Determining Bottom Time

[ksporry]

Guys,

On my last dive trip I got a bit confused about how to determine bottom time.
The way I learned was to use the Total Dive Time as Bottom Time when completing my logs. However, on my last dive trip I did a couple of specialties and my instructor there (different from when I got my OW and AOW), told me that my bottom time is the time I spend at the deepest point.
I would have expected the first method (Dive Time=Bottom Time) to be the correct way as it is more conservative (and thus also the reason why people use computers as there is a significant difference between tables that require the bottom time calculations, and computers that use algorythms that don't punish you for decent and ascent profiles).

Please note that I dove with a dive computer and was always aware of my limits. However, I do want to remove any confusion about bottom time calculations as in the end we all need to be able to use the RDP tables without computers.

Thanks.

 

[cephasiii]

You are going to get a few different answers to this question. However at your level of certification I would continue to count bottom time as the total time on the dive. If you want, you can subtract the 3 min safety stop, but why since it adds to the conservatism.


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[clownfishsydney]

Depends on what you are talking about. If it is for calculation of no deco time, then it is the time from when you leave the surface till you start the ascent. If it is for your log book, it is the total time underwater.

 

[kwinter]

Bottom time is the time from your initial splash until you BEGIN your ascent. Run time is the total from splash to surface. For recreational dives, the difference is insignificant. For decompression dives, there can be a huge difference. Your buddy wants to know your planned BT, so that you start your ascent together. The boat captain wants to know your planned RT, so he knows when to start worrying that you're not back on board. Deep dives can have a BT of a few minutes and RT of hours. Run time is usually what gets logged, but some folks also keep track of deco time separately.

 

[TSandM]

It depends on the tables you are using -- they define bottom time differently. The PADI RDP defines bottom time as starting with your descent and continuing until you begin "a direct ascent to the surface". This can be difficult to define if you are shore diving, and in fact, many shore dives simply won't fit on the tables.

It is always important, when you are using any decompression model, to know what the assumptions and definitions are that the model is using -- how to define bottom time, and what the expected ascent rate is for the model.

 

[boulderjohn]

It depends on the tables you are using -- they define bottom time differently. The PADI RDP defines bottom time as starting with your descent and continuing until you begin "a direct ascent to the surface". This can be difficult to define if you are shore diving, and in fact, many shore dives simply won't fit on the tables.

It is always important, when you are using any decompression model, to know what the assumptions and definitions are that the model is using -- how to define bottom time, and what the expected ascent rate is for the model.

That is correct, and that is how you do it if you use the RDP calculation graphic in the official log book. You enter the bottom time--the time you start your "direct ascent to the surface"--and the safety stop separately. It is that bottom time (without the ascent and safety stop) that is used to calculate surface intervals and residual nitrogen.

On the other hand, the current PADI materials also say that in logging dive time, it's up to you. They specifically say that a lot of people use total dive time, and they don't indicate that there is anything wrong with that. I believe that is part of the growing recognition that many more people are using computers than tables.

As for your last comments, if you are intending to somehow move from computers to tables between dives or even on the same dive, you will find that very hard. If you are doing a multilevel dive, which many people do, it may even be close to impossible. (Yes, I know it can be done....) I learned that on my first trip after certification. I went to Cozumel with my RDP in hand and learned that every dive was quite literally off the charts. If I wanted to do the dives everyone else was doing, I had to stow the RDP and get a computer.