Water Barrier Device?

[ramius-ii]

Hi Guys!
I am not exactly sure what to call what I am looking for or where to find it! I have a pressure sensor for measuring depth that only wants to see dry air. How or what would I use to "isolate" water from the sensor? I am using 1/8" tubing and would like it to be as small as possible.

Thanks, Ed

[salmon]

Re: Water Barrier Device?

Ed,
I am speaking out of no knowledge on this topic, so please carry this suggestion with that in mind. You might be able to use a balloon or rubber sheet to cover the end of the tube. The rubber will, in theory, bulge in as you go deeper and allow the creation of the pressure in the tube. That being said, if you are using a piston or a system that adds pressure in your wtc, then this pressure unit will need to be isolated from those effects or the balloon or rubber dam will bulge out.
Peace,
Tom

[Ralph --- SSBN 598]

Re: Water Barrier Device?

If using a rubber balloon or sheet to protect the sensor you may need to take in to consideration the pressure needed to deflect the rubber as well as the water pressure.
This may be as simple as recalibrating the sensor to include the pressure needed to deflect the rubber.

Working with in a 1/8" tube will be difficult.
Could make a chamber .... say 1" in diameter and as long as needed to put the sensor in.
Connect the chamber to the outside with a small 1/8" hard tube.
Divide the chamber in two with a rubber sheet keeping one side with water and the other with air.

[ramius-ii]

Re: Water Barrier Device?

Thanks Guys!
Good suggestions! The main problem is I am working with very low pressures, less than 1 psi and I am not sure the ballon will even bulge! Is there something that uses maybe a mineral oil barrier?

Best, Ed

[salmon]

Re: Water Barrier Device?

I was informed by a person whom I respect and has been doing this a long time and he said "Fill the tube with silicon oil; make the tube at least eight inches long -- water will never reach the sensor itself, but will compress the air between the water and the sensor to the ambient pressure. Capillary action keeps the water bunched up within the tube."
Peace,
Tom

[Guest]

Re: Water Barrier Device?

You can also just run the tube dry, and configure it into a u-bend, like a manometer spun 180 degrees.

No good if the boat is going to do aquabatics though!

[ramius-ii]

Re: Water Barrier Device?

Thanks!
Both are good ideas! The sensor needed to be low pressure as it is to measure the speed of the sub. I am using a differential sensor with the high pressure side in the bow and the low pressure side inside the bow to compensate for depth. There do not appear to be low pressure sensors that will measure water pressure in PSI.

Ed

[JWLaRue]

Re: Water Barrier Device?

Perhaps instead of measuring a pressure for determining speed, you could count prop shaft revolutions using a Hall Effect sensor. Some relatively simple trial runs would establish the ratio of turns-to-speed.

-Jeff

[ramius-ii]

Re: Water Barrier Device?

Good idea and it is a Typhoon with two motors. I have everything programmed into a PIC just concerned about sensor damage.

[mucklestone]

Re: Water Barrier Device?

I would think some sort of wheel in a tube would be better, and this way it is not effected by depth. Measuring the props may not be accurate, they may cavatate in different conditions and give unrealistic readings. This is how it is done on full size boats.

[ramius-ii]

Re: Water Barrier Device?

Thank you and with a differtial sensor you measure the incoming pressure (velocity) on one port (high side) and the "static" pressure on the other port (low side). The incoming pressure (velocity) is basically pushing down on a diaphram while the static or pressure at depth pushes up thus compensating for depth! What is great is PSI = speed!