Dive computer basics

[torrey]

Plug it into the simulator mode on your computer and find out...

Advanced Dive Planning Simulator
While AERIS manuals and cue cards are simple to understand, some people learn better through hands-on experience. The on-unit Simulator Mode allows you to run the XR-2 through actual dive profiles including Air Dive, Nitrox, Gauge Mode, and Decompression. You will see the dive happen on the screen as you would in the water.

While this is not a substitute for reading and understanding your Owners Guide, it provides detailed dive planning, is an excellent auxiliary learning tool, and it is a great way to refresh your memory on the way to the dive site, particularly if you have not been diving for a while. A side benefit is that you can always show your dive buddy what a cool new dive computer you have!

 

[SailNaked]

Thanks all for replying and If I understand the answers it seems to me that I will be bringing some tables with me on my dive and use the computer as a back up. I have no intentions of doing deco dives until I have my advanced training, I was really more curious about the safety factor that a computer might provide, and it seems that the computer is still a toy. and that decomp must still be planned on the boat. I cant think of a time that I would go past its limits, and since I dont dive with anything over my head no reason I would stay down longer than a NDL.

as a secondary question I noticed that the NDL limit on the computer at 60 ft is 7 min shorter than the tables. Why? law suits? is all the "computing" that conservative?

 

[Anti-Hero]

Could be that you have a conservancy factor set on the computer. Read the manual cover to cover. Then go back over any parts that are not completely clear.

I have an Oceanic Atom 2.0, during my AOW class on the deep dive, I got a high P02 alarm at 105'. This freaked me out, even though I knew my MOD for the gas I was using was 111 ft @ 1.4 PP02. I began a controlled ascent to the surface to figure out why I was getting the alarm. I later reviewed my manual (which I had read completely) and found a single sentence I had missed!!!

The sentence said: "A warning will be displayed when the PO2 reached .2 from your max P02 in the settings. "

Moral of the story: A single sentence can ruin a perfectly good dive!

 

[ianr33]

as a secondary question I noticed that the NDL limit on the computer at 60 ft is 7 min shorter than the tables. Why? law suits? is all the "computing" that conservative?

Tables have an inherent conservatism built into them by using the deepest depth to calculate NDL. Plan a 60 foot dive but drop down to 61 feet for 10 seconds , if you are doing it the "official" way then you have to use the NDL for 70 feet.

In practice a computer will give you much longer NDL's than tables because most dives are (to a greater or lesser extent) multi level.

 

[Charlie99]

what I am interested in is if a dive computer can give you useful information after it goes past its limits, my manual says it will warn me for 10 sec then go to guage mode, and after 5 minutes on the surface will effectively lock up if I go above the deco schedule. So I was curious if I am not paying attention (miss the 10 sec beeping) at say 140 feet and I go over whatever it is that im supposed to be at by a minute and then go above the stop for a few seconds will the computer ever give up on my stupidity and just flash "shouldnt have done that" at me? or will it allow me to get back under the deco stop and calculate how long I should stay there for my own good and happiness.

You should reread that section of your computer manual carefully.

Your dive computer will "give up on your stupidity" only after your profile is so wacko that it no longer has any chance of giving you reasonable guidance. It's not going to go into gauge mode at 140'. It will go into gauge mode only if you have violated the decompression stops (typically 10', 20', 30' sort of range) by ascending above the "ceiling depth", and staying above it for several minutes. Your computer will NOT give you 10 seconds warning that you are going into decompression and then stop giving you info. It will NOT just give you just 10 seconds above the ceiling before stopping to calculate decompression for you.

The ceiling depth when first entering decompression on your Aeris computer will be 10'. That is true whether you are at 80' or 140' or 200'. Only when you go shallower than your ceiling depth is your computer going to complain. Even if you violate that ceiling by ascending above it, or even surfacing, the computer will still try to guess what is needed for decompression stops. A common method of handling that violation is to add in 1.5 or 2 minutes of additional decompression stop for every minute that you have been above the ceiling depth. After 5 or 10 minutes of violating the decompression model by being shallower than the ceiling depth (such as by popping up to the surface), the computer is no longer able to make any sort of reasonable adjustment for that violation --- you may have already developed enough bubbling in your tissues that normal offgassing no longer applies. THAT's the point at which your computer will go into gauge mode and stop giving decompression info.

 

[Charlie99]

Could be that you have a conservancy factor set on the computer.

Since the XR2 doesn't have user setting for conservatism, it is more likely that he is at an altitude of a few thousand feet, and the automatic altitude correction has reduced the the NDL.

SailNaked ---- what altitude are you at when you get the reduced NDL???

 

[nereas]

Thanks all for replying and If I understand the answers it seems to me that I will be bringing some tables with me on my dive and use the computer as a back up. ...

Normally people do the opposite. They use their dive computers as their primary tool and have tables for pre-dive planning and use the tables as a backup, in case the computer fails during a dive or breaks or the battery dies.

...

as a secondary question I noticed that the NDL limit on the computer at 60 ft is 7 min shorter than the tables. Why? law suits? is all the "computing" that conservative?

All computers will be a little bit different from each other and from various tables. There are also different tables: USN tables, NAUI tables, PADI tables, Canadian tables, etc.

It is a good idea to memorize a set of basic NDL times, so that when you go underwater, you have a general idea of how long you can stay. It depends on your breathing gas, whether air or nitrox, etc.

Here is a useful guide for air (at sea level):

50 ft / 65 mins
60 ft / 55 mins
70 ft / 45 mins
80 ft / 35 mins
90 ft / 25 mins
100 ft / 15 mins

Note that when you add these pairs of numbers together you get 115. Easy to remember this way.

Here is a useful guide for Nitrox I (EAN 32) at sea level:

50 ft / 70 mins
60 ft / 60 mins
70 ft / 50 mins
80 ft / 40 mins
90 ft / 30 mins
100 ft / 20 mins

Note that when you add these pairs of numbers together you get 120. Easy to remember that way.

The NAUI basic open water manual explains how this procedure works. If you are wearing a back-up diving watch with your dive computer, then you can use these values if the computer fails. Make sure you keep to a good safe 1 hour surface interval between dives. Remember also that this method is primarily as a backup, and that a dive computer or else tables are better.

I do not recommend that you dive deeper than 100 ft. You can get into trouble in the range of anything deeper, because NDL times are very short, narcosis from N2 is very strong, and it is a long way back up to the surface.

Most sport diving is normally done within the range of 50 ft to 75 ft anyway. Within this depth range, your allowable NDL time is about equivalent to the air/nitrox in your tank as well.

 

[Zoomer]

First off the computer is going to warn you when you get past a certian depth (diver configurable setting)... but as suggested read the manual, put the computer in simulation mode and try it...

 

[perdidochas]

Thanks all for replying and If I understand the answers it seems to me that I will be bringing some tables with me on my dive and use the computer as a back up. I have no intentions of doing deco dives until I have my advanced training, I was really more curious about the safety factor that a computer might provide, and it seems that the computer is still a toy. and that decomp must still be planned on the boat. I cant think of a time that I would go past its limits, and since I dont dive with anything over my head no reason I would stay down longer than a NDL.

as a secondary question I noticed that the NDL limit on the computer at 60 ft is 7 min shorter than the tables. Why? law suits? is all the "computing" that conservative?

Different models for safety. The computer can calculate for multiple level diving. The tables assume a square dive profile.

 

[SailNaked]

Altitude? well my GPS says about 100 ft most days but that may have been on the 4th floor of my office. Houston is almost at sea level.

I would think that if the computer is calculating for multi-level then it would allow at least the table time however i can also understand it useing a more conservative number to allow for moderate fudge factor in the multilevel calculation. however if i just zoom to 60 feet hang still for the NDL time then the computer will have me surface 7 min earlier, not a big deal at all and I am not complaing about 7 min, just thinking about if the computer is say 12% more conservative than the tables what does it mean that I am say 10 sec over the NDL and now have to do a decomp stop when the tables say I have 7 more minutes and of course the tables have built in safety for nubkins like me any way.

all that said, I do not think there is any 140 ft water in BVI (im sure someone will correct me and say "uh huh you can at xxx") However after reading over 100 posts on the subject of computers and diving in General I plan to use the tables and use my dive computer as an easy way to time the dive since it starts when I hit the water, I dont always remember to start my watch so that is now covered. had guages and tables in 1982 when I started diving and they still work now.