Mental Tricks for Mixing
I often walk into shops, check the pressure on a tank, do some mental math and tell them how much gas i want. They're often shocked when the gas comes out to what I want. I thought I'd let people in on the secret.
Let's say you're at 1000 PSI of 32% and you have to take an air fill. You don't want below 28%. How much gas should you ask for?
Here's the trick. The difference between 32% and 21% is 11. Everything factors around the 11.
28% is 7/11 of the way between these gases, and closest to 32%.
Hence, a mix of 7/11 32% and 4/11 air will give you 28%.
OK, so you want a mix where 1000 is 7/11, how much air? Answer, 4* 1000/7 (the 11's cancel out). That's 4000/7, or 570 psi'ish. Say 600.
Easy!
Another example. You have a mix that's 1000 psi 32% and 2000 psi 21%. What's the final mix?
It's 2/3 of the way in between, closer to air. Remember the diff is 11. So 11/3 = 3.67. the mix will be (21 + 3.67) or 24.67, or roughly 25%.
Once you get the differencing trick, and if you're reasonably facile with mental arithmetic or (God forbid) a pen and paper, you can reliably predict what mix you'll get when topping up with dissimilar gases, or give the filler a reasonable extimate of how much gas to shove in.
Works for other gases too.
Want a 50% mix using air and O2? To make life simple pretend air is 20% O2. So now the difference is 80 between the gases (100-20). You want to go UP 30 from air and DOWN 50 from O2. The mix will be 3/8 oxygen and 5/8 air. For a 3200 fill (remember 32=4*8 ), that's 1200 O2, 2000 air.
Simple!
And here's the thing. If you can predict mixes, then when a surprise happens, you know that something in the chain is broken. This should start you checking EVERY gas at that point. Perhpas the O2 sensor is off - I've managed to catch that error before this way. Or perhaps the banks don't contain what the store thinks they do. So being good at predicting what a mix will turn out at is really useful.
Let me suggest this point ( tying back to the last post) - testing with an analyzer isn't much use unless you know what to expect out of the analyzer. Do the math - using formulae if necessary - so that the O2 monitor is a tool to VERIFY your mix rather than to DETERMINE your mix. The monitor may be miscalibrated, the gas poorly mixed, and you may end up with a different gas from what the O2 thingie says.
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Originally Posted by BgDadddy
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