High SAC reported at the beginning of dive

[Andrei Astra]

I'm using a Suunto Eon Core computer with the transmitter for several months by now. I noticed that it reports a very high SAC rate at the beginning of every dive. Sometimes, the initial SAC rate is as high as 80 l/min. I wonder what can be the reason for that?

Here is an example of a pool dive. The left side scale shows the dive profile (depth), the right side scale shows SAC. The deep blue line is SAC. As you see, the initial SAC rate was 44 l/min, and it dropped to my normal 16 l/min within 2 mins.

Also, I went to the surface twice during the dive. However, the SAC rate was not as high at the re-descent.

What could be the reason for such a high rate at the beginning of the dive? Could it be:

- An artifact. The computer does not calculate SAC correctly during the descent;
- Because I'm inflating my BCD during the decent at the beginning of the dive;
- I'm breathing heavily at the beginning of the dive;

What are your thoughts about it?

 

[crcobb]

What is the temperature difference between the air and water?

Cooling the air in cylinder will cause a drop in pressure, that drop in pressure will appear to your computer to be you using gas. Guessing you talked about Charles Law during your OW class which explains this behavior.

edit: Wrong law.

 

[ChuckP]

It could be some of the above.

A Shearwater computer doesn't display the SAC rate for the first few minutes of the dive as it's doing averages:

"During the first few minutes of a dive the SAC value is not
available, while the initial data is being collected for averaging
calculations. The SAC display will show “wait” during this time."

I don't really pay attention to SAC during a dive so I can't say for sure what it looks like under water.

Inflating your BCD or drysuit is going to skew that calculation. The closer to optimum weighting you get the less air you'll be putting in your BCD.

Initial high air consumption at the dive beginning is common - anxiety. It could also be from physical effort of getting to the divesite........

SAC rate is a great tool to look at over the whole dive, maybe portions of the dive where there was some exertion etc... I will say, it's not something to focus on during the dive if you are looking for improvements - relax and enjoy.

 

[Graeme Fraser]

Looking at your profile, it appears there are two distinct upward spikes that coincide directly with your descent. To me, this would suggest BC usage.

Realistically I would only concern myself with overall sections of the dive (ie working portion or deco). Micro data analysis will throw up too many anomalies to provide useful additional gas planning information.

If you were to watch SAC during a fast tech descent with drysuit / heavy gear, I'm sure it would be through the roof.

By descent I mean the two subsequent descents, not the initial one. Sorry, wasn't clear from my original post.

 

[Neilwood]

I would wonder if the initial spike is not, at least in part, due to cooling of the tank. I know that here if I start a dive that the pressure reading from the AI Perdix can drop by around 10 bar (depending on air temp/water temp) as the tank enters the water. It was particularly noticeable during the summer where air temps were 20+C and water temps were around 12C.

Add in BCD use and it all mounts up.

As others have said, I wouldn't worry to much about minute by minute use (too much can change minute by minute such as hitting a current etc). Look at comparing overall performance trends between dives.

 

[2airishuman]

I found, when I was new to diving, that my SAC rate was much higher during the first few minutes of the dive, even after compensating for any temperature change to the cylinder contents.

Gearing up and entering the water is inherently physical. With time I have become intentional about pacing myself, and typically don't descend until I'm relaxed and my breathing is normal. This means a combination of resting on shore (or aboard) and at the surface after entering the water.

 

[Andrei Astra]

If you were to watch SAC during a fast tech descent with drysuit / heavy gear, I'm sure it would be through the roof.

Here is the tec dive with the dry suit and heavy gear at Blue Hole in Dahab:

Again, I see that the high SAC rate coincided with the descent. We spend more then 10 minutes at the surface before starting the descent. I think that rules out the drop in the tank pressure due to the temperature.

 

[Kharon]

Mares Icon does the same thing. I'm pretty sure it's tank cooling. In any event it's an artifact and not real.

 

[RyanT]

In addition to what others have suggested (tank cooling, BC additions, etc.) it's not unusual for your SAC to be high at the start of the dive. Just the process of gearing up and getting in is going to elevate your breathing rate. When I'm gearing up on the boat and hot in my wetsuit, lifting doubles and stages, shuffling over to the back of the boat, stepping off and then swimming or pulling down a line, I'm always breathing harder. I commonly just take a minute or so on the bottom to relax and allow my breathing rate come down.

 

[Andrei Astra]

I'll check the tank cooling next time in the pool breathing from the regulator on the surface for 5 min before descending.