LI-er
Contributor
After decades of diving with a back mounted stony bottle (19cf and 30 cf) I made the switch to slinging it and I wish I had done so much sooner. With the back mount I could never get the trim right, either too much or too little lead in the opposite pocket to offset the bottle caused a roll tendency throughout the dive and the available weight increments on the charter boat often made it impossible to get just the right amount in that opposite pocket. Sometimes when I didn't require any weight such as no wetsuit and steel tank, I'd have to add up to 4 lbs to offset the bottle, which meant too much gas in my BCD and an annoying bubble that would rumble between the BCD wings, which would mess up my trim as much or more than the bottle itself, plus the increased drag from the partially inflated BCD and the extra lead. To make matters worse the backmounted tank interfered with the BCD wing on the same side and it would not properly inflate, thereby worsening the roll issue.
With the bottle slung under my arm there is a negligible tendency to roll which I can offset with a very small amount of weight (less than a half pound).
With the valve within reach I can dive with it off, eliminating any loss of gas during the dive. Which also means I don't need to monitor the gas in the tank after a predive check unless I use it to extend the dive, in which case it becomes a STage bottle rather than a Pony bottle. Hence the term "stony" bottle. If you post on ScubaBoard that you use a pony bottle to extend a dive you get a lot of flack but if you call it a stage bottle and monitor the gas, it quiets the masses. I use a second transmitter and receiver (which functions as a backup computer as well) for the stony bottle, but you could easily use a short hose or even just a first stage mounted mini spg depending on how you position the tank.
If the valve is left on, a freeflow or other leak would be quickly discovered. In the case of free flow the valve position will allow for feathering (turning valve on and off) as breaths are taken.
Even if mounted upside down, que @happy-diver, if dived with valve off it might be difficult to reach, especially under the stress in an emergency situation or during a possible entanglement situation.
Backmounted cylinders have caused accidents when the diver confuses it with the primary tank. This possibility is almost completely eliminated with a slung configuration which not only makes the reg and valve visible, but also gives the ability to bungee the reg to the tank and have easy access to it (I prefer the reg on a necklace).
It's easily removed to hand up to the crew when diving off a zodiac, other small craft, or in rough conditions when climbing the ladder, or any other time it's beneficial to lighten the rig. As I get older, being able to remove the bottle and not carry extra offsetting weights around a boat or up a ladder is very helpful. The bottle can be unclipped and placed at the splash point (typically at the stern boat rail) rather than carried back to your station.
A back mounted stony can make it difficult to stow your tank next to others on a boat in the typical rack arrangement.
Easier to swap tanks between dives as you aren't lifting the additional weight of the stony bottle and possibly extra offsetting weight. With a stony tank bracket you must remove the BCD then the stony bottle then reverse the process on the next tank. BCD mounted stony requires lifting the entire bulky rig or removing the bottle from the bracket and maybe also swapping tank positions to keep the stony bottle at the end of the rack so it fits without twisting the rig making it difficult or impossible to put on without removing it from the rack.
If tank mounted, and the main tank slips out of its strap while diving, sliding it back in may be more difficult. I used both types, and found my BCD strap mounted bracket to be more convenient but not all that secure. The stony slipped out of the thin strap a few times, so I added a ring and double ender clip around the valve which is essentially the same as slinging the top of the bottle.
Both types of brackets are expensive and add bulk to your gear bag. A sling requires nothing more than a ring, 2 clips and a short length of bungee.
It can be handed off to another diver to give safe space between the donor and recipient in a high stress situation where the recipient is OOA.
If I travel to a remote destination and prefer to solo dive (if permitted) but I don't want to lug all my gear, I can take my mask, computer & torch as usual but also my stony bottle (if rental not available) and clip it to a rental BCD (if it's equipped with D rings in the right place).
There are some tight squeezes I couldn't get through on wrecks because my back mounted stony valve would just catch an edge, which also exposes it to damage. Slung stony is out of the way and presents no such obstacle.
In case of a bad entanglement and low on gas, a diver could abandon their rig and ascend on the stony bottle, which itself when backmounted can present an entanglement hazard inside a wreck.
Slung bottle under the arm = less drag.
It looks way more cool and professional.
The single advantage of back mounted pony tank is that it's out of the way so there's no interference with a camera, hunting or other gear. During this past week when I dived with the slung configuraton for the first time (other than pool trials), I made slight adjustments by using different sized upper clips, tightening up the lower bungee, and sliding up the lower tank bracket so the bottle sits further back under my arm. It felt strange and bulky at first, but after a few dives I didn't even know it was there. It didn't interfere with my camera and provides a convenient arm rest.
Also worthy of note. This past week involved not only inter-continental travel but also local plane hops around Australia. I brought my whip so with each flight and required valve removal I could easily refill the stony bottle at my destination by siphoning off an available cylinder without requiring a visual inspection. None of the operators had an issue with it. Note that it takes at least 2 fills from two different full cylinders to get close to a full stony tank and don't use an HP cylinder for the second fill for reasons that should be obvious.
With the bottle slung under my arm there is a negligible tendency to roll which I can offset with a very small amount of weight (less than a half pound).
With the valve within reach I can dive with it off, eliminating any loss of gas during the dive. Which also means I don't need to monitor the gas in the tank after a predive check unless I use it to extend the dive, in which case it becomes a STage bottle rather than a Pony bottle. Hence the term "stony" bottle. If you post on ScubaBoard that you use a pony bottle to extend a dive you get a lot of flack but if you call it a stage bottle and monitor the gas, it quiets the masses. I use a second transmitter and receiver (which functions as a backup computer as well) for the stony bottle, but you could easily use a short hose or even just a first stage mounted mini spg depending on how you position the tank.
If the valve is left on, a freeflow or other leak would be quickly discovered. In the case of free flow the valve position will allow for feathering (turning valve on and off) as breaths are taken.
Even if mounted upside down, que @happy-diver, if dived with valve off it might be difficult to reach, especially under the stress in an emergency situation or during a possible entanglement situation.
Backmounted cylinders have caused accidents when the diver confuses it with the primary tank. This possibility is almost completely eliminated with a slung configuration which not only makes the reg and valve visible, but also gives the ability to bungee the reg to the tank and have easy access to it (I prefer the reg on a necklace).
It's easily removed to hand up to the crew when diving off a zodiac, other small craft, or in rough conditions when climbing the ladder, or any other time it's beneficial to lighten the rig. As I get older, being able to remove the bottle and not carry extra offsetting weights around a boat or up a ladder is very helpful. The bottle can be unclipped and placed at the splash point (typically at the stern boat rail) rather than carried back to your station.
A back mounted stony can make it difficult to stow your tank next to others on a boat in the typical rack arrangement.
Easier to swap tanks between dives as you aren't lifting the additional weight of the stony bottle and possibly extra offsetting weight. With a stony tank bracket you must remove the BCD then the stony bottle then reverse the process on the next tank. BCD mounted stony requires lifting the entire bulky rig or removing the bottle from the bracket and maybe also swapping tank positions to keep the stony bottle at the end of the rack so it fits without twisting the rig making it difficult or impossible to put on without removing it from the rack.
If tank mounted, and the main tank slips out of its strap while diving, sliding it back in may be more difficult. I used both types, and found my BCD strap mounted bracket to be more convenient but not all that secure. The stony slipped out of the thin strap a few times, so I added a ring and double ender clip around the valve which is essentially the same as slinging the top of the bottle.
Both types of brackets are expensive and add bulk to your gear bag. A sling requires nothing more than a ring, 2 clips and a short length of bungee.
It can be handed off to another diver to give safe space between the donor and recipient in a high stress situation where the recipient is OOA.
If I travel to a remote destination and prefer to solo dive (if permitted) but I don't want to lug all my gear, I can take my mask, computer & torch as usual but also my stony bottle (if rental not available) and clip it to a rental BCD (if it's equipped with D rings in the right place).
There are some tight squeezes I couldn't get through on wrecks because my back mounted stony valve would just catch an edge, which also exposes it to damage. Slung stony is out of the way and presents no such obstacle.
In case of a bad entanglement and low on gas, a diver could abandon their rig and ascend on the stony bottle, which itself when backmounted can present an entanglement hazard inside a wreck.
Slung bottle under the arm = less drag.
It looks way more cool and professional.
The single advantage of back mounted pony tank is that it's out of the way so there's no interference with a camera, hunting or other gear. During this past week when I dived with the slung configuraton for the first time (other than pool trials), I made slight adjustments by using different sized upper clips, tightening up the lower bungee, and sliding up the lower tank bracket so the bottle sits further back under my arm. It felt strange and bulky at first, but after a few dives I didn't even know it was there. It didn't interfere with my camera and provides a convenient arm rest.
Also worthy of note. This past week involved not only inter-continental travel but also local plane hops around Australia. I brought my whip so with each flight and required valve removal I could easily refill the stony bottle at my destination by siphoning off an available cylinder without requiring a visual inspection. None of the operators had an issue with it. Note that it takes at least 2 fills from two different full cylinders to get close to a full stony tank and don't use an HP cylinder for the second fill for reasons that should be obvious.
