Questions on RNT please help.

[CrowyCrow]

I am learning on PADI's Recreational Dive Planner and I have difficulty understanding RNT.

Lets assume that I had surfaced after my first dive and I am going to do my 2nd dive.

From what I understand of Residual Nitrogen time is the left over nitrogen that is inside me after my first dive.

so lets assume that this residual nitrogen time is X now just before I make my 2nd dive.

Table 3: repetitive dive time table:
I am at Pressure Group E.
if I dive for 16m my RNT is 21 minutes
If I dive for 14m my RNT is 24 minutes.

Question:
Before my second dive, there is X mins of nitrogen time in my body.
why does the X mins of nitrogen time changes according to my depth during my second dive?
If there is nitrogen time left inside my body before my 2nd dive. Isnt the RNT suppose to be a fixed amount?

If anyone can clear my doubts. Thank you.

 

[tbone1004]

the nitrogen time changes according to depth because it is based on a theoretical tissue loading percentage. I.e. if your tissues are still saturated with say 50% of the allowable nitrogen per the RDP, you will only have 50% of the allowable dive time remaining.

The reason they use residual nitrogen time is because the dive tables are based on time as that is a measurable variable. This is very simplified, but the way dive tables work for nitrox mixes are PN2+time=% theoretical tissue loading. We know PN2, that is a function of depth, and we know time because it is measure on whatever time keeping device you use, but the theoretical tissue loading is not a measurable variable. When we get to the end of the dive and are calculating repetitive dives, the only way to easily express that % theoretical tissue loading is to equate it to a residual nitrogen time based on your next planned depth

 

[Colliam7]

Table 3: repetitive dive time table:
I am at Pressure Group E.
if I dive for 16m my RNT is 21 minutes
If I dive for 14m my RNT is 24 minutes.

I am not entirely clear on where these numbers are coming from in Table 3. I see the 21 min RNT, for a diver in Pressure Group E, if they dive to 50 ft. But, I am not seeing a RNT of 24 min for any depth, for a diver in Pressure Group E. (I do see a 24 min RNT for a diver in Pressure Group F going to 50 ft.)

Question:
Before my second dive, there is X mins of nitrogen time in my body.
why does the X mins of nitrogen time changes according to my depth during my second dive?
If there is nitrogen time left inside my body before my 2nd dive. Isnt the RNT suppose to be a fixed amount?

Good question. In reality, for using the dive tables the RNT is NOT a fixed amount. Rather, the RNT is the relative penalty you pay for the nitrogen remaining in your body after a previous dive (or series of dives), in the context of the NDL for the 'next' dive.

For example, an 'E' diver has a RNT of 21 minutes going to 50 ft. But, that 'E' diver has a RNT of (only) 17 minutes if they go to 60 feet. On the surafce (OK, bad pun) it sounds like the RNT is less if you go deeper. BUT, the NDL for a dive to 50 ft is 80 min, and the NDL for a dive to 60 ft is 55 min. In fact, you are paying a bigger 'penalty' for residual nitrogen on the deeper dive. For the 50 ft dive, you give up 26% (21/80*100) of your potential NDL because of residual nitrogen. But, for the 60ft dive you give up 31% (17/55*100) of your potential NDL. The RNT 'penalty' is not a fixed number, but a proportional factor.

 

[CrowyCrow]

the nitrogen time changes according to depth because it is based on a theoretical tissue loading percentage. I.e. if your tissues are still saturated with say 50% of the allowable nitrogen per the RDP, you will only have 50% of the allowable dive time remaining.

The reason they use residual nitrogen time is because the dive tables are based on time as that is a measurable variable. This is very simplified, but the way dive tables work for nitrox mixes are PN2+time=% theoretical tissue loading. We know PN2, that is a function of depth, and we know time because it is measure on whatever time keeping device you use, but the theoretical tissue loading is not a measurable variable. When we get to the end of the dive and are calculating repetitive dives, the only way to easily express that % theoretical tissue loading is to equate it to a residual nitrogen time based on your next planned depth

Thanks for replying but any chance if you can explain it more layman?

 

[Pinecube]

I am not entirely clear on where these numbers are coming from in Table 3. I see the 21 min RNT, for a diver in Pressure Group E, if they dive to 50 ft. But, I am not seeing a RNT of 24 min for any depth, for a diver in Pressure group E. (I do see a 24 min RNT for a diver in Pressure group F going to 50 ft.)

The metric tables get you those numbers.
http://elearning.padi.com/company0/tools/RDP_Table Met.pdf

 

[Oldbear]

I find these videos helpful:

 

[Colliam7]

The metric tables get you those numbers.
http://elearning.padi.com/company0/tools/RDP_Table Met.pdf

Thanks, I was wondering if the 14m and 16m were meters. But, as an Imperialist, I wasn't sure. So, let me re-phrase my previous answer, in metric.

An 'E' diver has a RNT of 24 minutes going to 14 m. But, that 'E' diver has a RNT of (only) 21 minutes if they go to 16 m. On the surface, it sounds like the RNT is less if you go deeper. BUT, the Table 2 NDL for a dive to 14m is 98 min, and the NDL for a dive to 16 m is 72 min. In fact, you are paying a bigger 'penalty' for residual nitrogen on the deeper dive. For the 14 m dive, you give up 24.5% (24/98*100) of your potential NDL because of residual nitrogen. But, for the 16 m dive you give up 29% (21/72*100) of your potential NDL. The RNT 'penalty' is not a fixed number, but a proportional factor.

 

[tbone1004]

Thanks for replying but any chance if you can explain it more layman?

What I said before is about as simple as it gets, and apparently the PADI RDP doesn't hold linearity as you go deeper so there are extra fudge factors put in there. I work with NAUI so their dive tables are a lot different. Here is attempt 2

Your tissues have a theoretical maximum amount of nitrogen that they can hold, this is called saturation. All No Decompression Limit dive tables have you surfacing before the tissues hit this saturation level. This level is still theoretical and all of the variables depend on the algorithm that was chosen. I.e. this is why in general NAUI allows the first dive to be longer than PADI, but PADI says you offgas faster, so your repetitive dives are longer than NAUI, all assuming same dive profile.

Due to the pressure gradient across the tissues, the nitrogen is able to go in faster the deeper you go. This is why you have less time for no decompression diving as depth increases. Dive tables require two input variable, PN2 which is your Equivalent Air Depth calculated as a function of atmospheres absolute at depth, and the FN2 in your gas mix. I.e. with air at 66ft in sea water, your PN2 is going to be 2.37. If you are diving say EAN32, you can either dive to 82ft and have the same PN2, in this case you are diving for the same length of time but able to go deeper for the same nitrogen loading. Alternatively you can dive to the same depth with a PN2 of 2.04 so you can stay at the same depth much longer with the same nitrogen loading.

Residual nitrogen time equates to a percentage of this saturation that your tissues are currently in before you begin a dive. This is a function of how much nitrogen your body took in, based on PN2+time, and then how much your body got rid of, based again on PN2+2, but this time at the surface. The longer you are at the surface, the more you get rid of. This is still a volume of nitrogen that is in your body that shouldn't be, so when you dive again, you can only take in so much more.


I wish I had a visual example, but here is an explanation that I came up with to help students. These numbers are random and not representative of how fast anything moves, it is all relative motion.
Your body is a 5 gallon bucket that can be filled with "nitrogen" which in this case is water. At the surface, the bucket remains empty. When you dive to say 33ft, your bucket is filled at a rate of 1 gallon/hour so you can stay down for 5 hours before your bucket is full and you have to come up. When you are at the surface, your body loses water a rate of 1 gallon per hour. If you stay at the surface for 1 hour, you have 4 gallons left in the bucket, and if you go back to 33ft, you can stay down for 1 hour. Your "residual nitrogen time" here is 4 hours, or 80% of the limit of nitrogen you can take in.

Now, at 99ft salt water, your bucket is filled at a rate of 2 gallons/hour if you start with an empty bucket, you can stay down for 2.5 hours. So you stay down to your limit and surface with a full bucket. Your bucket still only drains at a rate of 1 gallon per hour. After 1 hour at the surface you still have 4 gallons in the bucket, same as the first dive profile. You go back down to 99ft, but instead of 2.5 hours, because you have 4 gallons in your bucket, and it's coming in at a rate of 2 gallons per hour, you can only stay down for 30 minutes. The maximum dive time based on an empty bucket is what the tables are based off of, so you have 2.5 hours=100%, but your bucket is 80% full, so your residual nitrogen time for this dive is 2 hours, leaving you with an adjusted maximum dive time of 30 minutes.

Third example. You want to balance opportunity on a boat and you have two wrecks, one at 99ft, one at 33ft, and they are 1 hour apart. The 5 gallon bucket is empty in the morning. If we dive the 33ft wreck first, we have 5 hours on that wreck, surface and because we have 4 gallons of water, 80% of the 2.5 hours maximum at 99ft is 30 minutes.
Alternatively we dive the 99ft wreck first where we have 2.5 hours, then because we have 4 gallons in the bucket for the second dive, 80% of 5 hours is 1 hour.

hopefully that makes more sense

 

[CrowyCrow]

Thanks, I was wondering if the 14m and 16m were meters. But, as an Imperialist, I wasn't sure. So, let me re-phrase my previous answer, in metric.

An 'E' diver has a RNT of 24 minutes going to 14 m. But, that 'E' diver has a RNT of (only) 21 minutes if they go to 16 m. On the surface, it sounds like the RNT is less if you go deeper. BUT, the NDL for a dive to 14m is 98 min, and the NDL for a dive to 16 m is 72 min. In fact, you are paying a bigger 'penalty' for residual nitrogen on the deeper dive. For the 14 m dive, you give up 24.5% (24/98*100) of your potential NDL because of residual nitrogen. But, for the 16 m dive you give up 29% (21/72*100) of your potential NDL. The RNT 'penalty' is not a fixed number, but a proportional factor.

sorry pardon me.

Still don't really get it.

Let me put it this way:
If I go to the supermarket (First dive)
I bought 20 apples ( Gained 20 minutes of Nitrogen time)
I ate 10 apples (Surface interval and left with 10 mins RNT)
Now I am left with 10 apples ( 10 mins RNT)
If I go supermarket again (2nd dive)
I will have 10 apples to carry forward to my next supermarket trip ( 10 mins of RNT)

Question: Why will there be other numbers of RNT instead of this 10 mins?

 

[CrowyCrow]

What I said before is about as simple as it gets, and apparently the PADI RDP doesn't hold linearity as you go deeper so there are extra fudge factors put in there. I work with NAUI so their dive tables are a lot different. Here is attempt 2

Your tissues have a theoretical maximum amount of nitrogen that they can hold, this is called saturation. All No Decompression Limit dive tables have you surfacing before the tissues hit this saturation level. This level is still theoretical and all of the variables depend on the algorithm that was chosen. I.e. this is why in general NAUI allows the first dive to be longer than PADI, but PADI says you offgas faster, so your repetitive dives are longer than NAUI, all assuming same dive profile.

Due to the pressure gradient across the tissues, the nitrogen is able to go in faster the deeper you go. This is why you have less time for no decompression diving as depth increases. Dive tables require two input variable, PN2 which is your Equivalent Air Depth calculated as a function of atmospheres absolute at depth, and the FN2 in your gas mix. I.e. with air at 66ft in sea water, your PN2 is going to be 2.37. If you are diving say EAN32, you can either dive to 82ft and have the same PN2, in this case you are diving for the same length of time but able to go deeper for the same nitrogen loading. Alternatively you can dive to the same depth with a PN2 of 2.04 so you can stay at the same depth much longer with the same nitrogen loading.

Residual nitrogen time equates to a percentage of this saturation that your tissues are currently in before you begin a dive. This is a function of how much nitrogen your body took in, based on PN2+time, and then how much your body got rid of, based again on PN2+2, but this time at the surface. The longer you are at the surface, the more you get rid of. This is still a volume of nitrogen that is in your body that shouldn't be, so when you dive again, you can only take in so much more.


I wish I had a visual example, but here is an explanation that I came up with to help students. These numbers are random and not representative of how fast anything moves, it is all relative motion.
Your body is a 5 gallon bucket that can be filled with "nitrogen" which in this case is water. At the surface, the bucket remains empty. When you dive to say 33ft, your bucket is filled at a rate of 1 gallon/hour so you can stay down for 5 hours before your bucket is full and you have to come up. When you are at the surface, your body loses water a rate of 1 gallon per hour. If you stay at the surface for 1 hour, you have 4 gallons left in the bucket, and if you go back to 33ft, you can stay down for 1 hour. Your "residual nitrogen time" here is 4 hours, or 80% of the limit of nitrogen you can take in.

Now, at 99ft salt water, your bucket is filled at a rate of 2 gallons/hour if you start with an empty bucket, you can stay down for 2.5 hours. So you stay down to your limit and surface with a full bucket. Your bucket still only drains at a rate of 1 gallon per hour. After 1 hour at the surface you still have 4 gallons in the bucket, same as the first dive profile. You go back down to 99ft, but instead of 2.5 hours, because you have 4 gallons in your bucket, and it's coming in at a rate of 2 gallons per hour, you can only stay down for 30 minutes. The maximum dive time based on an empty bucket is what the tables are based off of, so you have 2.5 hours=100%, but your bucket is 80% full, so your residual nitrogen time for this dive is 2 hours, leaving you with an adjusted maximum dive time of 30 minutes.

Third example. You want to balance opportunity on a boat and you have two wrecks, one at 99ft, one at 33ft, and they are 1 hour apart. The 5 gallon bucket is empty in the morning. If we dive the 33ft wreck first, we have 5 hours on that wreck, surface and because we have 4 gallons of water, 80% of the 2.5 hours maximum at 99ft is 30 minutes.
Alternatively we dive the 99ft wreck first where we have 2.5 hours, then because we have 4 gallons in the bucket for the second dive, 80% of 5 hours is 1 hour.

hopefully that makes more sense

Thank you!!! The bucket example helped a lot!