What Is The Difference Between Bar And PSI?

Ever had that debate about whether Fahrenheit or Celsius is the superior measurement of temperature? No? Well, either way, both are used commonly, and each is favored for different reasons. A similar conversation is happening with divers.

For scuba enthusiasts, knowing the difference between Bar and PSI, both of which are pressure measurements, is essential, as manufacturers produce equipment that is rated according to both units - this can be frustrating.

Referring to the amount of force applied to something in relation to its area, PSI or pounds per square inch is measured by lbs, whilst bar is measured according to kilograms per square centimeter.

Once again, this is simply a good old fashioned imperial versus metric argument, so depending on whether you buy your equipment from the US or order it in from across the world, you might see either measurement represented on its packaging.

How Do They Relate to Diving?

When it comes to using Self-Contained Underwater Breathing Apparatus (SCUBA) equipment for underwater diving, we’re interested in atmospheres, which is the unit of measurement for pressure whilst underwater.

Comparatively, 1 PSI is equivalent to 0.0689 bars of pressure, whilst one full bar would equate to 14.5038 PSI, though you don’t have to fret over these numbers - there are plenty of online calculators to consult and convert for you.

You’ll need to think about pressures when diving for two main reasons: the pressure remaining inside your oxygen tank, and the atmospheric pressure that is surrounding your body as you’re submerged under water.

Diving instructors will have their preferences for each, but why is it important to understand both and the difference between them, we hear you cry? Allow us to take you on a deep dive…

Pressure: What Is It And Why Does It Matter?

Back to those two important reasons: atmospheric pressure and tank pressure.

Both are equally necessary to bear in mind for a safe and secure dive, but each serves a different purpose...

Atmospheric

Measuring the total amount of pressure being exerted on your fragile human form at whatever sea level you’re at, atmospheric pressure simply displays the weight of water pressing down on you, which increases the further below the depths you go.

As you descend by a further 10 meters, an additional one bar of pressure is added, for instance at 30m underwater you’ll be experiencing 4 bars of pressure, or 58.02 PSI, and you need to keep an eye on this number.

As the pressure increases on our body, so too does the volume of air in our body decrease: at one bar of pressure, our body’s volume of air is 1. Once you reach 4 bars, that figure is just a quarter.

In order to prevent any perforation or rupturing of the eardrums, as well as protect the capillaries in your eyes, you need to equalize the pressure of your air spaces in your ears, mask, sinuses and lungs.

Tank

Tank pressure is imperative for reading your tank’s SPG, or submersible pressure gauge, which is hooked up to your tank using a specialist pressurized hose, measuring up essentially how much air there is left for you to breathe in there.

Your tank could measure this incredibly important, life-depending information in Bar OR PSI - a typical tank will have 200 bars or 300 PSI of pressure in when full, which depletes as you go, and you want to know when you’re about to run out, right?

Which Pressure Measurement Is Best?

The answer you receive to this question will be determined by who you ask, as it’s entirely subjective; again, depending on where you are in the world and whether that area uses imperial or metric measurements, one or the other will have precedence.

However, it is possible to argue that using the metric system to calculate, for instance, how much air remains in your oxygen tank, is much easier, because bar proves for simpler mathematics.

Typically, your tank’s capacity will offer 10, 12, 15 or 18 liters of air when full, and to work out how much air remains, all you need to do is multiply the total capacity with how much pressure you have left. 

If measured in bar, your calculation could be as simple as 12L multiplied by 200, much easier than trying to work out the equivalent in PSI, as this is measured in ft cubed. The equivalent of one liter is 0.0353147ft³ - try multiplying that in a hurry!

As a result, tanks which have a pressure gauge utilizing bars are preferred amongst diving amateurs and professionals alike, though again, we aren’t saying this is the case for everybody!

Nowadays, though, manufacturers have taken to displaying their tank pressure in PSI and Bar both, in order to provide divers with the best of both worlds, so there’s no need to panic, as it’s likely no maths will be necessary!

When Might PSI Prove More Useful?

Okay, so we’ve already established that bars are best in the eyes of most divers.

However, let’s say you’re a professional about to embark on a technical cave dive. You might want to use a tank that expresses your air measurement in PSI, because of the three-thirds rule commonly utilized in SCUBA diving.

What this basically means is that one third of your gas is for the outbound dive or descent, one third is for your return to the surface, and the final third is kept as a reserve just in case of emergencies, especially important on a technical trip.

A standard tank for cave diving has typically been filled to 3000 PSI at the start of your dive, so of course understanding each third as 1000 PSI is much easier and simpler than trying to convert anything to bars.

Because there’s more at stake and the potential for error is wider, being able to see at a glance exactly how much air you have left is much more important, so the quicker and easier calculation is of course preferred.

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