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Thread: WTF!?@ Strip inside pex pipe for arch is too long?

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    Default WTF!?@ Strip inside pex pipe for arch is too long?

    I am pretty adamant about the old saying measure twice, cut once. First time using strip inside pex pipe for an arch after just zap strapping bullets to the outside of the pex previously. I took my strip, cut it to length (dividing the strip roll into 3 parts for 3 arches). I measured the length a few times, then measured out the same length on the straight piece of pex twice as well, marked it, then I even took the strip and lined it up exactly with pex to make sure this was accurate before cutting to make sure I didn't waste the pipe. Fast forward to the pex now in an arch shape, I slide in the same strip used to mark originally.....And the strip is sticking out a good 3-4 inches on one side!!? Maybe I'm rusty on my geometry, but the length should not change, it's still the exact same piece of pipe. So what the hell is happening here? The only I could come up with is that i'm measuring it from the outside of the pex, but when it's bent into an arch shape, the strip is now almost 1" lower that the original measurement taken from the outside, so while the distance difference may not be large at any one point, the cumulative difference between the outside pipe measurement and the inner arc may add up to that extra slack. Having said that, I've never come across anybody stating you should cut the pipe longer/strip shorter to factor for this.
    arch.PNG

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    Default Re: WTF!?@ Strip inside pex pipe for arch is too long?

    Good thing PEX is inexpensive.

    Iíve never noticed this before, as my tubing was already fairly longer than the LED strip (to account for extra tubing for the connections and the part that is covered by a clamp).

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    Default Re: WTF!?@ Strip inside pex pipe for arch is too long?

    Quote Originally Posted by Bwinter View Post
    Good thing PEX is inexpensive.

    I’ve never noticed this before, as my tubing was already fairly longer than the LED strip (to account for extra tubing for the connections and the part that is covered by a clamp).
    I know the simplest explanation is mis measuring, but in this case, I know I did my due diligence. The more I think about it, the more I'm convinced that the distance on the outside of the pex pipe could be affected vs the distance traveled over the inside bottom of the pex about 1" lower, the very rough drawing in my post explaining. But yeah, I was trying to run the strips pretty close to the ends. Anybody else noticed this?

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    Default Re: WTF!?@ Strip inside pex pipe for arch is too long?

    Stretch & compression while bending pipe , Plumbing 102 or 103. The outer part of the arc will stretch while the inner part will compress .
    Voila !

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    Default Re: WTF!?@ Strip inside pex pipe for arch is too long?

    Quote Originally Posted by angus40 View Post
    Stretch & compression while bending pipe , Plumbing 102 or 103. The outer part of the arc will stretch while the inner part will compress .
    Voila !
    Bingo. A more eloquent way of describing. I had just never come across anyone mentioning to take that into account. It was always just measure and cut

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    Default Re: WTF!?@ Strip inside pex pipe for arch is too long?

    Quote Originally Posted by XmasinVancouver View Post
    Bingo. A more eloquent way of describing. I had just never come across anyone mentioning to take that into account. It was always just measure and cut
    I knew about this before, and math'd the heck out of it. I was still off.

    Now I'm like cut first, then measure and make it work after!
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    Default Re: WTF!?@ Strip inside pex pipe for arch is too long?

    Quote Originally Posted by deonb View Post
    I knew about this before, and math'd the heck out of it. I was still off.

    Now I'm like cut first, then measure and make it work after!

    Anyone have a formula or rule of thumb for this short of having your strip end farther inside ?

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    Default Re: WTF!?@ Strip inside pex pipe for arch is too long?

    My pipe (hdpe) was already bent but I put a strip in just to make sure if I cut in half, it would fit inside the pipe. No rule but circumference formula probably could help if you arch is a circle. If it is not a circle (or half) than you just have to guess and fix.

    I would imagine you are getting more compression than expansion. So, for arguments sake, let say you have have 1" pex and a 30" diameter. That is 47" for the half circumference. But you really have 28" (30 - 1" left - 1" right [ya ya - we are rounding]) on the inside of the pex so you have 44" for your circumference. There is your 3" difference.

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    Default Re: WTF!?@ Strip inside pex pipe for arch is too long?

    Quote Originally Posted by MikeKrebs View Post
    My pipe (hdpe) was already bent but I put a strip in just to make sure if I cut in half, it would fit inside the pipe. No rule but circumference formula probably could help if you arch is a circle. If it is not a circle (or half) than you just have to guess and fix.

    I would imagine you are getting more compression than expansion. So, for arguments sake, let say you have have 1" pex and a 30" diameter. That is 47" for the half circumference. But you really have 28" (30 - 1" left - 1" right [ya ya - we are rounding]) on the inside of the pex so you have 44" for your circumference. There is your 3" difference.
    Thanks Mike! If the circumference of the arch (half circle) is 47", where is 30 coming from? I mean 30- 1" clearance x 2=28", But what formula is giving you that 30?
    Last edited by XmasinVancouver; 10-19-2019 at 04:49 PM.

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    Default Re: WTF!?@ Strip inside pex pipe for arch is too long?

    Quote Originally Posted by XmasinVancouver View Post
    Thanks Dan! If the circumference of the arch (half circle) is 47", where is 30 coming from? I mean 30- 1" clearance x 2=28", But what formula is giving you that 30?
    Well I basically took the original idea, which really didn't require anything special as you pointed out. Took the diameter measurement from outside to the outside of each end (38"), then then inner diameter would be 36" from subtracting 1" for each side to account for the 1" pex pipe. When you plug those in to the basic circumference formula, treating the two diameters as separate calculations (3.14 x 36) / 2 , the end result is a little over a 3" difference, so there it is! For some reason I was thinking I needed to calculate something based on the pipe as a single unit with the two different numbers, instead of falling back to basic trig and treating them as if you had a separate arch that was slightly smaller inside the other. Do most people terminate their strip a little higher into the pipe? It just seems odd that with all the tutorials, videos etc... you find, I've never heard this mentioned. lol.

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