Choice of Pony Bottle Size
Given that the single function of the pony bottle is to provide a source of breathing gas for a controlled and prudent ascent to the surface in an emergency situation, it is most important that the pony bottle size is chosen to be sufficient for that purpose. Even when doing no decompression diving, the total reserves of breathing gas must still be sufficient to supply three phases of the ascent:
- enough gas to allow for a short period at depth to quickly sort out any problems, if necessary, before returning to the surface
- enough gas to make a safe gradual ascent to safety stop depth and
- enough gas to do a complete safety stop.
At the end of this time the pony bottle should still hold enough gas to provide sufficient pressure for smooth flow from the regulator first stage.
| Stage of ascent | Max depth 15 metres (49 ft) | Max depth 20 metres (66 ft) | Max depth 30 metres (98 ft) | Max depth 40 metres (130 ft) |
|---|---|---|---|---|
| Sort out problem: 2 minutes at max depth | 150 litres (5.3 cu ft) | 180 litres (6.4 cu ft) | 240 litres (8.5 cu ft) | 300 litres (11 cu ft) |
| Ascent from max depth to 5 metres (16 ft): ascending at 9 metre per minute (30 ft/min) | 67 litres (2.4 cu ft) | 113 litres (4.0 cu ft) | 229 litres (8.1 cu ft) | 379 litres (13.4 cu ft) |
| Safety stop: 3 minutes at 5 metres (16 ft) | 135 litres (4.8 cu ft) | 135 litres (4.8 cu ft) | 135 litres (4.8 cu ft) | 135 litres (4.8 cu ft) |
| Total | 352 litres (12.4 cu ft) | 428 litres (15.1 cu ft) | 604 litres (21.3 cu ft) | 814 litres (28.7 cu ft) |
| Air available at 150 bars (2,200 psi) | ||||
| Pony bottle 3 litre | 450 litres (16 cu ft) | 450 litres (16 cu ft) | 450 litres (16 cu ft) | 450 litres (16 cu ft) |
| Pony bottle 6 litre | 900 litres (32 cu ft) | 900 litres (32 cu ft) | 900 litres (32 cu ft) | 900 litres (32 cu ft) |
The table above is constructed to show gas consumed in just such a scenario: 2 minutes at depth for "sort-out"; a safe rate of ascent to 5 meters; followed by a 3 minute safety stop. Calculations are based on a heavy breathing rate of 30 L/min (1.06 cu ft/min) and an initial tank pressure of 150 bar (2,200 psi). In this particular scenario the 3 litre pony is just sufficient for diving at 20 meters but not 30 meters. It is important that anyone selecting a pony bottle should do such an analysis for his/her own breathing rates, cylinder pressure to be used, and required ascent profile, or take advice in the selection. Also it is important to note that since monitoring of remaining air is a key issue in ascent, a submersible pressure gauge readable at all times also needs to be attached to any pony bottle system.
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