Metric versus Imperial System for Diving?

[dfx]

The good thing about Farenheit is that a dive in water in the "upper 70's" sounds so much more appealing than the same dive "in the mid 20's" on the Celsius scale.

Which is why nobody actually says that :tongue:

 

[victorzamora]

You mean you can tell the difference between 77°F water and 78°F water? Or perhaps you would call both of them "upper 70s"?

No, I just mentioned it from a scientific standpoint. There's really rarely a benefit of working in Celsius.

 

[Akimbo]

What every traveling American Diver should have:

ScubaPro Dual Scuba Pressure Gauge

I’m normally against dual-scale analog instruments of any kind, but I do make an exception for SPGs. This is my fav… too bad OMS discontinued it.

 

[nimoh]

The good thing about Farenheit is that a dive in water in the "upper 70's" sounds so much more appealing than the same dive "in the mid 20's" on the Celsius scale.

to someone that uses celsius, upper 70s doesn't sound appealing at all

 

[agilis]

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.

 

[nimoh]

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.

sounds like another vote for the RJP Binary SPG.

The upgrade is not very expensive, you only need to buy 10 guages: 1 for your backgas and 1 for your pony.

 

[Akimbo]

… If a gauge was actually the metric equivalent of one measuring in PSI, it would be measuring in kilograms per square centimeter...

The proper current standard Metric (SI or Système International d'Unités) unit for pressure is the Pascal, which is a derived unit, just as PSI is a derived unit in the Imperial system. The SI base units for the Pascal are 1 N/m2 or 1 kg/(m•s2).

The Pascal is a very small unit of pressure so the Kilo and Mega prefixes are commonly used. 1,000 Pascals is approximately 1/7th of 1 PSI or 0.145037738 PSI. Of course Bars are still in common use, which equals 14.50377 PSI.

Pressure would be in Atmospheres if I were the god of diving units, which equals 1.01325 Bar or 14.69595 PSI. Bars are close, but Atmospheres are what we care about and is the same in Metric and Imperial. All other units would be SI/metric. One atmosphere of pressure is near-enough 10 Meters of sea water (10.0627586096078) — how cool is that. However, I would discourage the use of Centimeters. Kilometers, Meters, or Millimeters; which is allowable when you are the units god.

 

[agilis]

The proper current standard Metric (SI or Système International d'Unités) unit for pressure is the Pascal, which is a derived unit, just as PSI is a derived unit in the Imperial system. The SI base units for the Pascal are 1 N/m2 or 1 kg/(m•s2).

The Pascal is a very small unit of pressure so the Kilo and Mega prefixes are commonly used. 1,000 Pascals is approximately 1/7th of 1 PSI or 0.145037738 PSI. Of course Bars are still in common use, which equals 14.50377 PSI.

Pressure would be in Atmospheres if I were the god of diving units, which equals 1.01325 Bar or 14.69595 PSI. Bars are close, but Atmospheres are what we care about and is the same in Metric and Imperial. All other units would be SI/metric. One atmosphere of pressure is near-enough 10 Meters of sea water (10.0627586096078) — how cool is that. However, I would discourage the use of Centimeters. Kilometers, Meters, or Millimeters; which is allowable when you are the units god.

I understand all of the above. You seem to confirm my main point, which is that the so-called metric pressure gauges used in scuba are not really metric at all. Calling them metric is an error of convenience, because Atmospheres as a measurement are "the same in Metric and Imperial" (as you wrote), and are what is actually significant.

Atmospheres, as a measure, is outside the realm of either system. Identifying a unit of measurement as a Bar on some gauges is done because "Bars are close", but they are not the point in actuality. Available breathing gas, expressed as pressure within a container of known volume, is the point.

Being pragmatic, all the diver really is legitimately concerned with is the measure of remaining gas volume and the implicit measure of gas consumption by contrasting remaining gas volumes over time, depth, and other relevant variables. The relationship to pounds, kilos, cubic feet, Bars, atmospheres, or an appropriately graduated scale extending from full to empty is of no operational significance during a dive. These technical terms may be needed to write, communicate, explain and calculate, but that only reflects the limitations of human cognition.

The terms themselves are abstractions, nothing but words describing physical phenomena, things fascinating to contemplate at times, and needed for teaching, for analysis and research, but utterly useless during a dive.

All that is significant for a diver is that the data provided by a gauge enable that diver to understand what is happening to the amount of compressed gas he /she is relying upon. The data can be presented in any manner that effortlessly facilitates this understanding. Quasi-nationalistic perspectives or pretentious claims to superior science based on what is printed on the gauge are the stuff of infantile posturing.

Improvements in the nature of a gauge's design and measurement terms should focus entirely on the ease and accuracy of understanding the very narrow issues confronting a submerged diver breathing compressed gas. Simplicty in design should always be the goal.

 

[dmoore19]

3000 PSI = 20684 Kpa. I remember the first time I saw a pressure gauge in Kpa. (not scuba related) I looked at the gauge and started running the opposite direction. Someone pointed out that it was Kpa so I had to go back to work.

 

[dfx]

No, I just mentioned it from a scientific standpoint. There's really rarely a benefit of working in Celsius.

Scientifically there's no difference at all, because if you want more precision, you simply add more digits after the decimal point. Scientifically you wouldn't use Celsius anyway, you'd use Kelvin, which incidentally is based on the Celsius scale. The benefit of Celsius is that the low end and the high end of its scale make a lot more sense than those of the Fahrenheit scale.

---------- Post added October 10th, 2014 at 05:25 PM ----------

I understand all of the above. You seem to confirm my main point, which is that the so-called metric pressure gauges used in scuba are not really metric at all. Calling them metric is an error of convenience, because Atmospheres as a measurement are "the same in Metric and Imperial" (as you wrote), and are what is actually significant.

Atmospheres, as a measure, is outside the realm of either system. Identifying a unit of measurement as a Bar on some gauges is done because "Bars are close", but they are not the point in actuality. Available breathing gas, expressed as pressure within a container of known volume, is the point.

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