HANGLINE ARCHIVES
Issue #09-10 ____ __
Posted September 10th, 2009 .......................................Back to Archive Index
Blending Helium Gases
For those interested in what it takes to start blending helium or trimix gases - here's a VERY short introduction. Summarized highlights from a chapter in the IANTD Gas Blending Manual by Nadeau, Rutkowski, Mount and Townsend.
The preparation of Trimix and Heliox mixes is actual very simple. Once the basic principal behind gas blending theory and formulas have been learned the application of helium is only one extra step to meeting our objective. It is however important that Trimix Blenders thoroughly understand the underlying physics behind mixed diving as it is these same laws that determine what kinds of diving a diver can do with each mixture. 
Like nitrox divers, mixed gas divers rely heavily on Gas Blenders for accurate and well blended fills. As technology bounds so do the number of divers, both open circuit and rebreather. It is in this direction that our sport is heading and history shows there is no turning back. One thing we must consider is that the physiological aspect of mixed gas diving maintains a much greater complexity and therefore must be approached with demanding precision.
History
Helium has been used in diving applications for a very long time. It was first suggested by Elihu Thomson that helium could be used as a medium that could augment oxygen and nitrogen in a breathing mix Only small quantities were available and very costly. It was not until the early 1900's that the United States actually discovered vast quantities of helium in Texas giving them an 'exclusive dealership' and bringing the cost of the gas to within a reasonable rate. By 1924 heliox mixtures were being researched by the US Navy and the Royal Navy but quickly abandoned after a high number of DCI incidents occurred. Research resumed again after a number of naval disasters prompted rescue efforts in deep waters. Hans Hass, a Swiss mathematician along with a physicist named Buhlmann generated a set of dive tables to a 1000fsw using helium. The record setting dive made by Hass proved the validity of his tables and helium use. By 1970 Tom Mount at the University of Miami began a series of dive operations using helium and trimix mixtures and over a six year period recorded an outstanding safe and successful performance record based on hundreds of deep dives conducted by a large team of students and faculty.
Today thousands of divers around the world are venturing safely into new domains using trimix and heliox gases. It is proving that without a doubt mixed gas operations can provide a means to dive deeper, longer and with a higher functioning mind.
Characteristics
Like oxygen we first must understand the characteristics of helium. Helium is a very light gas and not very dense. Due to it's physical and chemical structuring it has very little narcotic effect on the body making it ideal for deep diving. By augmenting helium into our breathing mix we are able to reduce both the fraction of oxygen and the fraction of nitrogen thereby reducing a diver's susceptibility to hyperoxia and nitrogen narcosis.
Because helium is not a very dense gas it can be easily compressed, more so than oxygen or nitrogen. When partial pressure blending helium we must account for the effects of real gas methods as explained earlier in this manual. In small quantities the compressibility of helium is not really an issue however large volumes must be adjusted accordingly.
Helium can also be considered a fast gas or gas that likes to enter and leave our tissues before any other gas. Like nitrogen it is inert (our bodies can not metabolize it) and is stored in our tissues when under pressure. The fact that it enters and leaves our tissues ahead of nitrogen means that traditional off-gassing rates as calculated through the use of dive tables and air computers not longer apply.
Mixed gas dive tables take into account this phenomenon but rely on divers following a profile even more precise then that of air - especially since the nature of helium off-gassing requires deeper and more frequent stops.
Hazards
Unlike nitrox and oxygen gas blending where the emphasizes is on the prevention of operational hazards, the handling of helium for breathing has a weighting on the physiological aspect. Wrong mixes can serious expose a diver to oxygen toxicity, nitrogen narcosis and decompression sickness. Because mistakes in mixed gas blending can have some very serious physiological effects, our margin of error needs to be not just extremely narrow, but eliminated.
Historically there were no helium analyzers available in the diving industry so determining how much helium was in a tank was totally determined by math and physics. Our accuracy was heavily dependant on math calculations and principal rather than the reading we got from an analyzer. Today many still blend relying on calculations alone however the risks are not worth it - especially with the inexpensive costs of helium analyzers. When conducting any trimix or heliox calculations, double and triple check your math work and use accurate gauges - preferably digital.
A normoxic mix at the surface begins to become hyperoxic at depths below 218fsw (72 meters) so trimix for dives deeper than 180-200fsw (60-66 meters) will need to have an oxygen content of less than 20-22%. IANTD recommends that for technical dives bottom mixes not exceed PO2's of more than 1.5ATA. For this reason divers use trimix with FO2's that maintain safe PO2's. As a Trimix Gas Blender you can see the importance of providing accurate mixes.
The helium also provides a buffer against the nitrogen reducing the narcotic potential of a breathing mix. Air typically has 79% nitrogen but if a mix is prepared so that the nitrogen content is reduced, say by half (40% nitrogen) then that particular mix has only half the narcotic potential. A diver on a trimix with 40% nitrogen in their mix (a 18/42 trimix) will experience the same level of narcosis as a diver on air at half of his depth (all things being equal).
As mentioned earlier helium has a very fast on-gassing and off-gassing rate. This characteristic off sets traditional air and nitrox tables adding to the complexity of decompression profiles. The amount of helium in a mix and the depth it is being breathed will effect this rate. That is why it is very important that trimix divers know exactly what is in their tanks before they begin a dive. Following a table based on mix with a different amount of helium can lead to DCS.
A diver who begins a deep dive with an oxygen content higher than calculated or analyzed is predisposed to oxygen toxicity and a diver breathing a mix from the surface with less oxygen in their mix then calculated can suffer from hypoxia.
Defining Trimix and Heliox
Trimix is comprised of three (tri) gases; nitrogen, oxygen and either helium, neon or hydrogen. The third gas is nearly always helium as neon is expensive and hydrogen somewhat unstable. Heliox is strictly helium and oxygen. Heliox is not often used in recreational diving because of the price of filling a set of tanks with 80-92% helium does not outweigh the benefits of zero narcosis and reduced decompression. In exploratory depths (below 500fsw) heliox can manifest into a problem known as high pressure nervous syndrome.
The final mix when blended is labeled according the percent of oxygen and helium - this applies to both trimix and heliox mixes.
O2/He
For example a mix that contains 20% oxygen, 15% helium and 65% nitrogen is labeled as a 20/15 trimix.
20/15
Adding the Helium
Adding helium to any trimix or heliox mix is done using the same basic principals as nitrox gas blending. It is done based on the partial pressure filling formula. Which gas goes in first and how much we need to put in depends on the type of blending system we have and the mix we want. At a quick glance we can see two approaches to trimix blending;
|
Partial Pressure Only |
Topping with Continuous Flow or Pre-Mixed Bank |
Step One |
Slowly add calculated oxygen |
|
Step Two |
Slowly add calculated helium |
|
Step Three |
Top slowly with air |
|
Partial Pressure Only
To blend a system using only partial pressure methods you first need to decide whether to add the helium or oxygen first. It is common for blenders to add the oxygen first as it usually only requires a small amount. The helium is then slowly added and then the mix is topped with air. The entire process must be done slow as to avoid the heat of compression and because we are introducing more than 40% oxygen into the cylinder the tank and the air must meet oxygen clean standards. The process is slow and because each gas is introduced slowly the mix will require at least 12 hours to completely blend.
There are a number of disadvantages with using only a partial pressure method. As you can see is helpful to have a booster system because all of the gases or introduced into the tank solely through partial pressure filling. The tank will need to be kept oxygen clean and any air being added to the tank will also have to hyperfiltered. This also restricts where the cylinders can be filled limiting them to facilities with hyperclean air pumping capabilities.
Continuous Flow Trimix Blending
With a continuous flow blending system we have the ability to add the oxygen and the air together and eliminate all of the operational concerns and drawbacks of partial pressure filling pure oxygen. Blending trimix fills with a continuous blender also helps ensure a quick and homogenous trimix blend.We want to know what you think - send us an email and give us your thoughts. ~ DFP