US divers using metric?

[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???

 

[JMBL]

Surprise, surprise.. most of the world don't speak french - despite the obvious belief in the french population that everyone should be...

Well, I fear you can apply that to many a language and many a population.

 

[scubadada]

The metric system is much easier to work with. Every 10 metres you go down, the ambient pressure increases by 1 bar, making gas consumption calculations a doddle. Measuring cylinder pressure in bar makes things easier - cylinder volume X pressure in bar = actual contents. I know a lot of techies in the US use metric measurements for this reason, so getting a contents gauge in bar shouldn't be impossible. Many computers allow you to switch units, so there should be no problems there.

Doddle? Is that metric?

 

[tomfcrist]

I really don't see the argument of gas planning being easier in metric. I do just fine using imperial. To me, I believe you should use what is intuitive to you, and don't dive with a guide....you then won't have to worry about if they understand your preferred system( imperial/metric).

 

[scubadada]

Certified in 1970, imperial units are 2nd nature to me, OC only, I don't have any motivation to change. I'm quite sure I could dive with a metric diver, no problem.

 

[Kevrumbo]

I really don't see the argument of gas planning being easier in metric. I do just fine using imperial. To me, I believe you should use what is intuitive to you, and don't dive with a guide....you then won't have to worry about if they understand your preferred system( imperial/metric).

Here's another more comprehensive example:

Modified Thirds Planning and Calculating On-the-Fly Gas Needed to Do a Lost Buddy Search in a Wreck Penetration:

Using difficult & cumbersome US Imperial Units in PSI (the easier Metric calculation in comparison follows later down below):

Suppose you originally planned a wreck penetration starting with 3000 psi [double AL80’s] at the entrance, with an openwater Rock Bottom of 750psi; So 3000 minus 750 equals 2250psi usable for the penetration --Modified Thirds of this value is 750 psi (one-third of 2250 equals 750), so you would turn-around for egress when you consume 750 psi of gas with an actual SPG reading of 2250 psi. If you needed to do a gas-sharing emergency egress with your buddy at this point, you would both need 1500psi to get out of the wreck, with 750psi Rock Bottom remaining to get both of you to the surface (or your Oxygen deco bottle stop of 21&#8217.

Let’s say you used up 450psi already getting to the entrance of the wreck for a total of 2550 pressure available --can you quickly recalculate Modified-Thirds?

Well . . . 2550psi minus 750psi Rock Bottom yields 1800psi usable for the penentration; One-Third of 1800psi is 600psi which is your new Modified Thirds turn pressure value. Therefore you would turn the dive when you consume 600psi for an actual turn pressure SPG reading of 1950psi (2550 minus 600 equals the actual turn pressure SPG reading of 1950psi).

What if you lose your buddy at this instance, at the farthest distance inside the overhead your Modified Thirds value allows? How do you calculate the amount of gas to do a Lost Buddy Search?

[NOT] easy! At your turn around pressure reading of 1950psi on the SPG, simply add your Rock Bottom value to your Modified Thirds value (750psi Rock Bottom plus 600psi Modified Thirds equals 1350psi); Put a line-arrow pointing the way out on your mainline that you've laid, and take reference note of where you are inside the overhead at that exact point as well. Now go and search for your buddy with the understanding that you must be back at this line-arrow marker by the time your SPG reads this actual value (1350psi). So you would have from 1950psi down to 1350psi reading on your SPG, or 600psi delta of gas to search for your buddy --if you were to do a straight line search down a long corridor inside the wreck for example, tactically you should use 300psi out and 300psi back to your line-arrow marker for a delta of 600psi, and an actual end of search SPG reading of 1350psi-- you must start your egress whether you found your buddy or not when you use up this 600psi delta of gas, at the line arrow marker, with the actual 1350psi final reading on your SPG.

At any point before your Modified Thirds turn pressure, for a lost buddy search, the final egress pressure is figured just by adding your Rock Bottom value to the amount of gas you've consumed on the penetration up to that point --for example you start with 2550psi on your SPG and you lose your buddy with 2100psi SPG reading for a delta consumption of 450psi. 750psi Rock Bottom plus delta consumption of 450psi equals 1200psi. Drop a line arrow, and now you've got from 2100psi down to 1200psi (a tactical delta search pressure of 900psi) to look for your buddy, and be back to your line arrow to egress smartly when your SPG reads 1200psi.

At any point after your Modified Thirds turn pressure, all you need to do to figure out a final egress pressure for a lost buddy search is to subtract your Modified Thirds value from your actual pressure reading, and place a line-arrow pointing out at this point on your mainline. For example, if you're egressing and you lose your buddy with 1800psi actual reading on your SPG: Subtract the Modified Thirds value of 600psi from 1800psi -which equals 1200psi - and it is this actual reading that you must have on your SPG when you get back to your line arrow to successfully exit the wreck with all your Rock Bottom still available to reach the surface. Another way of looking at this, at your nominal turn-around point & afterward on egress, the amount of gas tactically available for a lost buddy search is always just your Modified Thirds value --in this case 600psi.

Remember that on a lost buddy search, you will deliberately encroach and use up the Modified Thirds Reserve Value needed for an emergency gas-sharing egress contingency (and possibly use up Rock Bottom as well) --in other words, if you do find your lost buddy and worst of all worst scenarios he happens to be out-of-gas in a silt-out . . .well dea ex machina. I hope you're in a 3-person Team, somehow make it out and run into other divers on the outside who can donate gas & assist. . .
_______________
Same thing as above but now using Metric Units:


Modified Thirds,Turn-Around Pressure and Lost Buddy Search Gas Availability Calculations are easier & more intuitive with a bar SPG too. . .

Suppose you originally planned a wreck penetration starting with 200 bar at the entrance, with an openwater Rock Bottom of 50 bar. 200 minus 50 bar equals 150 bar usable for the penetration --Modified Thirds of this value is 50 bar (one-third of 150 equals 50), so you would turn-around for egress when you consume 50 bar of gas with an actual SPG reading of 150 bar. If you needed to do a gas-sharing emergency egress with your buddy at this point, you would both need 100 bar to get out of the wreck, with 50 bar Rock Bottom remaining to get both of you to the surface (or your Oxygen deco bottle stop of 6m).

Lets say you used up 30 bar already getting to the entrance of the wreck for a total of 170 bar pressure available --can you quickly recalculate Modified-Thirds?

No problem with bar pressure metrics: 170 bar minus 50 bar Rock Bottom yields 120 bar usable for the penentration; One-Third of 120 bar is 40 bar which is your new Modified Thirds turn pressure value. Therefore you would turn the dive when you consume 40 bar for an actual turn pressure SPG reading of 130 bar (170 bar minus 40 bar equals the actual turn pressure SPG reading of 130 bar).

What if you lose your buddy at this instance, at the farthest distance inside the overhead your Modified Thirds value allows? How do you calculate the amount of gas to do a Lost Buddy Search?

Easy! At your turn around pressure reading of 130 bar on the SPG, simply add your Rock Bottom value to your Modified Thirds value (50 bar Rock Bottom plus 40 bar Modified Thirds equals 90 bar); Put a line-arrow pointing the way out on your mainline that you've laid, and take reference note of where you are inside the overhead at that exact point as well. Now go and search for your buddy with the understanding that you must be back at this line-arrow marker by the time your SPG reads this actual value (90 bar). So you would have from 130 bar down to 90 bar reading on your SPG, or 40 bar delta of gas to search for your buddy --if you were to do a straight line search down a long corridor inside the wreck for example, tactically you should use 20 bar out and 20 bar back to your line-arrow marker for a delta of 40 bar, and an actual end of search SPG reading of 90 bar-- you must start your egress whether you found your buddy or not when you use up this 40 bar delta of gas, at the line arrow marker, with the actual 90 bar final reading on your SPG.

At any point before your Modified Thirds turn pressure, for a lost buddy search, the final egress pressure is figured just by adding your Rock Bottom value to the amount of gas you've consumed on the penetration up to that point --for example you start with 170 bar on your SPG and you lose your buddy with 140 bar SPG reading for a delta consumption of 30 bar. 50 bar Rock Bottom plus delta consumption of 30 bar equals 80 bar. Drop a line arrow, and now you've got from 140 bar down to 80 bar (a tactical delta search pressure of 60 bar) to look for your buddy, and be back to your line arrow to egress smartly when your SPG reads 80 bar.

At any point after your Modified Thirds turn pressure, all you need to do to figure out a final egress pressure for a lost buddy search is to subtract your Modified Thirds value from your actual pressure reading, and place a line-arrow pointing out at this point on your mainline. For example, if you're egressing and you lose your buddy with 120 bar actual reading on your SPG: Subtract the Modified Thirds value of 40 bar from 120 bar -which equals 80 bar- and it is this actual reading that you must have on your SPG when you get back to your line arrow to successfully exit the wreck with all your Rock Bottom still available to reach the surface. Another way of looking at this, at your nominal turn-around point & afterward on egress, the amount of gas tactically available for a lost buddy search is always just your Modified Thirds value --in this case 40 bar.

Remember that on a lost buddy search, you will deliberately encroach and use up the Modified Thirds Reserve Value needed for an emergency gas-sharing egress contingency (and possibly use up Rock Bottom as well) --in other words, if you do find your lost buddy and worst of all worst scenarios he happens to be out-of-gas in a silt-out . . .well dea ex machina. I hope you're in a 3-person Team, somehow make it out and run into other divers on the outside who can donate gas & assist. . .

[Note: the above gas plan is taken from wreck penetration dives on the Yukon (San Diego); USS New York (Subic Bay Philippines); HMAS Perth/USS Houston (Sunda Strait Indonesia); and various wrecks in Truk Lagoon. Depth 30m using twin 11L/bar tanks (double AL80's) and Oxygen deco.]

 

[boulderjohn]

I'll dive with you in metric, it will just take a second for me to think about our dive plan. Or, I'll switch the units on my computer. It thinks faster than I do.

With you diving metric in an environment in which pretty much everyone else is diving imperial, Frank's statement above really sums up your biggest issue--communicating effectively with your buddy. Solve that minor problem, and you should be fine.

I once taught an AOW class for two students, and one of them was using metric because all the numbers were easier for him to work with. It was interesting because he was an American who had rarely left the country and did not otherwise work with metric. His buddy, the other student, was able to make the conversions, albeit slowly. The guy who went metric was hopeless. I told him that converting bar to PSI was roughly the same as figuring the tip in a restaurant. After a couple of failed attempts I wondered how many waitresses he had stiffed in his life.

 

[Kevrumbo]

With you diving metric in an environment in which pretty much everyone else is diving imperial, Frank's statement above really sums up your biggest issue--communicating effectively with your buddy. Solve that minor problem, and you should be fine.

I once taught an AOW class for two students, and one of them was using metric because all the numbers were easier for him to work with. It was interesting because he was an American who had rarely left the country and did not otherwise work with metric. His buddy, the other student, was able to make the conversions, albeit slowly. The guy who went metric was hopeless. I told him that converting bar to PSI was roughly the same as figuring the tip in a restaurant. After a couple of failed attempts I wondered how many waitresses he had stiffed in his life.

Actually, the savvy frequently traveling diver especially to overseas destinations in Asia, Europe and Middle East (Red Sea), would do much better by having one of these dual unit SPG's:

ScubaPro Dual Scuba Pressure Gauge

 

[Centrals]

I was brought up in Imperial System and then have to change to metric. No problem whatsoever. Just forget the "old" unit. Conversion is not necessary unless it is a mathematical question!!
1kg is 1 kg rather than 2.2 lbs and 1m is not 3ft + a bit etc etc.
However, I still like the unit of stone(=14 lbs) for body weight!!!!

 

[Tigerman]

Well, I fear you can apply that to many a language and many a population.

However, unlike the french, all other populations will try to be understood by using a 2nd language. French wont even try even when theire not in france...


Centrals, thats just cause 10 stones sound less, isnt it?

Sent fra min GT-I9300 via Tapatalk