SAC vs RMV, revisited

[Kensei]

Thanks, I'm late to this party but it still is good background on these calculations.

I dive often with a person using a Suunto and it literally says on the display, for example, that his SAC is "0.4 cu ft". Are they using a different calculation for SAC or are they just using the RMV formula but calling it SAC?

 

[tursiops]

Thanks, I'm late to this party but it still is good background on these calculations.

I dive often with a person using a Suunto and it literally says on the display, for example, that his SAC is "0.4 cu ft". Are they using a different calculation for SAC or are they just using the RMV formula but calling it SAC?

I hope what it says is 0.4 cu ft/minute, otherwise it is meaningless. Your guess is reasonable, but Suunto does strange things without explanation.

 

[Kensei]

I hope what it says is 0.4 cu ft/minute, otherwise it is meaningless. Your guess is reasonable, but Suunto does strange things without explanation.

What I'm looking at is output from his Suunto application, so exactly what is displayed on the computer could be different. According to this link, they do mean cu. ft. per minute.

Suunto EON Steel - Features - Gas consumption

The part I found strange was displaying SAC in vol. per minute. Is there a formula for calculating SAC in vol. per minute that isn't the formula for RMV?

 

[scubadada]

Thanks, I'm late to this party but it still is good background on these calculations.

I dive often with a person using a Suunto and it literally says on the display, for example, that his SAC is "0.4 cu ft". Are they using a different calculation for SAC or are they just using the RMV formula but calling it SAC?

I have a friend with a D5 and helped her set up her computer. The gas consumption field is added through DM5 and the cylinder characteristics are chosen. Gas consumption is reported as RMV in cu ft/min or L/min depending on units chosen. If you change cylinders, you must change the settings to maintain accuracy. This allows for calculation of the k (tank) factor in the @tursiops post you cited above (psi/min x cu ft/psi = cu ft/min. I believe this was called SAC in the owner's manual, perhaps that is the cause for the confusion?

I would imagine other AI Suunto computers work the same way, others can verify. @Diving Dubai has considerable experience with the Eon Steel

 

[Storker]

Suunto does strange things without explanation.

They sure do. My SAC (OK, RMV) numbers are more plausible when I get them from my Suunto DM3 program than when I get them from Diving Log (which obviously uses the ideal gas law and doesn't use compressibility factors). They still don't match - nor seem as plausible as - my own numbers, calculated using real world compressibility factors, delta P and average depth.

 

[EFX]

I would like to see the breathing rate converted to a surface rate described as SACP or SACV, i.e. Surface Air Consumption Pressure or Surface Air Consumption Volume. RMV is not descriptive enough. It is based on per minute (the M) and volume (the V) but respiratory (R) is superfluous and there is nothing indicating it is a surface rate quantity.

 

[Storker]

I would like to see the breathing rate converted to a surface rate described as SACP or SACV, i.e. Surface Air Consumption Pressure or Surface Air Consumption Volume. RMV is not descriptive enough. It is based on per minute (the M) and volume (the V) but respiratory (R) is superfluous and there is nothing indicating it is a surface rate quantity.

I'm getting more than a little confused by all these factors and their acronyms. And I have an STEM degree.

I count surface liters per minute. I like to call that SLM. What you want to call it - be it SAC, RMV, or SACV - is Not My Problem.

 

[Kensei]

"I believe this was called SAC in the owner's manual, perhaps that is the cause for the confusion?"

Yes. What Suunto is calling SAC appears to be RMV. The formulas are clearly different and require different inputs. I thought there was agreement on how to calculate SAC vs. RMV... maybe I'm wrong.

 

[tursiops]

"I believe this was called SAC in the owner's manual, perhaps that is the cause for the confusion?"

Yes. What Suunto is calling SAC appears to be RMV. The formulas are clearly different and require different inputs. I thought there was agreement on how to calculate SAC vs. RMV... maybe I'm wrong.

See Post #1 min this thread. There is no agreement.

 

[tursiops]

I would like to see the breathing rate converted to a surface rate described as SACP or SACV, i.e. Surface Air Consumption Pressure or Surface Air Consumption Volume. RMV is not descriptive enough. It is based on per minute (the M) and volume (the V) but respiratory (R) is superfluous and there is nothing indicating it is a surface rate quantity.

I'd like to see people use units, not confusing groups of letters or acronyms.
You are an engineer; you'd really prefer SACP instead of psi/minute or liters/minute?