Dive Computer Ascent Rates

[lowviz]

Both my Uwatec and VR3 give me an ascent rate warning. The fine print says that the 'recommended' ascent rate depends on depth. Now for my backup plan:

For diving 'tables' (meaning SPG, depth gauge, and timing device), 30ft per min is the generally accepted value. -let's not quibble, if you like 60 that's fine by me.

So I would expect the re-branded Uwatec bottom timer that I'm looking to buy would either have no ascent rate indicator or just follow the simple 30'/min recommendation. But no, it too depends on depth.

Here's the question: What is the depth correction for ascent rate? In other words, does it matter?

(yes, I did the mandatory searches before asking)

Thanks,
lowviz

 

[Jasoncassanova]

Both my Uwatec and VR3 give me an ascent rate warning. The fine print says that the 'recommended' ascent rate depends on depth. Now for my backup plan:

For diving 'tables' (meaning SPG, depth gauge, and timing device), 30ft per min is the generally accepted value. -let's not quibble, if you like 60 that's fine by me.

So I would expect the re-branded Uwatec bottom timer that I'm looking to buy would either have no ascent rate indicator or just follow the simple 30'/min recommendation. But no, it too depends on depth.

Here's the question: What is the depth correction for ascent rate? In other words, does it matter?

(yes, I did the mandatory searches before asking)

Thanks,
lowviz

on my aeris manta it says that if you're diving more than 60ft. the maximum ascent rate is 30fpm and if you've gone down more than 60ft. then the maximum ascent rate is 60fpm.

 

[Tortuga68]

The Uwatec varies the ascent rate between 7 and 20m/minute depending on depth; there are 10 depth 'brackets between 6m and 50m in roughly 5m intervals, and one for >50m. It's due to the gradient ie the percentage change in pressure - going from 50m to 40m (6ATA to 5ATA) is much less of a change than going from 10m to 0m (2ATA to 1ATA), so you can do it faster safely

Incidentally it may interest you to know the origin of the 18m(60')/minute ascent rate - it was a compromise between what the old hard hat divers wanted (faster) and what the tenders who pulled them up on their rope wanted (slower). Nothing scientific about it at all

 

[lowviz]

..... there are 10 depth 'brackets between 6m and 50m in roughly 5m intervals, and one for >50m. ..........

Thanks! That makes sense, constant rate of pressure change. So, as I tend to do, boogie (60+ fpm) up to my half-depth Pyle stop and then start taking a much more deliberate approach to ascent. Saves gas...

 

[Tortuga68]

...and gets you off-gassing quicker too. You're welcome

 

[on_two_wheels]

Incidentally it may interest you to know the origin of the 18m(60')/minute ascent rate - it was a compromise between what the old hard hat divers wanted (faster) and what the tenders who pulled them up on their rope wanted (slower). Nothing scientific about it at all

Why did they want each? I can figure why the diver wanted faster but why did the tenders want to go slower?

 

[Tortuga68]

Because it's hard work pulling up a diver wearing one of these

 

[lowviz]

Thanks to Tortuga68's information, I was able to refine my search and came up with a bit more detail http://www.scubaboard.com/forums/1979863-post2.html

This presents a new problem for the SPG, depth gauge, and timer purists. It looks to me like the most sensible ascent rate would have to be continuously varying (as per Tortuga68 and Rotuner's info) so that the ATA (not ATM) rate of change is constant.

Obviously, bottom timers are more than just a depth gauge / timer combo. They offer you this ongoing calculation. Now I know.