Metric versus Imperial System for Diving?

[Freewillow]

Metric pressure gauges do measure bar (= 100 kPa) though, not atmospheres.

1 atm = ≡ 1,01325 bar , so it is the same

 

[agilis]

1 atm = ≡ 1,01325 bar , so it is the same

And equally irrelevant.

 

[dfx]

And equally irrelevant.

How is that?

 

[agilis]

How is that?

Because, as I explained at excessive length above, the unit of measurement being used has little or nothing to do with the actual needs of a diver who is using the pressure gauge for the purposes intended: calculating the amounts of gas being used and available. The gauge is a mechanical object operating on mechanical principles, but is measuring only one thing: pressure. This pressure reading is converted into more complex data by a combination of the gauge face and the diver's interpretation. The unit of measurement, whether it is PSI, bar,or a graphic, are relevant only in their ability to allow that specific diver to accurately assess the available gas situation.

The measuring units used, as long as they convey accurate information reflecting the actual volume of gas available, are irrelevant, and reflect only cultural prejudices and preferences. Anyone can repaint the face of the gauge. If the proportional declines in pressure are clearly and accurately reflected in the new paint job, it can be Kanji script or Mayan, so long as the diver recognizes the implications of those markings as they relate to the remaining gas in the tank to which the gauge is attached. The units used, be they atmospheres, barometric units, or psi are of academic interest only. They provide no special insights into issues of importance to a submerged diver. They are completely irrelevant in that context.

 

[dfx]

Because, as I explained at excessive length above, the unit of measurement being used has little or nothing to do with the actual needs of a diver who is using the pressure gauge for the purposes intended: calculating the amounts of gas being used and available. The gauge is a mechanical object operating on mechanical principles, but is measuring only one thing: pressure. This pressure reading is converted into more complex data by a combination of the gauge face and the diver's interpretation. The unit of measurement, whether it is PSI, bar,or a graphic, are relevant only in their ability to allow that specific diver to accurately assess the available gas situation.

The measuring units used, as long as they convey accurate information reflecting the actual volume of gas available, are irrelevant, and reflect only cultural prejudices and preferences. Anyone can repaint the face of the gauge. If the proportional declines in pressure are clearly and accurately reflected in the new paint job, it can be Kanji script or Mayan, so long as the diver recognizes the implications of those markings as they relate to the remaining gas in the tank to which the gauge is attached. The units used, be they atmospheres, barometric units, or psi are of academic interest only. They provide no special insights into issues of importance to a submerged diver. They are completely irrelevant in that context.

I disagree. Once you get into SAC/RMV calculations, the units of pressure do matter. Maybe not when you're doing the calculations in imperial, because you gotta jump through several hoops to get it right, but when you do it in metric, having a unit that closely resembles atmospheric pressure is extremely handy and makes things a lot simpler.

 

[agilis]

I disagree. Once you get into SAC/RMV calculations, the units of pressure do matter. Maybe not when you're doing the calculations in imperial, because you gotta jump through several hoops to get it right, but when you do it in metric, having a unit that closely resembles atmospheric pressure is extremely handy and makes things a lot simpler.

I though I had made it crystal clear that I was referring only to "a submerged diver" using the equipment during the dive to determine the amount of gas remaining, and gas usage patterns affected by such things as underwater activity and depth. The diver's immediate and accurate interpretation of the system employed is pretty much the only thing of primary importance.

Surface air consumption and breathing patterns fall into the category of research and analysis, which I had specifically excluded, along with training, from my two posts on this topic. While i'm at it, let me add resale value to the list of situations in which the kind of measurement system used is important. During the dive it is equally irrelevant.

 

[dfx]

I though I had made it crystal clear that I was referring only to "a submerged diver" using the equipment during the dive to determine the amount of gas remaining, and gas usage patterns affected by such things as underwater activity and depth. The diver's immediate and accurate interpretation of the system employed is pretty much the only thing of primary importance.

Surface air consumption and breathing patterns fall into the category of research and analysis, which I had specifically excluded, along with training, from my two posts on this topic. While i'm at it, let me add resale value to the list of situations in which the kind of measurement system used is important. During the dive it is equally irrelevant.

And that's where you're wrong, because in metric, it's piss easy to work out how much gas you have left even during a dive, without having to do any work ahead of time. Pressure times tank size, bam.

 

[Kevrumbo]

It seems to me that the issue between US style pressure gauges and those styled "metric" is not really a difference in units of measure. It actually is a more fundamental difference, with the different gauges measuring different things. If a gauge was actually the metric equivalent of one measuring in PSI, it would be measuring in kilograms per square centimeter. So called 'metric' gauges are measuring barometric unit equivalents, or some damned thing.

My bottom line is that I want to use a gauge that I can understand instantly, at a glance, without having to think about it. In the end it all comes down to compressed gas pushing on a spring. What is printed on the face of the gauge is merely a cultural conceit.

Objectively, practically and working smartly, the Bottom line for any Scuba Diver wanting to be proficient in Gas Management Calculations -especially on-the-fly/real time during the dive- is to cognitively keep a running total of gas consumption: checking the SPG should merely confirm the delta pressure which the diver has figured, evaluated & already knows. . .

These figures are much easier arithmetically using meters and bars, more so than the confounding & superfluous extra digits and non-intuitive quantities of US Imperial System in feet and psi.


---------- Post added October 11th, 2014 at 03:02 AM ----------

For example: 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 28deg 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 (Surface Consumption Rate) 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???

 

[agilis]

Objectively, practically and working smartly, the Bottom line for any Scuba Diver wanting to be proficient in Gas Management Calculations -especially on-the-fly/real time during the dive- is to cognitively keep a running total of gas consumption: checking the SPG should merely confirm the delta pressure which the diver has figured, evaluated & already knows. . .

These figures are much easier arithmetically using meters and bars, more so than the confounding & superfluous extra digits and non-intuitive quantities of US Imperial System in feet and psi.


---------- Post added October 11th, 2014 at 03:02 AM ----------

For example: 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 28deg 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 (Surface Consumption Rate) 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???

I think I repeatedly mentioned that, in addition to remaining gas volume, SPGs should be used to monitor gas consumption under changing conditions, things like depth, temperature, and activity level. Predictability is always interesting, and I became rather good at this, especially in connection with depth changes on deep dives that involved a thermocline during the last decades of the 20th Century, when I did a lot of wreck diving in New Jersey. I generally knew what the SPG reading was before I checked it.

Not being a total fool, I did check it frequently, and during the course of hundreds of dives I was occasionally startled to find the reading lower than anticipated, occasionally a little higher. Things like a high level of excitement or unexpected intense physical exertion, even of short duration, can burn through a lot of gas. In fact, one of the things I learned early was never to trust my assumptions; I always checked my SPG, and rechecked frequently. I've seen, over the years, a number of idiots with bulging fear-filled eyes who did not.

I find the exacting calculations of expected air consumption interesting, but of limited value during a dive. My belief is that recreational diving should not be an activity which attempts to extract the maximum number of seconds and minutes from either the air supply or the computer. Safety margins should be so wide that small amounts of extra time, or of unexpectedly reduced time, become insignificant when absorbed by the safety margin.

Deep penetration cave dives, decompression dives, using stage bottles, mixed gases, and other kinds of specialized scuba activity do require the kind of mathematically precise calculations and planning described in some posts, but they obviously should be done before a dive and analyzed afterward. Making approximate air consumption predictions during recreational diving can be great fun if that sort of thing interests you, but that is all it should be. Understanding the physics and physiology involved in air consumption is vital, but cannot replace an SPG. The face symbols on an SPG are irrelevant when using appropriate safety margins. Attempts to squeeze the maximum duration from a given volume of gas is best limited to things like cross-country automobile competitions.

---------- Post added October 11th, 2014 at 01:42 PM ----------

And that's where you're wrong, because in metric, it's piss easy to work out how much gas you have left even during a dive, without having to do any work ahead of time. Pressure times tank size, bam.

It's not crushingly difficult to 'work out' how much gas you have left by looking at the SPG either. I sense a nerd factor creeping into all this. I imagine automobile drivers estimating their speed by calculating the elapsed time between mile markers. I spend very little of my time underwater doing calculations or consciously anticipating SPG readings. I know, more or less, what they should be, intuitively, and I know when various circumstances and conditions are likely to change air consumption rates from long years of careful experience.

 

[Freewillow]

I disagree. Once you get into SAC/RMV calculations, the units of pressure do matter. Maybe not when you're doing the calculations in imperial, because you gotta jump through several hoops to get it right, but when you do it in metric, having a unit that closely resembles atmospheric pressure is extremely handy and makes things a lot simpler.

I agree with you