Why are two different DSAT-based computers providing different NDL times?

Please register or login

Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

Benefits of registering include

  • Ability to post and comment on topics and discussions.
  • A Free photo gallery to share your dive photos with the world.
  • You can make this box go away

Joining is quick and easy. Log in or Register now!

Ucarkus

Contributor
Messages
572
Reaction score
403
Location
Berlin, Germany
# of dives
1000 - 2499
I think it is only expected that they will be different. An algorithm and implementation of it are 2 different things. Implementer will have to fill in all the gaps that are not provided by the tables and will optimize around different areas. E.g. older dc are optimized around fewer computations due energy inefficient chips of the time.
 

tursiops

Marine Scientist and Master Instructor
ScubaBoard Supporter
Scuba Instructor
Messages
15,825
Reaction score
15,263
Location
U.S. East Coast
# of dives
2500 - 4999
very good idea!

but alas, the NDL plan times are *exactly* the same (compared the Oceanic to, both, the Atmos2 & React Pro). the annoying mystery continues.
Now you've proved the algorithms are the same and the computers are set the same. The only oither inputs are depth and tme. What do the dive graphs look like? It is not a mystery until you've exhausted the investigation! One other point: odds are the problem is the depth sensor on the Aeris. It is the old chronometer problem: three clocks dusagree, two are the same but one is diffferent. Occam's Razor says the one that is diffferent is wrong, not the two that agree.
 

scrane

Contributor
Messages
1,808
Reaction score
1,572
Location
Boise, ID.
# of dives
500 - 999
Might not be your problem, buy it might help someone else:
I had an Oceanic that was giving me low NDL times. In dive planner the NDLs from the computer matched the NDLs printed at the back of the computer manual for limits at a higher elevation. So, it was the atmospheric pressure sensor that was providing the incorrect value.
 

scubadada

Diver
Staff member
ScubaBoard Supporter
Messages
17,398
Reaction score
15,127
Location
Philadelphia and Boynton Beach
# of dives
1000 - 2499
@divedrewsf is correct, the Aeris Atmos 2, 180NX and 100NX all run DSAT. There are no alterations in DSAT when implemented on any brand computer built by Pelagic Pressure Systems. They should run exactly the same. The 180NX and the 100NX do not have a conservative factor setting.

I have been diving Oceanic computers since 2002. I have had Pro Plus 2s in which the pressure sensor eventually went out and the altitude setting changed to 2000-3000 ft from sea level (0-2000 ft). This reduced the NDLs, for example, 60 ft on air, from 57 min to 46 min. In general, this will show up when you run the NDLs on the planner, because the defect is fixed. On at least one of my old computers, the defect was initially intermittent, before it became fixed. It is very hard to believe both the 180 and 100NX computers have an intermittent defect that allowed normal NDLs to be displayed in the planner.

So, unfortunately, I have no additional bright ideas. I will still bet on a defective pressure sensor. I assume all the computers were indicating the same depth during the dive, that difference would have been very obvious. The 180NX can be downloaded, the 100NX cannot. Viewing the profile compared to the Aeris might be revealing, the PPS download also contains a table of dive values based on the sampling rate.

I hope the reason is eventually revealed. Best of luck
 

Ucarkus

Contributor
Messages
572
Reaction score
403
Location
Berlin, Germany
# of dives
1000 - 2499
Might not be your problem, buy it might help someone else:
I had an Oceanic that was giving me low NDL times. In dive planner the NDLs from the computer matched the NDLs printed at the back of the computer manual for limits at a higher elevation. So, it was the atmospheric pressure sensor that was providing the incorrect value.
Low battery will give lower readings as well. I had replaced my dc battery with non fresh one accidentally and readings were -4 meters of actual.
 

scubadada

Diver
Staff member
ScubaBoard Supporter
Messages
17,398
Reaction score
15,127
Location
Philadelphia and Boynton Beach
# of dives
1000 - 2499
Low battery will give lower readings as well. I had replaced my dc battery with non fresh one accidentally and readings were -4 meters of actual.
Interesting, I do not change any of my Oceanic computer batteries until the low battery icon is displayed. The NDLs have never been off. My family is currently diving 2 Pro Plus 2s, a VT4 and a Geo 2, and a VT3
 

Ucarkus

Contributor
Messages
572
Reaction score
403
Location
Berlin, Germany
# of dives
1000 - 2499
Interesting, I do not change any of my Oceanic computer batteries until the low battery icon is displayed. The NDLs have never been off. My family is currently diving 2 Pro Plus 2s, a VT4 and a Geo 2, and a VT3
In my case, it was very cold water diving, at home battery level tested ok on start up. I normally expect a dc to fail battery check on start up and never switch into the dive mode. Your low battery level is probably still within normal “low” spectrum.
 
OP
divedrewsf

divedrewsf

Registered
Messages
38
Reaction score
24
Location
Makati, Metro Manila
# of dives
5000 - ∞
i don't see how a 'low' battery would affect the readings of any computer! in my 26yrs of using dcs, they operate pretty binary...either on or off based on the battery. so i'd be careful about making it a blanket statement vs *that particular* dc was acting strange. if it were me - and i had bought it new and noticed that abnormal behavior - i'd immediately return it for a replacement. that's a pretty strong indication of a defect (most likely with the sensor...which is, ime, what fails most often with dcs).

i mean, if dcs were that glitchy based on battery level, we'd hear about it far far more often. a 4mtrs diff is a significant error and would surely be causing DCS hits. get a new computer man.
 
OP
divedrewsf

divedrewsf

Registered
Messages
38
Reaction score
24
Location
Makati, Metro Manila
# of dives
5000 - ∞
Now you've proved the algorithms are the same and the computers are set the same. The only oither inputs are depth and tme. What do the dive graphs look like? It is not a mystery until you've exhausted the investigation! One other point: odds are the problem is the depth sensor on the Aeris. It is the old chronometer problem: three clocks dusagree, two are the same but one is diffferent. Occam's Razor says the one that is diffferent is wrong, not the two that agree.
well this very well may be the best suggestion itt.

i have another Atmos2 (in addition to 2 other Pelagic DSAT computers...yes, i have way too many dcs..but for good reason). so, i will continue the 'experiment' using these other 3 non-Oceanic Pelagic DSAT dcs and see what happens.

thx
 
https://www.shearwater.com/products/peregrine/

Top Bottom