Proper LRT and Case Length

[Poseidon2000]

If I wanted 2 liters of PC volume, ho long would my Case and LRT have to be? I heard somewhere that 15.5 inches of LRT is good, Ben said. How about case? IDK.


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[Drenchenator]

This depends on the LRT's inner diameter. For standard 3/8 inch ID LRT, I'll assume that it expands 8 times in diameter and twice in length (these assumption may not be completely correct, but we'll still get an approximate and reasonable answer with them).

8 times 3/8 is 3 inches, which comes out to 7.069 square inches of internal area. 2 L is 122.047 cubic inches, so divide 122.047 by 7.069 to arrive at 16.040 inches in length when expanded. Assuming the LRT is only expanded 100% (twice the unexpanded LRT length), the unexpanded LRT would be at least 8 inches in length.

So, for standard 3/8 inch IDT LRT, to get a 2 L PC, you need a piece of LRT at least 8 inches in length, and a chamber at least 16 inches in length to contain it. Again, if you change the assumptions I listed at the top, you'll get a different answer, but this is at least in the ballpark.

[Poseidon2000]

hmm... Thanks! so I would have to follow that rule of 2L's cubic inches / the LRT's size?

[Fishfan]

I thought LRT expanded three times in length. If it expands only twice in length, then I only need a 2 1/2 ft case on my CPH, not a 3 ft case. My LRT is 1 foot long, so I would get a 2/12 length case to make up for the length of tee, barbs, and endcap.

[Poseidon2000]

It could, but that would kill it's lifetime.

[Drenchenator]

Fishfan, you're right. LRT can expand up to 3 times in length, but I assumed 2 times for a slight safety factor. That doesn't mean that 3 times in unsafe, though. 3 times is fine too.

Poseidon2000, yes, you can just follow the same principle that I used. I calculated the area of a circle with the LRT's ID in square inches, and then I calculated the total volume of 2 L in cubic inches (if you don't want to do the conversion, but type "2L in cubic inches" into Google, and it will convert for you). I then divided the volume by the area to get the length. But remember: this is all approximate. These calculations assume that the LRT is a perfect cylinder, which obviously isn't the case. That's why I said at least 16 inches, since you have to account for tubing barb and other factors.

[soakinader]

Wait, so what's the deal here? Let's say that, no matter what, you fill the chamber up to 2L. What is the difference between a 3 foot long chamber with X diameter and a 2 foot long chamber, which would prevent linear expansion, and force outwards expansion to Y diameter, which is wider? Does one variation give more power/ more life span? Also, is this based on CPS blaster tech? I think my CPS 3000 and Monster had around 3:2 expanded-unexpanded length.

[Drenchenator]

Neither case is wider. If you look at LRT as it expands, you see that it first expands in diameter, and then in length. The first few pumps into the chamber give a really bulbous, balloon-like figure. It expands mostly in diameter for those. After that, it starts to fill up the entire bladder and expand in length more and in diameter less. The expanded diameter of a standard 3/8 inch ID LRT tube is around 3.5 inches regardless of the length, though in theory a longer tube may taper off slightly (example).



A quick example. This chamber isn't completely full. But it's obviously expanded more in diameter than length. First diameter, then length.

I said I assumed 2 times in length for a safety factor. That's mostly to increase the life span. LRT has a limited number of full expansions, and 2 times will increase the life span somewhat. Neither 2 or 3 times expansion gives more power, but 3 times means 50% more water in the chamber. It's a trade-off. If you'd like more water, design with 3 in mind. If you'd like a longer life span, design with 2 in mind. I also like 2 times because it makes the gun smaller and helps you use less materials.

[soakinader]

That information is very helpful, thanks. So the LRT fills up just like a long balloon....

I keep hearing people mention "life span". I certainly don't know how long the life span of your average CPH is, but how long has a/several 2X expansion/s lasted for you?

I really would like to make a homemade, but if the LRT only lasts for 100 full shots, it would dampen the practicality a bit, and I would be less enthusiastic about it! I mean, I have probably got close to 50 full shots using my trusty CPS 1500, and I can't imagine it breaking anytime soon.

[Drenchenator]

Ben and I have had LRT last for years and hundreds of expansions. We've only had it burst twice: once from overuse, and a second time from twisting adding additional stress.

For the most part, I don't worry about the bladder's life span. It's really quite long. I don't have any data on how long the average LRT bladder lasts, but it's long enough to not matter too much. For a stock gun like a CPS 1500, I wouldn't be worried at all, since the bladder doesn't seem to expand all that much anyway.

[soakinader]

Sweet. Now if only I had the time... I will stick with my current project for now, but later I would like to mess around with some designs. Has anyone ever tried making a homemade with multiple seperate chambers? Like a Power Pak except more linear, less restricting - maybe even with 2 chambers or 3 in a triangle?

[SSCBen]

To add to what Drenchenator said, the common 3/8 inch ID latex tube used for water guns generally will not burst when not abused. This, however, is not true for all tubes. Generally, it seems the higher the pressure the tube, the lower its life. I've done tests with tubes that burst after a single use. I specifically suggest against using these tubes in the LRT guide on SSC for this reason.

However, full expansions of latex tubing can case the tube to degrade, slightly reducing its operating pressure with each shot. Water gun companies limit the expansion of their bladders to avoid this problem. You can directly feel the effect of the wear on the tube by feeling it until you reach a point that you never expanded. That point will be noticeably harder to squeeze. There's no data available on how quality a tube will wear out, so do some experiments and share the results if you want to know. It won't take more than a half-hour or so to fill an empty a tube with a pressure gauge attached to see how the pressure drops as the number of cycles increases.

Also, check out this research paper on rubber tubing expansion for more information about how these tubes expand. Some of you are familiar with Hooke's law for the force springs provide. This paper uses a more complicated model (the Gent hyperelastic model) for the rubber expansion as rubber doesn't follow Hooke's law. If you don't have a background in engineering, this paper might be useless to you, but if you're good with math then Wikipedia should be able to provide any other information you need to understand the paper.

[Poseidon2000]

Wow. Welcome back, Ben!


Thanks for all this info.

[Fishfan]

Wow, didn't realize Ben had posted here. Hey, I built a SuperCPS according to your guide. It was awsome, until the bladder broke. I used the wrong lubricant, teflon silicone from dupont, which had some petroleum byproducts in it. I'm trying to get all of that lube out so I can put some silicone grease on and not have that problem.
Nice to uh... "see" you Ben.