PSI or BAR in Tech diving?

[Tortuga68]

The origin of the duodecimal (base 12) system of numbering is from the 12 cycles of the moon per one solar rotation ie 12 months to the year

The Vikings may have used it, but it predates them by around 2000 years

 

[boulderjohn]

Interesting, I didn't know that history. If I watched football, would I get that last bit?

Yes. Poor joke.

 

[boulderjohn]

The origin of the duodecimal (base 12) system of numbering is from the 12 cycles of the moon per one solar rotation ie 12 months to the year

The Vikings may have used it, but it predates them by around 2000 years

I was trying to remember when I learned that tidbit, and I realized it was in a high school math textbook decades ago (base 10), in a lesson on different base systems.

It does predate the Vikings, but the significance for us is that it was the Vikings who started us on the imperial path. Europe was dominated by the Roman (base 10) system until the Viking raiders terrorized northern Europe for a few hundred years late in the first millenium CE. England got hammered from both ends. Viking raiders were constantly coming at them from the north, and if you look at the history of the Kings of England, you will see that several before the Norman invasion were Danish.

The Normans themselves were recent descendants of Viking raiders in France. Norman is just a variation of Norseman.

 

[Kern]

Not every measurement in the Imperial system is base 12. There were 20 shillings to a pound, 8lb to a cwt, 8 furlongs to a mile, 10 chains to a furlong, etc. How does base 2 sound.

I always thought Charlemagne, had something to do with the development of the Imperial system of measures.

 

[boulderjohn]

Like the entire English language, it is probably a mix of about 18 different cultures.

 

[Akimbo]

I am normally not a fan of dual scale instruments, however Bar and PSI are far enough apart that you are not likely to get confused — especially on an analog SPG scale. There are a few dual scale SPGs that are not excessively cluttered or hard to read on the market so you can choose the best unit for the estimate/mental calculation being made. As for setting units on your computer; I agree with other posts that familiarity rules — especially on a digital scale. A reasonable compromise would be to use PSI on a computer and Bar on an SPG.

 

[ijdod]

Not being used to them aside, the Imperial depth and pressure units are linear, so fairly easy to convert. That said, I do remember my first reaction when an American friend told be how the tank volume system works: "You're kidding, right?". The system isn't even Imperial per se, it could just as well have been an Imperial volume unit, say a 3 US Gallon tank filled at 3000 PSI. Us metric minded people would call that an 11 liter tank at 200 bar.

 

[Kevrumbo]

I decided to put this into the Advanced Scuba Discussions although it has probably more to do with Technical diving. . .
Now I know the formulas can quickly be found on the interwebs to do the conversion, but I feel personally that Metric and Tech go hand in hand, for several reasons.
1: Atmospheric pressure is easy to calculate. 10m = 2 atm and so on
1a: Because Atm Pressure is easier I find it quicker to calculate gas mixes (even trying to teach a student Nitrox in Roatan had me umming and erring, even though I could have the answer in seconds with Metric)
2: SAC rate easy to calculate.
3: 3 metre stops, compared to 10ft stops

How does everyone else feel on this subject?

1: Atmospheric pressure is easy to calculate. 10m = 2 atm and so on
Simply divide depth-in-meters by ten, and add 1 to give depth in Atmospheres Absolute (ATA). For example, 6m is 1.6ATA; 21m is 3.1ATA; 30m is 4.0ATA etc.

1a: Because Atm Pressure is easier I find it quicker to calculate gas mixes (even trying to teach a student Nitrox in Roatan had me umming and erring, even though I could have the answer in seconds with Metric)
Dalton's Law applications for nitrox, partial pressure of Oxygen (ppO2) and Maximum Operating Depth (MOD) faster to implement because of the quick & easy translation above of meters to atmospheres absolute, and vice-versa.

2: SAC rate easy to calculate.
My SAC/RMV in tropical warm waters is typically 30% better than it is in temperate cold SoCal homewaters. However, after a week now diving in Palau 30deg C water temp, I've lowered that to around 50% of my nominal cold water SAC (from 22 litres/min to 11 litres/min).

This is how I'm using this value with a 11 litres/bar tank (i.e. an AL80) here in Palau:
11 [-]litres[/-]/min divided-by- 11 [-]litres[/-]/bar equals 1 bar/min pressure SAC rate.

All my dives are averaging 20 meters depth going with the drift current; 20 meters is 3 ATA (divide 20 by 10 and add 1 gives a depth in atmospheres absolute of 3 ATA).

Therefore 1bar/min multiplied by 3 ATA equals a depth consumption rate of 3 bar/min at 20 meters. Checking my bottom timer every 10 minutes, I expect to consume 30 bar (3 bar/min multiplied by 10min equals 30 bar), and accordingly my SPG should read 30 bar less in that 10 minute time frame.

So by 30 minutes elapsed dive time at 20 meters, I expect to be down 90 bar or at half tank (AL80 full tank is 200 bar). At 40 minutes elapsed time, I'm ascending off the wall into the shallow coral plateau around 9 meters (down 120 bar from 200 bar total, or 80 bar remaining in tank). And finally at the 45 to 50 minute mark, I'm at 6m and my 3-5min safety stop with 60 to 70 bar left. I surface and I know even before looking at my SPG that I have around 50 bar remaining in my tank.

This is how you should actively use your SAC rate with your particular tank, knowing how much breathing gas you have left, not just only pre-planning, but also during the actual dive real-time-on-the-fly . . .additionally, you have a SPG that reads in units of pressure: why not convert your SAC rate to a Depth Consumption Rate (DCR) in pressure units to make use of it??? Much easier to utilize in bar with the metric system.

3: 3 metre stops, compared to 10ft stops
Simply reference counting numbers by convention for depth --in Imperial units usage, it's every 10' (i.g. 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 etc); if you're on a bottom-timer/non-dive computer at an average depth of 55' you could conservatively round this evaluation up to 60' for deco or gas consumption calculations on-the-fly, for example. In Metric units, it's every 3m (3, 6, 9, 12, 15, 18, 21, 24, 27, 30 etc.); similarly if you're at 16.5m you could round this up to 18m. No big difference between either equivalent metric or imperial usage other than being easier again to figure out depth in ATA from depth in meters.

Much easier to though, to time from 6 meters depth (O2 deco stop profile), a slow six minute ascent to the surface, as the ascent rate becomes an intuitively easy to follow 1m/min on your bottom timer/computer in metric units, versus 3'/min from 20' in Imperial Units.

 

[KY]

The origin of the duodecimal (base 12) system of numbering is from the 12 cycles of the moon per one solar rotation ie 12 months to the year

The Vikings may have used it, but it predates them by around 2000 years

ummm i think you'll find Imperial measurements are all based on the human body..

ie i can count to 12 on one hand (try it, use your thumb to count the parts of your fingers..) i can measure 3" with my fingers (a hand), 1 yard with my arm (yard-arm!??), 1 foot with my,, well, foot...

So in olden days, when building things, it was easier to rely on standard body parts than a standard unit... make sense???


as for why a 3m stop.. Haldane was English.. the English used Imperial measurements... If Haldane had been French, i expect it would have been 2m, 4m, 6m etc,,,

 

[Doc Harry]

Imperial is so-o-o-o-o much easier than metric.

It's too late to teach an old dog new tricks.