Shearwater Peregrine -Altitude Diving

[Litefoot]

I’m new to diving and new to dive computers. I have a new Peregrine and most of my local diving will be above 5000’ elevation. I know the computer will save barometric pressure readings every so often and then use these as the new baseline or surface pressure at the start of a dive. And I assume that it will save this reading as you ascend to calculate safety stops and then begin the End of Dive countdown when your pressure drops back down to the saved pressure at the start of the dive. Sound right?

Okay, then here’s the dumb question. How does the computer know when to start the dive at altitude? I know it turns on automatically when it detects an increase in pressure at sea level. Is there a way to manually start the dive?

 

[inquis]

A dive is started when the pressure increases X mbar from its current estimate of surface pressure. (X equates to about 90 mbar or 3 fsw.) No, you can't "force it". In short, everything is automatic.

 

[Robert H. Diver]

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[Tracy]

I’m new to diving and new to dive computers. I have a new Peregrine and most of my local diving will be above 5000’ elevation. I know the computer will save barometric pressure readings every so often and then use these as the new baseline or surface pressure at the start of a dive. And I assume that it will save this reading as you ascend to calculate safety stops and then begin the End of Dive countdown when your pressure drops back down to the saved pressure at the start of the dive. Sound right?

Okay, then here’s the dumb question. How does the computer know when to start the dive at altitude? I know it turns on automatically when it detects an increase in pressure at sea level. Is there a way to manually start the dive?

As they said above. You turn it on and go diving. It knows what altitude you are at. Dive computers made altitude diving as easy as ocean diving.

 

[inquis]

Ah, my mistake. It's not a fixed depth, but a fixed pressure of 1100 mbar of pressure. At 5000 ft altitude, it will turn on about 9 ffw depth.

 

[inquis]

The one thing not captured by your dive computer is that ascents at altitude should be SLOWER than "normal". Surface pressure at 5000 ft is 83.2% that of sea level, so an ascent rate of 30 fpm at sea level is equivalent to 25 fpm at your 5000 ft altitude.

BTW, consensus holds that the final ascent from your safety stop should be pretty slow (e.g., 10 fpm/sea-level or 8 fpm/5000 ft). In other words, pace your final ascent to take about 2 minutes instead of the 30 seconds you may have been taught in OW.

 

[Litefoot]

Thank you for those answers. I re-read page 6 (thanks Robert) and then read the note on page 41 and I think I’ve got it sorted out. The dive starts at 1100 mbar pressure, but the surface pressure (and hence your depth) is derived from the minimum of the recently saved barometric readings. Right?

And thanks inquisit for the heads up on the slower ascent rates.

 

[inquis]

Yes on surface pressure. Depth error would be a bit less than 6 ft if diving at 5000 ft and the computer thinks it's at sea level. Not terribly critical, really.

More importantly, surface pressure plays a fundamental role in the NDL computations (and deco if staying longer). At 5k ft, you lose about 4 mins (9% of the total time) from the sea-level 60 ft NDL time (at what I believe is the default conservatism of 40/85).