Advice on SPG

[elgoog]

I use a Bar SPG because it really is easier to do the math.

Do you use this with tanks measured in cubic feet? I'm curious how the math works out with that for air consumption calculations.

 

[JohnnyC]

I think you would be shocked at how wrong you are regarding people's comfort level. I ask my students to estimate the distance from the building to the end of the parking lot. They have no clue. Not in feet or yards or meters. And if you really want a fun time, give people a tape measure. They might be ok with 38 inches, and maybe some would get the mixed measurement of 3 feet 2 inches. But what about all those little markings in between the inch numbers on a ruler? Fractions are a total mystery to most. And keep in mind these are college students. It is much easier for most people to measure 1.894 meters or even 189.4 cm. People deal with decimals all the time with money. The only ones dealing with fractions on a regular basis are carpenters and chefs. And they struggle with it too. I often had students who work in the trades take my class specifically to learn how to deal with fractions. It took less than a generation for people to be comfortable buying a liter bottle of Coke while they still buy a quart of milk. Bite the bullet and make the change. People will gripe and moan for about 8 congressional election cycles and then it will be second nature. But no one is willing to sacrifice those 8 elections. iPhone. iTypo. iApologize.

I think you missed my point. I completely agree about using fractions, trying to use a tape measure or a micrometer, etc. But I can tell you from much experience, that singular measurement, the foot, is what keeps people in the US away from using the metric system on a daily basis. MOST people don't actually estimate distance, or measure things, on a regular basis (except maybe miles). However, almost everyone in the US will reference a foot semi-regularly. As ridiculous as it sounds, people use feet all the time, they reference it, they have a handy estimation tool (their actual foot) and it's just so common. That doesn't mean it's not utterly ridiculous and devoid of logic, but having that intermediate measurement as the cornerstone of our distance measuring system, makes it hard for those people who aren't constantly measuring things to make such a radical change, no matter how logical it should be. Remember, we're talking about everyday Tom, Dick, and Harry here, we're not talking about an engineer, or a tradesman like a welder/fabricator, we're talking about a soccer mom, etc. Simply using decimals when dealing with money doesn't mean someone can automatically equate 1.67 meters as a meaningful height when visualizing a person, however I bet if you said "5 foot 5" they'd instantly have a reasonable approximation. Hell, I bet 75% of the population couldn't pass high school algebra if you gave them a test tomorrow.

The foot measurement is just so prevalent in US society that until a generation is forced to go metric, it's not going to change. I'm 5'10", or 5.83 feet, or 70 inches, or 1.78 meters, or 178 centimeters. If you were to poll the US population, which do you think they would most easily associate with something tangible? Now our enlightened metric brethren would have no issue whatsoever with either meters or centimeters, but that's more a function of a base-10 system of measurement where decimals actually equate across the board (as opposed to my 5.83 feet example)

I don't think your liter of Coke example is very good because people aren't actually using them as a real measure of anything. It could be any volumetric measurement and they'd be fine with it as "a bottle of Coke" or "a big bottle of Coke" or a "small bottle of Coke." 3 liters is bigger than 2 liters, but the actual volume difference is moot. People just don't actually care. We still buy milk in gallons instead of liters. If your example rang true, we'd be buying milk in liters as well, instead of gallons and half-gallons.

You are right, we need to bite the bullet and make the change, because sticking with imperial is simply illogical. Could be worse though, we could be using cubits, which would either be good or bad when trying to figure out your property line, or even worse, a smoot, which doesn't fit evenly in metric or imperial. Also a metric foot. But that's just a British thing....weirdos.

 

[KevinNM]

Do you use this with tanks measured in cubic feet? I'm curious how the math works out with that for air consumption calculations.

You look up the volume of the tank in liters and use that. For example, the Luxfer S80 is pretty much the prototypical AL80. Luxfer's chart says it has a volume of 11.1 liters, which we'll round down to 11.

So at 210 bar an 11 liter tank holds 2310 liters. We'll assume my SAC is 20 so at the surface there is 115 minutes of air. Every 10 meters you increase pressure by an atmosphere, so at 20 meters (60 some feet) you have enough air for 38 minutes (If you have a death wish and maintain no reserves of any kind).

If I get a tank that has 180 bar it's easy to figure out how much air there is. It's 180 * 11L, which you can do in your head as 1800+180. Compare to trying to figure out how much air there is in an AL80 at 2300 PSI. The S80 has an actual capacity of 77 cubic feet at 3000 PSI, so it's (77/3000)*2300, which I can't do in my head at all. There is an approximate short cut using tank factors and the circle T method, but it still isn't as simple as using bar and liters.