For beginner scuba divers, it might seem like there’s so much new information to learn. SPGs and SMBs, first stages and second stages, PSI and bar…. What?!
Depending on where you did your open water course, it is likely you’ll have heard of either PSI or bar. Both are measurements of pressure.
What’s the difference between them?
Here we go into the differences between bar and PSI and why it’s important to understand both as a scuba diver.
Let’s dive in…
What is Bar and PSI?
Both PSI and bar are measurements of pressure, which is defined as “the amount of force applied per unit of area”.
1 Bar is equal to the atmospheric pressure at sea level, the equivalent of 100,000 pascals (100 kPa) or kilograms per square centimeter (kg/cm²). This is a metric measurement.
PSI, standing for Pounds per Square Inch, is the equivalent of 1 pound of pressure per every square inch of space (lb/in²). This is an imperial measurement.
In scuba diving, these measurements are relevant for two reasons:
- The atmospheric pressure surrounding your body
- The pressure inside your tank
Atmospheric pressure is the amount of pressure exerted at sea level. This is essentially the weight of the whole atmosphere pressing down on an object, for example, your body.
This is known as 1 atmosphere (or ATM) of pressure.
1 ATM is roughly equivalent to 1 Bar: 1.013 bar = 1 ATM. As a result, it is much easier to measure atmospheric pressure in Bars than PSI, which is around 14.7 PSI to 1 ATM.
As you descend into the water, which is much denser than the atmosphere, you accumulate atmospheric pressure more quickly. For every 10m descended, an extra 1 bar of pressure will press down on you. Whilst this doesn’t affect our mostly liquid bodies, it does affect our air spaces which is why equalizing is so important.
Check out the table below for a quick conversion.
Atmospheric pressure can also be measured in PSI but the numbers aren’t as simple. Check out the table below:
Not quite as easy to calculate, but still doable!
The most common place you will encounter bar or PSI when scuba diving is when looking at your SPG. Your SPG (submersible pressure gauge) is connected to your tank using a pressurized hose, so it can accurately measure the pressure in your tank, which is directly proportional to the amount of air you have left.
Depending on where your SPG was manufactured, it might display either bar, PSI or both.
A scuba dive usually begins with a full tank – around 200 bar or 300 PSI. From this measurement, you know when you have a half tank (100 bar or 1500 PSI) and when you’re low on air (50 bar or 750 PSI) and should end the dive.
Why is it important to understand both?
Now, you’re probably wondering why we have two measurements… And if it’s important for a scuba diver to understand both bar and PSI.
Well, we have two measurements as a result of the metric and imperial systems. The imperial system is mostly found in the US, with bar used in metric predominant Europe and Asia.
It is not vital to fully understand both systems, but it is a good idea to be aware of full tank measurements. If you are planning to visit other countries which use the other system than you’re used to – it’s good to not be taken by surprise once you’re on the dive boat! It’s no different than having to learn a rough conversion between miles and kilometers, or Fahrenheit and Celsius.
Is PSI or Bar Better?
Ah, now we get to the controversial bit! Divers tend to get very attached to the system they learned to dive with, which is only natural.
But could one be making your life easier?
It can’t be denied that the numbers on the metric system make calculations a whole lot easier.
Calculating the air left in your tank with bar is very simple.
Tanks usually have capacities of 10, 12, 15 and 18L. In order to work out the volume of air remaining, you just multiple the tank capacity with the remaining pressure:
12L tank x 200 bar = 2,400L of air
Doing this calculation in imperial measurements is more complicated – just try remembering that 1L is the equivalent of 0.0353147ft³ on the fly! These measurements can be used to help to work out your surface air consumption, or SAC rate.
Doing calculations for Nitrox partial pressures is also much simpler with the metric system. Have you taken the plunge with enriched air yet? Check out our Ultimate Guide to Nitrox here!
If you want to know more about the partial pressures of gases at depth, check out this super helpful video!
On the defense of imperial systems, they are often favored by technical cave divers. These divers dive using the three-thirds rule – one-third of the tank is used for the outward journey, one third is for the return trip and one third is reserved for safety. Tanks filled to 3000 PSI make this calculation much easier.
1 Bar is equal to 14.5037738 PSI
Bar to PSI Formula:
Multiply bar value by 14.5037738
E.g. How many PSI is 4 bar?
4 x 14.5037738 = 58.0150952 PSI in 4 bar
PSI to Bar Formula:
Divide PSI value by 14.5037768 or multiply by 0.0689475728
E.g. How many bars in 50 PSI?
50 ÷ 14.5037768 = 3.44 bar
50 x 0.0689475728 = 3.44 bar
To help you out, we’ve put together a helpful Bar to PSI conversion tool for you!
We hope you’ve learned a little more about the measurements of pressure in scuba diving and it’s important to know about both.
Wanting to learn more about the physics of scuba diving?
Check out our blog on thermoclines next!