As a scuba diver, you will certainly come across the terms ‘Bar’ and/or ‘PSI.’ But what exactly do they mean? Well, as a scuba instructor, I hear this question a lot, so here we are going to look at what is Bar pressure vs. PSI pressure, what they are, the differences between them, and how it affects us while diving.
First up is a bit of information on the two most commonly used ways of measuring pressure.
What is Bar Pressure? (Metric)
Pressure is the measure of force exerted on an object and bar is the measure of how much force is being exerted.
Bar is used by the metric system for measuring pressure; Kilograms per square centimeter (Kg/cm2). This is the most commonly used system around the world in scuba diving.
In fact, only three countries in the world don’t use this system (more on those later) so chances are you will be learning to use bar when you first start out scuba diving.
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What is PSI Pressure? (Imperial)
As well as the Metric system we have the Imperial system. Imperial measurements are now only used by The United States of America, Myanmar and Liberia. From these countries, the USA unsurprisingly has the largest diving community.
These countries use what is called PSI or pounds per square inch (lb./in2) for measuring pressure.
As you can see from above, what system you learn to dive with will very much depend on where you learn to dive (or where your instructor is from!).
What is Bar or PSI measurements used for?
As scuba divers, we are breathing from our SCUBA unit, and relying on the gas inside our tanks/cylinders. This means we need to know how much is in our tank at all times so we can plan our dive and decide when to go up safely.
A submersible pressure gauge (SPG) will tell us how much gas/pressure is in our tank. It will tell us using pressure measurements of bar or PSI. This is why it is so important to know what these terms mean, and how to read your pressure gauge underwater.
It is also very good to know both units of pressure measurement. I can vouch for this from firsthand experience. While working as a newly certified Divemaster, I asked my group of divers underwater how much air (Pressure) they had left, to which I got three expected replies and one very strange set of hand signals. After asking the diver to repeat their answer it was only then that I figured out this diver was using PSI hand-signals and not bar. Now, this is where the fun started, where I had to convert PSI measurements to bar in my head so I would know if the diver was low on air or if we could continue the dive.
Converting Bar to PSI
Hope you guys are ready for a bit of math.
1 BAR is equal to roughly 14.5037738 PSI (How’s your 14.5 timestables?)
As you can see with these numbers, trying to do conversions in your head can be a little bit tricky. Below are formulas to help you work these a little bit quicker.
|Bar to PSI Formula||PSI to Bar Formula|
|Multiply the amount in bar by 14.5|
Example: 100 bar x 14.5 = 1450 psi
|Divide the amount PSI by 14.5|
Example: 2300 psi ÷ 14.5 = 158.6 bar
If, like me, math is not one of your strongest skills and these formulas are still a bit tricky, then it might be helpful to try to memorise some numbers. As a diver, what I have done in the past is to have some of the more crucial numbers and conversions written down on an underwater slate.
These are the numbers I usually have written down: –
|150 bar||2175 psi||This number or above means there is plenty of dive time left|
|120 bar||1740 psi||Roughly halfway through the dive|
|100 bar||1450 psi||Time to head back to the exit point|
|70 bar||1015 psi||Start the safety stop|
|50 bar||725 psi||Time to end the dive|
Atmospheric Pressure in Bar
Thankfully the math part is over, so now it’s time for some science.
So far, we have been talking about pressure in terms of how much air or gas is in our tanks. One other type of pressure that is crucial in scuba diving is the atmospheric pressure and how it changes at depth while you are diving.
Atmospheric pressure is the amount of pressure exerted at sea level. This is the weight of the atmosphere pushing down on an object. You will see this written down as ATM.
1 atmosphere (ATM) is roughly equal to 1 bar of pressure; this makes measuring pressure in bar much easier.
1 ATM on the other hand, is equal to around 14.7 PSI.
When diving we descend to various different depths depending on the dive site and certification level. One thing that remains the same is the change in pressure, as water is much denser (heavier) than air, the pressure increase happens much faster underwater.
For every 10 meters (32 feet) of depth we will have another ATM of pressure acting upon our bodies. We won’t feel this pressure change on most of our body but we will feel it in our air spaces (such as our ears) which is why equalizing is so important.
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Below is a table showing the changes in atmospheric pressure with depth for Metric (Bar) and Imperial (PSI)
|Depth Meters||Pressure (Bar)||Depth Feet||Pressure (PSI)|
Not sure about you but I find working out the atmospheric pressure a lot easier using Bar, doing it with PSI can be a little more challenging as you can see with the numbers.
Diving Air Pressure
So what affect does the atmospheric pressure have on divers, other than needing to equalize our air spaces? The big factor it affects is air consumption and how fast we are using up our air/gas from the tank.
Due to the increase in pressure we will breathe more air from our tanks at an increased depth. A good example is, hypothetically, if a diver takes one hour to breathe a tank empty on the surface how long would it take to empty the tank at 20 meters?
|Depth (Meters)||Pressure (Bar)||Time to empty tank (minutes)|
As we see from the table at 20 meters we are under 3 bar (ATM) of pressure. This means we would breathe the air 3 x quicker than if we were on the surfacing. This results in the tank only lasting 20 minutes.
This shows that an increase in pressure will increase the rate at which the air is consumed. Bare in mind that this is an example with no other factors applied.
Difference between BAR and PSI
By now you have probably realized that there isn’t actually much difference between bar and PSI. They are both a way of measuring pressure much in the same way miles and kilometers measure distance. So the question should be, is one better than the other?
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This is a great way to start a debate amongst divers. Depending on where you have been taught, who taught you, and where you dive will impact your preference. Like defending your favorite sports team, divers will defend the system they use as this is what they know best.
Personally, each has their pros and cons, however, I do find bar is much easier when it comes to doing math and calculations on atmospheric pressure (with my math skills this is a big plus).
PSI is generally preferred by technical divers as it makes the rule of thirds easier to manage. It will really boil down to the individual on which unit of measurement they prefer.
At the start of this article, we asked what is bar pressure, and how does it differ from PSI pressure? Hopefully by reading this, you know understand bar pressure and PSI pressure and why we have the two systems in place.
Converting between the two units can be a little confusing. Luckily nowadays you can find many different SPG’s (submersible pressure gauges) with both bar and PSI dials on them, which makes doing the calculation in your head unnecessary. If you have an SPG with only one system, you can always use the calculations above or use the cheat sheet numbers I used on my slates to help you out.