Does Underwater IP (aka MP) Gauge Exist?

[beanojones]

Does Underwater IP (aka MP) Gauge Exist? The talking about 'over-balanced' regs got me wondering about this.

Anyone ever seen one, or better yet used one?

 

[Luis H]

I looked for one and even talked to Pete Wolfinger (the original owner of Scuba Tools/ Peter Built) about it, but was never able to find one.

My planned solution is to just use a low cost gauge with a QD adapter and instead of trying to seal it against ambient pressure (and salt water) I am planning of drilling some vent holes and use it until it corrodes.

In that configuration it will measure the relative intermediate pressure versus true gauge pressure. Gauge pressure is defined as the pressure above one atmosphere (the SPG measures gauge pressure).

By using a flooded pressure gauge I just have to take note of the depth (which is part of what I am doing anyway) and the pressure I am measuring is the pressure above the surrounding ambient.


This is the easiest solution that I have come up with. Hopefully I will get to try it next summer.

 

[ClayJar]

In that configuration it will measure the relative intermediate pressure versus true gauge pressure. Gauge pressure is defined as the pressure above one atmosphere (the SPG measures gauge pressure).

I hope it doesn't seem like I'm picking nits here, but gauge pressure is *not* defined as the pressure above one atmosphere, although that is customarily what is referred to as "gauge pressure" in diving contexts.

A gauge pressure is *defined* simply as a relative pressure compared to some reference pressure. There is no such thing as "true gauge pressure". (I understand to what you were referring, of course, but using a broken construct to describe it will inevitably confuse things... and it'll drive people with engineering degrees nuts. :biggrin


As for the original topic, a "non-submersible" IP gauge won't last very long, I'd guess, but if you "vent" it (so it won't implode), it'll certainly work. On an overbalanced first stage, you should be able to see the IP move with respect to depth. Assuming this is just an experiment, you can get a cheap gauge and the requisite fittings for not much dough, and it only needs to work in the short term. (Having an inflator-style fitting so you can pop it off if you break it seems like a decent idea, and you can reuse the fitting.)

If you want a durable one to monitor your IP on every dive... um... well, that's going to cost a few bucks to fabricate.

 

[Luis H]

Yes, you are correct. I was incorrectly calling "sealed pressure", gauge pressure. I am just used to gauges using one atmosphere as reference.

Note:
I will have to check tomorrow (at work where I have some references) if the term sealed pressure is actually defined the way I am thinking about it. I have seen the term, but now I should double check if it has a specific definition.

... and it'll drive people with engineering degrees nuts. :biggrin

Don't worry...this is not the only time I have made a mistake and yes it does bother me, but it won't drive nuts.

 

[beanojones]

So just to make sure I am thinking about this right:
A flooded IP gauge (like LuisH is suggesting) would show the IP as the same, regardless of depth, assuming the regulator is not 'over-balanced' (in the Aqualung/Apeks meaning of the word), and it is balanced (in the stable IP over tank pressure ranges).

 

[beanojones]

Also as a side question, how did Apeks check its dry envronmental system to see if it works? Just by using a pressure chamber with no actual diving to check it?

 

[Luis H]

So just to make sure I am thinking about this right:
A flooded IP gauge (like LuisH is suggesting) would show the IP as the same, regardless of depth, assuming the regulator is not 'over-balanced' (in the Aqualung/Apeks meaning of the word), and it is balanced (in the stable IP over tank pressure ranges).

The flooded gauge should read a constant IP at any depth for a balanced (or unbalanced if the tank pressure stays constant, like in a pony test bottle). Hopefully it is not very exciting.

Also as a side question, how did Apeks check its dry envronmental system to see if it works? Just by using a pressure chamber with no actual diving to check it?

I don't know how Apeks test its regulators, but IMHO a chamber dive is still a dive. Back when I helped in the Advanced Scuba class (in the 70's), when it was available we always did a decompression chamber dive (in Cabo Rojo, Puerto Rico). We called it a dive like any other dive, just a dry dive.

The pressure effects are the same to the equipment.

 

[DA Aquamaster]

So just to make sure I am thinking about this right:
A flooded IP gauge (like LuisH is suggesting) would show the IP as the same, regardless of depth, assuming the regulator is not 'over-balanced' (in the Aqualung/Apeks meaning of the word), and it is balanced (in the stable IP over tank pressure ranges).

Exactly. The ambient pressure at depth will increase by about a half psi per foot of depth, but a properly vented pressure gauge would still record the same IP as the actual pressure inside the IP spaces of the reg itself will also be increasing at the same rate due to the ambient pressure compensation of the reg.

For example in a reg with an IP of 145 psi at the surface should have a pressure of 190 psi at 100 ft. If it does not, then the ambient pressure compensation of the reg is less than perfect.

I have an inexpensive pressure gauge mounted on a 1" nipple with a standard BCD QD fitting. If vented it could be plugged into an inflator hose to measure IP during the dive. I think if you soaked it in warm fresh water and then alcohol post dive the corrosion would be minimal.

As an aside I have noted when adjusting first stages with external IP adjustments that most will tolerate a significant increase in spring pressure before the IP actually changes if the system is closed. I am assuming this is due to internal friction resisting the opening of the valve to balance what amounts to the increased "ambient" pressure. So during the dive, if you and the reg on a pony you would want to breathe off the reg to cycle it consitently prior to each pressure reading.

 

[redacted]

My planned solution is to just use a low cost gauge with a QD adapter and instead of trying to seal it against ambient pressure (and salt water) I am planning of drilling some vent holes and use it until it corrodes

I've used an el cheapo water pressure testing gauge (about $5 at Home Depot) as a travel IP gauge. It could detect an IP stability problem but I doubt both its accuracy and precision. Still might work for you delta P needs.

Another alternative might be to seal a gauge inside an old dive light case, or other transparent plastic case. I'm sure there are plenty of larger dive lights that are past their prime and that way a quality gauge can be used without being sacrificed and the test can be repeated if necessary.

I just reread this and realized DUHHH that would block the ambient pressure ---- Never mind.

 

[Luis H]

Another alternative might be to seal a gauge inside an old dive light case, or other transparent plastic case. I'm sure there are plenty of larger dive lights that are past their prime and that way a quality gauge can be used without being sacrificed and the test can be repeated if necessary.

I just reread this and realized DUHHH that would block the ambient pressure ---- Never mind.

If you use a good pressure vessel, that would hold a constant reference pressure. In this case it would be basically one atmosphere. That would be a sealed pressure gauge (such as a standard SPG).

One issue with a sealed gauge is that the pressure inside a small container can change some with changes in temperature. This would can be easily calculated and adjusted for, but it becomes impractical if measuring low pressure magnitudes. I haven't looked at the error introduced by the temperature/ pressure change, but I think that would be small (I may do the calculations later).

I was thinking about using a clear pelican type case (I have seen some that will hold the pressure), but the water proof connection penetrating the housing was starting to look as more trouble than it was worth it. So I have basically given up on that idea.