Limited pressure drop design

[Silence]

Unforunately, clear PVC is expensive. Clear non-PVC won't really work either since the pipe actually holds pressure, unlike in CPS water guns. But if you have a dark-colored piston, then a faint line may show through.

If you want a completely transparent fill gauge, build your own using a plunger and a spring in a small piece of pipe. That pipe could be clear PVC, as the smaller size would make it much cheaper. Or it could be another type of clear tubing, as there's lots of availability and higher pressure ratings in that size.

Or just buy a pressure gauge. It'll have to show the range of pressures in high precision since the pressure only drops 20% throughout the shot (and even then, it's hard to compare to the actual fill amount).

[cantab]

Any pipe can be given a pressure rating, it just seems PVC is easiest to find.
For a clear option, maybe PET would work. It's what's used for a lot of drinks bottles so it can obviously hold pressure.

[aEx155]

Any pipe can be given a pressure rating, it just seems PVC is easiest to find.
For a clear option, maybe PET would work. It's what's used for a lot of drinks bottles so it can obviously hold pressure.

I've never seen PET pipe, seeing as it's not very rigid; a search on Google though makes me think otherwise.

Isn't the pressure rating on clear pipe a lot lower than regular PVC? I remember Ben saying that he was going to try it for his CP45, but the pressure rating made him think otherwise; pressure rating must make the whole gun in a precharger APH...

As for fill gauges, I had an idea of one based in bike brake lines:



A wire is attached to the piston and is fed through a small piece of clear pipe bent around and connected to the air side. A spring pushes the end of the wire to the end of the gauge (leftward in the picture) keeping it taut. When the piston moves left, it pulls on the wire, making the gauge seem empty, while as it fills, the spring pulls on the wire, filling up the gauge.

The small piece of clear pipe keeps the price down, while the wire allows it to be bent around to make it smaller. The only problem would be that you would have to find a way to put an elbow in a reducer or something similar, otherwise this wouldn't work.

[Silence]

Clear tubing is usually either much weaker or much more expensive. Clear PVC can be strong, but it's not usually as strong as regular PVC and it's very expensive. Other types of tubing are unsafe.

aEx, in your diagram, you could just use a rod that slides in and out of a clear tube. It doesn't need to double back, unless the chamber is too long.

[Specter]

I just got my new US Plastics Corp catalog today. 2008 Volume 6.
They have a huge selection of clear PVC. They have charts with size/diameter, Working pressure PSI @ 73*F, ID, OD, Wall Thickness, and Price. Just ask me what size you would use for the gauge and I'll let you know the info. Also comes in Schedule 80 or 40

[SSCBen]

With CP45 I was considering some clear 4 inch pipe, which has a pressure rating of 110 PSI according to Harvel Pipe (well, at least their clear PVC does, maybe what Specter's catalog says is different). For something like this I'm considering 2 inch pipe which has a rating of 140 PSI. Using this pipe would be working somewhat close to the pressure rating.

@aEx155: Interesting idea. I don't think a spring is necessary so that would simplify it. With the right tubing it could work and give you a better pressure rating.

Specter, what's the price and pressure rating for clear 2 inch pipe from US Plastics Corp?

[cantab]

Using this pipe would be working somewhat close to the pressure rating.

Pressure ratings normally have a safety factor of 1.5 or more anyway, to guard against minor damage or transient spikes. So I wouldn't worry too much about working close to, or even at, the rating.

[SSCBen]

I'm not worried much about explosions or anything similar. While I'm unsure as to what "burst pressure" means (I assume it means pipe failure), for PVC the burst pressures are significantly higher than the pressure rating, so the pipes are definitely safe. My main problem with clear PVC is the price but at 2 inches I think it'll be more reasonable.

I suppose the main thing that's wrong with this is that I've never really gone close to a pressure rating before, though, I have pressurized non-rated things (vinyl tubing, DWV PVC, etc.) so there's really nothing to worry about.

[Specter]

Specter, what's the price and pressure rating for clear 2 inch pipe from US Plastics Corp?

I made a chart in Excel with all the info. I'm just about to make a thread with it.

[C-A_99]

2 in. seems too small to get any useful capacity in. Is gauging the volume of the water absolutely necessary? Will trying to pump the water beyond the PVC bend break something? Or does pumping just stop? If this actually displaces the piston at the bend, we need a better solution.

[SSCBen]

With 20 inches of pipe you can hold about 1 liters of water, so I think 2 inches is adequate for a simple design. It's what I'm planning for the contest's water gun, which will be a test for the most part.

Gauging the water volume isn't completely necessary but it's helpful. You of course can still use the water gun without a gauge but I think it's helpful to know if you have to pump.

The way I'm going to design this water gun the pump will literally become rock solid when the piston reaches the end of the pipe. I'm going to use a simple bushing to block off most of the pipe so it literally can't go any further.

[aEx155]

2 in. seems too small to get any useful capacity in. Is gauging the volume of the water absolutely necessary? Will trying to pump the water beyond the PVC bend break something? Or does pumping just stop? If this actually displaces the piston at the bend, we need a better solution.

You could always just add something to stop the piston, otherwise, when the piston reaches the bend, it's probably going to start leaking as it's not completely on a flat, cylindrical surface anymore. You probably could drill, tap, and put in some bolts (with teflon tape and/or rubber o-rings/washers to seal is necessary) to act as stops for the pistons.

When the piston reaches then end of travel, whether it be stops or something else that prevents it from moving further, the pump would instantly get solid, and you wouldn't be able to pump any more. You would have to fire some water to push the pump in further if it was extended too much.

EDIT: Didn't see Ben's post; we pretty much said the same thing (just differently)

EDIT #2:

With 20 inches of pipe you can hold about 1 liters of water, so I think 2 inches is adequate for a simple design. It's what I'm planning for the contest's water gun, which will be a test for the most part.

It may seem stupid to ask this, but what contest are you talking about? I haven't heard of anything recently, and I was wondering if I could get some details on it.

[Silence]

It's a water gun review contest in the News forum. We only have one submission so far, so you've got a good chance of winning if you send something in. Sending in multiple entries will probably help too.

[cantab]

I suppose having an opaque pressure chamber and/or reservoir with no gauge isn't that bad, given there are plenty of guns around that are such. Especially in the UK, where I think laws are different - lots of guns where the US version has a clear reservoir the UK version is opaque, such as the XP240.

[C-A_99]

I'm mostly used to using guns w/o pressure gauges and personally don't think they're really necessary unless the gun requires long pumping (over 4-5 seconds or about 3 pumps) to recover from just a tap shot. That said, it could be useful on the LPD if one decides to pre-charge it to a higher pressure level, and though the number of pumps would stay the same (given one pump length, diameter, etc.), each would be slower and more difficult. This could get to be a problem here since I have had terrible luck getting custom pumps to seal and move easily at the same time. Its never happened before here, the PVC often inconsistent has inconsistent diameters, and I haven't really figured how to cut notches into the pump rod for the O-rings. (which will be pretty much absolutely necessary)

For stopping the piston, the method mentioned by aEx is a bit cumbersome and could be unreliable and/or easy to break, and isn't good for the pipe since it creates weak points. Perhaps it would be possible to use a coupler and place a washer between the 2 pieces of pipe. I'm not exactly sure what Ben means by the brushing though. Either way, the method should maintain air flow and be reliable.