A Better SPG?

[Akimbo]

A Better SPG?

There have been several recent posts that have me thinking about mechanical SPGs (Submersible Pressure Gauge). All of the units on the market now are Bourdon Tube based instruments in a waterproof housing that maintains one atmosphere or sea level pressure inside. As a result, they display the pressure in your tank the same as on the surface, regardless of your depth.

The two issues under discussion touched on the safe operating depth of the SPG housing and calculating usable gas left in your tank. The minimum pressure you can actually use equals your bottom pressure plus the Intermediate Pressure (IP) delivered by your first stage regulator. Note that your first stage automatically maintains the IP above or Over Bottom Pressure (OBP). Example:

Assume you are at 100 FSW (Feet of Sea Water)
Your IP is 135 PSI
There is .445 PSI per foot of sea water​

So the minimum pressure a first stage regulator can deliver is:
(100' x .445 PSI) + 135 PSI or 179.5 PSI as you would see on a standard SPG. So, if you run your tank down to 500 PSI at 100', you can only use 320.5 PSI (500 – 179.5) of it, at best. Not all first stages can deliver a stable IP pressure at such low tank pressures.

It has occurred to me that there is an argument for an SPG that displays the usable pressure automatically. Computers can easily be programmed to do that, but the low cost, simplicity, lack of batteries, and reliability of a mechanical SPG has value.

A conventional SPG will display OVB pressure instead of surface or gauge pressure simply by drilling a hole in it and let water flow in. Obviously that is hard on the delicate gear in a Bourdon Tube mechanism to say nothing about salt build-up, corrosion, and nasty stuff growing in the housing.

A small flexible diaphragm could be installed in the SPG housing and filled with a non-corrosive fluid to protect the mechanism and prevent water intrusion. Industrial pressure gauges are commonly filled with fluids to protect the mechanism from vibration and minor pressure surges.

Pressure Gauges | General Purpose Gauges | Wika® Glycerine Filled Commercial Gauges - GlobalIndustrial.com

Fluid filling also eliminates concerns over imploding and leaking since there would be no pressure difference acting on the SPG housing. It also occurred to me that the pressure gauge needle could have two pointers, sort of a snake tongue. The distance between the forks would equal the IP and the other indicating the OBP. The lower needle would indicate the minimum usable pressure in your tank. Illustration attached.

I am interested in your comments and suggestions for a better SPG design. I have no intent to claim ownership of any of these ideas or profit in any way beyond the possibility of being able to purchase a better product. I doubt that anything I have presented is patentable and even if it is, it is now in the public domain and is "prior art".

Edit: A fluid filled SPG would have a lot in common with a fluid filled mechanical depth gauge. The main difference is the Bourdon tube would sense pressure from the HP port rather than from pressure inside the housing. Also, I am not suggesting increasing gauge accuracy, though that may justify a higher-end version even on current SPGs.

 

[Cave Diver]

The first fly I see in this ointment is that if you're trying to be this precise in measuring your usable gas, is that you'd need to know what the IP of the first stage is set at. So your SPG would have to take readings from both the high and low pressure side to be able to take that into account, as well as determining the ambient pressure at depth.

The design you're proposing has some merit in taking into account the usable pressure differential that you have available to you at depth, but how deep do you need to be before that actually becomes a significant variable?

An interesting mental exercise, but not sure on the practical application.

 

[Cave Diver]

Assume you are at 100 FSW (Feet of Sea Water)
Your IP is 135 PSI
There is .445 PSI per foot of sea water​

So the minimum pressure a first stage regulator can deliver is:
(100' x .445 PSI) + 135 PSI or 179.5 PSI as you would see on a standard SPG. So, if you run your tank down to 500 PSI at 100', you can only use 320.5 PSI (500 – 179.5) of it, at best. Not all first stages can deliver a stable IP pressure at such low tank pressures.

BTW, at 100', I come up with 4.03 ATM (100/33+1) which equals 59.24 PSI (4.03 * 14.7) + 135 PSI intermediate pressure totaling 194.24. That leaves you only 305 usable PSI.

If we're gonna be precise 'n stuff.

 

[kanonfodr]

Nice idea, Akimbo!

One issue I see, especially with your IP indicator, is why do we necessarily need to be that precise? I'd say that in 95% of diving applications, your SPG doesn't need to tell you that you have exactly 1135 PSI of usable gas remaining at your current depth, simply because the recreational diving world doesn't work on that level of precision. I would feel comfortable that my SPG was within 50 PSI of my actual tank pressure, because through dive planning I'm being conservative enough that the 50 PSI I may not have is not readily needed.

But I do like your idea for filling the SPG with oil or water to protect the unit itself from possible implosion due to water pressure at extreme depths, that would be super useful to some of the guys like Nuno G and others who are pushing super extreme depths. Hell, it might even be better for the average joe recreational diver such as myself, if nothing else I am carrying a more robust product for a pretty important task like measuring my remaining gas capacities (I to this day don't get how the vintage guys could be just fine without an SPG, since I generally have two products that measure my remaining backgas).

I definitely like your concept, though. I'm sure somebody would buy it, if it were ever manufactured.

Peace,
Greg

 

[captndale]

Consider that the typical SPG is not likely more accurate than +/- 200 psi. Consider also that it sells at wholesale for about $50 to $60 and at retail for about twice that. Consider that the wholesale cost of most items are about 4 to 5 times actual manufacturing cost, before overhead, insurance, marketing, sales and other costs are added. Consider that your suggestions offer little added value but a great deal of cost. Would you make the investment in this idea. If so, go at it.

 

[Akimbo]

The first fly I see in this ointment is that if you're trying to be this precise in measuring your usable gas, is that you'd need to know what the IP of the first stage is set at. So your SPG would have to take readings from both the high and low pressure side to be able to take that into account, as well as determining the ambient pressure at depth.

My intent is not to be that accurate rather than a give "closer" indication of usable gas. Keep in mind that a typical SPG uses a 2½% accuracy industrial mechanism — that would be +/- 125 PSI right off the top on a 5000 PSI gauge.

Most people don’t do the gas management exercise in recreational pre-dive planning, in fact probably never considered this concept at all. A needle spread at the high end of IP would probably be better, like 150 PSI. Think of it as a closer estimate of reality without mental math. The greatest value may well be just getting more people thinking about gas management and applying the physics.

The design you're proposing has some merit in taking into account the usable pressure differential that you have available to you at depth, but how deep do you need to be before that actually becomes a significant variable? ...

To many people, 180 PSI less at 100' than they have ever considered would be significant. You can do the math until you find a depth that is significant to you. I my mind, gas management is the lesser of the two advantages.

I first began with the fluid filling concept to eliminate SPG housing leaks and the implosion potential. That automatically results in an OBP reading instead of a conventional "gauge" reading. My first concern was how to fix that "problem" and said, "Self, this could be a good thing". That revelation lead me to the second needle.

My idea of lobbying manufactures to put safe working depth ratings on their SPGs like on diving watches becomes moot with fluid filling — mentioned in other posts.

 

[spectrum]

It's an interesting idea and a good discussion about what is "usable.

One problem with trying to put a fine point on any of these instruments including the electronic versions is that none of this gear gets calibration maintenance. Part of the 500 PSI logic is that you may need the 300 to get air and the gauge may be 200 PSI off, especially when you're working in the lower 10% of full scale.

Pete

 

[Akimbo]

BTW, at 100', I come up with 4.03 ATM (100/33+1) which equals 59.24 PSI (4.03 * 14.7) + 135 PSI intermediate pressure totaling 194.24. That leaves you only 305 usable PSI.

If we're gonna be precise 'n stuff.

You are an atmosphere off because we are talking gauge pressure, not absolute pressure. (100/33+1) = 4.03 ATA (Atmospheres Absolute), not ATM or standard atmospheres. SPGs and standard pressure gauges used on fill stations read in gauge pressure, rather than absolute pressure. So that would be (100/33) * 14.7 PSI = 44.5 PSI + 135 PSI = 179.5 PSI.

You have to make an assumption regarding the average weight of sea water. I use 64.1 Lbs/Ft³ for gas calculations. Here are some handy conversions suited for spread sheets and computer programs:

SI (Système International d'unités) or Metric
SI Units
101,325 Pascals
101.325 KPa or Kilo Pascals or 1000x
0.101325 MPa or Mega Pascals or 1 Million x
1.01325 Bar
10.0627586096078 Meters of Sea Water
Imperial Units
14.695948775 PSI
33.899524252 Feet of Fresh Water at 4° C
33.014299900156 Feet of Sea Water​

Rounding errors may not matter at 100 FSW, but can cause a lot of confusion in my business designing for 1,650'/500 M.

 

[captain]

I never use tanks that are above 3000 psi. 3500 is about the max for HP tanks so why is a 5000 psi gauge necessary being the most accurate part of the gauge range is the mid pressure or 2500 psi on a 5000 psi gauge. I use an older 3500 or 3000 psi gauge which gives me more accuracy in the range I care about.

 

[redacted]

You are looking to improve accuracy of an SPG in a range that I mostly don't care about the inaccuracy of not compensating for true ambient pressure. Well before that error can become an issue, I'm on my way to the surface. And I suspect the same is true for most divers.