A Better SPG?

[Akimbo]

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).

No SPG designs being discussed will change the way any regulator currently functions. Many balanced first stage regulators can deliver nearly consistent inhalation resistance down to, or even a little below, the IP.

The basic question is do you want your SPG to display the actual gauge pressure in the tank (like all SPGs do now) or the pressure you can actually use at depth?

A fluid filled SPG would automatically subtract your bottom pressure, weather you want it to or not. Fluid filling also eliminates the possibility of the SPG housing imploding on deep dives. Since the fluid filled gauge would display tank pressure at the surface minus the ambient or bottom pressure, the "forked" needle part of the design would also subtract the IP pressure — weather it be 135 or 150 PSI for a little added safety margin.

For discussion, let's assume your regulator starts to breathe hard when your tank pressure is down to your IP which is 135 PSI. Also assume your SPG is very accurate, which or course it is not. At the surface you could breathe the tank down to 135 PSI, which equals 135 PSIG (gauge pressure, just like standard SPGs) and 135 PSIA, (higher than ambient pressure or Over Bottom Pressure) since they are the same at the surface.

At 200' (sea water), ambient pressure is 89 PSI higher than at the surface. Therefore the regulator will breathe hard when a standard SPG displays 224 PSI (89 + 135). Effectively, you have 224 PSI less usable air than your SPG displays at 200'. It will still display 224 PSI on the surface, but you could suck an additional 89 PSI out.

Now make the same dive with a fluid filled SPG with a "forked" needle. The upper needle will display 135 PSI on the bottom and the lower fork of the needle will display zero/empty/you don’t want to be here!

Now let's say you are breathing off your buddy's Octo and swimming toward the surface. A fluid filled SPG will display 44.5 PSI more pressure or 179.5 PSI in your tank, and it is usable since the IP pressure is 135 PSI above ambient or Over Bottom Pressure. Back on the surface the upper fork of the needle will read 224 PSI and the lower fork will point at 89 PSI.

I hope this makes sense.

 

[Akimbo]

BTW, here is a photo of a standard industrial Glycerin-filled stamped stainless steel pressure gauge:

WIKA Instrument Corporation: Products - Pressure Gauges - 23X.30

Valley Instrument 2 1/2in. Stainless Steel Glycerin Gauge — 0-3000 PSI | Filled Gauges | Northern Tool + Equipment

I figure it could look a lot like this except the glass would be tempered and thicker. The thicker glass would be for impact resistance only since the pressure differential would be zero.

 

[hudson]

I certainly didn't mean the SPG design would change the way the regulator works

What I meant was that many regulators including every single model I own WILL deliver air almost all the way to 0 psig (and balanced models most comfortably) therefore I would have no use for a forked needle and would prefer a simpler display without it. Granted the inhalation effort may increase sharply with some regulators, but that is not complete failure.

 

[Akimbo]

I certainly didn't mean the SPG design would change the way the regulator works

What I meant was that many regulators including every single model I own WILL deliver air almost all the way to 0 psig (and balanced models most comfortably) therefore I would have no use for a forked needle and would prefer a simpler display without it. Granted the inhalation effort may increase sharply with some regulators, but that is not complete failure.

Even with a balanced flow-through piston first stage, inhalation resistance jumps so high when supply pressure drops below IP that it almost immediately develops a dangerous respiratory work load. Granted, a little bit of air is better than zip, but for practical purposes it is not usable beyond slowly inflating a BC.

Balanced flow-through piston first stages are usually a little better than diaphragms at supplies below the IP just because the gas orifice is larger on most designs. You can often detect the difference without instruments. Pull the first stage plug or cap off the supply and suck on the second stage. You can usually tell the flow-through piston delivers a little more air at the same suction.

Is this a reason to choose a balanced piston over a diaphragm? Probably not, it is only a characteristic inherent in the design. In fact, I argue that an unbalanced first stage piston design is probably better for less experienced divers… but that is a whole different conversation.

…In my view, balanced diaphragm first stages have gotten a little too good at low tank pressures. A distracted diver can breathe a bottle down to 100 PSI before noticing abnormal resistance. Most modern unbalanced pistons produce perceptible resistance at 300-400 PSI, yet still allow sucking the bottle down to 100 PSI if really necessary. ...

Back to the regularly scheduled program.