Mailing List Archive Mailing List Archive
>In a message dated 95-10-25 22:10:08 EDT, DVanzandt@ao*.co* (David M. VanZandt) >writes: > >>chris. > >Several companies manufacture industrial scales for gravimetric mixing, >Mettler, Setra and I believe Toledo scales. > Setra is who we are looking at right now. > I presently use gravimetric mixing techniques for combustion experiments >that I perform on NASAs KC-135 and DC-9 aircrafts and the upcoming MSL1 >spacelab mission on the shuttle(no I'm not flying this one, though I wish I >was). We mix to K-bottles using a variety of gasses(N2, O2, SF6, CO2, H2, CH4 >etc.) to exacting specifications i.e. +/- 2% of each constituent (ex. for a >5.00% H2, 20.00% O2, 75.00% N2, the acceptable mixture range for H2 would be >4.90 to 5.10%, pretty tight huh!). We confirm our mixtures using a gas >chromatagraph calibrated over our range of interest. We use a SETRA scale >that has an accuracy of 0.2 grams. I will have to wait until I back to >Cleveland to get you the model number if you need it.(Presently I am working >the STS73, USML-2 spacelab mission at Huntsville, Al , working the grave yard >shift. So if any of this response makes any sense I will be suprised.). > I Agree fully. gravametric mixing IS THE way to go. No ands ifs or buts!! >Chris I think you are going about setting up your gas mixing program the >correct way. You will probably get little in the way of help from the >commercial gas mixing companies. We have already tried when we were doing >partial pressure mixing and all their data is proprietary. Their gas mixing >programs are based on empirical data collected over their years of gas mixing >and it is still an art. If the moon is full and its raining outside and 5:00 >they know to add just a little squirt more to hit the exact mix target. We >know this for a fact, we've watched them hit target mixtures right on the >nose using this technique. > Again, you are quite right. They have Solid tables that they have developed over the yeas through trial and error. And they are not about to share them with anyone else. >I would disagree with what you said about partial pressure mixing being >undestood and that all you were trying to do is to take into account >compressibility, they all go hand in hand. The first approx for partial >pressure mixing is to use the ideal gas laws. They work good if you are at >low pressures(tens of psi). Once you exceed about 20 to 50 psi the ideal gas >law assumptions begin introducing errors into the final target mix. The >second approx for pp mixing is to include compressibility into the equations. >This gets you a little bit closer to your target mix, but in my opinion not >close enough. The third appox is to use full blown equations of states for >the apprpriate constituents which gets you even closer, but these are >normally only good for a small range of pressures and are very hard to >determine, as you have to do teh experiments for each gas and combinations to >get anything that is useful. The fourth method is to gather empirical data >for alot of mixes and crunch it down to workable equations and use them. > I did not mean to say PP was either good or fully understood. Only that PP blening is what stores are using, for better or for worse. There are a "few" stores that i know of that have gone to the trouble of setting up a weight based system, but they are the exception and not the rule. For the moment what we have is PP blending with O2 testing, and thats pretty much it. >We gave up on pp mixing because of its errors and went to gravimetric mixing >because you don't have to mess with temperature, compressibility, cylinder >size, etc. All you need to do is calculate the weights of each of the >constituent gasses, throw a cylinder on the scale, and mix it. (If you have a >good setup & take care of drafts, line flex etc, etc) > >I am really conerned with whats being mixed in the diving community. Everyone >seems to be concerned with O2 tox, 1.4, no, 1.6, well, 1.8 or how about >1.549. Well not to beat that point but if you do not know whats in your tank >how can you calculate ppO2???? > The other problem is we have no realistic means of testing anything BUT the O2..... Their are strong rumors that Oceanic has contracted with a medical cupply company to use their portable gas spectrum analyser in the Phibian. If the information I have is correct, the unit costs about $500.00 and is thermal and humidity compensated and performs a broad spectrum analysis yeilding N2/He/O2/CO2 etc results!! This would be a Hell of nice tool to have at our disposal. >The measuring techniques I have heard discussed and the industries use of the >Miniox makes me glad I am not diving mix yet. If and when I do I will >definately mix my own!!! or have a decent analyzer so that I know what I >have. Well I am digressing now so I hope this info helps you. > Thanks for your kind reply. > >David M. VanZandt >Senior Project Engineer >Aerospace Design and Fabrication Inc. >216-977-0347 Christopher M. Parrett, President, Abysmal Diving Inc. Makers of ABYSS, Advanced Dive Planning Software. 6595 Odell Place, Suite G. Boulder CO, 80301 Ph, 303-530-7248, Fx, 303-530-2808 ftp://abysmal.com/users/abysmal http://www.emi.net/gulfstream/abyss/abyss.html
Navigate by Author:
[Previous]
[Next]
[Author Search Index]
Navigate by Subject:
[Previous]
[Next]
[Subject Search Index]
[Send Reply] [Send Message with New Topic]
[Search Selection] [Mailing List Home] [Home]