Subsurface

[GlennL]

I use subsurface to download my dives. My computer is AI, so after selecting the tank it will give a "sac" rate. For example on a recent dive using an al80 it shows for a 63 minute dive i used 51.23cuft of air and had a sac of 0.39 cuftmin. Max depth 96 ft.
Is there way to get psi per minute from subsurface? From my understanding the cuftmin is really the rmv, not sac. I see hand calcs for going from a psimin sac to rmv but not the other way around. I'm sure I could use the same formula and solve for the psi, but my curiosity for trend analysis does not extend that far.

 

[rhwestfall]

Yes, the nomenclature is off.....

RMV (cu-ft/min) is SAC (psi/min) x "k" factor

"k" factor is the tank's: volume (rated) / pressure (rated)

example: AL80 : 77.4/3000 = 0.0258

so,

0.39 = SAC x 0.0258

SAC = 0.39 / 0.0258

SAC = 15.11 psi/min


Edit Post by rhwestfall

 

[GlennL]

is there not a way to get subsurface to display it in psi? I know it has to have the data in there somewhere......

 

[AlexL]

is there not a way to get subsurface to display it in psi? I know it has to have the data in there somewhere......

AFAIK, no.

PSI as a reported unit is not very useful on it's own, as it depends on both cylinder size, and depth. SAC in CFM/LPM is independent of either, and can be used to figure out PSI/min relatively easy using tank factors as rhwestfall mentioned.

my SAC typically sits around .40~.45cfm, but for dive planning purposes call it .5cfm to make the mental math easy and pad the conservatism. with the common AL80 in imperial, round the tank factor to 2.5, which results in 20psi/min/ATA, then convert over into 5 minute segments of 80psi/5min/ATA. planned depth of 20m/66ish feet on an AL80? thats 3 ATA and 240psi/5min, call it 250psi/5min. planning using this method has always put us back on the anchor line with plenty of gas to spare.

subsurface does have a planning mode, which you can plug in your SAC rate in CFM, depth, and tank, and it'll spit out an estimated gas usage over the time of the dive. IIRC, when in rec mode, it'll extend the bottom time until it hits rock bottom based on your preferences. using ZHL or VPM modes, you can specify how conservative you want your rock bottom calcuations done.

 

[stuartv]

is there not a way to get subsurface to display it in psi? I know it has to have the data in there somewhere......

It has the data. I don't know of any way to get it to display it. That would be an enhancement request to send to the developers.

Edit: And I doubt the developers will do it because of the complexity involved in displaying it for dives that involve more than one cylinder. Subsurface can display RMV (which they call SAC) for a whole dive, regardless of the cylinders involved because that calculation only depends on the amount of gas used and the average depth. It doesn't depend on cylinder sizes. SAC depends on the cylinder size you're talking about (as well as the portion of the dive during which that cylinder was used). So, calculating the SAC for any dive with more than one cylinder would be pretty difficult. I am skeptical that the Subsurface developers would want to code in something that was an exception that only worked/happened when a dive only had 1 cylinder defined in the Cylinders pane of the Equipment tab.

 

[GlennL]

regardless of the cylinders involved because that calculation only depends on the amount of gas used and the average depth. It doesn't depend on cylinder sizes. SAC depends on the cylinder size you're talking about (as well as the portion of the dive during which that cylinder was used).

If i change the cylinder size in subsurface it alters the listed "sac".

 

[stuartv]

If i change the cylinder size in subsurface it alters the listed "sac".

Yes. And?

It uses the cylinder size and start and end pressure to calculate how much total gas volume you used from that cylinder. It can add up all the gas used from all the cylinders to get the total gas consumed during the dive. Divide by average depth over the whole dive and you've got your RMV.

To figure out SAC, you have to be talking about a specific cylinder. There is no SAC that applies to multiple cylinders, unless they all just happen to be the same size.

To figure out SAC for a specific cylinder in a multi-cylinder dive, you would have to know the average of the depth during just the portion of the dive where that particular cylinder was in use. As far as I know, Subsurface does not currently track the average depth during dive segments that are delineated by gas switches. So, for it to calculate the SAC, for a particular cylinder, it would be a lot of work. I think.

Note: When I say SAC, I mean psi (or bar) per minute. When I say RMV, I mean cu-ft (or liters) per minute. I do not agree with Subsurface's usage of the term SAC. What Subsurface reports is what I would call RMV.

 

[tursiops]

If i change the cylinder size in subsurface it alters the listed "sac".

All subsurface can *measure* is psi and time, so calculating psi/minute is trivial. Once you tell it a cylinder size, it can then *calculate* cuft/min. Give it a different cylinder, get a different cuft/min....but the psi/min stays the same. So if you were to tell it that your cylinder has a tank factor of 1.0, then cuft/min and psi/min will be the same. More realistically, give it a tank factor of 0.01, for example a 20 cuft tank at 2000 psi, and when it tell your "cuft/min" is 0.28, for example, it means your psi/min is 28.

 

[tursiops]

Note: When I say SAC, I mean psi (or bar) per minute. When I say RMV, I mean cu-ft (or liters) per minute. I do not agree with Subsurface's usage of the term SAC. What Subsurface reports is what I would call RMV.

You have your favorite definitions, others have theirs. There is no "right" answer. Best to just say what the units are and ignore the names.

 

[stuartv]

You have your favorite definitions, others have theirs. There is no "right" answer. Best to just say what the units are and ignore the names.

RMV stands for Respiratory Minute Volume. I don't see any other way to interpret that than as a measurement in volume per minute. E.g. cu-ft per minute. Expressing Respiratory Minute Volume as psi per min does not make sense (to me, of course), as psi is not a unit of volume.

That seems like the "right" answer for RMV (to me).