Metric vs imperial system in diving: what are the differences?
When you go diving as a European diver in Florida, Mexico or parts of the Caribbean, you soon notice that not everyone uses the same units of measurement. Where we are used to BAR, meters and liters, many dive centers there work with PSI, feet and cubic feet.
This is because many destinations are strongly geared towards American divers and therefore use the imperial system instead of the metric system we are familiar with in Europe. In this article we explain what the differences are, where you will encounter which systems and why you usually don’t notice much of that underwater.
Metric vs imperial: the main differences
In diving, different items are displayed differently depending on which system a country or dive center uses.
| Part | Metric system | Imperial system |
|---|---|---|
| Tank pressure | BAR | PSI |
| Depth | meters | feet |
| Tank capacity | liters | cubic feet (cu ft) |
In Europe, we almost always use the metric system. Many American diving destinations, on the other hand, work with the imperial system.
BAR versus PSI
The biggest difference that most divers notice right away is the tank pressure. In Europe you usually hear, “I start at 200 bar,” “We turn over at 100 bar,” “Reserve is 50 bar.” In American systems, it sounds more like, “My tank has 3000 PSI,” “We turn over at 1500 PSI,” “Our reserve is 700 PSI.”
BAR and PSI both simply measure pressure, just in a different unit. This table shows a handy comparison:
| BAR | PSI |
|---|---|
| 50 bar | ± 700 PSI |
| 100 bar | ± 1450 PSI |
| 150 bar | ± 2200 PSI |
| 200 bar | ± 3000 PSI |
Where do they usually use PSI?
Destinations with a lot of American influence often use the imperial system:
- Florida, Puerto Rico, Bahamas, Cayman Islands, Belize, Honduras (Roatán & Utila), Mexico (Cozumel, Cancun, Playa del Carmen), Panama, Costa Rica
Here, dive centers often default to PSI, feet and cubic feet.
Where do they usually use BAR?
Destinations with European influence usually operate metrically:
- Europe, Bonaire, Curaçao, Aruba, Saba, St. Eustatius, French Caribbean
There, briefings usually proceed in bar, meters and liters.
Tank sizes: liters vs cubic feet
The contents of dive tanks are also displayed differently in American diving areas. Whereas in Europe we usually talk about liters, many dive centers in Florida, Mexico and parts of the Caribbean use cubic feet (cu ft).
The standard rental bottle there is often an 80 cu ft tank. That works out to about 11.1 liters, which is just slightly smaller than the 12-liter bottle many European divers are used to.
If you book a larger tank, you often get a 100 cu ft bottle. This has a capacity of about 13 liters. That size is actually not used very often in Europe, but is actually a popular upgrade in American diving areas for divers with higher air consumption.
A really large bottle of about 15 liters is equivalent to 130 cu ft. However, that type of tank is harder to find in many U.S. diving areas. Many diving schools and liveaboards only have a limited number of them available, which often makes them more expensive or requires advance reservations.
Good to be aware of this, so you won’t have any surprises when you think you’re booking a “big tank” but end up getting a bottle smaller than the 15-liter tanks common in Europe.
| European size | American size |
|---|---|
| 11.1 liters | 80 cu ft |
| 13 liters | 100 cu ft |
| 15 liters | 130 cu ft |
The best known rental bottle in American diving areas is the:
- AL80 → aluminum 80 cubic feet
This is comparable to a bottle of about 11 liters. So it doesn’t quite match the sizes you’re probably used to, so keep this in mind.
Bring your own gear? Then you’ll notice almost nothing. Do you use your own dive computer and pressure gauge? Then everything usually just keeps working in BAR as you are used to. Under water, little changes. Only during briefings or communication with guides it is smart to coordinate which hand signals you use. American guides often automatically think in PSI, while European divers are used to BAR signals.
It is also good to consider the sizes of the tanks. So the “12L tanks” are slightly smaller.
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