Article: Liters and Meters and Bars: OH MY!!!

[Jeff Morelock]

You can view the page at http://www.scubaboard.com/forums/content.php?r=412-Liters-and-Meters-and-Bars-OH-MY!!!

 

[Hickdive]

Metric in diving is actually much easier than imperial.

For example, to calculate the ambient pressure at any depth in metres simply divide it by ten and add one (that's the one bar from the atmosphere) so at 25 metres the ambient pressure is 3.5 bar.

How much gas does a cylinder hold? Simply multiply the cylinder capacity by the cylinder pressure e.g. a 10 litre cylinder at 200 bar contains 2000 litres.

How long will that cylinder last at 10 metres (allowing for a SAC rate of 25 litres per minute)? 2000 litres (cylinder capacity) divided by (25 (litres per minute) x 2 (bar - the ambient pressure at 10 metres)) = 40 minutes not allowing for any reserves. In metric you don't have to allow for the fact that a 80 cu.ft. cylinder only actually holds 80 cu.ft. if pressurised to exactly its working pressure.

How much water do you need to displace with a lifting bag to raise a 50 kilogramme object? Not allowing for seawater or the object's displacement then you need to displace 50 litres of water. How much air will that take at 30 metres? 4 bar (ambient pressure) x 50 litres = 200 litres of air or 20 bar out of a 10 litre cylinder.

I've never seen a SPG in Europe with a red quadrant that isn't up to 50bar. That's actually 725psi or 45% more than the equivalent 500psi red quadrant on typical US gauges.

Within the European Union diving equipment can only be sold if it meets certain MINIMUM standards (most equipment actually exceeds the minimum by a considerable margin) so even if you are presented with an unfamiliar brand you can be confident it will operate well provided it has been maintained correctly i.e. precisely as you would expect a more familiar name to operate if it was properly maintained and if you were diving with rental gear in the US.

If you want a really easy ready-reckoner to convert temperatures from degrees Celsius to degrees Fahrenheit then simply double it and add 30 for example 8 deg C is roughly 46 deg F. Not super precise but close enough.

Inflatable boats (usually actually RIBs) are actually popular for diving throughout Europe - not just Greece and Croatia. In fact small fibreglass whalers, such as often used in the US for diving, are virtually unknown as diving vessels over here. Getting out of a RIB is easy but you will normally be expected to remove your weightbelt, BCD and cylinder in the water before 'bouncing' back into the boat, there's a bit of knack but it isn't difficult; ladders are usually only found on larger dive boats.

Things are a little different in Europe but not so different that you should be put off, ultimately the only one of the 'Ten European Diving Tips' that isn't something that should be standard practice anyway is Tip #3 about language. However, having heard some utterly impenetrable accents on my few visits to the States I think that could be debatable!

 

[henrik242]

Check out my scuba cylinder size and buoyancy calculator, it's quite helpful in understanding imperial vs metric scuba tank sizes.

 

[Foxfish]

Dove with an American tourist in Australia a few months back. Wondered why he was doing his safety stop at 7.5 m. I eventually had to go down and get him. When he surfaced it came out that he didn't know the conversions. Safety stops are done between 6 and 3 metres as you know.

 

[agilis]

An instantaneous reflexive understanding of and reaction to a system of measuring other than the one learned in childhood requires considerable time and exposure. Otherwise, a deliberate conversion process must be done, a thought process that takes time and is subject to error. The advantages of the metric system are obvious. It was created to be logical and clear. It's the best system by far, but only if it's the system you learned first and continued to use over time.

However logical and elegant a system may be, it is an impediment for anyone that usually operates within a different system. It took quite some time for Britons to be weaned off their feet, yards, and miles, not to mention their rods, furlongs and other arcania.

Years ago I spent a summer living in a small fishing community in Jamaica. Their usual measure for estimating moderate distances was a 'chain'. A boat might be described as anchored two chains from shore. That's 132 feet, since a chain is one tenth of a furlong. There are about 80 chains in a mile. A statute mile, that is. The original mile was somewhat less, since it was the distance covered by highly disciplined Roman troop formations in 1000 (milia) steps. 'Military' is derived from the same Roman word, as is millenium.

Sometimes even now, decades later, especially when on a small boat, I 'm startled to find myself calculating distances in chains. Old sailors measured speed in knots because speed on sailing ships was calculated using a knotted rope with one end tossed overboard. That knots continue to be used in naval and aviation applications is from pure tradition. There is no rational reason.

I use 'Imperial' (gotta love it!) gauges because I instinctively and immediately understand the implications of those numbers. This, I think, is true for anyone who grew up using that kind of system.

 

[Kevrumbo]

It's 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). 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???

 

[ttew]

Great post!

There are a few other hystorical and practical reasons behind the use of the metric system and the nautic relations between degress, nautic miles, and knots.

Speed on boats are measured in knots, which is equals to nautic mile per hour.
One nautic mile is a bit more than a common mile, because it represent the length of a minute of arch on the equator, 60' = 1 degree.
So it simplifies conversions of bearings, angles and distances to compute navigation times. e.g. we want to move north one degree sailing at 20 knots, it will take 60'/20knots = 3 hours (1 deg = 60' ).

The conversions in the metric system are also somehow much easier since they usually requires multiplications or divisions by 10.
1 liter = 1 dm^3 = 1 kg of fresh water.
1000 liters = 1 m^3 = 1 metric ton of water, which is a quite useful relation to estimate e.g. fresh water loads on a boat
1 bar ~ 1 ata = 10 meters of depth in salt water = 1000 kg / cm^2, which easily relate surface of water tight instruments with the minimal mechanical properties of the materials.

It's 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). 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???

 

[Doppler]

Nice post... interesting article.

Have to be able to "speak" both but always default to SI units since they are the easiest to use. And, have had pretty good success teaching CCR programs in metric alone. Makes life sooooo much easier when calculating oxygen volume limits.