Scuba Diving Cylinder FAQ

1. What mix of gases do air tanks contain?

This will depend very much on what type of diving you are doing and also your own personal preference.  Standard tanks come with roughly 79% nitrogen and 21% oxygen which replicate the air we normally breathe on land.   There are also different mixes available such as 'bottom gas'.  This is used when divers are descending to more extreme depths (at which point the standard mix becomes toxic due to compression).  Bottom gas is a normally a helium based option which has less than 21% oxygen.

2. Should you rent or buy gas tanks?

If you have just started diving then you may not know if this sport will suit you yet.  It can be a good idea to rent equipment in the beginning (usually available direct from the diving school).  Once you have decided you like diving and want to do more of this sport then you can invest in your own equipment.  If you are diving on holiday then you will not be able to take bulky diving equipment with you but in most cases so you can rent everything you need once you have arrived. 

3. Does a bigger tank hold more air?

When you are buying air tanks the size does not always relate to how much air the tank can hold when it is full.  To know this you have to find out what the maximum pressure of the air tank is.  Aluminium tanks usually hold around 3,000 psi and steel tanks can be either low pressure (2400psi) or high pressure (3440psi).  

4. How do I work out what size tank I need for my diving session?

How long a tank lasts will depend on two main factors.  Firstly how much air the diver consumes and secondly what depth the diver is descending too.  On average a diver will consume around cubic foot of air per minute.  However if you are exerting yourself this can go up significantly.  Also the further you dive down the more the air will be compressed and the faster you will use it up.  When you complete your diving course your instructor will go into more detail about calculating air tanks size and dive times. 

5.  Why should you never empty your tank completely on a dive?

Firstly you need to be able to calculate your dive time accurately so that you never get close to an empty tank.  This can be dangerous as anything could happen to delay your ascent to the surface and you need to know you have some spare air to last you.  Also if you completely empty your air tank then there is no pressure left inside to keep out moisture.  This can cause moisture to leak inside the tank resulting in an increased likelihood of corrosion therefore reducing the life of your air tanks.  If you leave a little air in your tanks when you come back to the surface you can partially open the tank valve and blow out any droplets of moisture that may have formed.

6. Do I need to get my tanks professionally checked?

If you buy your own air tanks for diving then you do need to make sure you get them checked over at least once a year by a professional technician.  This service is usually available from local dive schools.  This will ensure that there is no damage to your tank or the valve that could result in it bursting under pressure.  Tanks are crucial items of equipment when you are on a dive so you do need to make sure they are working perfectly before you get into the water.  Always personally check your tanks before every dive and don't forget to lift off the cylinder boot to see if any corrosion has formed underneath.

7.  What are scuba air tanks made from?

Scuba air tanks are typically made from aluminium or steel.  Each of these metals has their own advantages and disadvantages:

  • Steel – steel is much harder than aluminium and is more resistant to dents and other damage.  It is also slightly heavier than aluminium which means you do not need to wear as much weight on your belt/suit.  Steel is more prone to rusting though and once these tanks have started to corrode they are no longer safe to use.  Steel tanks can costs a lot more than aluminium tanks but in general they last much longer (if looked after properly).
  • Aluminium – aluminium is much softer than steel and therefore is more likely to get dents and scrapes.  Although aluminium does rust it is much less prone to this damaging form of oxidation than steel and this mean these tanks require less maintenance.  For this reason you will find nearly all hire shops use aluminium tanks. Aluminium tanks are less expensive than steel tanks but they do not last as long and they are lighter, which means you will have to wear more weights on your belt/suit.

8.  Can Kevlar air tanks be used for diving?

You can get air tanks for diving made from a Kevlar composite.  These are actually made for use on land for situations such as fire fighting applications.  They are not used for diving as they have a high positive buoyancy.

9. Why is tank capacity measure in both metric and imperial?

Metric and imperial techniques do not actually measure tank capacity in quite the same way.  Metric describes the tank capacity in terms of how many litres of water can fit inside and since a litre of water also weighs one kilogram this could be listed in either litres or kilograms.  The Imperial method rates tank capacity based on how many cubic feet of compressed air can be held in the tank at sea level.  Both these systems are used by tank manufacturers to it is important you understand the differences.

10. Why do tanks get hot when you fill them?

Basically when you fill an air tank gas (typically nitrogen and oxygen) will be compressed.  When you put gas molecules under pressure they start moving around more quickly and this causes a reaction of heat.  Therefore the more pressure you put your tank under the hotter it will get.