American and english units

[SparticleBrane]

I don't understand why you'd ever need to know how much a tank holds empty

I was under the impression that the metric way of measuring tank volume is how much water the tank would hold in liters...

 

[Benny-c]

I was under the impression that the metric way of measuring tank volume is how much water the tank would hold in liters...

Yup

A 12 litre tank is the most commonly used tank in metric(AFAIK), simply multiply the volume by the pressure e.g.

12 litres x 200bar/atm = 1200 litres of air/gas at surface pressure

1 X Litre is a little less than 2 pints

 

[bperrybap]

Unless one is severely brain damaged, it's easy to tell how much air a tank has at any given time in either system. I don't understand why you'd ever need to know how much a tank holds empty, yet that how the metric system defines its tanks. If I want a particular volume tank, in imperial, I can simply state the size tank I want. In metric, I must state both the tank size and the pressure. Yes, imperial makes more sense.

Uh.... is that why an Aluminum 80 usually holds 77 cubic feet of air...

And a metric guy would argue: Why do you need to know
the volume at an arbitrary service pressure when knowing
the volume at any given pressure is much more important.

Ignoring all the metric/imperial math conversion arguments,
one of the problems with imperial that really annoys me,
is that the volumes quoted are often with the overfill pressures
and not the service pressure or what most people think of as the
"full tank" pressure.
Example: many folks think of 3000 PSI as the working pressure
or the pressure when you get 80 cuFt of air in an aluminum 80.
However, many tanks don't really get 80cf of air at 3000 psi
or they have a working pressure of around 3100 PSI which brings
that "80 cuFt" at 3000 PSI down to around a real 77 cubic ft.

Both systems use a working pressure and volume. The difference
is that the working pressure for ALL metric tanks is the same
(atmospheric pressure) so the volume calculation can be simplified,
especially since the metric pressure units are based on a
multiple of the tank "working pressure".

Unfortunately with Imperial, the volumes quoted for tanks
are not all at the same working pressures so it makes the volume
calculation for a given pressure a lot less intuitive.

Another example:
Lets say I tell you I have a 80 cuFt tank and I have 1500 PSI
in the tank. How many cubic feet do I have?
You don't know because you need to know the working pressure
of the tank. It could be a low pressure tank that fills to 2400 PSI
or a tank that fills at 3000 or even 3180 PSI.

On the other hand, If I tell a metric guy he has a 11 litre tank
with 100 bar of pressure in it, he can calculate his volume
quickly in his head.

So while it may be "easier" to specify a volume in imperial
for ordering the tank, it is MUCH easier to calculate volumes
for arbritrary pressure readings in metric.

An imperial guy needs to know a tank volume, a tank working pressure
and his current tank pressure to calculate a volume for a given
pressure while a metric guy only needs to know the tank volume
and his current pressure.

Imperial guy does: V = Pressure * TankVolume / WorkingPressure
Metric guys does: V = Pressure * TankVolume

The metric guys gets a "freebie" because in his system, the
volume is at a working pressure that is 1 unit of his pressure units
which is based on atmospheric pressure.

And that is why you want to know how much a tank holds
when empty.

It normalizes ALL tanks volumes to the same working pressure which
makes the volume calculation for an arbritrary pressure
simpler especially when your pressure units are a multiple
of atmospheric pressure.

---------------
All this imperial complexity PITA stuff becomes readily apparent
if you are dilligently calculating and tracking SAC rates while using
rental tanks which are all different sizes and have different
working pressures.


BTW, I am still doing things in imperial units.
I often ask myself why.....

-- bill

 

[bperrybap]

I was under the impression that the metric way of measuring tank volume is how much water the tank would hold in liters...

I think water is used/quoted because its volume is much
less subject to pressure and temperature changes not to
mention its really easy to measure a tank volume this way.

You can think of the metric tank volume as the volume of the tank
with a working pressure of atmospheric pressure.

So if you look at the imperial vs metric volume calculation they
are in fact exactly the same:

Volume = Tankvolume/WorkingPressure * TankPressure.

Imperial Example:

Volume = (80 cuFt ) / (3100 PSI) * 1500 PSI
Volume = 80 cuFt / 3100 * 1500
Volume = 80 cuFt * .4838701
Volume = 38.71 cuFt

Metric Example:

Volume = (12 Liter)/(1 Bar) * 100 bar
Volume = 12 liter/1 * 100
Volume = 12 liter * 100
Volume = 1200 liter

------------------------------------------------
I think it is easier to ensure your conversion calculations
are correct if your write down all your units.
You then can see units dividing out and going away and
you will be left with the proper units.
If you are left with the proper units, then you have done the
conversion correctly.

Too many folks try to jump directly to the math and ignore
the units and end up making a mistake.

Above you can see that PSI or BAR divides out and you are
left with cuFT or Liters. If you draw it on paper using
horizontal lines for the divide it is a bit more obvious.
(I tried to do this but leading spaces get truncated)

I always treat "per" as a divide. example:
"PSI per working pressure", "cm per inch", "feet per mile"
Then simply flip it over depending on which units you need
to divide out.
I have always done unit conversions this way. It's simpler
for me and I know when I get it right.

--- bill

 

[Nemrod]

Well, whoever asked, since we in America do know the operating pressure of the tank, if it were a 2400 psi tank and at 2400 psi it held 80 cubic feet then at 1200 psi it would hold 40 cubic feet. This is not rocket science.

It is much easier to operate in fractions and halves and half again and half again etc. I suppose it is like comparing an analog watch with hands to a digital watch with numbers and decimals. I will take the hands, I don't need absolutes in everyday measurements such as carpenter work or for that matter the volume of my scuba tank.

Let me ask this, if you and your dad were building a house and you picked up a piece of lumber and your dad said, "cut that board into four pieces or by half." Are you going to measure it in milimeters and centimeters or you just going to eyeball one half and then cut it again each for four boards now 1/4 the length of the original? You see the American/English system is intuitive and practical, the metric system is for number crunching. My dad did not care about a centimeter, all he wanted was four boards from the original.

N

 

[Packhorse]

Let me ask this, if you and your dad were building a house and you picked up a piece of lumber and your dad said, "cut that board into four pieces or by half." Are you going to measure it in milimeters and centimeters or you just going to eyeball one half and then cut it again each for four boards now 1/4 the length of the original? You see the American/English system is intuitive and practical, the metric system is for number crunching. My dad did not care about a centimeter, all he wanted was four boards from the original.

N

Well my dad is a builder and when I used to help him he told me to use a ruler or tape to messure (messure twice, cut once) the wood to be cut so we know it was cut to the right length.
If you cut it "by eye" then 99% of the time your pieces of wood are going to be diffrent lengths. That may be OK with you but not in a house built by my dad.

Im not sure what relevance this has to the disscusion anyway. Does your SPG say 1/2 full, 1/4 full , 1/8 full? Mind sure doesn't

 

[bperrybap]

Well, whoever asked, since we in America do know the operating pressure of the tank, if it were a 2400 psi tank and at 2400 psi it held 80 cubic feet then at 1200 psi it would hold 40 cubic feet. This is not rocket science.

It is much easier to operate in fractions and halves and half again and half again etc. I suppose it is like comparing an analog watch with hands to a digital watch with numbers and decimals. I will take the hands, I don't need absolutes in everyday measurements such as carpenter work or for that matter the volume of my scuba tank.

N

For the most part I agree with you. You might do a dive
plan that accounts for gas usage in volumes. Then do all the
conversions back to pressures. But eventually you are probably
only going to keep track of certain pressures during the dive,
that may represent fractions of the total like 1/2, 1/3, 1/4, etc
for determining turnaround times, time to come up etc.

But what I was trying to show for Walter was that I believe that if
for some reason you are trying to calculate exact volumes, it is
much easier using metric units which depend on tank volumes
based on empty tanks rather than Imperial tank volumes
based on tanks at some service pressure.

With Imperial, the numbers quickly get too hard for my brain
to calculate with in any precision.

--- bill