Desat Time - Have I Got It Right?

[dkatchalov]

Hey Guys

Just want to make sure I got something right...

I just got a cressi archimedes II computer. After completing a dive, the comp. displays desat time in hours and minutes.

I understand that this is the time it takes for all of the residual nitrogen to leave my body - is this correct?

Also, desat time is not the same as surface interval is it? SI is the amount of time I need to stay out of the water in between dives. Is this correct?

I dont think my cressi comp displays SI time, but I can work it out using tables. As long as I stay within the no-deco limits, my SI time is zero, right? (I need to confirm this from tables which are at home, so if I am wrong on this, please dont flame me!!)

Thanks.

 

[jeckyll]

Ok, let me take this one at a time:

SI: The time you spend out of the water between dives. You computer will may well display it, read your manual. It's not the time you _need_ to stay out of the water. It's the time you _did_ stay out of the water between dives. The only time you'd need to work this out is if you know you want to do repetitive dives and know the length at depth X you want for dive 2. Isn't all this covered in your Open Water manuals?!?

Desat time is what your computer calculated as the "No fly" time, i.e. time for residual nitrogen to leave your system.

Remember, computers use models. It doesn't know how much sleep you had, how hydrated you were etc etc.

HTH

 

[dkatchalov]

Ok, let me take this one at a time:

SI: The time you spend out of the water between dives. You computer will may well display it, read your manual. It's not the time you _need_ to stay out of the water. It's the time you _did_ stay out of the water between dives. The only time you'd need to work this out is if you know you want to do repetitive dives and know the length at depth X you want for dive 2. Isn't all this covered in your Open Water manuals?!?

Desat time is what your computer calculated as the "No fly" time, i.e. time for residual nitrogen to leave your system.

Remember, computers use models. It doesn't know how much sleep you had, how hydrated you were etc etc.

HTH

Like I said, I remember how to work out SI using tables.

I was just making sure I was interpreting what my comp was telling me correctly.

I think SI is built into cressi's "plan" function.

 

[dkatchalov]

Ok, let me take this one at a time:

SI: The time you spend out of the water between dives. You computer will may well display it, read your manual. It's not the time you _need_ to stay out of the water. It's the time you _did_ stay out of the water between dives. The only time you'd need to work this out is if you know you want to do repetitive dives and know the length at depth X you want for dive 2. Isn't all this covered in your Open Water manuals?!?

Desat time is what your computer calculated as the "No fly" time, i.e. time for residual nitrogen to leave your system.

Remember, computers use models. It doesn't know how much sleep you had, how hydrated you were etc etc.

HTH

Like I said, I remember how to work out SI using tables.

I was just making sure I was interpreting what my comp was telling me correctly.

I think SI is built into cressi's "plan" function.

Thanks.

 

[bleeb]

I understand that this is the time it takes for all of the residual nitrogen to leave my body - is this correct?

As jeckyll said, you understand correctly.

One correction though: desat time is not necessarily the same as no-fly time. Desat times can be several days to a week, for example if you've been doing multiple days of deep diving. No-fly recommendations vary between certification agencies, DAN and computer manufacturers, but generally run between 12 and 36 hours, depending on dive profiles and repetitive diving. Some computers incorporate desat information into their no-fly calculations, while others simply count down. Don't know what Cressi does, but it'll be in the manual.

Also, desat time is not the same as surface interval is it? SI is the amount of time I need to stay out of the water in between dives. Is this correct?

No, desat time is not SI. I'm not familiar with your computer, but as the other poster said, it'll be displayed somewhere, since it's just a timer since the end of the last dive. Note that the models used in many computers (and cert agency tables) assume that if a SI is less than a few minutes (often around 10), then so-called second dive is actually treated as a continuation of the first, rather than as a new dive.

SI is what you decide, but most cert agencies recommend at least an hour. Like jeckyll said, this is the basic Open Water training. The computer isn't forcing or recommending any particular SI. It's just treating it as a datum to incorporate into it's calculations.

 

[BarryNL]

I dont think my cressi comp displays SI time, but I can work it out using tables. As long as I stay within the no-deco limits, my SI time is zero, right? (I need to confirm this from tables which are at home, so if I am wrong on this, please dont flame me!!)

I think we might need to flame your instructor - the OW course should be teaching this stuff. The SI (Surface Interval) is simply the time you are out of the water between dives. Your planned SI will depend on your intended profile for your following dive(s). The longer and deeper you intend to dive the longer your SI will need to be to ensure adequate off-gassing of nitrogen beforehand.

Your computer probably has a 'plan' mode which allows you to input your planned dive and it can then tell you the no-deco limit (NDL) for this dive. If the time is shorter than you want you can increase your SI to increase your dive time.

Having said all that, my rule of thumb is an hour and a half for SI's - with a minimum of one hour. I do this because the brain and spinal column off-gas fairly slowly and shorter surface intervals increase the nitrogen build up in these sensitive areas.