Does Underwater IP (aka MP) Gauge Exist?

[PhilEllis]

........ and the pressure I am measuring is the pressure above the surrounding ambient.

I am not quite sure about that. In a bourdon tube gauge, when the tube housing is flooded, the water exerts the EXACT amount of pressure on the tube on all sides. Therefore, the depth in the water should have no impact on the deflection of the tube; therefore, there would be no "water pressure" effect on the reading on the dial.

Pictures of the Inside of a "Brass and Glass" SPG

I have actually measured IP pressure of several regulators at depth. Even in the Apeks "overbalanced" designs, the increase if not nearly what you might expect. At 125 fsw, the increase was only about 10 psi if I remember correctly.

By the way, you can use a standard, inexpensive IP gauge under water. It will not last very long, and quickly the salt water will degrade the ink printing on the dial. But, it will work and work quite well, even though it is flooded.

Phil Ellis
Discount Scuba Gear at DiveSports.com - Buy Scuba Diving Equipment & Snorkeling Equipment

 

[beanojones]

I have actually measured IP pressure of several regulators at depth. Even in the Apeks "overbalanced" designs, the increase if not nearly what you might expect. At 125 fsw, the increase was only about 10 psi if I remember correctly.

So the gauge was reading 205 psi then?

135psi base IP
+ (4 atm*14.7psi) Ambient
+ (Apeks magical overbalancing number +10 psi?)

 

[Luis H]

I am not quite sure about that. In a bourdon tube gauge, when the tube housing is flooded, the water exerts the EXACT amount of pressure on the tube on all sides. Therefore, the depth in the water should have no impact on the deflection of the tube; therefore, there would be no "water pressure" effect on the reading on the dial.

Pictures of the Inside of a "Brass and Glass" SPG

I have actually measured IP pressure of several regulators at depth. Even in the Apeks "overbalanced" designs, the increase if not nearly what you might expect. At 125 fsw, the increase was only about 10 psi if I remember correctly.

By the way, you can use a standard, inexpensive IP gauge under water. It will not last very long, and quickly the salt water will degrade the ink printing on the dial. But, it will work and work quite well, even though it is flooded.

Phil Ellis
Discount Scuba Gear at DiveSports.com - Buy Scuba Diving Equipment & Snorkeling Equipment

All bourdon tube pressure gauges react to the differential pressure from the inside to the outside of the bourdon tube. The balloon analogy in your link is not a bad one. And like the balloon, a bourdon tube reacts to differential pressure.
All gauges by design only measure a differential pressure of some sort.

It doesn't matter if the fluid on the outside of the tube is a liquid or a gas. Both liquid and gas transmit pressure in the same form; it is transmitted in all directions.

The only issue in question here is whether the gauge is exposed to the outside ambient or to a constant environment in a sealed container.



I am very interested on the results from your test of the Apeks "overbalanced" designs. How did you perform the test? Did you use a dry chamber?

 

[beanojones]

So if I am following the discussion here, unless I have the IP gauge in a sealed 1 atm container, I cannot see how the actual IP is tracking?

That is what I am actually interested in, the actual IP. (Of course the 1 atm will be what it is.)

 

[Luis H]

So if I am following the discussion here, unless I have the IP gauge in a sealed 1 atm container, I cannot see how the actual IP is tracking?

That is what I am actually interested in, the actual IP. (Of course the 1 atm will be what it is.)

If you are using any pressure gauge open to the ambient, with a conventional first stage you should see it tracking as a constant pressure. That is if the piston or diaphragm is working properly and properly exposed to ambient, the IP should stay constant at any depth (the same amount above ambient).

To obtain the absolute pressure you will have to add the surrounding ambient pressure read from your depth gauge (just like you would for any similar calculation).


With a so called overbalanced first stage (as defined by Apeks) you should see just the pressure increase created by the area ratios of the inner and outer diaphragms.



Note: in a conventional first stage with a sealed gauge you would see the pressure increase corresponding to the depth.



I hope this is clear; I apologize if I am not.

 

[Couv]

Edit: Luis nailed it, unfortunately I was typing my boring post at the same time.

Hello Beano,

I have to admit, I got a little lost somewhere in this thread, how about telling us what you are trying to do and we can make suggestions from there. If I repeat anything that has already been stated, please excuse me…I'm old.

Here are the basics as I (mis)understand them:

Edit: (The info below is not exactly correct, see post 3 and 4 by ClayJar and Luis; but I did qualify it with an option of my misunderstanding the terms.)

IP= gage pressure (psig)
Psig + ambient pressure = pressure absolute (psia)

So if you have a regulator that you want to test at different ambient pressures you can:

A: Seal the case it is in, take it to the pressure/depth you want to check it. That will give you the gauge pressure at that depth. A funny way to look at this would be a gauge on the end of a long hose that is read at the surface. *

B: Modify the case (drill a hole) so that the ambient pressure can act on the outside of the bourdon tube. That will give you the gauge pressure at that depth minus the ambient pressure. However, all you need to do is add the water pressure to your reading and there you have it.

To get the ambient pressure at depth multiply .445 psi X the depth. For instance you have a regulator that has an IP of 135 and you take it to 10 feet.

The gauge in A should read: 139.45 because 135 + (.445 X 10)
The gauge in B should read: 135 because the ambient pressure is acting on the bourdon tube….so you have to add (.445 X10) to get the gage pressure at depth.

To get the absolute pressure for any of the above, simply add the atmosphere (14.7) to equation.

Couv


* This is not the same thing and a bit off topic, but a pneumofathometer is used in commercial diving, we use a long hose that is open on one end. Air is pumped through the hose then shut off. As the water pressure compresses the air in the hose a pressure gauge (marked off in feet of sea water) tells the top side the depth of the end of the hose. A capillary pressure gauge works similar to this.

 

[Couv]

All bourdon tube pressure gauges react to the differential pressure from the inside to the outside of the bourdon tube. The balloon analogy in your link is not a bad one.

As an instructor, I used a party favor to illustrate how a bourdon tube works. When you blow into the tube on one end, it unfurls; when you stop blowing it rolls back up. Add a pointer and a scale...there is your gauge.

c

 

[meesier42]

I believe the OP was trying to determine how much effect the overbalancing has.

why don't you just wrap the IP gauge in a balloon or condom and fill with some air and seal it to the LP hose. sure the balloon will compress at depth, but so will the air inside the balloon and the IP gauge "should" stay dry and hopefully will survive the trip. You'll need just enough air inside the balloon such that it won't form fit about the body of your IP gauge at depth. I would still use one of the cheapos, so if your "seal" fails your not out a $50 IP gauge ( Harbor Freight Cheapo)

 

[ianr33]

My guess is that an IP gauge would implode at any significant depth. A regular spg is designed to withstand external pressure but an IP gauge is not. The one I have looks very fragile compared to my glass/brass spg's

 

[PhilEllis]

If you are using any pressure gauge open to the ambient, with a conventional first stage you should see it tracking as a constant pressure. That is if the piston or diaphragm is working properly and properly exposed to ambient, the IP should stay constant at any depth (the same amount above ambient).

To obtain the absolute pressure you will have to add the surrounding ambient pressure read from your depth gauge (just like you would for any similar calculation).


With a so called overbalanced first stage (as defined by Apeks) you should see just the pressure increase created by the area ratios of the inner and outer diaphragms.



Note: in a conventional first stage with a sealed gauge you would see the pressure increase corresponding to the depth.



I hope this is clear; I apologize if I am not.

I think I may have made a confusing post. I agree with this post completely. As you submerge, you IP gauge will read the difference over ambient (assumimg both the regulator and gauge are exposed to the same ambient, which they are). So, if your IP gauge shows 140 psi at the surface, it will show 140 psi at depth. Agreed.

Phil Ellis
Discount Scuba Gear at DiveSports.com - Buy Scuba Diving Equipment & Snorkeling Equipment