allow us to test the gas in isolation it's basically a piston a free-floating piston and a cylinder and we can put whatever gas we want in the cylinder and then the idea is that I'm going to drop a weight on it from a known height and at the bottom of this cylinder there's a load gauge to connect it up to the oscilloscope through a differential amplifier filtering and then right into the scope I had a good time putting this video together because I learned quite a few new things myself if a few weeks ago someone said that you could build a gas spring and tune its behavior by putting different mixtures of gases into it I would have thought that's probably not true right I mean they're all ideal gases it's the whole point of having the ideal gas law but there's actually quite a bit more going on here and this proves it so let's take a look at the results the bottom three traces show force experienced over time for three different gases so we've got argon air and sf6 and negative means more force downwards so we start off at zero and then the impactor hits the top of the piston starts compressing the gas it's compressing it the pressure is going up that means the force is going up and then eventually it hits a peak where it's compressed it as much as it can and then it starts expanding again and eventually pushes the impactor off the top of the piston it is a spring after all and we get back to zero so you can see there's a huge difference here there's no worries about you know slight measurement errors it's a really very large distance or a very large difference between these gases and if we put them side-by-side and look at it with the high-speed camera we can see that there's also a difference in how far these things travel so the sf6 compressed the most you might even even bottom doubt I'm not sure but it did compress a lot and the argon was the least compressed and air was intermediate so let's see what this says about the qualities of sf6 being bouncier or a better shock absorber gas this first set of three traces show that sf6 would actually be a terrible choice as a shock absorber gas right because for a given load falling on it in this gas spring setup it has by far the highest peak force it's a pretty big difference to if you're compressing the gas very far so if you want a shock absorber gas argon is actually a much better choice it's smoothing out this peak force how about the claim that sf6 is bouncier now it's getting pretty interesting so these top three graphs are the integral of these over time so red is the integral of sf6 purple is the integral of air and this light green color is the integral of argon and it's literally just the area above these curves basically and this is the same drop event for each one and if you look carefully what's interesting is that these end up at different values so red is the lowest meaning the highest absolute value and light green has the least change so this means that sf6 actually pushed harder against the ground integrated over time than argon did which it first seems like a measurement error how could that be true if it was the same weight falling on it but if we go back to the high-speed video it's true that the the impactor actually bounced to a higher height with the sf6 than it did with the argon or the air and in fact it's lined up just as it looks on the screen here the sf6 balanced highest air is intermediate in argon was lowest so it is true sf6 makes bouncier gas Springs and we know that because it pushed against the ground harder for this rebound and the weight came up higher so it's it's literally true let's take a look at the physical reason for why these gases perform differently and then we'll finish up with a cool fire piston demo so if we compare a gas spring to a regular old metal coil spring we noticed that the force versus distance graphs are very different so for a metal spring it's linear until it hits the end in which the spring is like fully compressed and then it's basically just infinite force with no movement right but the point is that when the spring is actively working it's got this nice linear relationship but that's not how gas Springs work a gas spring has a hyperbolic profile where it kind of starts out less and then it gets to be higher and higher and if we measure sf6 air and argon we'll find out that they actually have different curves that have different amounts of hyperbola City to it basically so that argon behaves a little bit more like a metal spring where it's a little bit flatter whereas sf6 behaves much more radically where it's softer at the beginning of its travel but then it gets to be much firmer at the end and we've seen this in the experiment here that for a given input force the following weight it actually compresses the sf6 further so you could say that it's sort of softer but then the peak force is higher because we get further along down this curve so what's the physical reason for this change in behaviors what makes sf6 so different this is where it gets really cool so we know that when you compress a gas it heats up and when you heat up a gas it's volume increases or if it can't because it's contained then the pressure will go up and the answer is that when you compress sf6 it heats up less than argon or other gases for a given set up and we're assuming that you're compressing it quickly so that the gas doesn't have time to exchange heat with its surroundings and the reason for this is really cool if you have a single atom gas like argon for example pretty much the only way for it to store energy is to have these atoms moving around in the gas right so the speed of the atom is basically what temperature is and if you raise if you add energy to the gas the only thing it can really do is have the pressure go up and have the temperature grow up and temperature going up just means that these things are pinging around a lot faster but if you have a diatomic gas molecule like nitrogen pretend these are connected by a spring then the gas can store energy by having the spring oscillate like this or maybe even curving around like this or twisting against each other basically the more atoms that you have in your gas molecule the more ways that can store energy that don't contribute to just fast-moving pinging around so that when you add energy to the gas it can store that energy without heating up and if it doesn't heat up then the pressure doesn't rise as quickly so sf6 is actually a huge molecule it's got all kinds of different modes of storing energy and it's known as a low gamma gas because it has all these other modes of storing energy so when you put it into it the heat it doesn't heat up as much and the the constant the hyperbole lissa T is actually higher because it has this low gamma coefficient I know it sounds a little bit screwy here but I made some excel plots and was really happy to see that the theory lines up perfectly with the results that we collected here so low gamma gas means very hyperbolic behavior it's softer at first but then becomes much stiffer later on in its travel and for a given impact energy you'll actually get a higher peak force pushing back against that impact load higher gamma gases like argon for example behave much flatter they're less hyperbolic in this graph and for a given impact energy the peak force will be lower because it behaves a little bit more like a linear metal spring pretty cool that all this traces back to just molecular bonds basically is the basis for all this behavior now I should add that if you compress the gas slowly and it has time to exchange heat with its surroundings then everything behaves as if its gamma coefficient is 1 because that's basically this you know the ideal way that this happens and there's no sort of momentary effects caused by heating of the gas the cylinder that I'd built to test the different gases is actually very similar in construction to what's called a fire piston it's a pretty cool demo if I can get it to work the idea is that you take a little bit of really finely divided fuel like this piece of cotton that I pulled off the swabs there and put the fuel all the way to the bottom and then just put the piston in here and just press on it really hard just with arm pressure and we'll see if we can get the cotton to ignite pretty good actually I'm surprised that worked on the first try but that was pretty nice have to admit I put a little bit of oxygen in here too just to help my chances out but it did work and that was a really nice flash and a great way to end the video okay I hope you found that interesting see you next time boy

Channel: Applied Science

Views: 420,353

Rating: 4.9383612 out of 5

Keywords: sulfur hexafluoride, sf6, gas, physics, tennis balls, fill gas, nike air, ben krasnow, applied science, adiabatic, gamma, low gamma, compression, gas spring, hexafluoride, sulfur, science, tennis, balls, gas molecules, soft gas, spring

Id: TjiP8QIPews

Length: 21min 5sec (1265 seconds)

Published: Wed Jul 03 2019

Reddit Comments

[u/spoonguy123]

If you could pull a vacuum in a sufficiently large rigid structure, you could make a vacuum balloon airship!

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I wonder how magically light and ridged a material would have to be to get close, and whether anything in reality is close enough? Aerogel made in vacuum might fly? Neat to think about!

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Maybe Aerogel surrounded in a thin plastic layer to support a structure, then pulling a slight amount of air out.

[u/Stadiametric_Master]

SF6 is 3 times as dense as Argon, I wonder if that means it can absorb 3 times the heat energy, contributing to the differences in bouncyness. A comparison of all the noble gases (including Radon and Oganesson) would be interesting!