Question Definitions for SAC and RMV?

[tursiops]

though more commonly, we talk about "X psi per five minute segment"

?? Never heard this, not even once.

 

[tursiops]

Well Galileo was condemned for his theory that the sun revolved around the earth, so.....

?? Backwards on purpose?

 

[Lorenzoid]

?? Never heard this, not even once.

It's a GUE thing.

 

[Belzelbub]

For all of you who define SAC as volume/time/atm, what do you call gas consumption in pressure (psi or bar)/time/atm, or do you not use it at all?

I don't use it at all. I could see a pressure / minute being useful during a dive, but as I frequently use different size tanks, it wouldn't work for me. Instead, I prefer to use the GTR calculation. I don't have SAC displayed on my Perdix as GTR gives me what I need in a more consistent manner.

 

[Ome_Bart]

In Holland most people call the RVM a SAC but as long as you can calculate a good dive planning you should be fine

 

[harvalen]

I use SAC, mostly as volume-based liters/min, but pressure-based bar/min can sometime have its uses. Since the unit is important I think SAC works fine as the term for both cylinder-independent volume-based and cylinder-dependent pressure-based metrics. Although RMV might have a strict medical definition I think it mostly adds confusion, since it doesn't say anything about being at surface. If I had to switch then I would probably switch to SCR/DCR mentioned earlier in the thread, since those terms seem clearer.

Subsurface has been mention as a program that uses SAC as volume-based metric. For reference there are some other examples too:

Garmin has pressure-based SAC by default, and lets you use volume-based SAC if you've set your tank size. If you've set your tank size you can also pick "RMV", which in this case is the volume-based consumption at ambient pressure:

Descent T1 - Dive Terminology

www8.garmin.com

Suunto uses volume-based SAC, if you've set your tank size:

Suunto D5 - Features - Gas consumption

www.suunto.com

 

[Wibble]

How can Surface Air Consumption ever be pressure related???

It’s one Bar/ATA at the surface. It can only be volume / time

 

[mboden77]

How can Surface Air Consumption ever be pressure related???

It’s one Bar/ATA at the surface. It can only be volume / time

It is bar usage from the bottle at surface that the computer is telling you, not at current pressure.

To me this is a silly question, liter / min or bar from bottle / min is the same thing just different units meaning the same thing. Like 32 fahrenheit is the same as 0 celsius.

Shearwater displays sac in bar so they remove a potential user mistake if you enter the wrong cylinder volume which would cause the computer to give you wrong sac rate if it was displayed in liters / minute.

 

[Belzelbub]

To me this is a silly question, liter / min or bar from bottle / min is the same thing just different units meaning the same thing.

It’s only the same if you alway use the same size cylinder. If you use SAC as pressure, and switch to a smaller tank, you’ll run out of gas faster than you’d expect. Using volume, it doesn’t vary between different cylinders.

Shearwater displays sac in bar so they remove a potential user mistake if you enter the wrong cylinder volume which would cause the computer to give you wrong sac rate if it was displayed in liters / minute.

I think that’s appropriate for a dive computer.

SAC in volume is most useful pre and post dive. During a dive, the rate of pressure drop is most important. Or, actually what I find most useful is the GTR calculation. No need to do any math, it just tells me how many minutes I could remain at that depth. And time is the same in imperial or SI.

 

[UofMI_Divegeek]

Responding to:

Well Galileo was condemned for his theory that the sun revolved around the earth, so.....

Galileo was NOT primarily condemned for his belief in the Copernican heliocentric theory of the solar system.

Actually,,
He was condemned for his insistence that his views were "Absolute Truth" (a philosophical construct for that which is timeless, universal, necessary, and certain; that which can never be wrong and never changes.) But, science is fallible (can be wrong) and is constantly changing (evolving with new data and understanding). We use science, not because it's absolute truth, but because its useful.

At the time, there were serious problems with the calendar (Easter was moving more and more towards winter and its tough to celebrate new life when buried in snow). The heliocentric model was useful in that it was a better model for consistency in assigning major church holidays.

Galileo was told by the church to simply use the Copernican model to explain holiday dates and he would be left alone. However, he falsely insisted he was promoting "Absolute Truth."

and
he wrote numerous scathing personal attacks on the clergy and especially the pope. (Not wise at a time when the church was all powerful). It was mostly his arrogance, not his science (called natural philosophy at the time) that created so much ire. While belief in the Copernican heliocentric model was most definitely a component, it was his unrelenting attacks on the church that lead to his trials and condemnation.

He basically challenged the authority of the church and the church responded by banning his books and several trials for heresy. So, much of the controversy was power politics.

Galileo was given house arrest; many others who expressed belief in the Copernican heliocentric theory were burned at the stake. (At the time, anything contrary to Aristotle or Ptolemy was considered heresy.)

300 years after being convicted of heresy, the church reversed Galileo's heresy conviction.

Galileo 's major significant contribution to science was not so much promoting the heliocentric theory that had been previously proposed by others, but his insistence that nature was best understood by observation / experimentation, not conjecture. (Greek philosopher's mostly did not believe in observation). So, this insistence on experimentation (as well as developing a workable astronomical telescope) is a primary reason why Galileo is considered the "Father of Modern Science."

Now, back on topic

Many moons ago, I did a simplistic view of

Estimating Gas Consumption

and

Calculating Gas Density