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Reposted with header. pr > > Since some questions about temperature changes in filling > have been addressed to me here is some clarification > about scientific backround and difficulties of faster mixing. > > (If You have not read previous postings about compressibility > please read them for backround) > > > "Thermodynamic Factor" > > When compressing gas it heats up. This causes troubles in > gas mixing. Errors caused by deltaT's are significant > indeed. If we want to eliminate error caused by > gas temperature changes in system in addition to compressibility > following calculation approach could be applied: > > The warming of gas in isentropic compression can be calculated > by using formula > > T2/T1 = (p2 / p1) ^ ((k-1)/k), k=cp/cv > > cp and cv are gas specific thermodynamic > constants and can be found in references > specified in previous articles. > > Because there are at least 4 case specific factors involved and > we are not designing a specific system (by modelling it) it is more > fruitful of finding the solution some other way. > > > A possible simplified method could be: > > In fast mixing, if you measure the temperature of a cylinder during > fill, you may calculate schedules of filling for "standard" mixes by using > formula presented in article "Compressibility 1". Instead of > using stable 290 K, use variable temperature in formula because > you don't wait the cylinder to cool. > > Remember that > a) compressibility changes a bit with temperature > b) fill allways the same number of bottles a time > c) temperature of helium source may vary and cause additional error > d) you have to standardize the procedure, because there is a time > factor involved > e) these experimental results are good for your facility, fill, > and bank starting pressures, others have different parameters. > > In method described above is still some noise. Probably the easier > way to go is to fill slowly or: > > > Better results in fast mixing could be achieved by measuring the > gas by weight, not by pressure. > > For this you need a setup of filling panel, and high accuracy > digital scale with "tare" function. > > To mix 2 * 12 / 200 bar bottles for 65.9 % He, 24.2 % N2, 9.9 % 02, > > need 65.9 % He 3163 Litres 0.162 kg/m3 512 g > 30.6 % Air 1469 Litres 1.176 kg/m3 1727 g > 3.5 % O2 168 Litres 1.400 kg/m3 235 g > > Z= 1.068 (at 290 k), therefore deduct weigts accordingly. > > Setup bottles on scale. Tare to zero. Fill helium 479 g. > Tare to zero. Fill Oxygen 220 g. Tare to zero. Fill air > 1617 g. > > Gas should test OK. Pressure should be 200 bar after cooling, > if not, suspect error in procedure. Whole collection of cylinder > can be filled simultanously. Effects of deltaT is eliminated here > by not measuring heat affected parameter. Although cylinders should be > similar for best results... > > Thanks to Juha Flinkman, Matti Leinio, Raimo Turunen > > Pekka Raty > > PS. Using imperial unit system is clearly strokery by it's definition. > If in doubt ask NASA what pros use. > > > > -- > Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. > Send subscription/archive requests to `techdiver-request@aquanaut.com'. >
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