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Is one allowed to pump air into an inflatable mattress on Shabbos? Is this a problem of tikkun cli, makeh b'patish, etc? (Or any other problem.) What about if the mattress was inflated Erev Shabbos and some air came out and now one wants to add some more air to make it "harder/stronger" on Shabbos. Would this be any different (if the first case is ossur.)

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One may not inflate a mattress for the first time on Shabbos, as this would violate makeh b'patish. However, one may inflate or reinflate a mattress on Shabbos as long as it is not the first time the mattress was inflated, since deflating a mattress that is not being used is the standard mode of use. Therefore, the mattress is not considered broken when it is deflated (Shmiras Shabbos K'Hilchasah, 34:24). Note that this does not apply when you seal the air in with a knot, as that would violate kosheir, although you may use a rubber plug or the like to seal the opening (ibid., 16:8).

See also this article that cites opinions variously agreeing and dissenting with this view; some opinions forbid inflating mattresses because "new air" is used each time, which they maintain introduces a makeh b'patish problem any time it is reinflated. R' Shlomo Zalman Auerbach, disagrees with this rationale and writes that all of the air in the world is considered one entity (see footnotes 2,5, and 7 in the article, as well as Shmiras Shabbos K'Hilchasah, 34:24, footnote 95, for citations of various opinions).

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But a bow-knot or some other non-double-knot might still be ok (ie he doesn't say this is a unique case of kosheir)? –  Double AA Sep 23 '12 at 17:15
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Regarding balloons, even a knot that is not a kesher shel kayama would be assur rabbinically since knots on balloons are typically left tied for several days (Igros Moshe, Choshen Mishpat 2:47:3; see also Magen Avraham 317:20). However, since inflatable mattresses are frequently deflated within a day, I would assume that a bow-knot that is made with the specific intention to last less than a day would be acceptable. Practically, however, I question whether acceptable types of knots would be strong enough to keep the air in, especially with the weight of people on the bed. –  Fred Sep 23 '12 at 17:42
    
Great sources, thanks! How did you find the article from the Daf Digest? –  Yehoshua Sep 23 '12 at 19:29
    
@Yehoshua Google the keywords "melacha", "inflatable", and "pillow". –  Fred Sep 23 '12 at 19:39

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