There are three major schools of thought for a SPG on a slung pony or deco bottle.
1. Use a full size SPG on a 6" or 7" HP hose bungeed to the tank valve.
- Advantages = full size gauge is easy to read before during and after the dive so you always know how full the tank is.
- Disadvantages = More clutter during the dive, cost, and the bent HP hose will quickly fail if stored in the bungeed to the valve position. Ths approach also adds a high pressure spool that can fail and unless it is made of brass, the non boot protected SPG itself is vulnerable to damage.
2. Use a button gauge on the first stage
- Advantages = Easy to check pre dive to ensure tank is full/nearly full, no extra clutter underwater, no hose to fail. Even the traditional very small button gauges can be read under water if the need arises (with some difficulty, if adequate light is available) but the current crop of larger sized button SPG's (with a face between nickel and quarter in size) are much easier to read underwater and are still low profile. Plus button gauges are inexpensive at under $40.00. They are also durable and require no extra o-rings, hoses or HP spools that could fail/leak.
- Disadvantages = harder to read underwater than a full size gauge, especially in low light situations and the small scale on the button sized face is less precise than a full size SPG.
3. Don't use an SPG on the pony reg at all.
- Advantages = no clutter, no extra parts to fail or break.
- Disadvantages = You still need a tank presure gauge of some sort to check the tank pre-dive. If you use your regular regulator, it takes time and each time you check the tank by connecting your primary, you use a volume of air equal to a beath or two of air at safety stop depths. You can buy a dedicated gauge designed to check tank pressure on the surface, but they tend to cost as much as or more than a button gauge or SPG. You cannot check tank pressure under water - but at the same time, if you are using the pony or deco bottle you should be on your way to the surface and at that point you either have enough gas or you don't and knowing the pressure during the dive will not solve a gas planning failure.
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Of the three approaches I prefer the button gauge approach as it is easy to check the pressure on the surface with no need to connect and disconnect other regs or gauges with the associated loss of additional gas that occurs over time. And while a button gauge is not as precise as a full size gauge it is accurate enough to tell you if the tank is full or not. The newer larger sized button gauges also give you a reasonable ability to verify the contents underwater if you just have the urge to know without the associated failure points and reduced streamlining of a gull size SPG on a 6" or 7" HP hose.
