Metric vs Imperial

[BlueDevil]

For those of us that live in countries that adopted the metric system many years ago the imperial system just seems like complete craziness! Sure it takes a little while to get used to metrics at first, but once you have the hang of it you would never go back. I am not just referring to diving here, but a whole range of measures where metric is so much easier.

I guess the fact that this forum has a large US following makes it a bit frustrating for us metric fans when we encounter Fahrenheit, feet, psi, pounds etc. I started my diving career in the imperial age and did take a little while to come to terms with metric, but now it is so instinctive that I struggle to understand what people are talking about when they use imperial.

And by the way, the fact that 10m of seawater just happens to coincidentally equal 1 atmosphere of pressure is really handy.

My only gripe with the metric system as it relates to diving is tank sizes. When people went to metrics they didn't change over to an equivalent measure, but somehow decided that instead of reporting the tank size when it is full that they would report it when it is empty. This means that saying "I have a 12 litre tank" is meaningless unless you also specify the working pressure of your tank (ie it could be a 230 bar tank or a 300 bar tank, or some other pressure). I wish we had simply converted from the capacity of a full tank in cu ft to the capacity of a full tank in litres. For example a Faber 85 cuft tanks is referred to as a 10.5 litre tank, rather than [FONT=&amp]its full capacity of 2436 litres[/FONT] - admittedly 2426 litres doesn't roll off the tongue as easily as 10.5 litres, but no doubt it would be abbreviated to something like "24 hundred".

Anyway. what inspired this little rant is a photo I found on Facebook that shows how silly imperial is compared to metrics. I found it amusing so thought I would share it it with you. This thread is just a light hearted dig, so I hope our US forumites don't get offended.

 

[Akimbo]

Unfortunately, the US is a victim of bad timing. The metric system was barely conceived and virtually unknown when the US Congress had to make a decision (1789). They actually debated decimalizing common Imperial units. I believe the US was the first to have decimal money. France was the early adopter in 1799. The US market size and relative isolation made it possible to put off the inevitable for much longer than everyone else.

Here is an interesting article:
Units: The Metric System in the U. S.

I was in the UK when they converted in the 1970s. It was not easy and metric is still not fully engrained. It appears the power of cultural momentum is capable of overpowering logic for extended periods.

I do take exception to the date methodology. Year, Month, Day padding single digits with a zero makes more sense. Representing today's date with 2012-12-08 allows easy sorting, manually or by computer. It also can't be confused like 12/8/12 or 8/12/12 can. To eliminate any confusion, I always write 8 Dec 2012 regardless of who I am corresponding with. The world should have learned its lesson in Y2K that being lazy about designating the 100 year mark, let alone the millennia was imprudent.

 

[Fishpie]

Next Wednesday should keep everybody happy.
12/12/12

 

[Tigerman]

When it comes to tank sizes and empty/water capacity volum its actually got some very good upsides as well..
a 12l 200 bar tank would be 2400 liters of air, if you happen have a sac of 12l/minute - 150 minutes before the tank is at 50 bar, just adjust for depth.

 

[Kevrumbo]

It's much easier to work with pressure units for gas planning & management in the Metric System, especially if your pressure Surface Consumption Rate (SCR) turns out to be a convenient integer.

I have a cold water reference Surface Consumption Rate (SCR) of 22 litres/min(roughly 0.75 cuft/min in US Imperial units).

The common AL80 Tank holds 11 litres volume at the surface standard of 1 ATA(or 1.01 bar), for a metric rating of 11 litres/bar.

22 litres/min divided-by 11 litres/bar equals 2 bar/min SCR (Surface Consumption Rate) in pressure units --a more useful quantity to utilize during the dive since your SPG reads in bar pressure units. [And 2 bar/min is much easier & quicker to arithmetically manipulate in your head than 29 psi/min equivalent in US Imperial Units.]

Your depth in meters, which converts easily to ATA (simply divide-by-10 and add 1) becomes your multiplier depth factor for your 2bar/min pressure SCR.

Example: 30 meters depth is 4 ATA (divide 30 by 10 and add 1 equals 4); your 2bar/min SCR at depth -or Depth Consumption Rate (DCR)- now becomes 8bar/min. [4 times 2bar/min equals 8bar/min]. So 10 minutes at depth 30m on an AL80 (11L/bar) tank in nominal conditions, you would expect to consume 80bar of gas and your SPG reading to be down or show a delta of 80bar. . .

What if your SCR, or Depth Consumption Rate (DCR) is lower or better than the example above? --Just scale it as a percentage result:

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

This is how I used this value with a 11 litres/bar tank (i.e. an AL80) in Palau:
11 litres/min divided-by- 11 litres/bar equals 1 bar/min pressure SCR.[Compare: how much easier & intuitive is it to work with "1 bar/min" vs "14.5 psi/min equivalent" in US Imperial Units???]

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, therefore half-tank is 100bar remaining). 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 SCR with your particular tank, knowing how much breathing gas you have left not only on pre-planning, but also during the actual dive at depth, real-time-on-the-fly --all with easier to use metric units . . .additionally, you have a SPG that reads in units of pressure: why not convert your SCR to a Depth Consumption Rate (DCR) in pressure units to make use of it???​

 

[Insta-Gator]

I remember in the 1970' the US was supposed to convert. Was first introduced to metric measures in high school physics. Made perfect sense to me and was anxious for the change. But if you think the Y2K change was expensive, imagine not only changing computer programs, but also changing every piece of measuring equipment or any device that has measurements inscribed or even books of every kind, even cook books. I can understand why the decision was made to cancel the change.

 

[Tigerman]

It's much easier to work with pressure units for gas planning & management in the Metric System
...​

Only if you dont change tank sizes. If I use 2 bar/minute from a 12l tank I wont use 2 bar/minute from a 15l tank..

 

[Ayisha]

Canada is all metric except for seemingly everything to do with diving - tank size, tank pressure, depth...

 

[Kevrumbo]

Only if you dont change tank sizes. If I use 2 bar/minute from a 12l tank I wont use 2 bar/minute from a 15l tank..

It doesn't matter if you can do simple arithmetic --it's still relatively easier to manage Surface Consumption Rate (SCR) for initial gas planning in metric:

22 litres per minute -divided-by- 11L/bar tank (AL80): 2 bar/min
22 litres per minute -divided-by- 13L/bar tank (AL100): 1.7 bar/min
22 litres per minute -divided-by- 22L/bar twinset (double AL80's): 1 bar/min
22 litres per minute -divided-by- 34L/bar twinset (double Pressed Steel 104's with a cave fill): 0.7 bar/min

Just so you understand that a pressure SCR is dependent on tank size, while your volume SCR (my 22 litres/min above) - is constant across all tank sizes. . .

 

[Darnold9999]

I wish Canada was all metric. :depressed: Because our largest trading partner is the US a fair amount of stuff is still imperial. I have two sets of hand tools and I still mostly use the imperial set. But need both. Buy groceries in lbs - get temperature in celcius - wind speed in miles per hour (or even worse, knots if I am on the water), car speed in kilometres per hour. Very confusing - wish the imperial system would just go away, but until the US makes the change it is simply not going to happen.

Dive in PSI - could dive metric but inertia has left me in the old system.