You do this in 2 steps.

The helium content is not critical. You can't die from too much or too little helium, but you can die from too much or not enough O2. With helium, at worse you might wind up a little more narced than you want to be on a light fill. There is no consequence with a heavy fill. One experienced diver said that if you inhale some of your mix and talk like donald duck, your He is OK.

It is a good idea to check your mix after you add the helium and then check it again after you top off with air. It is recommended that you have some sort of backup analysis system (a second sensor and voltmeter) should there be any doubt as to the results of your reading. Your sensor measurements and your mix chart should coincide. When in doubt, drain your tanks and start over. Analyze your tanks on the boat before you attach your regulators and jump in the water. Remember, it's your life.

To analyze your mix you don't need to purchase a $375 minox. You can examine your own mix by simply purchasing an 02 detector from anywhere and hooking up a volt meter to it. A digital voltmeter will give the best results. Most 02 sensors are simply fuel cells. They burn oxygen and produce electricity. The more 02 in the gas, the more volts they pump out in linear fashion. Most can do between 1-2% accuracy, so throw a 2% safety factor in your tables and be done with it. If you try to be anal with mixing, you will promptly loose your mind.

The formula for using a calculator is (A/B)*C where:

A= the ambient percentage of air, almost always 20.8

B= the millivolt output of the sensor in ambient air

C= the millivolt output of the sensor in your mix

Shaken Not Stirred?

A common misconception is that you have to mix gas by shaking or rolling the bottle. When you spray gas into a cylinder, it does not just gently float out of the valve into the tank like pouring a Black & Tan. It is violently being shot in there like a firehose from a small orifice. Fill a bucket with water from a small orifice nozzle, you will get the idea.This will do all the mixing you will have to do.

What does effect your readings is temperature. Filling tanks with gas will heat them up and you will get different readings from a hot tank than from a cool one. So it is a good idea to let your tanks cool off to the point that you are getting a stable reading over 5 or 10 minutes time.

Flow Meters

I have seen some really involved flow meters for running the gas past your sensor. Some have little balls bouncing around in plastic blocks, regulators etc. While these will work, all you need is a metered orifice, the neatest I've seen is made by GMC Equipment Company, for $45.00. You plug it into your inflator LP hose and run a small hose to your sensor, very simple. Another good source is Northeast Scuba Supply, he has the bouncing ball type meter for about the same price. I have seen quite a few tanks mixed by just holding a sensor up to a cracked tank valve, and yes those divers are alive today.

The combination of a guage and a sensor will give you a safe mix. I would not dive any tanks which have been filled using only one of these techniques. And I would never, ever dive a tank which I had not personally analyzed.

Oxygen Analyzer Sources: