Americans, let's take it as a given that the whole rest of the world uses metric. Whether you like it or not is irrelevant... it just is. You don't see me screaming about how gravity is corny and that we never got a sequel for Buckaroo Banzai, do you? No. Because I've simply accepted that it is so.
So today I wind up with a sidemount pressure gauge in bar and one in psi. Because I forgot my Teric (again) and, therefore, couldn't use my fancy transmitter. Had to resort to the spare sidemount SPG on my left tank. Which is in a different format than the SPG on my right tank.
What's more, in our team we had some people who are used to metric and some used to imperial.
I've grown very comfortable switching between the two for gas planning purposes, so talks on the surface were a piece of cake. But it's rare that I have to convert so frequently on the fly as I did today, constantly switching sidemount regs and making sure that my tanks were balanced.
I could have just gone lazy about it and just trusted that my 5 minute switches were keeping the tanks even enough... but that's not how I roll. Instead, every 20 minutes or so, I wound up doing math in my head. Which is not my strong suit.
If it was my strong suit I'm sure I would have noticed the pattern much, much quicker than the two 90 minute dives it took me today. A pattern I'm pretty annoyed no one ever told me about.
So I'll tell you about it here.
1 bar = 14.5 psi
But for math in our head let's call it 1 bar = 15 psi. Because fuck it; pressure gauges aren't that accurate anyway.
So it stands to reason that if 1 = 15 then 20 = 300. That is, 300psi = 20bar. Those are actual numbers that an SPG can read. And gives us a super-easy 2:3 ratio.
All you need to do to go from bar to psi: add 50% then tack a zero onto the end.
160 bar +80 (which is 50%) = 240 tack on a zero and you get 2400psi.
(actual conversion is 2320... but who cares? You ever have one of those buddies with an AI computer who tells you their pressure to the single digit? Yeah. Me too.)
To go the other way, from psi to bar: divide by three, take two of those thirds, and take a zero off the end of it.
1800 psi ÷ 3 = 600 x2 = 1200 remove a zero 120 bar
(actual conversion 124 bar. But, again, who gives a shit?)
Or you can further streamline this, as I did, because working in smaller numbers makes me feel safer because I am a mathematical nincompoop. Just work through the 2:3 ratio in whichever direction you need to go and then assume that there's two zeros if you're going into psi and only one if you're going into bar.
And, obviously, just squidge it by 100psi or 10 bar in whichever direction you need to go if you wind up working with a number that isn't divisible by 2 or 3. And you'll be close enough for government work.
170 bar We'll work from 17. We need to add 50%. We'll call it 8 because that's half of 16. So 25 (I know because I just now counted it using my fingers). We're looking for psi so we just add two zeros. 2500. In my head (and on my fingers) I just now sorted that 170 bar = 2500psi
Actual conversion using conversion software: 2465 psi
1700 psi. Again, let's just go with 17. Divide by three? Yeah, fuck that, I'm gonna divide 15 by 3 and get 5. I'm gonna take two 5s and have a 10. I'm looking for bar, so I'l just add a zero to the 10 and have 100. But, you know, I divided by way less than what I actually had, so I'm just gonna throw another 10 bar at it for good measure. Again, in my head (I didn't need my fingers for this one) I just sorted out that 1700 psi = 110 bar.
Actual conversion using software: 117 bar
Just how the hell about that, huh?
Seriously, I'm pretty bummed no one ever showed me this.
But now I've shown you.
And now we can wait for the Americans to start screaming about how the whole rest of the world should change to THEIR system. Because a base 10 system that even works across different types of measurements is apparently far inferior to a system based on who knows (or cares) the antiquated fuck what. In about 3... 2... 1...