Introduction, First Question, and APH Design

[Ben]

Sounds good. I'd be interested in seeing some 3D model.

Drenchenator measured out the dimentions of many PVC couplers and such with a digital caliper. Perhaps I could ask him for the dimentions to post here. I am sure this information will aid in many designs.

I think after a while you'll think that the 2-inch pipe is much too small. After I became more familiar with PVC, even the 4-inch pipe doesn't seem that big to me anymore. 6-inch, yes, is very big! But 4-inch seems a lot smaller for some reason, likely because I have more experience with it.

[all_washed_up]

Well, I don't know, maybe I'm going too small, but the thing is still going to be pretty big for neighborhood water fights. Here is a pic, with three elements still needing input:


For a sense of scale, the gun is pretty much all 2-inch and 1/2-inch pipe. The grey pipe is threaded nipples, which probably added a fair bit to the cost, but made dry fit easier and will make it easier if I want to disassemble for modification.

I have yet to complete the pump because I'm not sure how long to make the pump chamber, and I have not put the top on the pressure chamber, because I'm not sure how big to make it. Smaller would be better for the overall balance of the gun but I want enough volume for a good shot. Also, I have put a short barrel on, both for the look of the gun and because I see no reason for empty pipe in front of the trigger.

So, any tips on 1) pump length, 2) pc size, or 3) barrel length?

Thanks,
a_w_u

PS: I suppose I could replace the second water chamber (in the rear of the pic) with a 3-inch or 4-inch version. That would not increase the profile of the gun but would significantly enlarge tank volume.

[Ben]

PC SIZE: The reservoirs are rather small. With about a 6 inch chamber, you could probably get 4 to 5 full shots off without refilling. I would suggest making the reservoirs larger in length or diameter or perhaps adding a few more (you know what I mean already). Then, you could make a larger pressure chamber. A single 6 inch chamber 50% filled (accounting for the air in the chamber) is only about 150 mL of water. Of course, you can get the water level higher and probably get maybe 250 mL of water in if you try hard (don't quote me on that though because it's just a guess). Two 6 inch chambers would make a respectable 300 mL at 50% and three of course would make 450 mL. I personally would shoot for pressure chambers in the 500 to 700 mL range in air pressure water guns, but that is my personal opinion at the moment. Just do what you prefer.

PUMP LENGTH: The pump should be at least a little less than a foot long. I never make the pumps I use shorter than 10 inches. I typically make them about 14 inches long at first and test them out, cutting pipe off if it feels too long. Experimenting to find what you are comfortable with is important here. Remember, in a pump you can cut off pipe, but you can't add pipe on as easily. You can cut the pipe at the end and add a coupler, but it would be best not to do that because that adds dead space.

BARREL LENGTH: Not longer than 6 inches on pipe diameters of 3/4 inch or less (and of course longer on the larger diameter pipes). The barrel right now looks fine. Long barrels are a bad idea. There is a disadvantage, mainly lost flow. I did test long barrels and noted slightly reduced performance, though I did not do a very in-depth study. There is no advantage to using a barrel aside from a minor stream "straightening" effect. For the most laminar (straight essentially) flow, smaller pipes should be used. That is why some nozzles have what appear to be a honeycomb in them. Those smaller pipes line up the water better than a larger one. And they're not very long.

[Silence]

I actually prefer somewhat shorter pumps if they help the design. You're still moving the pump the same distance to fill the PC...more frequent, but shorter, cycles.

I agree the PC is fairly small. You could add another PC if you want the low profile. You could replace the top tee with a cross and add a PC above it. Whatever floats your boat.

I agree that barrel length is pretty much insignificant. Longer barrels don't really help with lamination unless the tubing is small.

Looks like a fun water war!

[all_washed_up]

Great feedback, thanks. The pc pictured is just the base. I'll play around with measuring volume before I add the pipe and cap on top. I'd rather not go too high, but I'd also rather not add a second pc.

I am thinking that I'll enlarge the second water chamber, in order to add more volume. Either 3-inch or maybe 4-inch.

Thanks again for the great feedback. I'm pleased to be making improvements before actual construction.

a_w_u

UPDATE: Sheesh, the 3-inch fittings are expensive! $10 just for two endcaps. Anyway, I'm going to use 3-inch for the second water chamber. I thought about switching to that for the pc too, but it's too expensive and I don't want to have to change that many fittings. I'm hoping for about 500 ml in the pc. I hope that'll be good.

I'll probably change out the dry fitting today, if I can find some time. Maybe this evening. Thanks again for the tips.

[all_washed_up]

Okay, the gun is finished! It looks like the pic above, except that the PC is about 7-8 inches high, the rear water chamber is 3-inch and there is a pump rod and handle sticking out the front.

I'm pretty happy with the result. I'm hoping to get more laminar flow with a mod I'm hoping to get done in the next day or two. If it improves things, I'll post about it.

Also, if I get to it, I'll post some stats. Range and laminar flow not as great as I'd like, but all in all, I'm happy with my first effort.

[all_washed_up]

Well, I've got a couple of things to add. First, a caution to others. The ID of 1/2-inch threaded PVC nipples is narrower than the 1/2-inch pipe. So I had a terrible time making a decent pump, becuse I could not find o-rings to fit the ID well. I ended up cutting off the nipple and gluing a connector and some pipe on in its place. Afterwards, I was able to build the pump just fine, using 3/8-5/8 o-rings and some 3/8-inch copper pipe epoxied in place to hold them.

I said something earlier about modding for a more laminar flow. I tried adding some straws, but it actually made the flow less laminar. So I removed them.

So things are good, except that my range is only 27 feet or so; I expected more. And I can only pump about 7 or 8 times before I can pump no more. I suspect that's because the PC size is no more than maybe 6 or 7 times the size of the pump chamber.

Any suggestions for increasing range or shot time? I'm thinking maybe I'll put on a second PC, right behind the first. But that probably won't increase range, will it? I don't know...

Anyway, thanks for any additional suggestions.

[Silence]

What are you using for the nozzles? That's my primary concern, although I'm also curious as to how you did the pump.

[all_washed_up]

The nozzle is a threaded cap on 1/2-inch pvc nipple, as in picture earlier in this thread. I tried three different hole sizes, with 5/32 doing the best. As for the pump, it's pretty much as described by Doom.

[Ben]

Could you take a picture of the finished product? I think that would help in fixing your range problem.

Typically errors are made in designing the pump or pressure chamber systems that result in poor range.

Indeed, in the first "modern" homemade water gun design made by Pepper in 2003, he got poor range due to the use of a 1 inch diameter pump. With a pump that wide, building up pressure is difficult unless you are a bodybuilder (if you are, that might work great for you!). Smaller pumps build up pressure more easily due to the force of pressure (smaller areas mean less force needed). The problem is that smaller diameter pumps build up pressure more slowly. People look for a balance between difficulty and speed. That area is around 5/8-inches for most people.

If the pressure chamber is too small, pressure can be built up quickly, but the water stream is not substantially enough sized, resulting in a stream that is easier for drag to break up. The solution is obvious: a larger pressure chamber.

It doesn't sound to me that you have either typical problem however. I'll ask a few more questions though.

How strong would you say you are? An ideal nozzle diameter of 5/32-inches isn't too big. That indicates to me that the pressure is low. One fairly common way to create low pressure is a lack of strength.

Also, are you getting any leaks from the pump? I've been told the electrical tape holding O-rings in can leak.

Hopefully we can help you diagnose the problem to either fix in this water gun or your future ones.

[all_washed_up]

Ben, I'll follow up on this during the last week of July. I am otherwise committed till then.

Thanks for your reply.

D