CAUTION

Before we jump in, it's important to first consider the safety aspects of compressed gasses. Pressurised tanks can be hazardous if not handled with respect. You should always follow the correct safety practices when handling scuba tanks, as you would with any other pressurised tank.

• Do not overfill the tank past it's ratings.
• Keep moisture out of the tank. Internal corrosion could become an issue.
• Don't leave your scuba tank in a hot car. Pressure will build up inside the tank when it gets hot.
• Have your tank regularly inspected and hydrostatically tested. There are standards on how often this should be done.
• Do not drop the tank onto the valve. In fact, don't drop it at all! You don't want to know what happens when a tank valve is broken.
• Transport your tank horizontally and secured it to prevent it from rolling around.
• Store your tank vertically.

Other Important Safety Considerations

• If you adjust the intermediate pressure on the 1st stage regulator, Label the stage "Not for breathing" to make sure someone doesn't come along later and try to dive with it only to discover they aren't getting enough air 100 feet below the surface..
• Use only high pressure (HP) hoses for connecting components to the HP port of the 1st stage regulator. These need to be able to handle at least 3000psi.
• Avoid making your own adaptors for any HP connectors, Only use commercial components rated for the pressure.
• Fill your rockets slowly. Filling them quickly can cause excessive internal heating weakening the plastic and causing them to fail prematurely.

What you need to set up a Scuba tank air supply for your water rockets

Before looking at how we can connect everything together let's have a look at some of the components that you may need for the air supply. You may want to jump ahead and see some of the configuration options so that things are clearer where the different components fit in the whole scheme of things. Click on the links below to find out more about each component,

• A scuba tank - Holds the compressed air. No surprises there!
• A 1st stage regulator - Reduces the tank pressure to a manageable level.
• High pressure gauge - Shows you the tank pressure.
• Low pressure gauge - Shows you the pressure in the rocket.
• Pressure release valve - Allows you to depressurise the system after the tank is closed.
• Hose - Connects your 1st stage regulator to your launcher
• Hose adaptors - Used to connect different hoses together.

Scuba Tanks

Tank Sizes

Scuba tanks come in quite a few different sizes and are usually made from steel or aluminium. Larger tanks store more air but make transportation a little more difficult if you have to carry them to the launch site. The following table shows a list of some common tank sizes and their operational pressures:

Here is some more info on tank sizes: http://www.huronscuba.com/equipment/scubaCylinderSpecification.html

Tank valves

There are two main types of tank valve used on scuba tanks. The DIN valve has an internal thread for connecting the regulator, while the K-valve uses a yoke. There are also two kinds of DIN fittings: 200 DIN fitting which is rated for 200 bar and 300 DIN which is rated for 300 bar. You can connect any equipment with 300 DIN fittings to the 200 DIN valve, but not the other way round.

It's not important what kind of valve the tank has for our purposes, but it will affect what type of regulator you should buy for your tank. There are adaptors available to connect one to the other but these just add extra cost. DIN fittings are popular in Europe while the Yoke type are more common in the US. There really isn't a price difference between the two.

K-valve or Yoke

DIN Valve

Here are two examples of 1st stage regulators, one with a Yoke fitting and one with a DIN fitting.

Yoke fitting (left) and DIN fitting (right)

Regulators

Standard scuba equipment uses two pressure regulators. The 1st stage regulator goes on the tank and converts the tank pressure down to an intermediate pressure (IP) of around 140-150 psi (10 bar). The 2nd stage (demand valve) connects to the 1st stage via a hose and goes in your mouth. The 2nd stage converts the intermediate pressure to a pressure you can breathe which varies with depth. You generally do not need the 2nd stage regulator for water rockets.

Terminology Note: For clarity in this article we will often refer to the Intermediate Pressure (IP) as Low Pressure (LP).

There are two main types of 1st stage regulators - one uses a piston and the other uses a diaphragm. Piston first stages tend to be cheaper and easier to service while the Diaphragm first stages are sealed, and can have their output pressure adjusted. If you have a choice of which regulator to get, I would recommend the diaphragm type for it's pressure adjustment capability.

A typical 1st stage regulator will have 1-2 two HP ports and 2-5 LP ports available for you to connect various equipment to, The HP ports are typically marked on the regulator body. Any unused ports need to be plugged with LP or HP plugs.

Adjusting pressure

You can adjust the pressure of a diaphragm regulator usually with an Allen key. NOTE: for most purposes you probably will not need to do this, but it is a way to increase the intermediate output pressure of the 1st stage regulator. If you decide to change the set pressure, label the regulator "not for breathing" as mentioned in the safety considerations. The range over which you can adjust the pressure depends on the regulator model, but in general you should be able to adjust it from 0 to about 200psi (14 bar). The default setting is usually around 140 - 150psi (10 bar)

An Allen key can be used to adjust pressure on a diaphragm regulator

This regulator has a slot instead of a hex head for adjusting pressure

Pressure gauges

You would normally use two pressure gauges on your scuba tank water rocket air supply. The high pressure gauge tells you how much air is left in the tank, and the low pressure gauge tells you how much pressure is in your rocket/launcher.

A typical HP gauge will show pressures in the range of 0 - 4000psi ( 0 - 300bar ) while a low pressure one will show 0 - 300psi (0 - 20 bar)

High Pressure gauge

Low Pressure gauge

You can also buy pressure checking gauges that fit on the scuba tank instead of the 1st stage regulator. They come with their own yoke or DIN fitting and have a pressure release valve. Here are a few examples:

Pressure checking gauge with a yoke fitting

Pressure checking gauge with a DIN fitting

Pressure Checking gauge attached to a tank

These are useful when your 1st stage regulator doesn't have a high pressure gauge.

Pressure release valves

A pressure release valve (also know as a bleed valve) is used to depressurise a pressurised portion of your air supply. These can take any number of forms. Here are a few examples:

2nd Stage Regulator When connected to the 1st stage, pressing the purge button will release the pressure.
Bleed valve on LP gauge Sometimes the low pressure gauge will have an integrated pressure release valve.
Separate valve You can pretty much use any valve rated to the pressure as a release valve. Just use the correct adaptors to connect it to a hose or air manifold.

Hoses

There are two different types of hoses commonly used with scuba equipment. A high pressure (HP) hose connects your HP gauge to the HP port of your 1st stage regulator. Low pressure (LP) hoses connect to the 2nd stage regulator and a buoyancy compensator (BC) for example. For water rockets you need to be able to connect your launcher to one of the LP hoses. There are several different ways of achieving this. You can use a standard air hose that you may already have on your launcher, and then use an adaptor that will allow it to connect directly to the LP port on the 1st stage.

Typical LP and HP hose fittings	Two ends of LP hose fitting used to connect to a 2nd stage	BC quick connector fitting

The HP hoses usually use a 7/16 UNF male fitting and the LP hoses usually use 3/8 UNF male fitting.

Alternatively you can use a standard scuba hose with a quick connector (normally used for connecting to a BC) and then attach a quick connector adaptor to the end of your hose. The one advantage to using the quick connector is that it has a valve in it, and when disconnected it closes so that you can measure the tank pressure.

Typical LP and HP pressure hoses	A scuba hookah hose

Typical scuba hoses are available in short lengths usually around a meter with connectors at either end. Since you will need a longer hose for your launcher you will need to connect a scuba hose to your longer air line. Unless of course you use a number of adaptors and connect the short hoses together. Long scuba hoses are also available called "hookah" hoses that allow you to connect directly to the LP port but are less common and more expensive. 

Hose Adaptors

Hose adaptors come in lots of different varieties, and which ones you use will depend on your specific set-up. Here are just a few examples of the different kinds of adaptors you may find useful when connecting to scuba gear. Use your favourite search engine to find the adaptor you need. In some instances you may need to use a couple of adaptors to get the desired conversion.

1.	2.	3.
4.	5.	6.
7.	8.	9.

As you can see adaptors (7,8,9) are available with hose barbs so that if you can't find the right fitting you can connect them directly to a hose. You would then use a hose clamp over the top to make sure they don't come off. You can use a combination of something like 2. and 5. to connect a standard air hose used for air powered tools to connect to the 1st stage's LP port. Adaptors like 3. are available that connect a scuba quick connector to a bicycle valve that you may already have on your launcher.

Putting it all together

When setting up your air supply you have several configuration options to choose from depending on your preference and what equipment you may already have. Here are some suggestions for connecting your scuba tank to your launcher:

Blue represents LP scuba hoses, Red represents HP scuba hoses, and Green represents other air hoses that are a part of the normal launcher.

The quick connectors referred to here are the kind used to connect to a Buoyancy Compensator (BC). They have an internal valve that closes when disconnected.

Option A 1st stage regulator with a LP gauge, hose with quick connector and 2nd stage connected to 3 LP ports. A HP gauge is connected to a HP port on the 1st stage. To measure the HP pressure you need to disconnect the quick connector, and then open the tank. After taking a reading, press the purge button on the 2nd stage to release the pressure.
Option B 1st stage regulator with a LP gauge and hose with hose adaptor connected to 2 LP ports. A HP gauge is connected to a HP port on the 1st stage. The LP hose is connected to a manifold with a pressure release valve and an isolation valve. These could be any kind of valves and may be present on the launcher already. To measure the HP pressure, you need to close both the isolation valve as well as the pressure release valve and open the tank valve. After taking a reading, close the tank valve and then open the pressure release valve. Open the isolation valve for normal launches again.
Option C 1st stage regulator with a LP gauge and hose with quick connector connected to 2 LP ports. A HP gauge is connected to a HP port on the 1st stage. The LP gauge has an integral pressure release valve. To measure the tank pressure you need to disconnect the quick connector, and then open the tank. After taking a reading, release the pressure by opening the release valve on the LP gauge.
Option D 1st stage regulator with a LP gauge and hose with quick connector connected to 2 LP ports. You can use a separate pressure checking gauge to check the tank pressure. In order to do this you must first disconnect the 1st stage from the tank valve.
Option E 1st stage regulator with a hose with quick connector connected to 1 LP port. This option is useful if your launcher is already equipped with LP gauge. You can use a separate pressure checking gauge to check the tank pressure. In order to do this you must first disconnect the 1st stage from the tank valve.

NOTE: When setting up your own configuration keep the following in mind: To take a HP reading you need to open the tank valve to allow the full tank pressure to reach the HP gauge and at the same time you need to isolate the rest of the system to prevent air escaping. After you close the tank you need to use a pressure release valve to depressurise the connection between the tank and the HP gauge. If you fail to include a pressure release valve in your design you will have a really hard time removing the regulator from your tank.

Pressurising your rocket

When you have everything connected, you are ready for launch. Because the intermediate pressure set on the regulator may be too high for your rocket, you can fill your rockets to lower pressures simply by opening the tank valve slowly and letting the pressure build up in the launcher and rocket. You need to be looking at your low pressure gauge. When the pressure reaches the desired level, turn the tank valve off.

CAUTION: If you don't turn the tank valve off when the rocket reaches the desired pressure, the pressure will continue to build until it reaches the regulator's IP setting. This pressure may exceed the burst pressure of your rocket and/or launcher. The same thing would happen if you fail to turn off your compressor when it reaches the desired pressure.

With a diaphragm type 1st stage you can preset the pressure to a lower value so that it is at the desired launch pressure. You can then open the tank valve and air will stop flowing when the set pressure is reached. Then you can turn the valve off again.

Before you launch your rocket, always make sure the tank valve is turned off so you don't waste air.

How many launches do you get out of a tank?

The number of launches you can get out of a single tank depends on several factors such as the tank volume, the capacity of rockets you are launching and the pressure at which you launch them. The calculator below will give you a rough idea on how many launches you can expect.