Since I took my first plunge into the big blue back in 2009 I have been diving in the metric system. But I am comfortable with both, up to a point.

The Metric System

Depths are measured in metres (on my dive computer to the nearest 10cm). Tank sizes are measured in litres – this is the amount of water it would take to fill the tank. And tank pressure is measured in bar. Which makes calculating the volume of the gas in your cylinder easy. Just take the number of litres and multiply it by the bar and you are done. Most of the calculations can be done in your head.

The Imperial System

Depths are measured in feet (on my dive computer to the nearest foot, which is approximately 30cm so it doesn’t feel as accurate). Tank sizes are measured by the amount of gas that they hold ie 80 cubic feet. Pressure is measured in psi. And working out many of the calculations requires a pen and paper if not a calculator.

I prefer the metric system, to me it makes sense. The imperial system feels overly complicated and it doesn’t fit nicely.

Metric or Imperial RDP?

However, not only do we have these two different systems of measurement but due to them we have RDP tables in both metric and imperial. When we view these tables we can see that there are some slight differences. In fact some of the differences can be really big. I remember from my open water course that calculating the minimum surface interval in metric for a particular dive gave a significantly smaller time than for the calculation using imperial.

Why are there these differences? Well it is all really just down to the rounding that has been employed. For example using PADI metric RDP/eRDP a dive to 10m/33ft has NDL of 219mins, using PADI Imperial/US RDP/eRDP the same dive has NDL of 205 minutes.

Which one should you follow? Just choose whichever unit you are more familiar with, and stick with it. Don’t change tables between dives. Stay on either metric or imperial. Better yet, get a dive computer and it will track everything for you making your diving safer and a little easier. There is absolutely no difference in how one table operates compared with another. The methods that you use on the metric are exactly the same as those on the imperial table.