Question about sac rate

Discussions about Diving Log 6.0 - questions and hints
Data
Posts: 4
Joined: Thu Dec 07, 2006 05:56

Question about sac rate

Post by Data »

I'm still waiting for my atom2 to arrive, until then I'm playing with various dive logs to make a decision which to use in the future.
I have a few questions, but let's take this one first.
looking at the sac rate some things I couldn't find the answer to are:

how does the program know if I used a LP or HP tank?
there is a huge difference if I use a 120cf tank rated @2600psi and filled to 3000psi (~140cf) or if I used a 120cf tank rated at 3400psi and started with it filled to 3000psi (~105cf)
shouldn't there be a field that asks for the rated pressure of the tank?
also, when adding a new dive manually, how can I add the sac rate to this dive?
let's say I haven't used a computer on a dive, but know my starting and ending pressures, the tank size, it's rated pressure and my average depth and time.
I can easily calculate my sac rate, but how can I add it to the log calculations?
what is happening to my sac rate if I use multiple stage and/or deco tanks?
thanks

divinglog
Site Admin
Posts: 5192
Joined: Sat Feb 08, 2003 21:02
Location: Coburg
Contact:

Post by divinglog »

Hello

Diving Log calculates internal with metric values, so the SAC rate is calculated with this formula:

Code: Select all

SAC = (AirUsage * Tanksize) / (Divetime * (AvgDepth / 10 + 1))
Maybe someone can explain this a little bit because I'm not familiar with the imperial calculation. Multible tanks are in the current version unfortunately not fully supported. I'm planning a better support of technical diving and multible tanks in version 5.0

The SAC rate is automatically calculated. The only thing which is missing in manually entered dives is the dive profile. To workaround this you can enter one dive profile point which has the depth of your average depth. This way Diving Log should calculate the SAC rate for manual dives and use it also for the statistics. You can enter the dive profile in the profile data tab of the logbook window.

Data
Posts: 4
Joined: Thu Dec 07, 2006 05:56

Post by Data »

divinglog wrote:Hello

Diving Log calculates internal with metric values, so the SAC rate is calculated with this formula:

Code: Select all

SAC = (AirUsage * Tanksize) / (Divetime * (AvgDepth / 10 + 1))
Maybe someone can explain this a little bit because I'm not familiar with the imperial calculation.
here is the imperial version:

SAC = [((Pi - Pf) / Pw) * V] / [((D + 33) / 33) * T]

Pi = initial tank pressure
Pf = final tank pressure
Pw = tank working pressure rating
V = actual tank volume
D = average depth during dive
T = time in minutes

would be nice if you could add this to your program in the future.
divinglog wrote: Multible tanks are in the current version unfortunately not fully supported. I'm planning a better support of technical diving and multible tanks in version 5.0
sounds good
divinglog wrote: The SAC rate is automatically calculated. The only thing which is missing in manually entered dives is the dive profile. To workaround this you can enter one dive profile point which has the depth of your average depth. This way Diving Log should calculate the SAC rate for manual dives and use it also for the statistics. You can enter the dive profile in the profile data tab of the logbook window.
ok, tried that and it works.

Data
Posts: 4
Joined: Thu Dec 07, 2006 05:56

Re: Question about sac rate

Post by Data »

HoooDooo wrote: Well, Sven is from a modern civilized country, a place where they don't use achaic forms of measure that are left over from the middle ages.
The program is written to use a tank size of 'liters' - in that form the "full" pressure is irrelevant. Only us basakwards Americans have a problem with that. In the metric form the tank is rated X Liters @ 1Bar. I suggest you use that instead of cuft for the tanksize and all will be well.
so in other words, you want me to write my own software or rewrite this one or start converting all numbers related to my dive gear into the metric system to use this software? :?
and here I thought this software would make my life easier.
HoooDooo wrote:
Data wrote:what is happening to my sac rate if I use multiple stage and/or deco tanks?
If you are qualified to use "multiple stage and/or deco tanks" - then you really don't need the Program doing that calc do you?
guess what, I'm even qualified to count the number of dives I had and still would like a piece of software to do it for me. what's your point?

Data
Posts: 4
Joined: Thu Dec 07, 2006 05:56

Post by Data »

also sven,
when inserting a dive in imperial, there are many sample tanks from 13 to 100cf to chose from.
since one is never asked for the working pressure of a specific tank size, all calculations to sac rates could therefore be wrong.
maybe until this changes you should warn your customers of that.

James Connell
Posts: 103
Joined: Fri Dec 08, 2006 05:33
Location: Alaska, USA
Contact:

Re: Question about sac rate

Post by James Connell »

[quote="Data"]so in other words, you want me to write my own software or rewrite this one or start converting all numbers related to my dive gear into the metric system to use this software? :? [/quote]

No, just enter the size of the bottle in metric. Nothing else changes. File->Preferences->Miscellaneous->Volume = liters. You get your SAC in L/Min that way, but it's a simple conversion in the provided "Units" converter to get cuft. You may even find you like doing it like the rest of the world - you can get used to seeing a whole number instead of a small fraction.

You would also be doing your part in helping to convert the USA to the metric system. When a leader in Europe asked the person in charge of the US' conversion plan how the conversion was progressing, he was told -" things are going slowly, We're just inching along"
Last edited by James Connell on Fri Dec 08, 2006 21:11, edited 1 time in total.
JConn Inv.
Software Tools for Suunto Dive Computers
http://LiquidImagePhoto.com

James Connell
Posts: 103
Joined: Fri Dec 08, 2006 05:33
Location: Alaska, USA
Contact:

Post by James Connell »

[quote="divinglog"]Maybe someone can explain this a little bit because I'm not familiar with the imperial calculation. [/quote]

Sven, what seems to be confusing you is the way bottles are sized in the US.

We don't rate them with a liquid volume like europe does. Here a Tank is given a 'Maximum' pressure by the government, and it will hold X cuft of gas at that pressure. so it's a X cuft bottle. It makes the calculations Much more complex - you need the rated working pressure, the start pressure and the 'size' - not just the size and pressure change to find volume consumed.
JConn Inv.
Software Tools for Suunto Dive Computers
http://LiquidImagePhoto.com

divinglog
Site Admin
Posts: 5192
Joined: Sat Feb 08, 2003 21:02
Location: Coburg
Contact:

Post by divinglog »

OK, thank you both for the explanation! :)
That was quite new for me, I hadn't heard that before. I'll see if I must adjust the way Diving Log works with imperial tank sizes.

bperrybap
Posts: 4
Joined: Sat Jan 20, 2007 20:25
Location: Dallas, Tx USA

Post by bperrybap »

In reality the calculation for metric or imperial is EXACTLY the same.

The final answer is Volume/minute

In Imperial you get volume by

Pressure used * Tanksize / Working Pressure

The same is true in metric.

The difference is that in metric the working pressure of ALL
tanks is always the same and is 1 BAR. In Imperial it can
literally be anything and is in PSI.

But since the working pressure in metric is 1 BAR it is often ignored
in the metric equation.
The problem is that if you truely performed the full equation
ignoring the working pressure, you end up with the wrong units
in metric.
You have to include dividing by the working pressure in order
to get the pressure units to go away and be left with only a
volume unit of liters.

--------------------------------------------------------------

For example lets look at the full equation:
vol = ((Start Pressure-End Pressure)/Working Pressure) *Tank Volume

In Metric, many folks will tend to short cut this into:

vol = (Start Pressure-End Pressure) *Tank Volume

However with real values AND units you will immediately
see the problem.

If you take the short cut
vol = (200 BAR - 50 BAR) * 15 Litre

You end up with:

Vol = 150 BAR * 15 Litre
Vol = 2250 BARLitre

What the heck is a BARLitre ?
You want to end up with Litre (L) not BARLitre or BARL

But if you use the FULL equation which includes the working
pressure you end up with the correct units.

Vol = ((200 BAR - 50 BAR) / 1 BAR) * 15 Litre
Vol = 150BAR/1BAR * 15 Litre
Vol = 150 * 15 Litre
Vol = 2250 Litre
Vol = 2250 L

The units all divide out properly.
You need that working pressure to divide out the pressure
units to leave you with just a proper volume unit.
So many people try to take short cuts and make mistakes
by leaving off the units and then slapping them on the end number
because they know what the units of the final answer should be.

The safest way to to so this is to ALWAYS divide by the working
pressure. In metric, that pressure is always the same and
is 1 BAR.
In imperial it is not always the same and the user must enter it in PSI.

You can cheat and can get away leaving off the working pressure in
metric; however, in Imperial you cannot.

For me, I always do equations and conversions with full units.
If the equations are correct, the proper units will be left.


--- bill

divinglog
Site Admin
Posts: 5192
Joined: Sat Feb 08, 2003 21:02
Location: Coburg
Contact:

Post by divinglog »

Hi Bill

Thank you for the further explanation. I'll add a field in the next update for the working pressure. I think for the compatibility I'll set this field to a standard working pressure until the user enters a correct value or what do you think? What should be the standard pressure? Or should this field be blank and all imperial users must enter a valid working pressure to see their SAC rate? With the template manager this can be added very easily for a lot of dives if the pressure is the same.

AndyDragon
Posts: 110
Joined: Tue Nov 14, 2006 08:38
Location: Penetanguishene, Ontario
Contact:

Post by AndyDragon »

According to IANTD, there are some misconceptions in this thread...I'm not sure if ACUC Europe or other agencies use the formulas differently.

SAC (surface air consumption) is NOT dependent on working volume or working pressure, RMV (respiratory minute volume) is.

Imperial:

SAC = (air in psig) / [(average depth in ATA) x (time in mins)]
SAC units are psig / min

Metric:

SAC = (air in bar) / [(average depth in ATA) x (time in mins)]
SAC units are bar / min

Imperial:

RMV = [(SAC in psig/min) x (working volume in cuft)] / (working pressure in psig)
RMV units are cutft / min

Metric:

RMV = [(SAC in bar/min) x (working volume in L)] / (working pressure in bar)
RMV units are L / min

SAC will change from tank to tank as working pressure and volume change and is a straight measurement from one tank used to calculate RMV. It is simply a depth-related air consumption value for a given tank. The RMV is not dependent on any particular tank and should be used as a gauge of the air used over time at depth. To determine breathing time at a given depth for a given gas supply, you use the SAC or RMV depending on how you calculate your gas supply. If the gas supply is in psig or bar, you would calculate using the SAC for that particular tank (as long as you have the working pressure and volume for a tank, you should be able to calculate the RMV from the SAC for that tank or the SAC for that tank if you have your RMV). If you have the gas supply in cuft or L, you use the RMV directly.

Sven...one thing I noticed in the 4.0.7 release, the pop-up for the working pressure shows 'bar' for the units in Imperial mode even though the values are in psi (3000 and 4400). Also, for some reason, if there is no profile, the SAC is not computed... Can the application take the max depth and divide by two in this case, or is that likely to be too far off?
Andrew Forget
PADI IDC Staff Instructor 212158

divinglog
Site Admin
Posts: 5192
Joined: Sat Feb 08, 2003 21:02
Location: Coburg
Contact:

Post by divinglog »

Sven...one thing I noticed in the 4.0.7 release, the pop-up for the working pressure shows 'bar' for the units in Imperial mode even though the values are in psi (3000 and 4400). Also, for some reason, if there is no profile, the SAC is not computed... Can the application take the max depth and divide by two in this case, or is that likely to be too far off?
I fixed the bar labeling. It's only a label bug, the calculation in Imperial units is correct. The profile is needed for the average depth calculation. I'm not sure if it's a good idea to use half of the max depth when no profile is available. Any other opinions?

sublub
Posts: 41
Joined: Sat Aug 06, 2005 23:44

Post by sublub »

You could add a field where you can manualy put a average depth value when the profile is missing and use this for the calculations. This is more accurate then half the maximun depth :)

reefhound
Posts: 54
Joined: Mon May 07, 2007 01:10

Post by reefhound »

Maybe I'm missing something here. I have some manually entered dives without the profile. To calculate SAC, I need Average Depth. To get Average Depth, I enter one profile point with the average depth.

Fine. But then I get a SAC rate with a humongous number like 20cuft3. I duplicate the profile point a few times, average depths remains correct, now SAC rate is like 10cuft3. It appears the calcs are pulling dive time from the profile not the logbook field. What's going on?

Post Reply