SPG in bars

Three more reasons diving metric is better

Of the 197 countries in the world, only three do not use the metric system. These are Myanmar, Liberia…and the United States. Myanmar and Liberia use the old Imperial system of measurements once common throughout the UK and Commonwealth. The USA uses a close cousin of the Imperial system called United States customary units.

Ironically, were it not for pirates, the USA would likely have joined the rest of the industrialized world in using the Système international two centuries ago. Doing so would undoubtedly have made our life as divers — and especially as cave divers — easier. Here are three more reasons why.

A happy coincidence

Before readers in inundate us with critical comments, let’s be clear for the sake of accuracy:

  • You will often hear or read that ten meters of seawater equals one atmosphere of pressure. That’s not entirely accurate. The more precise value is 10.09 meters — but that’s still pretty damn close.
  • You will also hear or read that one bar and one atmosphere represent the same amount of pressure. They don’t. One atmosphere is actually 1.01325 bars — close enough that bars and atmospheres are essentially the same.

What’s most important is that, for our purposes, we can assume water pressure increases by one atmosphere for every ten meters of depth in saltwater. The fact it does so is coincidental — but it’s a very happy coincidence indeed. Here is are some examples of why:

  • Despite the fact there are only three countries on Earth that do not use the metric system, most diver training agencies base their training standards on ten-foot increments.
  • As an example, the recommended depth limit for unsupervised dive teams with entry-level training is 60 feet. That’s 18 meters. This must strike most divers (who grew up with the metric system) as bizarre. Why 18 meters? Why not a round number like 20 meters?

Now consider this:

  • Divemaster and instructor candidates must often solve physics problems which require them to convert depth to atmospheres.
  • To do this, non-metric users must divide saltwater depth in feet by 33, then add one atmospheres to the result. Some people can do this accurately in their heads; most of us cannot.
  • In contrast, doing this calculation when working in meters chiefly involves just moving a decimal point. If the seawater depth is 18.0 meters, you move the decimal point one place to the left, giving you a value of 1.80. Now add one to this and you discover the depth in atmospheres absolute is 2.8 atm. This is the sort of math anyone can do easily in their head.

Let’s face it: Our life as divers would be easier if we all just went metric.

A better way to measure cylinder capacity

Divers who rely on non-metric values to measure cylinder capacity are severely handicapped in doing so. This is due to the counter-intuitive way we measure tank size in the USA.

How often have you run into recreational divers using different size cylinders who nevertheless believe they each have the same amount of air because both cylinders are filled to 3,000 psi? This type of thinking only works when divers all use identical cylinders.

In contrast, metric divers measure tank size based on a cylinder’s liquid capacity. Doing so means:

  • The standard aluminum 80 has a liquid capacity of roughly 11 liters.
  • Steel LP 85s and HP 100s have a liquid capacity of roughly 13 liters.
  • Steel LP 95s and HP 120s have a liquid capacity of roughly 15 liters.

Knowing how much gas you actually have at sea-level pressure is simply a matter of multiplying cylinder pressure in bars by tank capacity in liters. In other words, an 11-liter cylinder filled to 200 bars holds the equivalent of 2,200 liters a sea-level (200 times 11). As we will discuss shortly, this can make the process of gas matching much easier.

To illustrate just how screwed up the non-metric approach to cylinder capacity is, consider a question which appeared on a certain training agency’s instructor exam for several years. The question read: How much air does an aluminum 80 hold at a depth of 66 feet?

  • This question was intended to fool instructor candidates into thinking that, because the cylinder was at a depth of three atmospheres, it would have one-third of its rated capacity or a little under 27 cubic feet.
  • The answer the test was looking for was 80 cubic feet. This assumed that, because a scuba cylinder is relatively inflexible, it holds the same volume of air at depth as it does at the surface.
  • Except this is not accurate. The cylinder in question never had 80 cubic feet of air in it, regardless of ambient or internal pressure. What it had was the equivalent of 80 cubic feet at sea level — but only if filled to its rated capacity of 3,000 psi at an ambient temperature of 22° C/72° F. Regardless of depth or internal pressure, the cylinder will always contain just eleven liters of air — albeit at varying pressures and densities.

And, should you want to split hairs, most aluminum cylinders, when full, actually contain the equivalent of 78.8 cubic feet at sea-level pressure. So there.

Metric system users never have to worry about this sort of nonsense.

Gas matching in your head

For cave divers using Imperial measurements, gas matching isn’t a lot of fun. Most students learn to do this using baseline multipliers. Doing this accurately requires either exceptional math skills or a calculator. The catch is, you can’t take a calculator with you in the water. And that’s really where this sort of gas-matching calculations need to take place.

For cave divers using the metric system, gas matching is so easy you can often do it in your head. For example:

  • Hans is diving a twinset consisting of two 15-liter cylinders (30 liters total). Dietrich is using twin 13-liter cylinders (26 liters total). Both sets are filled to 210 bars.
  • Multiplying cylinder capacity by fill pressure, Hans’ starting volume is 6,300 liters. Dietrich’s is 5,460 liters. This makes Dietrich the controlling diver.
  • Dietrich’s usable gas is one-third of 210 bars or 70 bars. That’s the equivalent of 1,820 liters (70 bars times 26 liters). His turn pressure is two-thirds of 210 bars or 140 bars.
  • Hans’ cylinders hold this much gas at a little over 60 bars. You get this by dividing Dietrich’s usable gas volume of 1,820 liters by Han’s twinset’s liquid capacity of 30 liters. Subtracting 60 bars from 210 bars, Hans gets a turn pressure of 150 bars.
  • This is easier to understand and much less prone to error.

Should you switch?

If you live or dive someplace where the metric system is the norm, you’ve likely read this far with the smug assurance your life is infinitely easier because of it. But what if you are not so fortunate? Should you make the switch? After all, doing so will likely entail no more than resetting your dive computer and investing in new SPGs.

My own situation is probably typical of that of many readers. As much as I might like to go metric, the fact remains the students and others I dive with are stuck with the Imperial system. If nothing else, gas matching becomes problematic if we do not all use the same measurements.

However, if you generally dive with the same buddy or buddies, you might collectively decide to go metric. It will make your life easier.

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