23 June, 2009

Mixing Trimix - Trimix Part 2


The following post and any pictures used in this post originally appeared on DivingAnarchy.info.DivingAnarchy.info and Diaries of a Frog Man have recently entered into a post swapping agreement where posts which are deemed of relevance to the diving community are shared / swapped to further improve access and distribution to relevant content.


Part 1 of the trimix series appeared earlier on at:



----------------

Article by: Jim Cobb


Trimix consists of helium, oxygen and nitrogen. Nitrogen is free and pretty much all over the place. Helium and oxygen are available at your local gas supplier. What kind of helium does not matter, just ask for an analysis of the type you have access to, if it is 99.9 percent pure, it does not matter what the designation is.
The purpose of this web site is not to get into how to obtain O2 and He, there are too many regional differences to pigeon-hole this topic. Suffice to say, where there's a will, there's a way. A small tip: there are many uses for 02 and He which do not involve scuba diving, such as welding and medical use, and it would behoove you to approach your acquisition from this angle rather than stating outright "I wanna homebrew trimix, got any helium?".
An excellent reference on this topic is Vance Harlow's Oxygen Hackers Handbook. The OHH covers information on O2, tanks, laws, designations, etc. The recently released version 3 added info about how to procure helium and detailed info about O2 cleaning. This book is a must have, in my opinion.


O2 Cleaning


O2 cleaning your tanks seems kind of odd to me. After all, why would you want to breath anything out of dirty tanks? So you want to inspect your tanks on a frequent basis to make sure that your compressed air source is not blowing stuff into your tanks. If there is the slightest doubt, then it's time to break down your doubles and have a look. You don't have to be a certified tank inspector to unscrew your valve and drop a light into your tank. Dirt, grime and other contaminates can be visually spotted.
Cleaning your tanks simply means flushing them out with a degreaser like 409 or Simple Green and rinsing thoroughly with hot water. Rust has to be rolled or whipped out.
As far as O2 combustion is concerned: what I've learned over the years is that for a fire to take place you need 3 things: Fuel, Oxygen and Heat. If you remove any one of these three, you won't have a fire. It is almost impossible to remove all the fuel,which is what O2 cleaning is supposed to do. So lets remove the heat. When filling a tank with O2 you can generate heat by filling your tank too fast. To avoid this, fill your tank very sloooowly. You can do this by opening your valves slowly, using a restricted orifice or a needle valve.


Equipment


To mix your gases you need some plumbing. The easiest thing to do is to purchase a pre-made whip from Northeast Scuba Suppy , Lloyd Baileys or any other source and save yourself the trouble of piecing one together. A whip consists of an adapter for the tank, a length of hose, a gauge and a scuba tank adapter with a bleed valve. Global Mfg. makes a pre-made whip, part number #45245. The gauge is not a super-accurate type, but combined with an O2 analysis system, it is good enough for our uses. To use an O2 whip on a helium bottle you need an adapter, as the valves are different. Again, you can get this from Bailey's. If you would like to build one yourself, here is a parts list


Fill 'Em Up!


The idea is to drain your tank empty, attach it to your He tank, fill it to the required pressure. Then hook up to your O2 tank and add that amount. Why do you add the helium first?
Simple, typically when mixing trimix you don't have that much O2. Even for a 20/20 trimix you're only squirting in about 100psi of O2. The needle on most of the analog pressure gauges that are sold out there on O2 whips does not move very well in the first 150 psi range or so. Yes, it may work on some gauges, but for the most part it doesn't move.
However, going from 700psi to 800psi is pretty much 100psi on any gauge. It's just the first 150psi or so where the gauge will probably stick. If you throw the He in first, then blow O2 on, the needle on the gauge should already be out of the "problem zone" and you won't put too much O2 in there.
Since you should already be adding some extra He any way to account for temperature (this is more of an issue when filling helium then gas compressibility, helium being a small molecule heats up REAL quick and you'll lose a lot more pressure from temperature drops then you will from compression), it's just the O2 portion that's the big concern. A fairly good rule of thumb is go a little high on the helium, a little low on the oxygen.
Also helium is more expensive than your oxygen, so it behooves you to get as much out of the He bottle as possible by gassing with it first.
Then test your mix with your 02 analyzer. When working with O2 it is a good idea to throw in a large safety margin. Too much O2 will kill you. Not enough, you hang a little bit longer. What would you rather do? Once you top off your tank with air, test it again.
When it comes time to top off your mix with air and you don't have a compressor, off you go to the local dive shop. It is recommended that you be up-front with your shop. Tell them you want to top off some mix, therefore it's important that they don't drain your tanks and that they have to fill to the pressure you entered into your mix program.
If they give you a hard time, politely let them know that you are more than willing to take your business elsewhere and spend your money at a shop which does not have idiots running it. I have found that if you are loyal to a dive shop, and spend a reasonable amount of dough there, they will pretty much let you do anything you want.
What is the required pressures? There are several mix programs available out there, and there are a couple on this site. There are also several mixing spreadsheets out there for cross-platform use.
Basically they give you the breakdown as to what partial pressure is needed for the mix. For example a dive to 140 with a pp02 of 1.4 and a equivalent nitrogen depth of 100 in a 3000psi tank you would add 377 psi of He. Next you need 317 psi of 02, so you would hook it up to your O2 tank and bring the total pressure up to 694 psi. Then top it off with air. The result is 27% 02, 13% He and 60.7% N2. Or what is referred to as a 27/13 mix.
Helium has a compressibility factor that you might want to consider. Typically you need to add 50-100psi more gas than the charts call for, and some mix programs compensate for this. So if you find your target PPO2 running a little high, this is probably what's going on.
Dealing With Low PPO2 Mixes by George Irvine
The 2 percent accuracy issue is not too bad when you are partial pressure mixing for middle readings, like 35%, for example. You have to double check the partial pressure and the reading, and the use of the gas is not a problem from either a toxicity or a decompression standpoint.
Where this gets tougher is mixing the real deep mixes, but then the partial pressure still applies and the totals are still valid. The PPO2 are low, so that the amount of O2 added is small. You can start to see where rebreathers, with their small air tanks, can become a problem. The smaller the volume of your mix, the harder to hit your PPO2 mark. A solution to the rebreather problem is to create your mix in a set of doubles and decant into the smaller flasks.
The big trick is to be sure you actually added the gases when mixing, and did not have a valve off while you THOUGHT you added 35 psi of oxygen, or some such number, when in fact you did not, and then the miniox reading, at such low PPO2 seems "acceptable."
A real good anal way of doing this procedure is what is needed to do it right. Bill Mee and I do it together, and we have a whole checklist to go through before disconnecting the tanks, and for the whole process.
Adding oxygen to a high helium mix can feel like you are adding it when you are not. Just pressurizing a big fill line for a small increment higher, even with a very accurate digital gauge, you can be fooled since you can hear the gas moving even when the tank is not in fact open to accept it - a real dangerous situation. You have to depressurize the line afterwards and note the tank change on the same gauge that the system was on to begin with , and you must do it quickly. There is no way the pressure has risen without the addition, even if the number is thrown off by the cooling - it can not decrease.
Little double checks like that, and then immediately throwing the analyzer on the result will give you some comfort. Then analyze the pure helium to be sure the reading is not offset. A lot of work, but you are always betting your life with this stuff.
-special thanks to the WKPP for help with this subject


Managing Your Gas Supply


A problem with partial pressure mixing is that you will have to turn in your 02 or He tank with quit a bit of gas left in it. For example if you need 20 cuft. of gas for your lowest mixture and you only have 19 in the tank, then you throw away 19 cuft. of 02 or He.
There are several ways to get around this:
Haskel pump- This is an air powered pump where 100psi of air is used to create 4000psi. Essentially a Haskel pump is a great big piston connected to a very small piston. You put air behind the big piston and it pushes the small one. The drawbacks of this unit is that it's very expensive.
Air compressor- If you have your own air compressor you can feed helium into the intake and let it pump it into your tanks. You can do this with oxygen, but your compressor must be specially prepared for this service, as compressors generate heat. A British method for pumping helium is to attach an old single stage regulator to the bottle, and hook the supply hose to the air compressor intake.
Cascade system- This is where you take several tanks and hook them up via a hose manifold. The idea with a cascade is you fill your tank with the lowest psi tank first then work your way up to the highest psi cylinder. Using this technique you can practically drain your cylinders before turning it in. You can extend your manifold to include the helium tanks if you wish, or you can manually move your whip from tank to tank without the manifold.

1 comment:

Trimix diver said...

Hi !
A french diver who loved Trimix. i don't really understand all the things.
See you