This and several other discussions gave me some ideas on an improved SPG and I would appreciate your comments.
http://www.scubaboard.com/forums/advanced-scuba-discussions/343217-better-spg.html#post5355256
"Comments?" Okay, but you're probably not going to like what I have to say.
Your ideas and design for a better, more accurate SPG I think are spot-on... And in seeing the other designs that I assume you created before, I know you could build an impressively beautiful piece that would function better than today's units that are available in the marketplace.
...However, like a lot of things scuba, I don't believe that your better, more accurate, and deeper-capable SPG is a solution for a problem that really exists. That is, today's simpler SPGs aren't particularly problematic. Sure, if you're going to 800 feet, the problem (possible failure at depth, a reading that may not be a totally accurate representative of available gas) may be exasperated and therefore an issue... But for almost any kind of diving (say, anything above a rational 300' or 400', the "inaccuracy" caused by ambient pressure and the worry of implosion is probably not the biggest concern. It's arugable that divers going to those extreme depths are even aware of these types of risk... Much less, the everyday diver going to 60 fsw.
I would much prefer to see an SPG with a more flexible bourdon tube... Which would be affected by pressure at depth, and therefore show you only the usable gas available in your tanks. Such a thing would also alleviate the risk of implosion of the bourdon tube... Which I'm not convinced is a real problem either, since I've never seen it happen. Maybe at 800 feet...
Add an oil-filled face, and the gauge would be very practical to just about any depth - circumventing the whole "depth rating" suggestion.
...But it's my understanding that Thermo's bourdon tubes are already somewhat flexible, which is to say that this problem has already been solved, and probably the reason why nobody seems to be experiencing SPG failures at depths around 500 feet.
...And while oil-filling the face would definitely make the SPG even more depth-tolerant, there's no sense in creating a fix for a problem that doesn't exist. If they're not blowing out on these dives as they are now, why make things more complicated?
If you're looking for a real problem to fix, address this one: Many divers fall out of the sport because they find themselves "married" to a local dive shop for gas fills. If the dive shop goes out of business (common today) or their local dive shop is a significant distance away, they often fall out of the sport due to the logistical complications of obtaining fills. If they don't own their own tanks, it's even worse, as doing one dive will take two trips to their distant, local dive shop - one to pick up tanks and one to return them.
A simple solution would be to create a cheap and easy fill station - one with a CFM of like 0.1. It could run on AC110v (household current), with comparatively small electric motors that would be quiet and run very cool. Filtration systems at such a low rate (as well as low temperature) could be very inexpensive, effective, and easy to maintain. The compressors might not even need oil, which would make them simple to maintain, portable, clean, and would not risk the contamination of the breathing gas. It wouldn't matter that it would take days to fill an AL80... Most people have to work anyway, and the bottom line would be that every weekend, they'd have cool, clean, healthy breathing gas without sucking up the entire weekend getting tanks.
Available accessories could be auto on/off electronics and "pigtail" fill whips that would allow a series of tanks to be connected to the whip rather that one at a time - so that the compressor fills two or four tanks by next weekend, not just one.
The market supports such a product in the $700 range, new... Can you develop something like that?
...Another product that I think could be built is a modified automotive shop air tool, hooked to a propeller. This device would attach to an AL80, and create a practical DPV. Avoiding today's electronics, offgassing batteries, sealing issues, and cost, this type of system would be comparatively cheap, simple, portable and powerful... And use as a fuel something that divers assumedly have a strong supply of: compressed gas. This would make a lot more sense than the batteries, electronics, reed switches, and sealed hulls that we're using now.
Lastly - and here's a no-brainer... I have an electronic toothbrush (SonicCare) that, when sitting on it's base, charges without any metal contacts on the bottom of the brush... It's too cool. Why not combine that and a wireless SD card to create an underwater camera system that does not require the user to repeatedly open and close their camera case? Between the price of underwater photo and video equipment, the maintenance of these items, and the risk taken every time the case is opened and resealed, underwater photography is a true pain in the neck. Such a camera - itself sealed rather than in a housing of some sort - would greatly simplify the entire process and would be worth a great deal on the market. If a dive shop and/or boat had WiFi internet access, photos could be on Flickr before the diver was even rinsed off. And maintaining the camera - an issue only because the diver's already got so much to maintain - would be as simple as dunking it in freshwater and setting it down on it's base (or mat).
Comparatively, addressing an SPG to make it "more accurate" doesn't seem to be a very fruitful endeavor... Even if you are successful in making something like that.
