Can anyone suggest a computer for returning to altitude after a dive

[kirstyjay]

Your case is peculiar, as you live at an altitude. So your amount of nitrogen which is stable in your body is smaller than the amount which people living at sea level have.
In practice you start your dive with less nitrogen than your buddies, and hence at the end of the dive you will have slightly less nitrogen in your body than what your computer evaluates.
This is particularly true for the slowest tissues. This provides you with an additional safety factor, which I suppose the computer is not evaluating. So, if the computer says that you can climb up to 700m, do it without fear, you are on on the safe side.

How interesting. I didn't know that. All these posts are a great read. It's like being back at school

 

[dmaziuk]

The more I think about this (not for the OP, I think her situation is pretty safe), the more I think watching GF99 in real time as you drive up to altitude might be the way to go.

Off the top of my head I'd say GF99 and the rest of it is based on being in uncompressible liquid where ambient pressure changes linearly with depth. I.e. in air GF99's going to be off.

 

[RonR]

Off the top of my head I'd say GF99 and the rest of it is based on being in uncompressible liquid where ambient pressure changes linearly with depth. I.e. in air GF99's going to be off.

If the ambient pressure is being measured, then the rate at which is changes in a particular medium is irrelevant and there would be no problems. Pressure is just a direct input to the calculation.

... Perhaps that is a bit confusing but one atmosphere in the air is perhaps 60 thousand feet so if you were fully off gassed at sea level, you would have to go up to 30 thousand feet to again hit that 2:1 pressure difference to have a problem.

Gasses are compressible, so the change in pressure with altitude is not linear. You reach 50% of sea level pressure somewhere around 17,000 ft/ 5,100m if I recall correctly. Your basic point that the OP has nothing much to be concerned about going to 700M is valid, though.

 

[Michael Jastrzebski]

If the ambient pressure is being measured, then the rate at which is changes in a particular medium is irrelevant and there would be no problems. Pressure is just a direct input to the calculation.

Exactly, I was going to write the same comment but got lazy ..

 

[dmaziuk]

If the ambient pressure is being measured, then the rate at which is changes in a particular medium is irrelevant and there would be no problems. Pressure is just a direct input to the calculation.

Maybe I'm missing something here. Like the point of watching GF99 as you drive up the mountain.

1. Surfacing M-value at sea level is calculated with the assumption you're getting (in and) out of the water at sea level. Surfacing M-value at 600 m is calculated based on the assumption you're getting out of the water in a mountain lake in Switzerland at 600 m altitude. M-value line is a straight line because the pressure changes linearly.

In the air M-value line is no longer straight, but your dive computer has no idea. If you're getting out of the water at sea level and driving up to 600 m, the surfacing M-value at 600 m calculated by the computer is wrong for you.

2. Similarly, computer calculates off-gassing between sea level and 600 m up based on the assumption that ambient pressure drop is linear. If you're driving up in air, it isn't, but the computer does not know this. Your computed gas loading is going to be off.

3. GF99, as I understand it, is the ratio of calculated gas loading, as per #2, to the surfacing M-value as per #1.

To me this sounds like two wrongs. They may largely cancel each other out and make an almost-right, but personally I'd take Scubapro's little yellow mountains over GF99 any day.

 

[rsingler]

If the computer calculates based upon measured mB pressure, then changes won't be linear on ascent, will they?

 

[dmaziuk]

If the computer calculates based upon measured mB pressure, then changes won't be linear on ascent, will they?

Schreiner equation assumes that inspired gas pressure changes at a rate corresponding to the rate of ascent or descent. (Erik Baker, Derivation of Gas Loading Equations, Derivation of the Schreiner Equation). That is not the case in air.

I was wrong about M-value, though: I was hung up on Workman for a second there. Buhlmann's M-values are in absolute pressure, so M-value at 600 m should be correct. It's only the tissue loading calculated using Schreiner formula will be off.

Now, as everyone else says, at 600 m up it probably doesn't matter unless you do several dives right up to NDL and jump in the car and drive up straight from your last safety stop. Still, if I were to trust a computer with this, I'd take G2 over GF99.

 

[rsingler]

I knew I could rely on you for the math. Hope Shearwater chimes in.

As for relying on Uwatec instead, I have no idea how it clears each mountain segment. Is it, "if your compartments are this full, wait this long", or is it something specific based on your actual dive and projected offgassing? It's nice to see the little mountain, but it's a proprietary black box.

 

[dmaziuk]

As for relying on Uwatec instead, I have no idea how it clears each mountain segment. Is it, "if your compartments are this full, wait this long", or is it something specific based on your actual dive and projected offgassing? It's nice to see the little mountain, but it's a proprietary black box.

Tables from #2 (or some agency equivalent) would be the way to do it: it knows your current "pressure group", all you need is add a table of altitudes and wait times and a little look-up function.

But we'll never know what's really inside the box.

 

[rsingler]

But tell me your thoughts on this: IF the computer actually recalculated the predicted tissue overpressure in real time based upon measured barometric pressure as you ascended, then would watching leading compartment overpressure (GF99) be a good way to assess your safety?