fast ascent ?or wrong computer ?

[Wolfy]

hmmm have a quandry :)

on a dive at the weekend a diver had a slight panic and made a slightly quick ascent, they were wearing 2 computers ( one wrist mounted new flash thing, and one trusted console mounted aladin pro)

the pro showed SLOW at the surface as did the new flash thing..

now the good old aladins profile showed a lift from 14M to 6M that took slightly over 1min...

now in my book thats a rate of 8M/min and as such safe as its below the 15m/min BSAC taught rate....

the new flash thing though showed an ascent rate at 18M/min ???
what am i missing ?

regards :)

[Dave]

hmmm have a quandry

on a dive at the weekend a diver had a slight panic and made a slightly quick ascent, they were wearing 2 computers ( one wrist mounted new flash thing, and one trusted console mounted aladin pro)

the pro showed SLOW at the surface as did the new flash thing..

now the good old aladins profile showed a lift from 14M to 6M that took slightly over 1min...

now in my book thats a rate of 8M/min and as such safe as its below the 15m/min BSAC taught rate....

the new flash thing though showed an ascent rate at 18M/min ???
what am i missing ?

Well, one thing you are missing is that emoticons don't work in this forum for some reason.

One reason I can think of for the wrist computer showing fast ascent rates at points in the ascent could well be the diver moving their arm about inflating BCs et al whilst the console one is maintaining the same relative position to the body.

Dave

[PeteM]

:=hmmm have a quandry
:=
:=on a dive at the weekend a diver had a slight panic and made a slightly quick ascent, they were wearing 2 computers ( one wrist mounted new flash thing, and one trusted console mounted aladin pro)
:=
:=the pro showed SLOW at the surface as did the new flash thing..
:=
:=now the good old aladins profile showed a lift from 14M to 6M that took slightly over 1min...
:=
:=now in my book thats a rate of 8M/min and as such safe as its below the 15m/min BSAC taught rate....
:=
:=the new flash thing though showed an ascent rate at 18M/min ???
:=what am i missing ?


One reason I can think of for the wrist computer showing fast ascent rates at points in the ascent could well be the diver moving their arm about inflating BCs et al whilst the console one is maintaining the same relative position to the body.

Another issue could be sampling rate: I have a Suunto Vyper which has a sampling rate of 20 seconds, if we where talking about this and another computer that sampled at 1 minute then if we have a brief fast ascent say 6 meters in 20 seconds but get it back under control and in the next 40 seconds only went up another 2 meters you would get the figures you quote

HTH

Pete

[Nigel Hewitt]

Another issue could be sampling rate: I have a Suunto Vyper which has a sampling rate of 20 seconds, if we where talking about this and another computer that sampled at 1 minute then if we have a brief fast ascent say 6 meters in 20 seconds but get it back under control and in the next 40 seconds only went up another 2 meters you would get the figures you quote

Agreed. Freediving with a VR3 it totally missed the bottom five meters of a touch-and-go on the dump and showed me hanging about at 20 meters for a several seconds. It just so happened that I went through 20 on the way down when it sampled and again on the way up.

Scuba computers tend to assume nothing changes too fast and if it does they just beep and whinge. Since their job is decompression any spikes don't effect the algorithm so they save the battery and sleep between samples. In this case both computers are probably calling what they saw and if you want a better record get something like the Reefnet datalogger, set it to 1 second samples and ask him to do it again.

nigelH

[Matt]

Another issue could be sampling rate: I have a Suunto Vyper which has a sampling rate of 20 seconds, if we where talking about this and another computer that sampled at 1 minute then if we have a brief fast ascent say 6 meters in 20 seconds but get it back under control and in the next 40 seconds only went up another 2 meters you would get the figures you quote

Quite often the fast ascent caution will be displayed immediately or shortly after the ascent rate is exceeded. If the condition remains for a certain number of sample periods, the computer judges the event to be significant and upgrades the caution to an alarm, which is recorded in the log book.

You would need to read the manual to find out how a particular model does things.

I have yet to come across any computer that discriminates between say a 50m 50min profile and a 5m 5min profile. If the ascent alarm condition is met the alarm is recorded. The former would of course be a damn site more serious.

Regards
MattS

[Philip Smith]

the pro showed SLOW at the surface as did the new flash thing..

now the good old aladins profile showed a lift from 14M to 6M that took slightly over 1min...

If you have downloaded the profile, it may indicate in the bars at the bottom of the profile display when it gave ascent alarms during the dive. However, the profile itself is recorded in 20 second increments, but the the computer was sampling much more frequently during the dive. Also, (depending on the model) the maximum ascent rate is quite slow in the shallowest part of the dive and it is easy to trip the alarm in the last few metres. If I remember correctly, an alarm is logged if the ascent rate exceeds 200% of the maximum at any time and if it is over 100% for a certain time.

Philip Smith

[nick kay]

Whilst the BSAC-88 tables may state (Max):
a) 15m/min up to 6m, and then
b) 6m/min to the surface

Most decompression theory/practise nowadays seems to be based on (Max)
a) 10m/min up to 6m and then
b) 6m/min to the surface

Further, if you do the BSAC ERD course, you're likely to be told that the last 3m should take 2mins - that 1.5m/min

Also, most (new / new algorithm) dive computers seem to use 10 (or 12)m/min as the maximum ascent rate
The Suunto Cobra (and Vyper) uses:
a) 10-12m/min as "fastest" - anything faster than that gets a "slow"
b) 10m/min continuous as "fastest" - anything faster than that gets a "slow"
That effectively means that if you're doing a constant speed ascent, your max ascent speed is less than 10m/min...

[Khaled Alwassia]

Wolfy,
The problem is most likely based on the following.
The Aladin Pro, even the older once, have a function in their algorithen (probably misspelled) which changes the allowed acent speed. That counts especially for the depth range you mentioned. The accesd speed warning changes depend on things like water temp., dive profile - possible hacksaw or secondary decents, earlier ascent speeds and soem other things I fergot.
Most newer flashy computer have similar functions.

Hope that helps

Khaled