Flexible Ballast Valve

[captain nemo12]

Hey guys,

Earlier in this forum I started a thread discussing the flow rate of peristaltic pumps. However, after doing some calculations I've decided to go with a gear pump instead. After locating a suitable unit, I ran a test today with a blood pressure cuff bladder:



Since for this test I did not use a check valve between the pump and the bladder, after shutting the pump off I expected water to come gushing out due to the bladder's elasticity wanting to it deflate but to my surprise, it stayed in this state for a quite a long time... Examining the pump intake and the inner diameter of the rubber hoses, the latter being about 1/8'' (3mm), it seems that water is leaking very slowly out of the hose.

I realize that this is a pretty nice built-in safety system since it will cause the sub to eventually rise to the surface, if no pumping input is applied for a long time. The question is, if the ballast intake is to be located about 1 inch below the water surface through the WTC endcap, will I even need to install a valve to prevent water from entering the bladder when it is empty? The inside of the WTC will be at standard atmospheric pressure (101.3 kPa), will that be enough to counter the outside water pressure at 1 inch below the surface?

[JWLaRue]

Re: Flexible Ballast Valve

I think that you'll want something to prevent water from coming in the intake. (been there, done that....got the water in the ballast system to prove it) The intake will allow water to seep into the tube...not a lot, but it will happen.

-Jeff

[captain nemo12]

Re: Flexible Ballast Valve

Hi Jeff,

Do you remember how much water got into the ballast tank by any chance? Was it enough to upset the trim of the boat?

Thanks!

Nate

[JWLaRue]

Re: Flexible Ballast Valve

After two dives there was about a half a teaspoon. Whether that upsets the trim of the boat depends on the size of the boat. In my case it did not (a 1/32nd scale Type II), but I was not happy about the water being pushed through the pump and the possible damage it could do.

-Jeff

[Guest]

Re: Flexible Ballast Valve

A geared pump will offer a lot of restriction to back flow. But your bench test leaves out an important factor- as you fill the bag in a sealed wtc, the air inside will compress, and that pressure will push the water out through the pump, which is why you need a valve to check the flow. The valve isn't there to prevent water getting in, not near the surface at any rate.

Try putting a couple of bags of sugar on top of the bag, and repeat your test.

BTW, don't run those pumps dry, and try and keep the voltage to under 5 volts. I trialled a couple of those pumps and found the plastic used for the gears isn't really up to the job- it's rather soft and tends to melt if the gears aren't kept wet. If you run it slower they work okay. Shame they didn't use brass gears.

[JWLaRue]

Re: Flexible Ballast Valve

Ahhhh...just realized that you are pumping water not air. In my case it was air.....so it doesn't apply to your original question.

-Jeff

[captain nemo12]

Re: Flexible Ballast Valve

Thanks for the input guys!

I did a little bit of research and it seems that my pump is the same as the one sold by Caswell. I was lucky enough to find a picture showing the internals of the pump on their website:



Indeed, the gears are plastic and from the picture, it looks like the idler gear is spinning on a plastic shaft too. The website recommends running on 6 to 9V but I guess that's a little bit too much? I was testing the pump on 7.2V and it was pumping quite fast!

Nate

[Guest]

Re: Flexible Ballast Valve

Yes. IMO these pumps are designed for use on 4-5volts, no more. They run on higher voltages, but if they run dry, they get into trouble.

it's a shame, because the design of the pump is compact and they shift a lot of water, they just chose a lousy material for the gears!