SAC decreasing with depth
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Hrmm, I've never seen that "Do you want to simulate" question. I'm using a D9, so maybe there is some difference there, but I would say when you say "no," if you are getting a graph like I describe, that should be an actual SAC graph. At least in mine the SAC graph does not mimick the dive profile like the pic you posted.
I notice, like Puffer described that my SAC is lowest at my most relaxed moments of my dive. In my case this is the few minutes just before my turn on a double profile dive (my standard profile). That's because I've checked my buddies air consumption, estimated his/her air use, made a decision on the turn/ascend time, set my alarm, and just get to relax at the deepest part of my dive for a good 5-10 minutes without any worries.
On my safety stops my SAC goes through the roof, but that is usually because I am blowing air bubble rings, and sucking down a Capri Sun, popping my purge between gulps
Happy Diving!
Jeff
I notice, like Puffer described that my SAC is lowest at my most relaxed moments of my dive. In my case this is the few minutes just before my turn on a double profile dive (my standard profile). That's because I've checked my buddies air consumption, estimated his/her air use, made a decision on the turn/ascend time, set my alarm, and just get to relax at the deepest part of my dive for a good 5-10 minutes without any worries.
On my safety stops my SAC goes through the roof, but that is usually because I am blowing air bubble rings, and sucking down a Capri Sun, popping my purge between gulps
Happy Diving!
Jeff
Hrmm, I've never seen that "Do you want to simulate" question. I'm using a D9, so maybe there is some difference there, but I would say when you say "no," if you are getting a graph like I describe, that should be an actual SAC graph. At least in mine the SAC graph does not mimick the dive profile like the pic you posted.
I notice, like Puffer described that my SAC is lowest at my most relaxed moments of my dive. In my case this is the few minutes just before my turn on a double profile dive (my standard profile). That's because I've checked my buddies air consumption, estimated his/her air use, made a decision on the turn/ascend time, set my alarm, and just get to relax at the deepest part of my dive for a good 5-10 minutes without any worries.
On my safety stops my SAC goes through the roof, but that is usually because I am blowing air bubble rings, and sucking down a Capri Sun, popping my purge between gulps
Happy Diving!
Jeff
Jeff, I spend the time sleeping...
Yeah, last time I tried that I had a little shrimp crawl in one side of my nose and out the other. After that I developed sleeping-shrimp-in-the-nose-aphobia and just can't seem to relax on my safety stops anymore.
I downloaded the Vytec DS manual from the Suunto website. In this regard, it's the same as my Cobra; the profile memory records only depth at each sample. See page 71:
Logbook/Dive Profile Memory:
Recording interval: 20 seconds (the recording interval can be set to
10s, 30s or 60s with the optional PC-interface unit and software),
records the maximum depth of each interval.
The starting and ending pressures are also recorded. See page 49 for a description of what is uploaded to the PC (i.e., to SDM), which also implies the profile is only depth info.
So it spite of what it's telling you, SDM cannot report a meaningful SAC rate at different parts of the dive; the raw data is not available to it. The graph of tank pressure shown in post #16 (blue line at bottom) is a linear interpolation between the start and end pressure, all the data that is available. This is a totally meaningless and misleading construct that should be used for nothing. At this point, nobody knows what the tank pressure was at any point within the dive, other than the start and end pressures. Not SDM or even the Vytec DS itself.
Compounding the confusion by claiming to compute a changing SAC rate within the dive is just rampant bogosity. The only thing you can compute is one "average" SAC rate for the entire dive, based on average depth, and start and end pressures.
So I think the original question is just asking for interpretation of something that lacks any meaning. Not the OP's fault, it's Suunto's fault for promulgating nonsense.
All that said, I won't discount the possibility that SAC rates may actually be lower at deeper depths, or cruising at the bottom vs. ascent/descent, for many of the reasons suggested by posters above. It is meaningful to talk about an instantaneous SAC rate, and that varying over a dive. But you can't obtain that information from a Vytec DS, or my Cobra.
Logbook/Dive Profile Memory:
Recording interval: 20 seconds (the recording interval can be set to
10s, 30s or 60s with the optional PC-interface unit and software),
records the maximum depth of each interval.
The starting and ending pressures are also recorded. See page 49 for a description of what is uploaded to the PC (i.e., to SDM), which also implies the profile is only depth info.
So it spite of what it's telling you, SDM cannot report a meaningful SAC rate at different parts of the dive; the raw data is not available to it. The graph of tank pressure shown in post #16 (blue line at bottom) is a linear interpolation between the start and end pressure, all the data that is available. This is a totally meaningless and misleading construct that should be used for nothing. At this point, nobody knows what the tank pressure was at any point within the dive, other than the start and end pressures. Not SDM or even the Vytec DS itself.
Compounding the confusion by claiming to compute a changing SAC rate within the dive is just rampant bogosity. The only thing you can compute is one "average" SAC rate for the entire dive, based on average depth, and start and end pressures.
So I think the original question is just asking for interpretation of something that lacks any meaning. Not the OP's fault, it's Suunto's fault for promulgating nonsense.
All that said, I won't discount the possibility that SAC rates may actually be lower at deeper depths, or cruising at the bottom vs. ascent/descent, for many of the reasons suggested by posters above. It is meaningful to talk about an instantaneous SAC rate, and that varying over a dive. But you can't obtain that information from a Vytec DS, or my Cobra.
Foxfish
Contributor
I take it that you have an air integrated system. Can you read tank pressures at various time intervals? If so, you could calculate the SAC based on the pressure change over an interval and average depth for that interval and the SAC calculated based on the same interval but using the linearized pressures. Compare those results with the SAC calculated by the software.
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I've not done a screenshot before, and I don't know how good the res will be, but I think reefduffer is right. If you look at the graph here you can see (at least I can on the full screen) that my air consumption is not really a flat line (starting and ending pressure). This is what i get with a D9, and I know as I scroll through my dive log on the watch, I see different pressure readings every 20 secs, perhaps other model Suuntos don't do this?
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I have the air-integrated verision. You are right, it does not transfer pressures. That's why it asks that question. The only way how to get pressure data in DM is that "simulation". I tried to re-calculate a profile from D9 and it didn't ask producing a nice realistic SAC profile with clearly non-linear pressure drop over time. Manual for D9 claims it transfers "real time tank pressure consumption (with optional wireless transmitter in use)".
I don't understand why they decided to implement this "simulation" feature since it has nothing to do with reality... Just misleading lies... The only realistic thing is the average SAC rate.
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