RDP Question

[911_abuser]

Ok. If you get PG "R" after your second dive then how do you calculate TBT's for your 3rd dive? Don't you have to factor in RNT?

 

[DiveTub]

So after Your second dive you are in PG R

So now you need to answer the following questions to work out your third dive


1: Whats your planned Bottom time and Depth for your third dive ?

2: Whats the minimum surface interval before you can do your this dive ?

3: Whats the Maximum depth and time you can do after your planned Surface interval ?

Here is a table for you

 

[AzAtty]

You do need to account for residual nitrogen in your body, but the dive tables take that into consideration on repetitive dives by requiring you to cut your actual bottom time based upon the residual nitrogen in your body. Table 3 on the RDP presents Residual Nitrogen Times and Adjusted No-Decompression Limits ("ANDL"). The ANDL reduces the NDL by the amount of residual nitrogen you have in your body after a dive.

It sounds like you're trying to work through the example presented in the RDP Instructions:

Dive 1
D: 45ft
ABT: 42 minutes
SS: 3 minutes
Ending PG: N

Dive 2
SI: 1 hour
PG after SI: D
D: 38ft
ABT: 50 minutes
RNT: 25 minutes
TBT: 75 minutes
SS: 3 minutes
Ending PG: R

Sydney set out the information you need for the third dive, so here's how you use that information:

At the end of the second dive, you are in pressure group "R". First, determine how long your surface interval is. Let's say it's one hour. Look at Table 2. An SI of one hour puts you in pressure group "F". The surface interval is where the magic happens. During the surface interval, your tissues off-gas and your body eliminates the residual nitrogen in your system. If your surface interval is not long enough, then the amount of time you can dive on a repetitive dive is reduced.

So here's how the dive plan looks so far:

Dive 3
SI: 1 hour
PG after SI: F

Now flip the RDP to Table 3. Determine what your maximum depth will be for the third dive. If we're using the example in the RDP instructions, your first two dives were to 45 and 38 feet, respectively. Following that profile, you should make a shallower dive on the third dive. Let's say 35 feet.

Dive 3
SI: 1 hour
PG after SI: F
Planned Depth: 35ft

Under the "F" PG column on the 35 foot row, you have a Residual Nitrogen Time of 36 minutes (the top number) and the ANDL of 169 minutes (bottom number). Now, by way of comparison, if you did not take a surface interval after the second dive, your RNT would be 100 minutes. Waiting an hour has reduced your RNT by 64 minutes. You still have an RNT of 36 minutes, though, which means not all of the excess nitrogen in your system has been eliminated.

So now you have your RNT and the maximum Actual Bottom Time (ABT) for your third dive. On a third repetitive dive to 35 feet, your ABT cannot exceed 169 minutes.

Dive 3
SI: 1 hour
PG after SI: F
Planned Depth: 35ft
ABT: no more than 169 minutes
RNT: 36 minutes
TBT: no more than 205 minutes
SS: 3 minutes
Ending PG: ?

Let's say you spend 37 minutes at 35 feet. 36(RNT) + 37(ABT) = 73(TBT). Thus, at 35 feet, your ending PG would be "N" (73 min at 35 feet = "N" pressure group). You didn't actually spend 73 minutes at 35 feet, but the tables force you to add RNT to your ABT to find the "effective" time at depth on a repetitive dive to acccount for the residual nitrogen in your system.

Dive 3
SI: 1 hour
PG after SI: F
Planned Depth: 35ft
ABT: 37 minutes
RNT: 36 minutes
TBT: 73 minutes
SS: 3 minutes
Ending PG: N

Does that make sense? Planning a third repetitive dive is identical to planning a second repetitive dive. All you need to do is figure out the ending PG on the second dive and the starting PG on the third dive and work the table accordingly. There are some special rules for surface intervals on 3+ dives if you come too close to the NDL, but those are set out on the table and in your instructions.