Always Surface with 50 bar
This is a gem of dogma we come across in nearly every dive briefing. To be back on the boat with 50 bar left in our cylinders is the ideal. It all comes from way back when military divers used to have a twin-set with isolator valve, single regulator and no pressure gauge.
The divers would keep the isolator closed, breathe one cylinder down until the regulator got sticky, balance the gas between cylinders by briefly opening the isolator, then repeat the process. They would then have 25% of their original gas left, and ascend.
While dive gear has progressed and pressure gauges are pretty much standard equipment, the balancing practice is still used by military, police and commercial divers working in zero visibility.
If the original cylinder pressure were the US standard 207 bar, this double balancing routine would result in divers beginning their ascent on 52 bar. This got rounded in numbers and time to become the sport-diving dogma of finishing a dive with 50 bar remaining.
Just how much gas a cylinder pressure of 50 bar really provides and what we can do with it all depends on the cylinder capacity. On a 12-litre cylinder, 50 bar is 600 litres, or 15 minutes’ diving at a depth of 10m and a surface RMV (Respiratory Minute Volume) of 20 litres/ minute. The underwater time would drop to 12.5 minutes for a 10-litre cylinder, or stretch to almost 19 minutes for a 15-litre tank. If the quantity of gas left on surfacing was really critical, the magic pressure should be more for a 10-litre cylinder and could be less for a 15-litre cylinder, but that’s not what we are told. On a shallow dive, or the shallow part of a multi-level dive, we could safely use a fair part of this remaining gas and still have enough left to handle problems and the final few metres of ascent. Pottering about at 5 or 6m while using up some of this gas will actually make our dive safer from a decompression point of view.
When we get on to more complicated diving, such as decompression, cave or technical diving, 50 bar does not enter into the equations.Divers learn to plan their gas requirements so that they have enough gas to return safely to the surface following any single equipment failure or the need to share with one other diver.
No-decompression cave-diving begins planning with a rule of thirds: 1/3 in, 1/3 out, and 1/3 reserve, or about 70 bar depending on starting pressure. But this is only a starting point. Detailed planning to allow for decompression may lead to a greater reserve pressure; or to less, where multiple stage cylinders are brought into the calculation.
So 50 bar is not a hard and fast rule. It’s just an arbitrary number with plenty of margin to keep beginners very comfortably safe.