A recent discussion on the facebook group 'Divers Forum' was prompted by a question as to whether or not divers should use an analogue console, in light of the fact that modern dive computers often include digital compasses and air-integration.
The short answer to that question – with regards to the analogue SPG, at least – is 'yes'.
They are not perfect – they are not always 100 per cent accurate. However, an analogue SPG, whether is used as your main source of information for your air supply, or merely a backup to your air-integrated computer, will never run out of batteries.
The advent of air-integrated computers is not necessarily a bad thing. They allow for a much more complete display of available resources including bottom time and no decompression limits – standard for all dive computers – but they also factor into the equation your remaining air supply and (in some cases) current air consumption rate, thereby giving a much more rounded, overall picture of the information required to complete your dive safely.
But the batteries are a problem.
In most of the scenarios I have dealt with over the years, the failure of a dive computer battery is not much more than an inconvenience. If the battery is user-changeable then it might be a simple matter of delaying the dive (assuming a spare battery kit is available), but if the computer is not air-integrated then personal and team judgement can be used to determine whether the diver can still participate in the dive.
This depends on the circumstances, dive site, conditions and the experience of the dive team but – for example – if the dive team has six members and one computer fails, then we still have up to five computers and / or timing devices which we can use to measure the dive profile. We can change the dive plan to err on the side of caution and remain at much shallower depths, revert to dive tables to ensure we don't exceed recommended limits, or perhaps change to a different dive site if necessary.
If the batteries on an air-integrated computer run out, however – and it's not possible to replace them – then there are only two alternatives: switch the regulator set or dive computer for a spare if available, or stay out of the water. Dive time and depth can be worked around; air supply cannot.
One comment in the discussion suggested that people should look out for the low-battery warning, but the power characteristics of the lithium Ion batteries are such that they remain at full charge until they get very close to the end of their lifespan. It may be the case that a diver begins their holiday with a working computer, but then it runs out half-way through. It might be possible to rent a regulator set from the dive centre, however if you're out on a liveaboard with limited spares, then this may not be an option.
Wireless air-integrated computers add an extra point of failure to the situation as there are now two batteries involved – one for the transmitter and one for the computer. It's also possible (but rare, in my experience) that incorrectly configured wireless computers may actually read another diver's air supply.
Analogue SPGs and depth gauges are not much more than stiff springs, which expand and contract depending on the pressure to which they are subjected. A series of gears moves the needle on the display to correspond with that pressure.
Like all springs, they deteriorate over time with repeated use, and even a new SPG can read within +/- 10% of the actual amount of air in the tank, but this means it might read 45 or 55 bar when it should be reading 50. This is not so much of a problem, assuming the diver is adhering to safe diving practices.
The main problem with defective readings from analogue SPGs arises from divers who allow water into the first stage of the regulator (often during cleaning without fixing the dust cap in place, hence the dire warnings at every dive centre in the world), which is then forced into the mechanism when it is put under pressure, corroding the SPG from the insides.
With proper care, however, they will last for hundreds, even thousands, of dives and remain a reliable source of air pressure readings. A small SPG and hose can be purchased for around £50 for a basic model, a fairly small price to pay for the added security and convenience of ensuring you are able to read your remaining air supply.