Brass and glass for SPG. Why?

[geoff w]

I'm not sure if this belongs here or in the instruments and gauges forum, but figured I'd ask here for a broader audience. I always read that brass and glass SPGs are the standard for tech diving. What's so special about brass housings and tempered glass faces? Why aren't an equally strong plastic stainless steel housing and some stronger more indestructible polycarbonate face as good? From what I can tell, there's just one company in Italy that makes all the brass and glass SPGs. Is it for standardization, or it there something special about those materials?

 

[PfcAJ]

The brass and glass ones just hold up well.

The plastic bodies ones tend to break, but the metal bodied/ polycarbonate face ones do ok.

 

[tbone1004]

acrylic while stronger and more impact resistant scratches very easily, so no go there. Polycarb is less bad, but still doesn't offer the scratch resistance of glass which is why the cheap spg's tend to be foggy on the lens from abrasion.

Chromed brass is relatively cheap and easy to mill while still being mostly inert to salt water *same reason the vast majority of regulators are chromed brass*, stainless steel is expensive in comparison and a colossal pain in the a$$ to mill. Chromed brass is the standard in the industry for regulators, and with the exception of a very select few regulators, all first stages are made out of it, so it's a proven process and it's also a very easy one, no other metals other than SS are really suited for it, but SS would up the costs tremendously both from a raw material cost, but also the expense of machining them, I could realistically see a true doubling of the cost for that.

 

[Lorenzoid]

There might very well be some sort of plastic material that would work just as well, but what you may not realize is that pressure gauges with brass bodies and glass faces were around long before they were first adopted by divers. They were and still are used in all kinds of harsh industrial environments and have proven themselves over decades and decades. You ask about stainless steel, and I will add that the exact same gauges ARE available in stainless steel for use in even harsher environments (like where they might be exposed to acid), but they are much more expensive than brass gauges. So I would say that "Why reinvent the wheel?" is as good an answer as any other.

 

[rhwestfall]

as the chrome plating process continues to dwindle at the hands of the environmentalists, I wonder if there will be a shift in product, and B&G goes through a metamorphosis.......

 

[Akimbo]

as the chrome plating process continues to dwindle at the hands of the environmentalists, I wonder if there will be a shift in product, and B&G goes through a metamorphosis.......

I see a lot of manufacturers (not specific to diving) switching to stainless due to the price of brass, issues with Chrome plating, and modern CNC machining centers with high horsepower and flood cooling.

I suppose it is a race between who gets to the finish first: stainless replacing Brass, reliable enough AI computers (wireless or not), or higher-end plastic housings. I would take a well-designed glass-filled Nylon case with a glass or Polycarbonate lens over brass any day. All the lenses are clear when wet and they all scratch.

 

[Keith.M]

Because they are bad ass. ;-)

 

[Nemrod]

B&G SPGs are durable and reliable. Absolute accuracy is not required. Toughness and reliability is which eliminates plastic types and electronic types. I have owned numerous plastic SPGs and they invariably get squashed, smashed, cracked, full of salt water etc. And usually at the worst possible time.

I would be okay with a stainless or titanium housing with either polycarbonate or glass lens. If I am going to have to pay for the stainless or titanium then give me a mineral glass lens.

And while at it, a straight SPG instead of the 90 degree so I could see it by looking down at my left hip clip rather than having to grab it, unclip it and twist it to face me. Like this (and BTW, that is a polycarb lens about 3/8 inch thick. They can be re-polished when scratched):

N

 

[geoff w]

Thanks for all the replies. So basically, it's environmentally-unfriendly plating on materials and technologies dating back to steam locomotives, not necessarily accurate, but reasonably reliable and durable. I personally wouldn't trust electronics for something as critical as an SPG, but it's clear from the comments that brass and glass are good enough, even though there might be better material options

 

[tbone1004]

you will likely see a slow trend to move chromed brass to SS as the environmentalists start cracking down on the chroming process though there are some more environmentally friendly options that are coming around to plate brass, but if the industry moves to SS it will require a substantial cap-ex for all of the manufacturers to upgrade their CNC mills. The accuracy of the SPG's is limited only by their size, larger the gauge the more accurate it is, though the lower the working pressure the more accurate it is where you need it. These gauges are most accurate in the middle of their range, so the 5000psi gauges are perfect for cave diving because 2400psi is generally turn pressure so that is when you want your most accurate range. The problem is in recreational diving with the 250psi increment gauges and the divers trying to turn at 700psi and those gauges can be wildly off.

Keep in mind though that proper chrome plating is MUCH more resistant to salt water corrosion than even 316 stainless. Go look at backplates that see lots of use in salt and compare them to the tank valves and first stages, the chrome holds up a lot better, so I think the only way you'll see them move away from the chroming process is if they are forced to.