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Ron, It's a simple procedure. If you have premixed OEA36 and have it banked in large enough quantities or you are using a membrane system it does not get much easier to make the 18/50 or other mixes . The key is to start with an empty cylinder transfill the helium slowly, and then let it rest a while until it is at ambient temperature, this way you know that you have the correct amount. Use a quality analog gauge (20 psi increments) or a digital gauge. Remeber the helium will dilute the oxygen from the OEA in direct proportions. Here is a chart I created for the NOAA Diving Manual 4th ed. that can help you. MOD 1.4 % Helium atm po2 to add % of NN36 Final FO2 Final Helium Trimix fsw 126 0.20 0.802 0.29 0.20 29/20/51 168 0.37 0.629 0.23 0.37 23/37/40 210 0.48 0.518 0.19 0.48 19/48/33 223 0.50 0.500 0.18 0.50 18/50/32 255 0.56 0.440 0.16 0.56 16/56/28 297 0.62 0.383 0.14 0.62 14/62/14 * 352 0.66 0.339 0.12 0.66 12/66/12* To use the chart identify the desired mix, then multiply the final desired cylinder pressure by the helium factor to find the amount of helium to add in psi. For example, a NOAA Trimix 18/50 is needed at 2600 psi. Multiply 2600psi by .50 resulting in 1300psi of helium needed in an empty cylinder, then top with NN36 to the final pressure. To make a trimix 29/20 (29% oxygen 20% helium) for a dive in the 130-150 fsw range less helium would be needed. For a final cylinder pressure of 3,000 psi, multiply 3000 psi by .20 resulting in 600 psi of helium needed in the empty cylinder, then top with NN36 to the final pressure. Refilling cylinders to the same mix that have a remainder of gas in them can be done by using the same mixing methods but will require a little more care. For example a NOAA Trimix 18/50 cylinder has 1500 psi remaining after a dive. The cylinder needs to be refilled with the same mix to 3,500 psi. The blender needs to add 2,000 psi of Trimix 18/50 to the cylinder set. The existing cylinder will have 1000 psi of helium added bringing the total pressure to 2,500 psi, (intermediate oxygen analysis of 11.2%,), the cylinder will be topped off with NN36 to the final pressure of 3,500 psi. The final oxygen analysis will be 18%. A gas booster may be needed to bring the helium up to the intermediate pressure, but in any event the handling of high pressure oxygen has been eliminated. I hope this helps you. regards, Joel Silverstein http://www.trimixdiver.com At 12:28 PM 3/26/00 -0600, Ronald D Thompson wrote: >Experienced tri-mix home brewers on the List: > >I'm considering blending some helium and EAN36 to yield a specific >tri-mix (this would be my first attempt at blending tri-mix), and I'd like >some guidance regarding fudge factors to use in order to compensate for >helium compressibility. > >My ideal gas calculations would have me first half-fill an "empty" >cylinder with helium and then top off with EAN36, to obtain a tri-mix >consisting of 18% O2, 50% He, and 32% N2. (At MOD = 225 fsw, this mix >has PO2 = 1.41 ata, END = 71.5 ata, and AED = 96 fsw.) > >Now, my admittedly naive understanding of helium compressibility suggests >that I really need to start with more than half-filling the cylinder with >helium to obtain the desired mix. My question is: How much more? >Specifically, if I'm using twin LP 112's with an ending pressure of 2640 >psig, what fudge factor should I use to end up with tri-mix 18/50? Same >question if, instead, ending pressure is 3500 psig? > >Thanks in advance for your suggestions. > >Ronald > > >-- >Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. >Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'. > -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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