What should I trust?

[The Kraken]

If you want to log your pressure groups in your log book, there are some ways to back into the pressure group using your computer.

If your computer allows, you can go into the set up mode for your next dive. When you put in the max depth for your second dive, your computer should give you the max dive time calculated on your current nitrogen loading.

Using the bottom time that your computer gives you, you can go to the tables and find the pressure group/bottom time at the max depth of your next dive. That will be then pressure group you're in when you end your first dive.

the K

 

[scubafanatic]

Never absolutely trust anything with your lilfe which is operated mechanically or electronically.

...umm, even the human brain operates 'electronically' ...an electro-chemical biological 'computer'...as it were.....

 

[RonFrank]

Most students learn to calculate NDL based on a flat profile. So if you have an NDL of 30 minutes on air at a depth of X, and spend 30 minutes at depth X, than you have reached the deco limit. However once you start to ascend, the profile is no longer flat.

One can spend 30 minutes at depth X, and then ascend 10feet, and spend another 5 minutes at that depth, and than ascend another 10 feet, and spend 5 minutes at that depth, and so on. This is called a multi-level profile, and this is how most of the world dives. All current computer models calculate NDL's based on multi-level depths.

It get's more complicated than that as most computers have *compartments*, and there are a few different models that are used (PMG, RGBM, etc). Different model are more or less conservative. As a new diver it is best to start out with a conservative approach, and push NDL's as you become more comfortable UW. The computer does not measure anything in your body, so these models are exactly that, theoretical.

I would trust a new computer over a new diver for calculating NDL on multi-level profiles, or even flat profiles (if there is really such a thing). Computers are cool as they measure exact depth, and exact time at depth, and put it through a constantly updated process to determine current NDL's not based on some predetermined theoretical profile, but on what the diver is doing during the dive.

WORD~computers ROCK! If you don't like the idea of trusting a device that can malfunction, than my suggestion is to purchase two!

 

[RonFrank]

Never absolutely trust anything with your lilfe which is operated mechanically or electronically.

Like Cars, Planes, Traffic signals, CO2 detectors, process foods, regulators, backup regulators, tanks... the list is endless. We trust our lives with mechanical and electrical equipment multiple times a day.

The alternative is of course to go live in a cave, but the water may make you sick if you are not acclimated to non-chlorinated bacteria laden water!

 

[Hatul]

Like Cars, Planes, Traffic signals, CO2 detectors, process foods, regulators, backup regulators, tanks... the list is endless. We trust our lives with mechanical and electrical equipment multiple times a day.

The alternative is of course to go live in a cave, but the water may make you sick if you are not acclimated to non-chlorinated bacteria laden water!

I think he meant never trust 100% mechanical/electric device with your life. If you see that the reading is off, don't blindly follow it. If you see that your car starts making a loud noise or is not handling right, stop and find out what the problem is. Etc.

There's a story about a guy who drives into a sand pile following his car's GPS.

Adam

 

[Interceptor121]

If you want to log your pressure groups in your log book, there are some ways to back into the pressure group using your computer.

If your computer allows, you can go into the set up mode for your next dive. When you put in the max depth for your second dive, your computer should give you the max dive time calculated on your current nitrogen loading.

Using the bottom time that your computer gives you, you can go to the tables and find the pressure group/bottom time at the max depth of your next dive. That will be then pressure group you're in when you end your first dive.

the K

if you are a fan of pressure group this method actually works, assuming you had the tables with you!

 

[The Kraken]

One assumes nothing.

Thanx, Icpt121

the K

 

[Interceptor121]

To expand on the idea of the K
If are interested in the pressure group put your computer in plan mode to a given depth (for example 18) as soon as your surface and take a note of the NDL then go back to side two of the DSAT and work out you PG
However that could be not practical so the procedure to calculate the PG some time after surfacing is:
1. Make a note of your surface interval
2. Put your computer in planning mode and then at a set depth calculate the adjusted NDL (note this depth does not need to be the planned depth of next dive but can be a set value just to calculate the PG)
3. Look up that NDL on the side two of the DSAT table and find the repetitive pressure group.
4. The pressure group found in point 3 is the current PG not the one at the end of the previous dive so you go to the surface interval credit table and with the PG you calculated you intersect with surface interval you noted in point 1.

You have now calculated the PG at the end of the previous dive and now you can use this information to calculate the PG after the full surface interval

 

[flyingvisit]

Thanks all - I think you've pretty much covered off everything I wanted to know - i.e. how to fill in log and whether I was ok to trust the computer.
Just as an FYI the two dives themselves were max depths of 63 and 64 ft but that had more to do with my not so brilliant buoyancy control... I know on the second dive I was mostly around 55-58 ft. The first was a total dive time of 35 mins, the second was 42 mins (both had safety stops at 15 ft for 3 mins). In reality I probably spent about 20 or so minutes at the bottom on the first dive, and maybe a little longer on the second. There was 1.14 between the two dives. My dive computer showed 4 bars nitrogen loading at the end of both dives - I set it to warn me when it hit the 4 bars. Call me paranoid, but I don't want to see it any higher than that! I was confused though, because if I go by the dive tables I have to log each dive as 70ft and I'm well over the limit.
Daft question - do these computers count the saftey stop into your total dive time? (I bet its in my instructions...off to read them again).
And just to prove you can't trust these computers totally - the thing is now showing a low battery warning! (I've only had it since August!) but I hear thats quite common with this make (Aeris Manta).
I'm going to take the Nitrox qualification next, but I'm not sure thats going to massively improve things anyway at the minute as I burn through the air so fast it limits my bottom time anyway!

 

[Jax]

Daft question - do these computers count the saftey stop into your total dive time? (I bet its in my instructions...off to read them again).

Yep . . . Most of them count all underwater time as the same dive, until you have surfaced for a set time, then it closes the log. Uwatec, for instance, gives you five minutes, during which time you can get oriented or swap tanks and continue the dive.

Real good idea to read those instructions well . . . I'd even say "Memorize them". That's your life you are trusting to that little computer.