SAC rate has been uncharacteristically high?

[sunnyboy]

The first rule I follow in tracking down something like this is: find out what changed.
First, check your calculation. Mine is in a spreadsheet so I only plug in the numbers and it calculates the same way every time. Second, check your numbers. Again, I get two tank pressures taken the same way, same place (at home, before and after at the same inside temp). My depth and time come off the Shearwater so they are the same.

Once you've eliminated numbers and calculation, it can ONLY be one of two things to account for increased SAC: something is leaking on your gear, or it's you.

Gear is equally easy to check - do a bubble check during the dive.

So that leaves you. If you can't track down a known reason (med change, stress change, known changes in your health), then maybe time to see a doctor and have a checkup. Normally it's pretty easy to find a good reason for SAC changes, so if you can't, that could be a sign somethings up.

 

[WantSomeScuba?]

Thanks you all for the advice. I went back over my numbers, and I'm confident in the math. That said, it could be a number of things that were mentioned. I'm definitely going to a doctor in the next week--it's been a couple years anyway, and this has me confused. Stress and health are definitely contributing factors (with health probably being a smaller factor than stress)--every time I've been diving, I've been in a "hurry" before I got on the boat or showed up at the pool.

 

[scubadada]

@WantSomeScuba?

Best of luck at your checkup, please give us some follow up regarding how this works out for you.

 

[Brandon]

Gas consumed/(exposure time x average ATA).

Could easily be explained by a simple (and common) math error:

Sample dive: 85CF consumed, 45' average depth (2.36 ATA), 45 minutes.​

SAC = Gas Consumed / (Exposure Time x Average Depth)​

SAC = 85 / (45 x 2.36)
SAC = 85 / 106.2
SAC = .70​

If you're multiplying exposure time x average depth first, leaving that result your calculator, then dividing by gas consumed, you've made an order of operations error and the math actually looks like this....

SAC = (45 x 2.36) / 85
SAC = 106.2 / 85
SAC = 1.25​

Interestingly enough, it produces an error in almost the exact magnitude as what you're experiencing.

As an aside, my offer stands to go diving whenever you're up for it. If the math is solid, can at least eliminate stress or equipment concerns from your puzzle.

-B

 

[stuartv]

Same exposure protection? Going from a wetsuit to a (possibly) baggy drysuit can make a notable difference. So can changing from a single tank to doubles, for that matter.

 

[WantSomeScuba?]

Same exposure protection? Going from a wetsuit to a (possibly) baggy drysuit can make a notable difference. So can changing from a single tank to doubles, for that matter.

Same exposure protection, same water, same tanks.

 

[Pearlman]

Two years back My SAC rate dropped mysteriously to the point I was struggling to extend a dive beyond 40 mins when 60+ mins was normally feasible in relaxed diving conditions on a 12ltr tank. Spanning more than a year diving internittently about 40+ dives, I could not figure it out and began losing my confidence. Until I got thinking my vacation habit of 10mg tadalafil per day was changing my pulmonary pressure and affecting my breathing. Indeed I began observing that my capacity to regulate my outflow/exhale in a slow steady consistent manner was affected. I just couldn't seem to build the strength to inhale strongly or even hold some pressure in my lungs (though I don't do a breath hold while diving), So I stopped the tadalafil habit and things seemed to improve - when I realized on my next vacation after that (albeit belatedly) that my habit of taking a pill of holy basil 400mg for stress relief was also impacting in a similar way. Holy Basil is a recommended herb for Asthma and Bronchitis problems and it seems to affect my lungs in a similar fashion - unable to hold pressure with a low threshold for involuntary exhale. Now I avoid both and it seems to have improved my SAC rate to almost normal. (I cant help taking tadalafil in the days before diving so it still affects me but skipping the doses is helping a bit for sure ...).

If its really pinching you intellectually as well, I would recommend you get a wireless air integrated transmitter. I use one and when I upload my dive log to subsurface, I get to see the curve for air consumption from start to finish superimposed with depth and you can see your SAC rate change at the difficult sections of the dive.

P

 

[2airishuman]

every time I've been diving, I've been in a "hurry" before I got on the boat or showed up at the pool.

You can blow through a lot of air if you're stressed when you first start breathing from the reg, enough to distort the SAC for the remainder of the dive. There is also a good deal of uncertainty in all the measurements. If you're in cold water on a hot day, for example, your apparent SAC will be higher because the cylinder pressure will go down when it cools from contact with the water.

I use my dive computer to determine runtime, but it does not count time spent above 5' deep, and in some freshwater lakes it is possible to spend a good deal of time at very shallow depths and again throw off the calculations.

 

[jstrang]

I would think it is a medical issue. I would make sure at minimum that you have a full cardiac workup/stress test.
Good luck,

John

 

[ams511]

Until I got thinking my vacation habit of 10mg tadalafil per day was changing my pulmonary pressure and affecting my breathing. I

Do Viagra and other PDE5 Inhibitors Increase the Risk of DCS in Humans?