A Better SPG?

[Akimbo]

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.

Actually, manufacturing cost would be the same or less. First, the housing no longer has to be pressure resistant. It could be stamped stainless steel instead of a machined brass and the glass could be rolled on instead of using a machined retaining ring. Chrome plating could also be eliminated.

There is already a resilient plastic blow-out plug that could be redesigned to serve a dual purpose as the sensing diaphragm with very little cost impact. Stamping the needle has no cost effect. The cost increase of industrial gauges for fluid filling is about $1. A good manufacturing engineer has the potential to do even more.

Most SPGs use standard industrial Bourdon Tube assemblies rated for 2½% accuracy. That would be +/- 125 PSI on a 5000 PSI gauge. That is before we smack then against rocks and the sides of the boat though.

It would certainly be less expensive than an SPG rated for a safe working depth in excess of 500', which is one of the other threads that started this discussion. There is nothing like subjecting an idea to a world-wide peer review. Thanks.

 

[Scott L]

What is the best pressure checking device typically available in which to check bourdon tube SPG's accuracy? In the absence of any future improvement in technology I am in favor of picking through an LDS's inventory for the most accurate gauge. I am a very simple fellow...

 

[Akimbo]

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.

I agree that you want a pressure gauge that does not have a higher range than necessary since accuracy of the mechanism is based on the max pressure. 2½% accuracy over 4000 PSI beats 2½% of 5000 PSI every time and is easier to read. A common guideline from manufacturers is to select pressure gauges with a maximum pressure that is at less 10% above your working pressure.

It could be any pressure a manufacturer wants. I used 5000 PSI for the illustration because that is the rating of most SPGs I have seen lately have. They do cover some 300 Bar/4,351 PSI cylinders available in Scandinavia and Europe. This all started with ideas for a high-end gauge for deeper work where higher pressure bottles are an advantage.

 

[Akimbo]

What is the best pressure checking device typically available in which to check bourdon tube SPG's accuracy? If the absence of any future improvement in technology I am in favor of picking through an LDS's supply for the most accurate gauge. I am a simple fellow...

SPGs are easy if there is a higher accuracy gauge on a fill station you can compare it with. Mechanical depth gauges need a chamber with a calibrated gauge. Simple is good.

 

[Akimbo]

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.

I am not suggesting any change in the accuracy of SPGs on the market now. The objective is more to eliminate leaks and implosion risks and possibly develop a more useful display. The over bottom pressure in my tank is actually more useful to me than the surface gauge pressure because I can’t suck the difference into my mouth.

 

[SangP]

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".

I completely support this idea although, the difference in pressures won't really matter much to most divers, the idea of a tougher and more accurate spg is very interesting.

I would like to add that a lot of spgs are rather bulky and the Cochran spgs are really nice but the plastic screen does make them vulnerable.

Make a gel filled spg that roughly the same dimensions of the cochran with a tough urethane outer boot and I'll change all of my current spgs.

SangP

 

[Akimbo]

…One issue I see, especially with your IP indicator, is why do we necessarily need to be that precise? …

You don’t need to be, but it is less misleading. Knowing you have 180 PSI less available at 100' than you though can give some people a little more pause without understanding any advanced concepts. Stamping a forked needle would have zero cost impact; it is more a question of how useful it might be.

…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…

I wish they were that accurate. It is more like +/- 125 PSI overall, worst case.
However, accuracy and repeatability are not the same thing. Your SPG may be 100 PSI off at 3000 PSI, but it will be very close to that reading every time the tank is at 3000 PSI.

…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)…

Filling the gauge with fluid is where the Over Bottom Pressure comes from. If you just fill a standard SPG with fluid you will reduce the risk of implosion. But unless you also install a bellows or diaphragm to transfer pressure into the fluid, your gauge accuracy will really be a crap shoot. You would have to ask yourself: Is it reading like a normal SPG or has the housing started to deflect canceling out some of the reading?

Unless virtually all pressure differential on the housing is equalized, you still have the potential of sea water entering the housing contaminating the fluid and corroding the mechanism. Compensating for temperature is also a problem without a diaphragm. Industrial fluid filled gauges have a lot of air in them for that reason, but most would have to be evacuated for the best effect in this application.

Just the process of addressing everyone's comments has helped make the concept clearer in my mind. I hope my replies are as useful to you.

 

[Akimbo]

… 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…

Good point. But their repeatability is far better than their accuracy. Reading Over Bottom Pressure instead of surface gauge pressure plus an indicator for IP overhead is relative to that instrument. I find that most Bourdon Tubes gauges are off in the same direction over their entire range (reading high or low) and usually by a similar amount over their range.

One of the first things they do to high-accuracy gauges is install a calibration screw that lets the operator rotate the gauge face to align Zero to the needle. Very often, that is the only differance between a 2½% and a 1% gauge. Lots more hand crafting goes into the ¼ and 1/10th % gauges though.

 

[Nemrod]

If you are deep enough to implode a brass and glass spg you are well beyond my capability on my best day, good luck.

N

 

[Cave Diver]

Just the process of addressing everyone's comments has helped make the concept clearer in my mind. I hope my replies are as useful to you.

These types of discussions are great mental exercise and provide some interesting points of view and ideas. Resulting spin off discussions can take on a life of their own, much like how this one did.