Vytec DS: Does it display nitrogen level between dives?

Hello,

Is there a way to display residual nitrogen level between dives with a Vytec DS?
I noticed other dive computer have this feature, but I'm unable to figure this out from a Vytec DS.

It seems the Consumed Bottom Time (CBT) graph only actives while in water and below certain depth.

Since the CBT is simply NDL time shown on a graph rather than numerically, it cannot show anything useful when on the surface; or even shallower than 30 or 40' in most cases. It is NOT a display of N2 loading.


You can verify this by looking at the CBT on your next ascent. When you get shallow (at a 15' safety stop for example) the CBT will immediately go way down low, well into the green -- not because you are no longer heavily loaded, but because the NDL at 15' is very long, even when you have a high N2 loading.

Thinking that the CBT is a reflection of N2 loading is a common misconception. A true N2 loading bargraph as seen on many other computers will only slowly go down as offgassing takes place during the ascent and stops. The Suunto CBT immediately drops as one goes shallow and NDL increases, perhaps misleading divers into thinking that they are no longer heavily loaded with N2.

Charlie Allen

Charlie99:

Thinking that the CBT is a reflection of N2 loading is a common misconception.
Charlie Allen

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That is exactly what I thought!
So what's the best way for a novice to calucate residual N2 loading between dives?

What do you mean by "residual N2 loading"? Why do you wan't to calculate it?

On the PADI RDP residual N2 loading means the N2 loading in the 60 minute halftime compartment. No other compartments are tracked for repetitive dive calculations. For the USN table and tables derived from it (SSI, NAUI, YMCA, etc.) the only compartment tracked for repetitive dive calculations is the 120 minute compartment. (Using the 60 minute compartment is more appropriate for NDL open circuit scuba and results in repetitive dive calculations that are closer to that of the underlying model and dive computers than does using the 120 minute compartment, but that's a different discussion. )

A dive computer tracks all compartments and therefore will come up with different results.

If your desire is to have a fallback calculation to use in the event of a dive computer failure the simplest method is, if you stayed within NDL limits on the previous dive, to assume that you surfaced from the previous dive at pressure group Z on the PADI table.

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Theoretically, you could look at the NDLs at various depths and from that infer what the computer calculates as the N2 loading in each halftime compartment, but that is more complicated math than I intend to do during a SI.

Charlie99:

What do you mean by "residual N2 loading"? Why do you wan't to calculate it?

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I’m curious at the carry over nitrogen level if I stay within my NDL but do not take the full 60 minute interval.

scuba_moron:

I’m curious at the carry over nitrogen level if I stay within my NDL but do not take the full 60 minute interval.

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Your Vytec will be calculating the residual N2 levels for all compartments, even though it doesn't show it.

If you are trying to track N2 loading for conversion over to tables in the case of a dive computer failure, if you have stayed within NDL, you can simply assume that you surfaced from the previous dive in Pressure Group Z and calculate your dives per the PADI RDP from that point.

With some computers, you could go through and look at NDLs at each depth and from that infer what the compartment loadings are. That won't work with Suunto computers, since they change the compartment limits (M-values) in ways that aren't publically disclosed. These reductions in M-values for things like reverse profiles, short SI's, rapid ascents,etc. are good in that they add conservatism to the computer in response to bad profiles, but what exactly the computer is doing is now a mystery.