Units of Measure in Diving?

[Akimbo]

Please let us know what units of measure you use most frequently, not necessarily prefer, in diving — for instrument readings and calculations.

[knowone]

Depends on the dive, gear and gauges. Metric for techish decoish type things, because after one comes two then three more easily and requires less fingers..

[Rhone Man]

I'm bi-numerical. Except when I am narc'ed.

[super7]

All the dive destinations I've visited in Asia was using metric measurement, with Bar for pressure. I haven't seen any SPGs or consoles in MPa yet.

#I was a kid living in Melbourne when Australia turned Metric. I still remember seeing on TV workers making the 100kph speed limit sign at a factory, and was amazed to see the digits hand painted.

[vladimir]

As an American abroad I'm ambi-unital; I still have more of a gut-level feel for Imperial units, however. I can multiply or divide by 2.2, 3.3, or 14.7, as needed.

[Kern]

Um, pascals & bars are sorta interchangeable. But sure, in the spirit of the thing, & in spite of the fact that Faber steel tanks in Oz are marked in mega Pascals, I answered; Meters, Grams, & Bars outside the US.

[String]

Suspect this poll is going to be havily geographical related. Also theres a mix in some places.

Most of the world barring N. America and Carribean use metres. However some places in asia still use PSI.

[Akimbo]

Um, pascals & bars are sorta interchangeable. But sure, in the spirit of the thing, & in spite of the fact that Faber steel tanks in Oz are marked in mega Pascals, I answered; Meters, Grams, & Bars outside the US.

For those unfamiliar with SI (Système International d'unités), commonly called the Metric System, here are some conversion factors:

One Standard Atmosphere equals:

SI Units
101,325 Pascals
101.325 KPa or Kilo Pascals or 1000x
0.101325 MPa or Mega Pascals or 1 Million x
1.01325 Bar
10.0627586096078 Meters of Sea Water
Imperial Units
14.695948775 PSI
33.899524252 Feet of Fresh Water at 4° C
33.014299900156 Feet of Seawater based on a density of 64.1 Lbs/Ft³

[Blackwood]

The 4th one

[bracko]

Um, pascals & bars are sorta interchangeable. But sure, in the spirit of the thing, & in spite of the fact that Faber steel tanks in Oz are marked in mega Pascals, I answered; Meters, Grams, & Bars outside the US.

it's a requirement under AS 1210 that the tanks are marked in MPa i believe... although it's been a while since I've had to read over the standard.

ummm... why isn't there a SI for inside the US? don't the US armed forces use SI?

[Kern]

For those unfamiliar with SI (Système International d'unités), commonly called the Metric System, here are some conversion factors:

One Standard Atmosphere equals:

SI Units
101,325 Pascals
101.325 KPa or Kilo Pascals or 1000x
0.101325 MPa or Mega Pascals or 1 Million x
1.01325 Bar
10.0627586096078 Meters of Sea Water
Imperial Units
14.695948775 PSI
33.899524252 Feet of Fresh Water at 4° C
33.014299900156 Feet of Seawater based on a density of 64.1 Lbs/Ft³

1 bar = 100,000 pascals
1 atm = 101,325 pascals

[awap]

Weights are always pounds. Depth is meters half the time and feet the other half (two computers, not setable). Pressure is usually psi but I do have one SPG in bars.

[Sas]

I use metres, bar and pounds. Most of my weights are in pounds so I tend to use that instead of kg when writing down my weight.

I have no problems switching to imperial though (even though metric is superior), and would depending on who I was diving with. In Fiji me and my buddy were the only non Americans on our boat most of the time and they gave dive briefings using imperial.

[thanksforallthefish]

I use metres, bar and pounds. Most of my weights are in pounds so I tend to use that instead of kg when writing down my weight.

I have no problems switching to imperial though (even though metric is superior), and would depending on who I was diving with. In Fiji me and my buddy were the only non Americans on our boat most of the time and they gave dive briefings using imperial.

+1 for metres, bar and pounds

[juanfr22]

Meters, Celcious degrees, Bars and grams.

[MartianBeerPig]

i use metric unless i'm diving in north america whereupon i switch to imperial during actual dive time. then convert to metric for my log. i remember seeing my first american dive buddy's eyes bulge out of his head when i showed my gauge displaying 170bar. it took him a second to realise it wasn't in psi

[Thalassamania]

Actually I prefer to have my volumes in megaparsec barns and my depths in rods and furlongs.

[nielsent]

As a scientist, I expect to see meters, kilograms, pascals/bar/atm, liters
As an Ameture machinist and mechanic, its all in feet, pounds, psi, and cubic feet.
As a barfly and occasional crazy. I prefer my measurments in slugs, hands, cubits, cables, leagues, fathoms, hogsheads, pecks, bushels, pints, quarts, and occasionally, eyeballed, as the keg's tapped, or as the crow flies.

[Thalassamania]

You forgot gills ... :D

[Tigerman]

Actually I prefer to have my volumes in megaparsec barns and my depths in rods and furlongs.
Been doing the kessel-run lately?

Im all metric, since well, I live in a part of the world thats not holding on for its bare life to feet and inches because we "dont have a relationship to meters" :eyebrow:

[=C=]

I use metres, bar and pounds. Most of my weights are in pounds so I tend to use that instead of kg when writing down my weight.

I have no problems switching to imperial though (even though metric is superior), and would depending on who I was diving with. In Fiji me and my buddy were the only non Americans on our boat most of the time and they gave dive briefings using imperial.

HOLD ON THERE!!!

Meters and Bar represent a lower level of accuracy in our sport. How can you call that superior?

[Sas]

. OK, OK, I guess were all bozos on this bus, or at least geeks!:D

You're also a Star Wars apologist ;) I think he said that because Lucas didn't know what a parsec was!


HOLD ON THERE!!!

Meters and Bar represent a lower level of accuracy in our sport. How can you call that superior?

Please explain how metres and bars are less accurate.

[Walter]

Why are the poll choices "inside the US" and "outside the US?"

[Akimbo]

Meters, Celcious degrees, Bars and grams.

Thermal units… hmmm. OMG, you must be more obsessive than me if you apply thermal compensation in your gas management calculations! ;)

Seriously, I stand corrected.

[Akimbo]

Why are the poll choices "inside the US" and "outside the US?"

From my experience in the US, the vast majority of divers use Imperial units. I was curious how many used them where their official national units are SI. Other countries in the Americas because of proximity to the US, and the British Common Wealth countries for example. Then there are also US ex-patriots who may or may not have made the transition.

[gsk3]

Holding your depth to 1 meter is a lot easier than holding it to one foot. On the other hand, if your depth gauge/computer shows you fractional meters, then you'd be better off for mid-water ascents, since you could more finely see when you're deviating up/down and correct sooner. Not sure if any computers do that.

As a side note, while we're being all technical, kilograms technically measure mass, not weight. Hardly makes any sense when you're talking about lift. So who measures their lift in Newtons? :-)

[Akimbo]

I use metres, bar and pounds. Most of my weights are in pounds so I tend to use that instead of kg when writing down my weight...

To be fair, accuracy is a measure of the instrument's precision. Units the instrument displays is irrelevant. I believe resolution betters describes your point.

Some units are courser in some base units such as meters and decrees Celsius, but are much finer in others like Grams and Pascals. That is what decimal points and prefixes like Kilo and Mega accomplish. In Scuba diving we rarely measure mass much closer than a pound. Half a Kg is 1.1 pound and is the smallest common diving weight in Europe as well.

Meters may seam a little course to people used to Feet, but given the accuracy of our depth gauges and computers; it is in the margin or error. Since Mechanical depth gauges read in 5 or 10 FSW increments at best, is it any different if the gauge displays units with hash marks at 1 MSW and numbers in multiples of 3 (at the shallow end anyway)?

However, many units are very convenient in diving. Notice how close all the SI units are to multiples of 10.


For those unfamiliar with SI (Système International d'unités), commonly called the Metric System, here are some conversion factors:

One Standard Atmosphere equals:

SI Units
101,325 Pascals
101.325 KPa or Kilo Pascals or 1000x
0.101325 MPa or Mega Pascals or 1 Million x
1.01325 Bar
10.0627586096078 Meters of Sea Water
Imperial Units
14.695948775 PSI
33.899524252 Feet of Fresh Water at 4° C
33.014299900156 Feet of Seawater based on a density of 64.1 Lbs/Ft³

[captndale]

Safety and practicality are what is important. It really does not make any difference what set of units are used as long as they are used consistently. All team members must be using the same units for optimal communications and optimal safety. Remember NASA and the Mars probe. Different contractors using different units of measure led to the lander's failure.

[bracko]

From my experience in the US, the vast majority of divers use Imperial units. I was curious how many used them where their official national units are SI. Other countries in the Americas because of proximity to the US, and the British Common Wealth countries for example. Then there are also US ex-patriots who may or may not have made the transition.

Australia, Canada (from what I've seen) NZ, India (i believe), Hong Kong... all use SI units.

the UK now actually use SI except for mph which is most likely a cost varient of changing street signs than anything.

[ditch-diver]

Australia, Canada (from what I've seen) NZ, India (i believe), Hong Kong... all use SI units.

Actually in Canada, while being a metric country the vast majority dive imperial units at the recreational level. Not sure about the techies...

We'll drive 5km to the 100' dive, and fill our log books with the air temp being 24C and the water temp 57F

[peterbj7]

I'm perfectly happy with both, and diving with mixed groups here (Belize) I often have to use both systems concurrently.

One aspect that concerns me is tank capacity measurement. Here what changes between the US-dominated sphere and the rest of the world is not particularly the units used, but what actually is being measured.

[vladimir]

Then there are also US ex-patriots who may or may not have made the transition.Hey, I'm an expatriate, but I'm still a patriot. ;)

[Quero]

All my gauges are in metric and I learned in metric, so it comes more "naturally" to me to use metric, and I prefer it for that reason when I'm doing my own diving. I can make the conversions, no problem, but as I rarely dive in N. America, imperial seems rather backwards/quaint to me.

When I'm teaching I generally find metric easier since it's just factors of ten (except when converting between fresh and salt water). I can and do teach both, especially if my students prefer imperial. For math-phobes, though, imperial just makes it more complicated having to remember seemingly random numbers for calculations. It really makes a difference for calculating lift/displacement, RMV/SAC rates etc.

I don't think it has anything to do with patriotism, but I do think it shows very little confidence in the intelligence of our citizenry that the country reneged on the plan to convert to metric back when I was a school girl.

[peterbj7]

There are actually strong reasons against metric as we have it. One is that the units bear no relationship to the physical world, whereas the older system grew up precisely because of such relationships. Secondly the metric system itself is of far less value than it could have been, being to the base of 10 which is not divisible by 3. A far more useful system would have been a base of 12, but that didn't come about because we only have 10 fingers (the binary system used by computers is of even less value to us in our everyday lives). And lastly, any metric system is prone to errors due to misplacement of the "decimal" point - such an error is easy to make and often not at all obvious.

[Akimbo]

Hey, I'm an expatriate, but I'm still a patriot. ;)

Oops, I never noticed it was spelled that way. I originally drafted "X-Pats" but figured not everyone would get it. I stand corrected… any icons around with a sheepish grin???

[JanK]

If you're just counting a small number of sheep and dividing them between yourself and your two brothers, I agree that divisibility by 12 is convenient (but what if you have 4 brothers).

As soon as you go to larger numbers and fractions, which mean the use of a positional system (which, BTW, has nothing to do with metric), i.e., number 523.23 means 5*100 + 2*10 + 3*1 + 2*0.1 + 3*0.01, and do mathematical operations, you have the same "decimal point problem", regardless of the base you're working in. In base 12 you might have a number like 3AB2.A9, divide it by 100 (i.e., 144 in decimal), mess up the decimal point position and be off by a factor of 12 - so you'd actually make a bigger error.

But with metric you have a great advantage that you can easily convert within a great range of numbers with very simple operations, e.g., metric tons/cubic metre = kilograms/litre = grams/cubic centimetre. Try converting between tons/cubic yard, pounds/cubic foot and ounces/cubic inch. More of less *any* conversion in the metric system is trivial, with only simple decimal point shifts.

Not to mention that as soon as you go above around 10,000 the scientific notation comes into play, so I can instantly convert 12.3 kilometres to 123*10^6 millimetres. Compare that with 12 inches in a foot, 3 feet in a yard, and 1760 yards in a mile... So, how many inches in 12.3 mile?

And BTW, binary is not as useless as you think. I can count to 1024 on my 10 fingers and to 32 with just one hand using binary. The latter I actually do sometimes, the former not.

Of course, one gets conditioned to whatever system they have been using since childhood and any system has its flaws, but, if someone tries telling me that having to know all the conversion factors listed on United States customary units - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/United_States_customary_units) is easier and more convenient for calculations than the metric system, I will just laugh.

[Herk_Man]

Stoichiometry rules! If it's a unit, I can convert it!

[Akimbo]

There are actually strong reasons against metric as we have it. One is that the units bear no relationship to the physical world, whereas the older system grew up precisely because of such relationships…

I am not promoting one over the other but in fairness, that is not entirely accurate. Just to name a few:

The meter was originally intended to equal one ten-millionth of the length of the meridian through Paris from pole to the equator. For convenience, it was represented by a metal rod measured at a certain temperature stored in France. The definition was updated in 1983 to equal the distance a certain light frequency travels through a vacuum in one second. You are right though, using the King's right foot and part of his finger also is a physical relationship.
1 Cubic Meter of fresh water weighs 1 metric ton or 1000 Kg
1/1000th of a cubic meter is a Liter, and weighs 1 Kg
0° C is the freezing temperature of fresh water and 100° C is the boiling point.

One of the few non-metric measurements that continues to dominate world-wide is the Nautical Mile's use in navigation, not to be confused with the Imperial Statute Mile, because of it's relationship to the earth — though that will become less relevant as GPS and computer time-to-target calculations take over.


…Secondly the metric system itself is of far less value than it could have been, being to the base of 10 which is not divisible by 3. A far more useful system would have been a base of 12, but that didn't come about because we only have 10 fingers (the binary system used by computers is of even less value to us in our everyday lives)…

The first official measurement system I am aware of using powers of 10 is the US Dollar. The Metric or SI system followed. I think powers of 10 was chosen because Base-10 numeric systems dominated the world rather than the number of human digits on both hands — though that may well have influenced the creation of the Base-10 system. I would agree with you if the world used a Base-12 number system. Binary is used by computers because there are only two states to semiconductors, off or on represented by 0 and 1.

Days, Hours, Minutes, and Seconds somehow slipped past both. :dontknow:


…And lastly, any metric system is prone to errors due to misplacement of the "decimal" point - such an error is easy to make and often not at all obvious.

In my experience, far more errors are made calculating Feet, Inches, and fractional Inches than any from an improperly placed decimal. Example: Multiply 1' 7-5/8" (which is 19.625") by two. How about Gallons, quarts, cups, and teaspoons? When it comes to precision work where the SI system is not used, the primary linear unit is Inches with decimal fractions where the same problem exists.

In the end, the most compelling argument may be that the USA and Liberia are the only two countries in the world where the Imperial System is dominant or official — the justification being the value of being on the same page for trade and communications rather than the majority rules.
:coffee:

[Herk_Man]

I do all my dive calculations in binary.

[GrumpyOldGuy]

I dive whatever units my buddy is using. It is easy enough to switch the computer to metric/imperial and I use SPG's labeled in both PSI/BAR. No conversion needed.

In the end, they are only numbers. By matching my buddy, I reduce the chance of misunderstanding each other. This is far more important the which system I use.

[DuboisP]

an easy question

I have a 80 cu tank
I begin at 2320 psi and end at 1160 psi
i dive at 32.8 feet during 20 min
i don't have a calculator
what is my SAC in imperial ?

now, do it with
a 11.1 L tank
i begin at 160 bar and end at 80 bar
i dive at 10 m during 20 min
i don't have a calculator
what is my SAC in metric ?

[peterbj7]

That's not an easy question. The first part can't be answered, since you haven't stated the nominal pressure of the tank (the pressure at which it contains 80 cu.ft. Consequently you have no way of knowing how much air is compressed into the tank at 2350 psi.

The second has a solution but I can't be bothered.

[SailNaked]

Beers per unit Dive > 6 = 0 dives
Wife agravation in blood pressure / weather.
tequilla is measured in Proof I believe so it is proof that you can or cant dive.
mellow = dives / days >= 3 is good.
$/dives <=$20 is great 40 is acceptable and >50 is not so good.
Time per dive is a great unit of measure, anything > 60 is awesome.
Other less used units in diving
family members per trip
sealife / dives
and the inverse relationship of psi to surface time.
:D
Edit: I forgot the unit of Sac. It is higher when you have to dive in bad weather or in a cave but usually low if women are diving with you. (no offense to women of course).

[DuboisP]

That's not an easy question. The first part can't be answered, since you haven't stated the nominal pressure of the tank (the pressure at which it contains 80 cu.ft. Consequently you have no way of knowing how much air is compressed into the tank at 2350 psi.

ok
i'm not familiar with imperial pressure
if I say 80 cu at 3000 psi, is it ok ?

in metrics, it's easier
by saying 11.1L beginning at 160 bar, it doesnt' matter if it is a low pressure (200-230 bar or a high pressure (300 bar)

[peterbj7]

It's not imperial, because Britain doesn't do it this way. It's just America and American-dominated regions (the Caribbean for example). Instead of measuring a tank's capacity according to how much water can be poured into it, the way that France, Britain and most of the world does it, they measure the volume of air at "standard" conditions (sea level, 20c, etc) that it takes in filling it to its nominal rated pressure. If the tank is rated for over-pressure (a "+" sign after the pressure stamped on the shoulder) then that rated pressure will be 10% higher than the value stamped. So for example, if you have an aluminium tank stamped with 3300 psi (or more likely just 3300) and there is a plus sign after the 3300 then that tank does not reach its stated rated capacity until it has been pressurised to 3300+10% = 3630 psi. If the tank has a stated capacity of 80 cu.ft. then that will only be achieved at that pressure of 3630 psi. If it is in fact pumped to 3000 psi it will only contain 80 * 3000 / 3630 cu.ft., or just over 66 cu.ft. Rather over 3/4 of what you thought it contained.

This has nothing to do with the units chosen, so in the above example the pressure could be denoted in bar and the volume in litres. But as it is a practice only found in the American sector it makes sense for normal American units to be used.

[GrumpyOldGuy]

It's not imperial, because Britain doesn't do it this way. It's just America and American-dominated regions (the Caribbean for example). Instead of measuring a tank's capacity according to how much water can be poured into it, the way that France, Britain and most of the world does it, they measure the volume of air at "standard" conditions (sea level, 20c, etc) that it takes in filling it to its nominal rated pressure. If the tank is rated for over-pressure (a "+" sign after the pressure stamped on the shoulder) then that rated pressure will be 10% higher than the value stamped. So for example, if you have an aluminium tank stamped with 3300 psi (or more likely just 3300) and there is a plus sign after the 3300 then that tank does not reach its stated rated capacity until it has been pressurised to 3300+10% = 3630 psi. If the tank has a stated capacity of 80 cu.ft. then that will only be achieved at that pressure of 3630 psi. If it is in fact pumped to 3000 psi it will only contain 80 * 3000 / 3630 cu.ft., or just over 66 cu.ft. Rather over 3/4 of what you thought it contained.

This has nothing to do with the units chosen, so in the above example the pressure could be denoted in bar and the volume in litres. But as it is a practice only found in the American sector it makes sense for normal American units to be used.

This is a unique take on American standards. Not true.

An AL80 pumped to 3000 psi will have 77 cu ft of air.
AL tanks do not use the "+" marks, nor do most of the newer steel tanks.

[FritzCat66]

One aspect that concerns me is tank capacity measurement. Here what changes between the US-dominated sphere and the rest of the world is not particularly the units used, but what actually is being measured.
...
That's not an easy question. The first part can't be answered, since you haven't stated the nominal pressure of the tank (the pressure at which it contains 80 cu.ft. Consequently you have no way of knowing how much air is compressed into the tank at 2350 psi.
...
Instead of measuring a tank's capacity according to how much water can be poured into it, the way that France, Britain and most of the world does it, they measure the volume of air at "standard" conditions (sea level, 20c, etc) that it takes in filling it to its nominal rated pressure... This has nothing to do with the units chosen...

I thought about this as soon as I read this thread. The three comments quoted above point out what is, to me, a bigger problem than just units of measurement. Sure, I wish the units were uniform, but at least I can easily convert between metric and imperial units and get a "feel" for each - for example, 33'~10m~1atm/bar is pretty intuitive to me. But tank capacity across systems is way out of whack.

Call me an Imperialist pig, but it just seems to make more sense to talk about a tank in terms of its capacity at rated pressure - because that's what we're using it for - than to talk about how much liquid it can hold - because the basic idea is to not get ANY liquid into them at all! I don't really care whether we're talking litres or cubic feet; might as well standardize on litres. But before we standardize the units, we should first standardize what it is we're measuring!




I do all my dive calculations in binary.

LOL! Yes, but even in binary, you still must assign units of measurement. However, I do have a t-shirt that says:

There are 10 kinds of people: Those who understand binary and those who don't.

:D

[Akimbo]

... Yes, but even in binary, you still must assign units of measurement. However, I do have a t-shirt that says:

There are 10 kinds of people: Those who understand binary and those who don't.

:D

I gotta get one of those! :rofl3:

[DuboisP]

Call me an Imperialist pig, but it just seems to make more sense to talk about a tank in terms of its capacity at rated pressure - because that's what we're using it for - than to talk about how much liquid it can hold - because the basic idea is to not get ANY liquid into them at all! I don't really care whether we're talking litres or cubic feet; might as well standardize on litres. But before we standardize the units, we should first standardize what it is we're measuring!

OK
I have a 15L filled at 230 bar and I breathe at 15L/min in surface.
So i can breathe 15*230/15 equals 230 min

give me the same calculation in imperials to demonstrate me the facility.
with a S100 at 3000psi

[DuboisP]

I gotta get one of those! :rofl3:
i'm a french IT using metric and binary :D

[peterbj7]

This is a unique take on American standards. Not true.

An AL80 pumped to 3000 psi will have 77 cu ft of air.
AL tanks do not use the "+" marks, nor do most of the newer steel tanks.

I'm interested to know what's not true, in any part of what I said but especially in the substantive part.

I have owned over 500 "standard" aluminum scuba tanks of three different makes, together with other sizes in aluminum and some 100 steel tanks of various sizes. The rated pressure for the aluminum tanks varies with manufacturer, so they cannot contain the same amount of air when pumped to the same pressure. And the "+" sign appears on many of these tanks, both aluminum and steel. Now, I haven't bought any tanks in the past six years, so if things have changed since then I may not be aware of it. But most tanks in service are at least that old.

Do note that my example of 3300 was just to illustrate the logic - I didn't say that that number appears on any of my tanks.

[Herk_Man]

LOL! Yes, but even in binary, you still must assign units of measurement. However, I do have a t-shirt that says:

There are 10 kinds of people: Those who understand binary and those who don't.

:D

My wife bought me that shirt!

[GrumpyOldGuy]

I'm interested to know what's not true, in any part of what I said but especially in the substantive part.

I have owned over 500 "standard" aluminum scuba tanks of three different makes, together with other sizes in aluminum and some 100 steel tanks of various sizes. The rated pressure for the aluminum tanks varies with manufacturer, so they cannot contain the same amount of air when pumped to the same pressure. And the "+" sign appears on many of these tanks, both aluminum and steel. Now, I haven't bought any tanks in the past six years, so if things have changed since then I may not be aware of it. But most tanks in service are at least that old.

Do note that my example of 3300 was just to illustrate the logic - I didn't say that that number appears on any of my tanks.

I think you tried to over simplify the US system and choose poor illustrations.

Here is my list per your request:

1) The "+" is not used on AL tanks in the US.

2) The 3300 rated pressure you mentioned is for a 100cu ft AL tank (not 80 cu ft).

3) The same tank achieves is capacity at 3300PSI, not 3630PSI.

4) Only some steel tanks are allowed a + rating
- Those that do are as you stated (capacity is at 110%)
- Those the do not achieve their capacity at 100% pressure rating

5) The variations in rated pressure for AL tanks are much less than steel. Most are 3000 with a couple of 3200/3300 rating for some oddball sizes. Steel tank pressures vary tremendously from 1800 to 3500PSI.

[goonsquad]

I only use American units for the same reason the French only speak French, I'm too lazy to care to learn anything else.

[SailNaked]

OK
I have a 15L filled at 230 bar and I breathe at 15L/min in surface.
So i can breathe 15*230/15 equals 230 min

give me the same calculation in imperials to demonstrate me the facility.
with a S100 at 3000psi

My Sac is .5F3/m so at 33F with an AL 80 I should have 2*.5=1 80/1=80 min.

or more likely at 99ft .5*4=2 with AL80 = 40 min.


Il serait si facile de le faire dans n'importe quelle langue dont le français.

[DuboisP]

My Sac is .5F3/m so at 33F with an AL 80 I should have 2*.5=1 80/1=80 min.

or more likely at 99ft .5*4=2 with AL80 = 40 min.

Il serait si facile de le faire dans n'importe quelle langue dont le français.
i would prefer the reverse calculation
calculate the time knowing the tank, the depth and the SAC

Félicitations pour cette phrase en français.

[DuboisP]

I only use American units for the same reason the French only speak French, I'm too lazy to care to learn anything else.
false
a lot of french people speak english, correspondingly more than US people speaking french or anything else.

[Blackwood]

A far more useful system would have been a base of 12, but that didn't come about because we only have 10 fingers (the binary system used by computers is of even less value to us in our everyday lives).

Hey, you've got two arms, don't you? You can count to 3!



in metrics, it's easier
by saying 11.1L beginning at 160 bar, it doesnt' matter if it is a low pressure (200-230 bar or a high pressure (300 bar)

That has nothing to with the metric system. It's simply a way of characterizing cylinders (in their 1ATM condition rather than their pressurized condition).


but it just seems to make more sense to talk about a tank in terms of its capacity at rated pressure - because that's what we're using it for

Not really. Technically, the rated volume is how much volume would come out (less 1ATM worth) if emptied at the surface. We typically breathe them under pressure. :D

[Blackwood]

i would prefer the reverse calculation
calculate the time knowing the tank, the depth and the SAC


The math is the same in metric units as it is in english units.


(Pressure Used)/time * (Volume/Pressure) = Volume/time.

Total Volume/(Volume/time)/Ambient Pressure = how long it lasts.


For "imperial" cylinders, that Volume/Pressure = Rated Volume/Rated Pressure (e.g. 100CF/3442PSI).
For "metric" cylinders, that Volume/Pressure = Volume/Liter (e.g. 11L/BAR).
Ambient Pressure is absolute (e.g. BAR or ATM).

[DuboisP]

Ambient Pressure is absolute (e.g. BAR or ATM).
may be i'm wrong, sorry...

so you also use bar or atm to characterize the pressure ?

I didn't remark this unit when I'was looking the weather reports on TV.

[Blackwood]

Weather reports tend to list barometric pressure in inches of mercury.

[DuboisP]

Weather reports tend to list barometric pressure in inches of mercury.
thanks
we quit "mm of mercury" a long time ago, to use millibar

except on my barometer, but it's easier
going right : ok
going left : not ok
:D

[SailNaked]

J'abandonne. Je ne comprends pas la question. quelle différence cela fait-il ce que vous appelez les unités de mesure si vous utilisez de la même manière. J'aime utiliser les métriques en raison de la conversion de la profondeur aux atmosphères est facile 10 = 1, mais ma jauge et l'ordinateur lire dans les pieds si je dois utiliser 33 = 1 même même. si elle est juste opinion sur aimer un ou l'autre alors je comme celui que j'ai appris quand j'avais 5 et je suppose que vous faites pour.:D

[SailNaked]

so in binary you can count to 32 on each hand and can add and subtract using your fingers. no idea why this never caught on.:D

[Krenath]

so in binary you can count to 32 on each hand and can add and subtract using your fingers. no idea why this never caught on.:D

Using both hands, you can count to 511. I just have some trouble holding up selected finger combinations for certain numbers. Assuming the thumb is the 1 bit, I have a lot of trouble with 11 (11010) as I can't seem to get my ring finger to stand up on its own if my middle and pinky fingers are supposed to be down and my thumb is unavailable because it's up too.

And we'd have some 'issues' with numbers like 4, 6, and 22. 17 and 19 would be cool, though. Counting to 1 underwater would end your dive :)

That sort of thing is probably why 10-bit binary didn't make it as the default finger-counting method.

[Blackwood]

Using both hands, you can count to 511.

Are you missing a finger?

[Tigerman]

Actually for 10-bit binary the highest bit would be 512 which means you could count to 1023..
Unless you want to go for signed int instead that is..

[peterbj7]

I think you tried to over simplify the US system and choose poor illustrations.

Here is my list per your request:

1) The "+" is not used on AL tanks in the US.

2) The 3300 rated pressure you mentioned is for a 100cu ft AL tank (not 80 cu ft).

3) The same tank achieves is capacity at 3300PSI, not 3630PSI.

4) Only some steel tanks are allowed a + rating
- Those that do are as you stated (capacity is at 110%)
- Those the do not achieve their capacity at 100% pressure rating

5) The variations in rated pressure for AL tanks are much less than steel. Most are 3000 with a couple of 3200/3300 rating for some oddball sizes. Steel tank pressures vary tremendously from 1800 to 3500PSI.


All this may or may not be true, but is irrelevant to the substantive point, which is that what Americans measure in a tank is not what others measure.

[GrumpyOldGuy]

All this may or may not be true, but is irrelevant to the substantive point, which is that what Americans measure in a tank is not what others measure.

I am pretty sure we are both measuring volume of gas in the tanks.:D

You are correct in that Americans do it differently than most of the world and this can lead to confusion when mixing systems. Which leads me back to my original post way back...

The best measurement system......is the one your buddy is using.

[peterbj7]

I am pretty sure we are both measuring volume of gas in the tanks

Well no actually.....

[Thalassamania]

so in binary you can count to 32 on each hand and can add and subtract using your fingers. no idea why this never caught on.:D

We do it all the time. On one hand you can count to 31 ... 11111 = 16+8+4+2+1. On two hands you can count to 1023 ... 1111111111 = 512+256+128+64+32+16+8+4+2+1.

[Tigerman]

I am pretty sure we are both measuring volume of gas in the tanks.:D

You are correct in that Americans do it differently than most of the world and this can lead to confusion when mixing systems. Which leads me back to my original post way back...

The best measurement system......is the one your buddy is using.
We dont..
In the US you measure the ammount of compressed air in the tank (in cu ft).
Most of the rest of the world measure tank sizes by actual volume (in liters), as in how many liters of water can you pour into it.
By doing that youll also now that if the tank is 12 liters and its compressed to 100 bar, you have 1200 liters of air to breathe. If its 15 liters and at 200 bar, 3000 liters..

[Akimbo]

Tigerman:

A little off topic, but are there any 300 Bar/4,351 PSI working pressure Scuba cylinders available in Norway?

[Akimbo]

We do it all the time. On one hand you can count to 31 ... 11111 = 16+8+4+2+1. On two hands you can count to 1023 ... 1111111111 = 512+256+128+64+32+16+8+4+2+1.

I find this works best on air at 50M/164' or more. That way I am never bored by the same answer. :dontknow:

[peterbj7]

In the US you measure the ammount of compressed air in the tank (in cu ft)

Actually you don't. You measure the amount of UNcompressed air in the tank.

[Kern]

Actually you don't. You measure the amount of UNcompressed air in the tank.

No, he's measuring the amount of incompressible water that will fit in the tank, & so knows how much gas it will hold at any given pressure. :mooner:

[Thalassamania]

I find this works best on air at 50M/164' or more. That way I am never bored by the same answer. :dontknow:Binary Four, Binary Four!:D

[Tigerman]

Tigerman:

A little off topic, but are there any 300 Bar/4,351 PSI working pressure Scuba cylinders available in Norway?
Yes, but for some reason not many use them..

[super7]

thanks
we quit "mm of mercury" a long time ago, to use millibar

except on my barometer, but it's easier
going right : ok
going left : not ok
:D

Japan Meteorological Agency have gone far as to use hPa (hecto-pascal) instead of millibars for weather forecasts.
I personally think that change wasn't necessary.:shakehead:

[peterbj7]

No, he's measuring the amount of incompressible water that will fit in the tank, & so knows how much gas it will hold at any given pressure. :mooner:

No, that's OUTSIDE the USA.

[Akimbo]

Questions:

(username) blueeyes_austin:
I noticed you specified "Other" on the poll. Care to elaborate?

(username) damage:
Thus far I see you are the only diver using Pascals. Does your computer support Bars and Pascals or do you have an SPG in Pascals?

[Tigerman]

Actually you don't. You measure the amount of UNcompressed air in the tank.
Fine, I can split hairs too..

In the US you measure the ammount of COMPRESSED gas IN the cylinder in cu. ft. when UNCOMPRESSED OUTSIDE OF the cylinder. Once its in that cylinder its compressed wether you split hairs or not..

[peterbj7]

We're talking the same language. I just want to make/keep it clear that what is measured in the USA and what is measured elsewhere are NOT the same.

[String]

Weather reports tend to list barometric pressure in inches of mercury.

Not outside the USA :)

QNH is given in millibar.

[vladimir]

Questions:

(username) blueeyes_austin:
I noticed you specified "Other" on the poll. Care to elaborate?

Probably metric in the United States; I'm surprised there aren't more "others" in that category.

[Akimbo]

Probably metric in the United States; I'm surprised there aren't more "others" in that category.

Duh, I should'a thunk of that. :confused:

I originally included that option for units like Fathoms, Atmospheres, and ounces.

[Thalassamania]

... and furlongs, chains, rods, pecks, bushels, hogsheads, slugs, etc.

[GrumpyOldGuy]

Ah, but if you are measuring in cubits, would it be compressed or uncompressed, in the good ole USA or in Belize?

It is rather silly, as divers we measure "pressure" via a gauge and convert it to volume using the appropriate formula... B.F.D. What we are really bitching about are the various methods used specify the size of the tank by the marketing folks and which is "better". Most of those marketing types are idiots, yet they have us running in circles chasing our tails.

[Belce]

I learned to dive in Imperial and later moved to metric on my dives. I love metric diving and think it works best

Really looking forward to the time the US goes to metric and drops British Imperial system for measurement.

[peterbj7]

Britain hasn't used most of the units in the "Imperial system" for many decades/well over a century, and happily mixes old units with metric. Some of what some Americans think is the British Imperial system was actually invented in America and has never been used in Britain. :D

But you're in Canada anyway. What does it matter to you what the Americans do?

[Atom]

Britain hasn't used most of the units in the "Imperial system" for many decades/well over a century, and happily mixes old units with metric. Some of what some Americans think is the British Imperial system was actually invented in America and has never been used in Britain. :D

But you're in Canada anyway. What does it matter to you what the Americans do?

Being so close to the states, all our equipment is imperial units (PSIs, feet, ...). It's a hard thing to change, everybody is used to imperial. I'm kinda used to both since I've learned abroad and continued diving here, but everybody who learned here is really more familiar with PSIs/feet.

I prefer metric though, makes computations simpler and my computer show depth in 0.1m increments instead of 1 foot increments when set in metric so I find it easier to hold a stop at a fixed depth without a visual reference.

[Akimbo]

Britain hasn't used most of the units in the "Imperial system" for many decades/well over a century, and happily mixes old units with metric. Some of what some Americans think is the British Imperial system was actually invented in America and has never been used in Britain. :D

But you're in Canada anyway. What does it matter to you what the Americans do?

I happened to be doing a lot of work in the UK when they started their official transition to SI in the 1970s. As you would expect, there was plenty of resistance, especially from the older and more skilled craftsman. One machine shop doing some work for me was so fed up by dozens of expensive errors due to conversions inventoried all the Imperial measuring instruments in the shop, including those privately owned. A few weeks later when all the Metric-only replacements arrived, the literally swapped all of Imperial instruments in the shop. The privately owned equipment was locked up for safe keeping in case someone wanted to leave their employ and didn’t want to keep their new "trade".

After about a week of grumbling, it was all over including that big uptick in errors. In all fairness, machinists deal a great deal more with inches and decimal inches so the conversion to Millimeters was pretty simple — decimal units is decimal units.

[peterbj7]

Britain had of course been using the SI system for decades before that, mainly in specialist scientific and engineering circles. It had been an integral part of school education for a long time, if only because it was the standard system only 22 miles away across the Channel.

I remember one job I had as an early consultant, working for a steel yard which had overnight to convert all its stocks from imperial to metric measure. They wanted to know the most efficient and least costly way of re-designating all their pre-cut lengths of steel. A very difficult job.

What started then, ordered by our new masters the EEC (later to be renamed the EU), was a crash programme of concerting people in their everyday lives to using the metric system. Quite unnecessary in my view and that of many others. At different times it has been illegal (yes, illegal, punishable by imprisonment) to sell apples by the pound or milk by the pint, and there are countless other examples. They never succeeded in ordering us to buy beer in pubs in litres, as I think there would have been mass riots, though they did try. Petrol (gasoline) is now sold by the litre, though since the price is comparable to what it used to be for the gallon people can relate to that more. :depressed:

I have to say the whole thing has been nonsense, dreadfully carried out, and has caused a vast amount of resentment. In schools they even tried suppressing information on the imperial system so that kids would grow up only knowing the metric system. but that didn't work because they saw imperial units all around them as they grew up.

Britain and most countries have changed units before, totally painlessly, the new system being adopted by common acceptance. What is so wrong about this is that it has been forced upon Britain by an unelected occupying power. I don't know why we bothered to defeat Napoleon at Waterloo as his ideas have won anyway.

Still, we in Britain escaped being forced to drive on the right, which he imposed on every country he conquered (the USA drives on the right because of Louisiana), and the whole world escaped the next change he intended forcing on people, the metrication of time.

The silly thing is that from a practical point of view the metric system has three major shortcomings - (1) the units of measure don't relate in any way to people's everyday lives, whereas the older units mostly came into existence by usage; (2) 10 is only used as the base because we have ten fingers. 12 would have been far better as it is divisible by 3; and (3) what is cited as a major advantage of the decimal system, a point you can simply move to change the order of a number, is equally a disadvantage as people make silly mistakes and have no idea they have made them.

[Atom]

The silly thing is that from a practical point of view the metric system has three major shortcomings - (1) the units of measure don't relate in any way to people's everyday lives, whereas the older units mostly came into existence by usage; (2) 10 is only used as the base because we have ten fingers. 12 would have been far better as it is divisible by 3; and (3) what is cited as a major advantage of the decimal system, a point you can simply move to change the order of a number, is equally a disadvantage as people make silly mistakes and have no idea they have made them.

Wow... gotta disagree...

1) It's all a matter of everyday use, of course switching between systems is confusing at first but I don't see how a mile relates more to everyday life than a km, why a PSI is more intuive than a bar.

To me celsius is very intuitive, 0 C is water freezing, 100 C is water boiling. A liter is 10cm x 10cm x 10cm of volume, which if occupied by water is also a kg. And 10cm is the lenght of the orange plastic sticks we had in primary school ; )

2) People commonly use base-10 numbers, so unless you want to express all measurements as fractions all the time you'll lose the precision anyway. And nothing prevents you from doing the same in metric, 1/3 of a meter is a perfect expression of a meter divided by 3.

3) If you can't do basic math in metric you won't fare better in imperial. Quickly... How many feets in 3 miles + 456 yards + 63 rods + 2 chains? How many inches in 13 feets and 4 inches? I've seen way more people screw the multiply by 12 required to convert feets to inches than people screw the simple movement of a decimal point, and I'm not even considering all the conversion factors you have to remember between the unit of different magnitudes.

[eponym]

Good stuff. Hijack continues.

I worked in London in the early '70s and was fascinated by the lumber yard conversions, first to SI-labeled plywood which replicated the Imperial sizing, which is of course what you need when doing a repair. "2.4 by 1.2 meter sheet? right here, sir."


I don't know why we bothered to defeat Napoleon at Waterloo as his ideas have won anyway.

You are correct that calculations in the Metric system make everything much less mentally challenging. But did you ever stop and ask yourself WHY this system was suddenly deemed necessary?Good rants, nice irony. Legend holds that Napoleon commanded his engineers to create a logical underpinning (hence "one-millionth of the distance from the equator to the pole") for a new unit of measure which had to be "slightly longer than the English yard." Talk about seeing whose is longest . . .

Using SI on my computer only when I want to practice really really slow ascents,
Bryan

[DuboisP]

The silly thing is that from a practical point of view the metric system has three major shortcomings - (2) 10 is only used as the base because we have ten fingers.
just one question
how long is 11.27 % of one once ?
can you measure it with a drawing-rule ?

[DuboisP]

Wow... gotta disagree...

1) It's all a matter of everyday use, of course switching between systems is confusing at first but I don't see how a mile relates more to everyday life than a km, why a PSI is more intuive than a bar.


a bar is approximatively 1 kg/cm2 (1.019716, in fact), as intuitive as a PSI

[Akimbo]

a bar is approximatively 1 kg/cm2 (1.019716, in fact), as intuitive as a PSI

I think it is fair to say that the vast majority of diving related calculations are based on Standard Atmospheres and linear water depth. It is hard to argue that imperial measurements are simpler overall if you allow for reasonable rounding.


…One Standard Atmosphere equals:

SI Units
101,325 Pascals
101.325 KPa or Kilo Pascals or 1000x
0.101325 MPa or Mega Pascals or 1 Million x
1.01325 Bar
10.0627586096078 Meters of Sea Water
Imperial Units
14.695948775 PSI
33.899524252 Feet of Fresh Water at 4° C
33.014299900156 Feet of Seawater based on a density of 64.1 Lbs/Ft³

There is some validity that the finer granularity of Feet for depth measurement is useful, until you consider that accuracy on most depth gauges and computers is not good enough to resolve closer than 1 Meter anyway. I find that argument only holds a little water in commercial diving where Pneumo Fathometers are used with large diameter precision gauges with ¼ to 1/10% accuracy gauges.

http://www.scubaboard.com/forums/5366555-post197.html

My story about the conversion to SI in the UK illustrates there is added risk in switching back and forth or converting. However, because the Bars and Atmospheres conversion is plenty close for estimating, dual scale SPGs are one of the few useful mixing of units on a single instrument that has value — IMHO. Arguing with a dive shop that your fill is 20 Bar short of full may not be so effective if all their gauges are in PSI.

However, your depth gauge should match your tables if you are not using a computer and dual-scale depth displays are more confusing and hard to read than helpful.

Keep in mind the poll question is:
What units of measure do you use most (not necessarily prefer) in diving?

I suppose the answer to the "best" units is the one you "think" in. Being off by a factor of 3.28 when mentally estimating gas consumption or decompression is not good for safety or getting the most our of your dives.

[Atom]

a bar is approximatively 1 kg/cm2 (1.019716, in fact), as intuitive as a PSI

Never said it was less intuitive. The way I relate to bar is that most of the time there's 200 of em in a Al80 ; )

[DuboisP]

Never said it was less intuitive. The way I relate to bar is that most of the time there's 200 of em in a Al80 ; )
i agree
only the numbers I need.
as this one : on a S40 of oxygen, I can do 4 decompressions dives, on the way i dive :D