SAC Rates

[vladimir]

SAC calculation method:
...

In other words, Suunto uses the same method as anybody who generates an average depth and does the arithmetic post-dive, except most people wouldn't bother to correct for temperature.

 

[piikki]

That does it. It's all Suunto's fault that my SAC is so bad When I have calculated SAC for tanks lolligagging on 80F surface water and then made most of the dive in 45F to 55F bottom temp, I've been cheated.

I always let my tanks warm up (sometimes cool) pre-dive to get my thirds/turn pressures but honestly I never thought of letting them toast post-dive. Sure they would cool during 90 minute dives in chilly water, and it would be fair to measure the end pressure from the same condition one took the start pressure.

 

[Mike Boswell]

In other words, Suunto uses the same method as anybody who generates an average depth and does the arithmetic post-dive, except most people wouldn't bother to correct for temperature.

Yes, but the temperature correction applied by Suunto to a dive in 84 F degree water in Roastan is - apparently - a big number.

Suunto's SAC numbers are typically 9-12 percent LOWER than what I calculate using their data without a temperature correction. In colder waters (54 degrees) here in San Diego, that difference is about six percent.

Which raises the question: Which SAC is the correct one to use? Should we use the one before temperature correction (to zero degrees C) or after?

 

[jashaw]

On my first wreck dive to 120' my SAC was .57 with a drysuit and aluminum 80. I was shocked when I saw this because at Gilboa my SAC has regularly been between .67 and .75.

 

[Mike Boswell]

That does it. It's all Suunto's fault that my SAC is so bad When I have calculated SAC for tanks lolligagging on 80F surface water and then made most of the dive in 45F to 55F bottom temp, I've been cheated.

I always let my tanks warm up (sometimes cool) pre-dive to get my thirds/turn pressures but honestly I never thought of letting them toast post-dive. Sure they would cool during 90 minute dives in chilly water, and it would be fair to measure the end pressure from the same condition one took the start pressure.

It IS Suunto's fault! We jump in with a tank that has been sweltering in the hot sun for hours. The pressure in the hot tank is hundreds of psi higher than if it were cool. The computer turns on and reads this high pressure. Throughout the dive, the tank cools and the pressure drops accordingly, and at the end of the dive the computer reads this lower pressure and computes your SAC.

Note that if you didn't use ANY air in the tank, the pressure drop caused by temperature changes alone would result in some air consumption shown by the computer.

Best thing to do, then, is to put the tanks on ice before the dive.

 

[ianr33]

Which raises the question: Which SAC is the correct one to use? Should we use the one before temperature correction (to zero degrees C) or after?

If you want to compare apples to apples then you need a standardized temperature.

In science the standard conditions are STP Standard conditions for temperature and pressure - Wikipedia, the free encyclopedia.

What does a gas integrated computer do when you put cold tanks in warm water (e.g. Cave Diving in Florida in February) does it give a negative SAC at the start of the dive as the tanks warm up??!!

 

[Mike Boswell]

If you want to compare apples to apples then you need a standardized temperature.

In science the standard conditions are STP Standard conditions for temperature and pressure - Wikipedia, the free encyclopedia.

What does a gas integrated computer do when you put cold tanks in warm water (e.g. Cave Diving in Florida in February) does it give a negative SAC at the start of the dive as the tanks warm up??!!

I would think - yes. If you did not breathe any air from a warming tank, it would show more pressure at the end of the dive, hence a negative SAC.

As I understand Suunto's procedure, it uses beginning pressure, end pressure, dive time, average depth, and minimum temperature. You tell it what size tank - volume and rated pressure - you are using, and it then computes SAC and adjusts it to zero degrees C.

This adjustment factor is due to the thermal coefficient of expansion of air and is approximately 0.003 per degree C. In very warm tropical waters that would produce an adjustment of (30 degrees times 0.003) or 0.09, meaning a 9 percent decrease.

What this means to me is that, to plan your SAC for a given dive, you must first take the SAC number provided by Suunto and adjust it (upwards, since you are unlikely to be diving in water colder than 0 degrees C) for the anticipated temperature of the actual dive.

 

[ianr33]

This adjustment factor is due to the thermal coefficient of expansion of air and is approximately 0.003 per degree C. In very warm tropical waters that would produce an adjustment of (30 degrees times 0.003) or 0.09, meaning a 9 percent decrease.

Coefficient of expansion is a term normally applied to solids.

For an ideal gas the volume is proportional to the temperature (Charles Law). The temperature must be the absolute (Kelvin) temperature.

 

[scarefaceDM]

deleted

 

[ajduplessis]

Calculating SAC using a Suunto transmitter.

This real-time information collected by the transmitter, and the data from the DC is calculated to give you a precise SAC rate at any giving point within your dive profile. You also don’t need to worry about remembering your start and end pressures. When I look at my data in Dive Manager I can immediately see where my workload increased, or where I was more relaxed. The smaller the data collection intervals are (1, 10, 20, 30, 60 second interval), the more accurate the data. Obviously there is a downside to smaller collect intervals as the dive logs get full more easily. This is not a problem is you download and backup you data on a regular basis. I love my transmitter!!!!