Diving & Nitrox Gas Blending
Diving & Nitrox Gas Blending FAQ
This article is based on Dive Centres in The United States of America by EnviroDive US.
- What is Nitrox?
- Why use oxygen enriched air?
- What does “Oxygen Service” mean?
- With The Nitrox Stik continuous gas blending equipment do I have to oxygen clean anything?
- What is the “40% rule”?
- Can I use an oil-lubricated compressor to make Nitrox?
- With the membrane or The Nitrox Stik continuous gas blending equipment how will the residual gas in the compressor’s lines and filters effect subsequent fills?
- What is it going to cost me to make Nitrox and how much can I sell it for?
- How many tanks of Nitrox can I make from one oxygen cylinder?
The term “Nitrox” was originally coined to describe a breathing gas used for shallow saturation diving in underwater habitats. The breathing gas was basically air, with a decreased oxygen level. This oxygen reduced gas, “Nitrox” was necessary for the long-term storage by divers in shallow saturation. The oxygen level in (normoxic) air would eventually cause lung damage (pulmonary oxygen toxicity) at the increased saturation pressure.
When divers started adding oxygen to air to extend their bottom times, they called it “enriched air Nitrox”. Eventually, this phrase was shortened to just “Nitrox”. Although it is inaccurate, it is generally accepted by divers to mean Oxygen Enriched Air, or OEA.
As working divers, we correctly refer to oxygen reduced air as “Nitrox” for use in shallow saturation, and “enriched air Nitrox” or “oxygen enriched air” for air that has oxygen added.
As divers for pay, our usefulness is measured in bottom time. It is our singular ability to get the job done, with the added handicap of being underwater, that differentiates us from our landlocked counterparts. Anything that allows us to safely remain on the job longer is a good thing.
With oxygen enriched air we can significantly increase our bottom times without increasing our decompression obligation.
This is a notable departure from oxygen enriched air for occupational scuba divers, where the primary reason for using oxygen enriched air is to limit nitrogen “on-gassing” to keep within no decompression limits.
“Oxygen service” means that any part of a gas containment system that is exposed to an oxygen concentration above forty percent is cleaned to a verifiable oxygen clean standard, has oxygen compatible parts for the systems rated working pressure and oxygen content, and is designed for oxygen use.
The only thing that has to be oxygen cleaned is the oxygen regulator and micro-metering valve that comes with The Nitrox Stik. However, the regulator and micro metering valve are factory cleaned, have oxygen compatible parts and are designed for oxygen service, so unless contaminated by the oxygen source, do not require further user maintenance. Once downstream of the oxygen regulator, the oxygen is low pressure (much safer) and once into The Nitrox Stik itself is completely mixed below 40%.
More of a North American diving industry guideline than a “rule”, Dr. Morgan Wells, who was the diving director at the National Oceanic and Atmospheric Administration (NOAA), needed to know what oxygen percentage in a mix presents an increased danger of fire or deflagration when making or using Nitrox. To find the answer, Dr. Wells visited a nearby fire department and perused their library. Finding a reference to 40% oxygen in a text or manual, the forty- percent rule was born. To this day, Dr. Wells does not know what book he referenced.
Ironically, and happily, this 40% guideline has proven to be a very workable number for the diving industry. However, it is important to understand that the fraction of oxygen in a mix is only part of the equation. Other factors that are equally important are the working pressure, temperature, equipment design, material compatibility, presence of condensed hydrocarbons, and gas flow velocity in the system.
At a Divers Alert Network (DAN) sponsored “Nitrox Workshop” in 2000, industry leaders from all aspects of the diving community got together and discussed Nitrox for the new millennium. Military, commercial, recreational, technical, diving medicine and diving equipment manufacturers were all represented. One of the main items discussed and agreed on was that any breathing gas mix with an oxygen concentration of 40% or less could be treated the same as air. Therefore “oxygen service” is not required for mixes with 40% oxygen or less.
Keep in mind that with the single exception of partial pressure blending, every method of mixing Nitrox either boosts or compresses the oxygen enriched air mixture. It is an industry standard to use oil lubricated high-pressure compressors. If an oil-lubricated compressor is used for partial pressure blending, the compressed air must be filtered to a modified grade “E” standard. This is necessary to keep the condensed hydrocarbons introduced into the system downstream of the filters to a minimum. Keep in mind however that eventually the amount of hydrocarbons in the system will exceed the allowable standard and will require oxygen cleaning. The main problem with this is that it is difficult to know when the build up of hydrocarbons reach an unsafe level.
With the membrane and continuous gas blending system (The Nitrox Stik) this is not an issue as the Nitrox mixture is kept at 40% or below prior to being compressed or boosted. Keep in mind that most dive shops that mix Nitrox use synthetic oil in their compressors.
Perhaps the most important guideline for compressors (air or Nitrox) is to keep the oil level to the manufacturer’s specs and run the compressor as cool as possible. This may include fans or air conditioners. Make sure that there is lots of air circulation in a well-vented room. If the filter towers can be remotely mounted a few feet away from the compressor this will help with cooling as well as increase the effective life of the filter due to isolating it from the vibrations caused by the compressor.
With the membrane or The Nitrox Stik continuous gas blending system, how will the residual gas in the compressor’s lines and filters effect subsequent fills? For example, if I just finished filling an air cylinder and am now going to fill a EANX 32 won’t the amount of air in the compressor’s lines and filters cause the nitrox mixture to be off?
We have found that even in really large systems with hundreds of feet of air lines and multiple compressor filters, it really does not affect the outcome as any residual gas is purged by the mechanics of mixing. When you are making Nitrox with The Nitrox Stik the first thing you do after turning on the compressor is to calibrate the analyzer for the oxygen content of air (20.9). To keep the compressor from automatically shutting down when it has reached maximum pressure, we usually crack open a bleed valve on a fill whip. The initial calibration takes less then 20 seconds. When oxygen is introduced, it takes perhaps another 20 to 30 seconds before the desired mix is registered on the analyzer readout. At that time we close the bleed valve and fill the cylinder. So technically, even though the mix entering the cylinder may not be the exact final mix, the small volume involved does not effect the final mix.
If you are compressing Nitrox into large volume bank bottles, it doesn’t matter if you purge residual gases as the larger volume takes care of the tiny volume of residual gas that is not at the desired oxygen content. Going the other way where there may be residual Nitrox in the compressor system and a customer wants air, it is easy to just crack a bleed valve on the fill whip for a couple of seconds before filling the customer’s cylinder.
What a lot of dive shops don’t realize is that they may be losing money on every air fill. A number of years ago, DEMA commissioned a study to find out just how much the average dive shop had to charge just to break even filling a 80 cu.ft. cylinder. The price was $6.00. The study took into consideration such things as the cost of annual compressor maintenance and amortizing the cost of the compressor over its life span as well as the obvious stuff such as wages, power consumption, etc. The problem is that due to competition many dive shops cannot charge enough for air fills to break even, much less make a bit of a profit. So how does this affect the cost/profit for Nitrox? Depending on where you live in America, your cost of oxygen will vary. However, as a dive shop will get between 24 and 32 fills per 330cu.ft. oxygen cylinder, the cost averages out at around $1.00 of oxygen per 80 cu.ft cylinder. The only additional cost to factor in with a continuous gas blending system is the actual capital cost of buying the Nitrox blender in the first place. An active shop can easily recover the capital cost within a couple of years factoring in an additional dollar per Nitrox fill.
|Cost to the dive shop for the base air fill||$6.00|
|The average cost of oxygen per fill||$1.00|
|Return of capital for system purchase||$1.00|
|Total cost per Nitrox fill||$8.00|
If our cost on average is $8.00 per Nitrox fill there is definitely room for profit while still offering our customers a good value. Please keep in mind that these numbers represent a Nitrox Stik continuous gas blending system. As the capital cost of a membrane system is considerably more and the labor cost for a partial pressure system (both for mixing and maintaining the system) is substantially higher, the cost for these systems are more.
That depends on the size of the oxygen cylinder and the desired Nitrox mix percentage. Download this spreadsheet to see how much oxygen is required to make the various Nitrox Mixes. It also has an area where you can determine the number of tanks you can fill from one oxygen cylinder.
Cleaning Standards for Oxygen and Nitrox Cylinders
A View of a Controversial Subject
The proper preparation of a scuba cylinder for use with Enriched Air Nitrox is essential for the safety of the cylinder user and the person filling the cylinder. The primary issue here is the introduction, even temporarily, of pure oxygen into the scuba cylinder. Pure oxygen can create flammable or explosive situations that would not be possible in regular atmospheric air. Substances that would not burn in atmospheric air burn freely in the presence of pure oxygen. Routine, everyday substances become ignition sources in a pure oxygen environment. This was demonstrated clearly in the space program fire of Apollo I, where Velcro, a popular and common substance, became explosive when used in a pure oxygen environment during capsule testing. In the scuba cylinder, the issue is the presence of hydrocarbons, or oil, in the scuba cylinder. When exposed to pure oxygen, hydrocarbons have a much lower flash point. To safely use a scuba cylinder for partial-pressure filling of Nitrox, special precautions must be taken to ensure that all hydrocarbons are removed from the cylinder prior to use in the Nitrox filling operation.
At Dive Sports, we think that proper cleaning of scuba cylinders for use with Enriched Air Nitrox is essential. In addition, we have clear opinions about the methods that should be used in this cleaning process. Many people, including some scuba store operators, feel that proper cleaning can be accomplished by simply filing the cylinder with commercial cleaning solutions and rolling the cylinder around for several minutes. The solution is then washed from the cylinder and the cylinder dried. The cylinder is then ready for Nitrox service. Actually, a little more is involved. First, we need to understand that the hydrocarbons we are trying to remove can easily hide in scratches, dents, and other imperfections inside the cylinder. In addition, hydrocarbons can stubbornly adhere to the cylinder walls by various methods, including static electricity, and may not be adequately broken down by simple exposure to the cleaning chemicals. The proper removal of these hydrocarbons requires special methods.
Proper oxygen cleaning requires three basic elements…..the use of special cleaning solutions designed to degrade hydrocarbons without introducing chemicals that would present a problem in breathing air, some method of agitation that would break the hydrocarbons from scratches, dents, and other areas where it might cling, and a complete and tested method of washing the oxygen cleaned cylinder after the cleaning process. At Dive Sports, we use special cleaners made by Global Systems, a specialist in supplies and chemicals for oxygen cleaning. These special cleaners are basically made of citrus juices, combined with other chemicals and concentrated just for hydrocarbon removal. We then introduce small glass beads into the cylinder to produce abrasion during cylinder tumbling. The cylinder is placed on a special rolling machine, where it is tumbled for 15 to 30 minutes. During this time, all stubborn hydrocarbons are removed from the cylinder. The cleaned cylinder is then placed on special washing stands, where it is flushed with hot running water for five to ten minutes. The cylinder is then tested for presence of the cleaning solutions using a bubble test, a simple test that involves taking wash water samples from the cylinder and testing it for the presence of soaps. The cylinder is dried and is ready for oxygen service. In addition, the valves used on the Nitrox cylinder must be cleaned and prepared with special o-rings, seats, and grease designed for exposure to pure oxygen. The cylinder is then banded with the familiar yellow and green Nitrox band, an inspection sticker is applied, and the cylinder is ready for use.
Maintaining the integrity of an oxygen cylinder is also critical. Any subsequent introduction of air that is not “oxygen-compatible” air contaminates the cylinder and it is no longer suitable for partial pressure filling. Most scuba stores will not fill a Nitrox banded cylinder with normal air, unless that air comes from a Nitrox production facility, in which case it is identified as 21% Nitrox. However, the practice of filling Nitrox cylinders with standard breathing air is becoming more common place. This presents a clear problem to a facility that subsequently completes a partial-pressure fill on that cylinder. We caution our customers to be very careful when obtaining Nitrox fills or when obtaining air fills in Nitrox cylinders. Cleaning is expensive, and we don’t want you to waste additional money because you get an “improper” fill in your oxygen-clean cylinder.