DIRECTORY
 
 
By Curt Bowen
 
Heliair, a term originally coined by Sheck Exley, is the simple process of adding helium to a scuba cylinder then topping it off with air. Heliair, sometimes called "poor man's mix" was implemented in the early deep air days of cave exploration for the reduction of nitrogen narcosis. At this time it was believed that narcosis was the cause of several deaths of divers using deep air. Today, we know that even though nitrogen narcosis may have been a contributing factor, most likely oxygen toxicity was the culprit. Heliair has multiple advantages and disadvantages for the extended range diver
 
Advantages
 

Nitrogen Narcosis: Heliair reduces the effects of nitrogen narcosis.

Oxygen Toxicity: Heliair reduces the oxygen PO2 exposure levels reducing the possibility of a 02 hit.

Ease of Mixing: Heliair mixture s are easily obtained by adding helium to a scuba cylinder then topping it off using a standard air compressor.

No Fire Hazard: Heliair mixtures don 't require handling of 100% oxygen, thus removing any possible fire hazard.

Analyzing: Heliair's nitrogen and helium percentage s are a mathematical constant according to the oxygen percentage thus providing the blender with the exact nitrogen and helium percentages.

Ease of Travel: Heliair minimizes the need to carry additional bank cylinders of oxygen to remote dive locations.
 

Continuous Blending:

 

With the proper mixing chamber, heliair can be pumped directly through a standard compressor.

 

Disadvantages:
 

Best Mix: In trimix, a PO2 of l .4ata and an equivalent narcosis depth of 130 feet are desirable. With heliair, to obtain an equivalent narcosis depth of 130 feet, a PO2 of 0.9 to 1.1 ata must be accepted – far below the desired l.4ata PO2. If a PO2 of l .4 ata is obtained then an equivalent narcosis depth must be accepted, much deeper than the desired 130 feet.

Increased Decompression: Due to the fact that optimal PO2s of 1.4 are not obtained, decompression requirements will increase slightly compared to that of the Best Mix.
 

Partial Pressure Mixing:
 

Two basic formulas are required to mix heliair. The first formula is used to determine how much helium in psi is needed for a desired mix. FHe x Ending Pressure, where FHe stands for fraction of helium.

Example: An ending helium percentage of 24% is desired with an ending pressure of 2640 psi. Formula: 0.24 x 2640 = 633 psi helium to be added. The second formula gives the oxygen percentage in the cylinder after topping off with air: 1 - FHe x 0.21 Example: The oxygen percentage in the above 24% mix would be
(1 - 0.24) x 0.21 = 0. 159 or 15.9%

The mixing process is relatively simple. First drain the scuba cylinder then bank the required p i of helium into the scuba cylinder. Allow time for the helium to cool and the pressure 10 drop then make the needed corrections. Fill the scuba cylinder with air to the desired ending psi. Analyze the oxygen content with an 02 analyze r. If the oxygen percentage is too low just add more air until the mix is correct. Always round the oxygen percentage down

(Example 15.6% to 15%) to the next lower number this will add a small amount of safety to the decompression tables.
 

This chart gives the Equivalent Narcosis Depth( END) in fsw, and the partial pressure of oxygen (P02) in atmospheres absolute for various heliair mixtures.
Match up the desired depth with the desired END or P02. Once located, the desired helium and oxygen percentages are in the left black box. The helium fill psi from 2600 -3600 psi are located in the same block just above the depth row.

This chart provides the amount of helium to add (in psi) to an empty scuba cylinder to create the various heliair mixtures. First, find the desired heliair mixture to the left, then the ending scuba cylinder pressure at the top, intersect these two rows to get the required helium (in psi). Then top off the tank with air. You can also use the chart to determine the amount of helium left in a cylinder after a dive. Many times this helium can be saved.
 

Analyzing:
 

It is imperative to analyze your gas right after mixing and then again just before the dive. You should calibrate the analyzer to air (20.9%) and not 100% 0 2. This will decrease the range of error for the analyzer.
 

Saving Helium:
 

Due to the high cost of helium, divers try to save every drop. If multiple heliair dives are to be conducted over several days the helium left in the scuba cylinder after a dive can many times be saved. This is done by calculating the helium psi still in the scuba cylinders then adding more helium on top of it Example: heliair 14% 02 / 33% was used on the first dive. Upon returning to the surface the diver has 800 psi of mjx left in his cylinders. The amount of helium in psi still left in the mix can be calculated by taking the fraction of helium times the pressure le ft in the cylinder. 0.33 x 800 psi = 264 psi of helium still in the scuba cylinder. Let's say the next dive requires a heliair mix of 15% 02 / 28% He. If the cylinders were empty, 739 psi (0.28 x 2640) of helium would be required to make the proper mix at 2640 psi but we have 264 psi of helium still in the cylinders from the first dive. Subtract the psi of helium still in the cylinders from the required psi of helium needed if the cylinders were empty. 739 - 264 = 475 psi of helium to be added on top of the 800 psi still in the cylinders then topped of with air to make the proper 15/28 mix. The only problem with remixing heliair in this fashion is the higher bank pressures needed to remix. If the pressures get to high you will not be able to obtain the psi without a haskel pump. The highest helium bank pressure available will determine if you will need to drain the left over mix in the scuba cylinders completely or just partially.
 

Transfer Whip:
 

A transfer or fill whip is required to partial pressure till helium from the bank cylinder to the scuba cylinder. All components of the whip must be designed for high-pressure use. If the whip is going to be used for transferring pure oxygen then all components must be oxygen compatible and free of hydrocarbons.

A whip can be purchased whole and ready for use or the components can be purchased and assembled. The following is a list of whip components.
 

Connector:
 

An industry standard CGA-580 style connector is required for the helium bank cylinder. Note a CGA-580 to CGA-540 adapter can be purchased to convert an oxygen whip into a helium transfer hose.
 

Line Filler Valve:
 
A needle valve is used to control the rate of flow from the bank helium to the scuba cylinder. Transferring any gas for mixing should be done slowly to help minimize heat build up which can cause an error in the final mix. The best in-the-field technique is to place your ear on the scuba cylinder and crack the needle valve until you can hear the gas flowing.
 

High Pressure Hose:
 

High-pressure hose 3-6 feet in length is sufficient.
 

Pressure Gauge:
 

Pressure gauges come in a variety of sizes, styles and pressure readings. Digital gauges are recommended but due to their high cost most brewers use analog. Digital dive computers with integrated pressure gauges work very well.
 

Scuba Yoke:
 

Scuba and DlN fittings are standard per the scuba industry.
 

Check Valve: (optional)
 

A check valve can be used to prevent back flow of gas into the supply cylinder in the event the scuba cylinder contains more pressure.
 

Flow Restrictor: (optional)
 

Flow restrictors prevent the till rate from exceeding a set value thereby preventing excessive heat build up.
 

Helium Storage Cylinders:
 

Helium comes in a variety of large bank cylinders from your local gas dealer. Cost seems to vary greatly according to your personal business relationship with your local gas company. You may need to shop around to find the cheapest price. The number of bank cylinders needed will depend on your diving but three seems to work well for the active deep diver and his buddy.
 
Safe Diving............