# Why are tennis balls sold in a vacuum-packed tube? **URL:** \ **Category:** Factual Questions\ **Created:** [July 1, 2000, 4:59pm UTC](https://boards.straightdope.com/t/why-are-tennis-balls-sold-in-a-vacuum-packed-tube/24286 "2000-07-01T16:59:41Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Ruffian](https://avatars.discourse-cdn.com/v4/letter/r/71e660/32.png) [@Ruffian](https://boards.straightdope.com/u/Ruffian)\ **Post date:** [July 1, 2000, 4:59pm UTC](https://boards.straightdope.com/t/why-are-tennis-balls-sold-in-a-vacuum-packed-tube/24286/1 "2000-07-01T16:59:41Z")
Is freshness that important for a tennis ball? It’s not exactly a flavor seal. So…why? ---
**Author:** ![Andy](https://avatars.discourse-cdn.com/v4/letter/a/aeb1de/32.png) [@Andy](https://boards.straightdope.com/u/Andy)\ **Post date:** [July 1, 2000, 5:45pm UTC](https://boards.straightdope.com/t/why-are-tennis-balls-sold-in-a-vacuum-packed-tube/24286/2 "2000-07-01T17:45:50Z")
On the few ocassions where I’ve opened a new tube of tennis balls, none were vacuum packed. Are many brands vacuum packed? ---
**Author:** ![Gunslinger](https://avatars.discourse-cdn.com/v4/letter/g/4491bb/32.png) [@Gunslinger](https://boards.straightdope.com/u/Gunslinger)\ **Post date:** [July 1, 2000, 6:01pm UTC](https://boards.straightdope.com/t/why-are-tennis-balls-sold-in-a-vacuum-packed-tube/24286/3 "2000-07-01T18:01:14Z")
After they’re exposed to air for a few months, they lose their bounce (They still bounce, just not as well). ---
**Author:** ![BobT](https://avatars.discourse-cdn.com/v4/letter/b/8e8cbc/32.png) [@BobT](https://boards.straightdope.com/u/BobT)\ **Post date:** [July 1, 2000, 6:37pm UTC](https://boards.straightdope.com/t/why-are-tennis-balls-sold-in-a-vacuum-packed-tube/24286/4 "2000-07-01T18:37:34Z")
Don’t they put new balls into play after seven games (I’m unsure on the exact number) during a tennis match. If someone like Sampras is serving with tennis balls fresh out of the can, they are almost impossible to return (or even see.) I’m pretty sure that the cans aren’t vacuum packed, but mainly designed to keep moisture and outside air away from the balls. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 1, 2000, 7:16pm UTC](https://boards.straightdope.com/t/why-are-tennis-balls-sold-in-a-vacuum-packed-tube/24286/5 "2000-07-01T19:16:28Z")
Tennis balls aren’t vacuum packed-- Actually, they’re pressure-packed. The idea is this: The interior of a tennis ball is at high pressure. Now, rubber is very airtight, but there’s still a small amount of diffusion of air through it. The rate of this diffusion is proportional to the difference in pressure of the different sides of the rubber, among other things. So, if you keep tennis balls under pressure, they’ll stay well-inflated indefinitely, but when exposed to the lower-pressure atmosphere, they’ll slowly deflate. ---
**Author:** ![jayron\_32](https://avatars.discourse-cdn.com/v4/letter/j/41988e/32.png) [@jayron\_32](https://boards.straightdope.com/u/jayron_32)\ **Post date:** [July 1, 2000, 9:27pm UTC](https://boards.straightdope.com/t/why-are-tennis-balls-sold-in-a-vacuum-packed-tube/24286/6 "2000-07-01T21:27:18Z")
Many high-end tennis balls aren’t filled with air, but with sulfur hexafloride gas. Why? Because it’s fairly inert (like nitrogen) plus it’s a MUCH heavier gas (one of the heaviest known inert gasses, IIRC). Any high school chemistry student should be able to tell you that according to Graham’s Law of Diffusion, heavier molecules diffuse at a rate inversely proportional to the square root of their masses; heavier gasses diffuse much slower, so to keep the balls inflated longer, they use this heavy gas. ---
**Author:** ![DAVEW0071](https://avatars.discourse-cdn.com/v4/letter/d/e19b73/32.png) [@DAVEW0071](https://boards.straightdope.com/u/DAVEW0071)\ **Post date:** [July 1, 2000, 9:45pm UTC](https://boards.straightdope.com/t/why-are-tennis-balls-sold-in-a-vacuum-packed-tube/24286/7 "2000-07-01T21:45:51Z")
Personally, I think they taste better fresh out of the can. ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [July 2, 2000, 9:05am UTC](https://boards.straightdope.com/t/why-are-tennis-balls-sold-in-a-vacuum-packed-tube/24286/8 "2000-07-02T09:05:12Z")
> [@](#): > > \*\* > Tennis balls aren’t vacuum packed-- Actually, they’re pressure-packed. The idea is this: The interior of a tennis ball is at high pressure. Now, rubber is very airtight, but there’s still a small amount of diffusion of air through it. The rate of this diffusion is proportional to the difference in pressure of the different sides of the rubber, among other things. So, if you keep tennis balls under pressure, they’ll stay well-inflated indefinitely, but when exposed to the lower-pressure atmosphere, they’ll slowly deflate. > \*\* Yup. They also sell little gadgets to put the bounce back in. They’re just cans with screw-on tops. As you screw harder and harder, your balls are subjected to increased pressure. Leave them in that state for a while, and you will once again have bouncy balls! Because air is forced back into them, I mean. What did you think I was talking about? You guys are sick. ---
**Author:** ![Chas.E](https://avatars.discourse-cdn.com/v4/letter/c/e99b99/32.png) [@Chas.E](https://boards.straightdope.com/u/Chas.E)\ **Post date:** [July 3, 2000, 8:43am UTC](https://boards.straightdope.com/t/why-are-tennis-balls-sold-in-a-vacuum-packed-tube/24286/9 "2000-07-03T08:43:51Z")
I should point out that there were brands of tennis balls that were sold without pressurized cans. I say _were_ because I haven’t played tennis in 25 years and I don’t know if they make them anymore, but I used to use them back when I was a kid. I think Dunlop made these balls, you just bought them in plastic boxes, not in pressurized cans, and they never lost their bounce. I don’t know how they did it, but they never caught on because they were kinda heavy and didn’t bounce quite right. They never lost their pressure, and they never lost their bounce, but the bounce was never quite right, even when brand new.