A Better SPG?

[SkimFisher]

Sounds a lot like the ol' "build a better mousetrap" concept. If it ain't broke - don't fix it.

 

[Deep South Divers]

You're likely to need more depth than that.

Yeah, I agree... But to do what, exactly?

I don't need any depth at all to establish that an SPG can be oil-filled, and any simple dive will do to establish that it'll work just fine.

It sounds like I would need to find the point of failure of a regular brass & glass SPG (implosion), then oil-fill one, duplicate the depth, and see how things change. For that, you're right... I'd need some serious depth.

...But we haven't yet established that there IS a practical point of failure for a non-oil filled SPG. What if the reliable failure point is 1200 feet?

Frankly, as much fun as this is as a mental exercise, we're creating a solution for a problem that does not exist... The downfall of many companies in the scuba industry today.

If we're looking to solve a problem, then why don't we pick one of the ones that we know exist? That seems like a better use of our time and money, and I'm sure it would be no less fun...

I asked JJ to come on here and give his two cents - maybe he'll have an input on whether or not 2" brass & glass SPGs are reliable at such extreme depths. I know that he's got a general philosophy to avoid internet forums (I'm sure most people here understand), but perhaps we'll luck out and he'll see it as a chance to plug a product and give an educated answer.

 

[Lead_carrier]

What's wrong with oil? I've got a ton of Bauer synthetic compressor oil sitting around... Could also use mineral oil or vegetable oil, or anything else transparent.

What's the concensus? Glycerine? Oil?

I need to find some sort of standardized rubber stopper so that whatever I do, it can be easily duplicated by everyone, assuming it's successful.

Do y'all think I can just remove the rubber burst plug and reinstall it? I think that it'd be enough to transmit pressure, assuming there's positively no air bubbles in the gauge...

I don't guess anything would be wrong with using mineral oil or compressor oil if you'd like. Most of the liquid filled pressure gauges that I've used contain glycerin. I guess you can use whatever is handy.

 

[Akimbo]

Sounds a lot like the ol' "build a better mousetrap" concept. If it ain't broke - don't fix it.

I also believe in "If it ain't broke - don't fix it" motto. What about the SPG implosions? What if manufacturing costs were reduced?

I consider the suggestion of displaying available breathable pressure, rather than standard gauge pressure, a convenient refinement. Not a big deal, but something you get for free as a result of SPG exposure to ambient pressure by liquid filling. The forked needle is one thought on how IP pressure can be subtracted without mental math or added cost.

This question is to everyone interested in responding.
So, let's say you needed an SPG and one came on the market with:

1. A stamped stainless steel housing
2. The same thickness tempered glass as current brass SPG
3. The glass was retained by a crimped edge of the housing instead of a screw-on retainer
4. Was oil filled and therefore automatically subtracted ambient pressure from the display
5. Had a safe working depth of… pick a number only a marketing weenie would love, say 1000 Meters/3,300'. Probably should avoid the Marianas Trench because the Bourdon tube could crush from the outside!
6. Had a forked needle that indicated the OBP at one point and the OBP minus about 150 PSI for IP or the max usable pressure in your tank at the other needle
7. Was 10% less expensive than a standard Brass & glass SPG, and was 20% more profitable to the manufacturer

Would you be interested? Are there features that would make it worth the same price, even if it were more profitable to the manufacturer? What features would be undesirable to you and why?

Perhaps even more important, are there any other features (besides maybe free) that you would like to see on an SPG? The whole idea is to discuss a better SPG even if my ideas suck whale snot.

 

[Akimbo]

What's wrong with oil? I've got a ton of Bauer synthetic compressor oil sitting around... Could also use mineral oil or vegetable oil, or anything else transparent.

What's the concensus? Glycerine? Oil?...

I don't guess anything would be wrong with using mineral oil or compressor oil if you'd like. Most of the liquid filled pressure gauges that I've used contain glycerin. I guess you can use whatever is handy.

As a matter of principal, the oil should be non-combustible and Oxygen compatible would be ideal. You also don’t want something that will chemically react with your wetsuit or latex neck dam if a little leaks — especially in the prototype stage!

Glycerin dominates the industrial fluid-filled pressure gauge business, and may actually be used exclusively. There may be tons more reasons or maybe it is just tradition. I figure at least some manufacturers would use something else if there was a cost of compelling advantage. In any case, Glycerin is a safe choice unless you are inclined to do a bunch of research.

 

[Slamfire]

This may a bit of a hijack, but thinking about the old J-Valves, how about a bypass valve inside the 1st stage that turns your LP port into HP whenever the pressure inside the tank = your IP? With said gizmo, now you would only have depth as the one factor that's keeping your spg from being totally accurate. And unless you are very deep, IP is a considerably larger contributor than depth.

 

[Akimbo]

This may a bit of a hijack, but thinking about the old J-Valves, how about a bypass valve inside the 1st stage that turns your LP port into HP whenever the pressure inside the tank = your IP? With said gizmo, now you would only have depth as the one factor that's keeping your spg from being totally accurate. And unless you are very deep, IP is a considerably larger contributor than depth.

I don’t understand? When the tank pressure drops to the IP, then the HP port equals the IP plus ambient or over bottom pressure on standard SPGs. I think I am missing the essence of your idea.

 

[Slamfire]

For simplicity's sake let's assume ambient pressure has no effect.

LP ports will not provide (adequate) airflow for the 2nd stage when tank pressure = IP pressure

HP ports do not have the above hindrance.

When your tank gets down to IP pressure, you're effectively out of air even though you still have ~150 psi in the tank. You cannot use those 150 psi. If at that point, you had a way to connect your 2nd stage to an HP port, wouldn't that make the last 150 psi usable?

 

[Akimbo]

For simplicity's sake let's assume ambient pressure has no effect.

LP ports will not provide (adequate) airflow for the 2nd stage when tank pressure = IP pressure

HP ports do not have the above hindrance.

When your tank gets down to IP pressure, you're effectively out of air even though you still have ~150 psi in the tank. You cannot use those 150 psi. If at that point, you had a way to connect your 2nd stage to an HP port, wouldn't that make the last 150 psi usable?

The HP port inside the first stage and in the SPG's hose fitting have tiny holes as a safety feature in case of hose failure. The flow would is too restricted to be of any value for breathing. The purpose of those tiny holes are:

• To prevent emptying your tank very rapidly with a failed hose
• Full flow at HP on a failed hose would cause the hose to whip around and beat the snot out of everyone around — on deck more than in the water.
• These small holes reduce shock to the gauge when the valve is opened quickly. You also see them on most industrial and hardware store variety gauges.

Unscrew the SPG hose from your regulator and look down both fittings and you can see the little holes. Thanks for the clarification, I was thinking something entirely different.

 

[hudson]

Akimbo,

I'd definitely be interested.

However, you are saying you won't get air below IP. While the effort of breathing will be regulator dependent, you will still get air and in most balanced designs without any noticeable decrease in quality until well below the IP level. So the forked needle isn't of any use (to me).