pressure gauges

[zetreque]

Does anyone happen to know of a cheap/low cost pressure gauge I can either buy or make that can measure pressure up to 900 psi under water?

[CanadysPeak]

Call Omega Engineering and ask to talk with an applications engineer. If they have no love, do the same with Dwyer.

[zetreque]

I'll check them out. Pretty sure it's going to be pricey though.

[Dave_Oblad]

Hi zetreque,

Are you trying to measure water pressure? Want a Gauge Meter or Electronic Meter? What will you be using this info for? Robotic submarine or wrist band for a Diver? Need more info. 1000 PSI is about 1/2 mile under water.

Regards,

Dave :^)

[zetreque]

reading pressure under water as the gauge gains depth. It can be visual (looking at it) or electrical digital signal can probably be an option.

[CanadysPeak]

I'll check them out. Pretty sure it's going to be pricey though.

Any inexpensive pressure gauge will give psig rather than psia. You could always use a reference pressure and 2 cheap gauges. You can get pretty inexpensive gauges off old pressure washers - they go up to 1800 psi or above. You can make a simple reference with a 1 atm cast iron sphere, like a Magdeburg sphere except clamped shut.

[zetreque]

I'll check them out. Pretty sure it's going to be pricey though.

Any inexpensive pressure gauge will give psig rather than psia. You could always use a reference pressure and 2 cheap gauges. You can get pretty inexpensive gauges off old pressure washers - they go up to 1800 psi or above. You can make a simple reference with a 1 atm cast iron sphere, like a Magdeburg sphere except clamped shut.

Not sure I understand. The gauge is submersed in water so the needle side is open to the water environment, and the liquid side is sealed to the iron sphere? A pressure washer gauge would be opposite. The hose side is connected to the fluid/liquid and the gauge side is open to the atmosphere.

Actually, it doesn't matter if the pressure is acurate at all. I just need to be able to tell an increase in pressure as depth increases, and it needs to be able to read up to about 900 PSI


How well would it work if I did something like encase the needle atmosphere side inside a clear acrylic epoxy or something and just left the liquid side open to the water?

[CanadysPeak]

I'll check them out. Pretty sure it's going to be pricey though.

Any inexpensive pressure gauge will give psig rather than psia. You could always use a reference pressure and 2 cheap gauges. You can get pretty inexpensive gauges off old pressure washers - they go up to 1800 psi or above. You can make a simple reference with a 1 atm cast iron sphere, like a Magdeburg sphere except clamped shut.

Not sure I understand. The gauge is submersed in water so the needle side is open to the water environment, and the liquid side is sealed to the iron sphere? A pressure washer gauge would be opposite. The hose side is connected to the fluid/liquid and the gauge side is open to the atmosphere.

Actually, it doesn't matter if the pressure is acurate at all. I just need to be able to tell an increase in pressure as depth increases, and it needs to be able to read up to about 900 PSI


How well would it work if I did something like encase the needle atmosphere side inside a clear acrylic epoxy or something and just left the liquid side open to the water?

You didn't give enough info. Based on what you just posted, go on ebay. You can buy a perfectly good piezo buzzer/pressure sensor for about a buck each. All you have to do is add a readout and calibrate it. In case you're actually doing production quantities, you can get pre-instrumented piezo sensors for the same price on alibaba.

[zetreque]

In the past, I've not been able to find one that ranges up to that high of pressure, additionally being able to waterproof it....

[CanadysPeak]

In the past, I've not been able to find one that ranges up to that high of pressure, additionally being able to waterproof it....

Again, you're not giving enough info. If you're looking for 1000 units, waterproofing and range is a piece of cake off-the-shelf. For a one off, you have to do the packaging yourself.

[TheVat]

This is a shade tree engineer asking this, so forgive my ignorance, but if you are where its 900 psi do you really want the cheapest option?

[zetreque]

I thought I gave plenty of information for my task.

Looking for cheap method to read pressure under water up to 1000psi (lets go with that nice round number instead).

Does it help if I say to pretend it is a scuba diver that wants to read a change in pressure up to 1000psi? And the scuba diver is not close enough to the surface to connect any sort of hose to the atmosphere.

[CanadysPeak]

I thought I gave plenty of information for my task.

Looking for cheap method to read pressure under water up to 1000psi (lets go with that nice round number instead).

Does it help if I say to pretend it is a scuba diver that wants to read a change in pressure up to 1000psi? And the scuba diver is not close enough to the surface to connect any sort of hose to the atmosphere.

The best divers in the world, using deep dive gases, might handle 400 psi or so, so that seems like a good scenario.

But, if you're doing engineering, designs always start with performance specs, budget available, and the business stuff (quantity, ROI, RONA, etc). It's important to remember that engineers expect to be able to solve a problem, not in theory, but in practice. So, when you specify that you want a low cost gauge, you have to know that you often see prices like: 1 through 499 = $25 each; 500 through 2000 = $4 each; 2000 through 50 000 = $1 each.

[Dave_Oblad]

Hi zetreque,

For a Pressure Gauge ranged 0-1000 PSI at just $12 click here.

This help? If not.. why not.. and maybe we can get some idea about your design restrictions.

Best wishes,
Dave :^)

[zetreque]

Thanks Dave,

I wonder though for that gauge. If you were to drop it in the ocean, and it sunk to a depth of ~2000 feet, would it still function since it was designed to work in the atmosphere and not underwater?

I'm imagining that if you took any ordinary gauge and put it deep down into water, the pressure is just going to be equal on both sides of the "diaphragm" at any depth. Correct me if I am wrong, but gauges like that work by starting out with the same pressure in two spaces (the atmosphere, and the container) and then changing the pressure of one space, and the gauge reads the difference of pressure.

If that thinking is correct.
One idea I just had this morning was to take a gauge like that and attach it to a hollow metal sphere. Charge that sphere up to a pressure of the gauge (1000psi) then as your gauge sphere combination sink into the water, the increase in water pressure is going to push the pressure back down, and you would able to read pressure, just in reverse.

[CanadysPeak]

Thanks Dave,

I wonder though for that gauge. If you were to drop it in the ocean, and it sunk to a depth of ~2000 feet, would it still function since it was designed to work in the atmosphere and not underwater?

I'm imagining that if you took any ordinary gauge and put it deep down into water, the pressure is just going to be equal on both sides of the "diaphragm" at any depth. Correct me if I am wrong, but gauges like that work by starting out with the same pressure in two spaces (the atmosphere, and the container) and then changing the pressure of one space, and the gauge reads the difference of pressure.

If that thinking is correct.
One idea I just had this morning was to take a gauge like that and attach it to a metal sphere. Charge that sphere up to a pressure of the gauge (1000psi) then as your gauge sphere combination sink into the water, the increase in water pressure is going to push the pressure back down, and you would able to read pressure, just in reverse.

Do not, under any circumstances, charge a metal sphere up to 1000 psi. That's called a pneumatic claymore.

[Dave_Oblad]

Aw Haw..

So you are reading the differential pressure between inside something and outside something?

Anyway, the specs should explain that.. but as you say, you can seal one side at standard pressure and it will display the difference between the sealed section and the high pressure side.

CP is correct.. but a Scuba tank is rated from 3000 to 4000 PSI, and they also have Gauges for charging them.

Some more (not too expensive) stand-alone Gauges:
http://www.ebay.com/bhp/scuba-tank-pressure-gauge

Regards,
Dave :^)

[zetreque]

Aw Haw..

So you are reading the differential pressure between inside something and outside something?

Anyway, the specs should explain that.. but as you say, you can seal one side at standard pressure and it will display the difference between the sealed section and the high pressure side.

CP is correct.. but a Scuba tank is rated from 3000 to 4000 PSI, and they also have Gauges for charging them.

Regards,
Dave :^)

end result. I just want to read pressure changes under water. It doesn't have to be fancy, it doesn't have to be true pressure. The smaller, the better. The cheaper, the better. And it MUST go between 0 and 1000 PSI

[Dave_Oblad]

Note: Added another link to my last post but got cross posted.. so here it is again. Some tiny cute $25 Gauges shown here:

http://www.ebay.com/bhp/scuba-tank-pressure-gauge

Regards,
Dave :^)

[zetreque]

Dave, those appear to be for scuba tanks. I'd rather not have a huge scuba tank, and I think those gauges are meant for reading tank pressure, and would not work for water pressure.

Here is another thought I had, and I am not sure if it would work. Take a regular pressure gauge and create two "spaces" between it. The one side is open to the water, while the other side has a different density liquid in it. As the pressure changes, perhaps the more easily compressed liquid will cause a change to register on the gauge?

[CanadysPeak]

Aw Haw..

So you are reading the differential pressure between inside something and outside something?

Anyway, the specs should explain that.. but as you say, you can seal one side at standard pressure and it will display the difference between the sealed section and the high pressure side.

CP is correct.. but a Scuba tank is rated from 3000 to 4000 PSI, and they also have Gauges for charging them.

Some more (not too expensive) stand-alone Gauges:
http://www.ebay.com/bhp/scuba-tank-pressure-gauge

Regards,
Dave :^)

Dave,
That is correct, but you should be familiar with the safety wrapping applied to those tanks, and to the inspection procedure required prior to filling them. Read this
http://www.divebuddy.com/forum/20194/sc ... aged-cars/
or look at
http://www.scubaengineer.com/tank_servicingx.htm

[zetreque]

of course it would be difficult to seal a plastic bag to a metal stem that can withstand large pressure differences. To have a clear bag to see the gauge ontop of all that is another challenge. Maybe if the liquid was chosen right, and the gauge read lower pressure so that it was more sensitive it might work. Then the pressure difference wouldn't be as great between the "spaces", and the gauge would pick up slighter changes.

I think the bag side of the space would need to be more dense, so that the needle moves in the correct direction.

[Dave_Oblad]

Hi,

I could be wrong.. but I am under the impression that the $25 cheap gauge shown just below is NOT a differential Gauge. I believe it is a spring tension type.. since it may have to withstand 4000 PSI.

Gauge from link provided on Ebay above

Thus the meter movement is sealed to just withstand normal sea level pressure. So, if true, we just need to make sure the meter movement part can withstand the 1000 PSI via a strong plastic box (1/2" thick Lexan?) with the pressure sensitive part exposed to the water. Air and/or Water pressure should not make much difference except rusting etc.

Pressure insulator box around Gauge

This might do the trick.. if this Gauge is not a differential type but rather a tension type.

Sort of your bag trick except the bag would not insulate the face of the meter from the pressure.. and it (the meter movement & glass) would implode.

Regards,
Dave :^)

[zetreque]

now you have the right idea :)

I admit that my first description in this topic wasn't the best. I wonder if the seal within that gauge is strong enough or the water would just leak to the interior compartment through the gauge.

Another idea.
If you had a digital pressure sensor/transducer inside of a dry sealed metal container, would the resulting pressure on the outside of the container transfer pressure inside the container that would be picked up on the sensor since cheap electronics sensors usually only go up to 10s of PSI?

In that case you wouldn't need a full 1000psi sensor, and you could possibly even build a scale conversion to the true psi if you knew the container properties.

and I don't know anything about pressure transducers, but maybe the companies CanadysPeak mentioned might enlighten me on if they would work underwater because I have seen a couple of them for cheap.

[zetreque]

If I could get my hands on a clear plastic or similar that can withstand the pressure, put a bit of water inside of it, then sealed it, maybe I could use gas laws to calculate how much it would compress the gas (and raise the water level) depending on the pressure to make a crude gauge. Similar to a thermometer.

[Dave_Oblad]

Hi again,

Yes, a cheap Divers Depth Gauge is just a soft plastic flex tube with an air bubble trapped inside. The greater the Pressure, the more compressed the bubble becomes. Good for about 100 feet.. but the concept should work for any depth.. but it will be hard to read with any accuracy with a non-linear scale.. as the deeper you go the less visible (smaller increments per linear depth) the bubble size change becomes.

At about 33 feet the water pressure is about double sea level and the bubble becomes half-sized. Don't quote me, as I'm pulling these numbers out of my butt as always..lol.

Regards,
Dave :^)

[zetreque]

Good stuff Dave,

Believe it or not, talking through this yesterday allowed me to make significant progress on this project. I ended up getting out an arduino I had purchased a long time ago but never got around to using. Figuring out how to implement it which was a challenge in itself. I found some cheap barometric pressure sensors that can be used with it ranging from $5 (ok ones) to $50 (for extremely accurate ones). The sensors can be used inside a container to detect slight changes in "cabin" pressure. As a backup since they are also cheap, I can have a force stress sensor which is inside the "cabin" and attached yet another compartment which will cause it to stress.

However, the tube bubble idea has me intrigued as well. Might just be a super easy method with the right kind of tube.