Computers for beginners

[Joneill]

I just dove (OC 47 dives) my Shearwater PERDIX AI and it's an awesome, fun dive computer but the battery is an issue. My Saft LS 14500 3.6 V lithium-thionyl chloride battery was brand-new, it's reading one bar at 1.5V after 16 days of diving. I dove the NEW Cochran Lifeguard side by side and I started with 3.1 volts on my Wrist Unit, PBAT & SBAT of my CPU and end with 3.0 on all units after 16 days of diving.
On a personal note I liked a lot of the features on both dive computers but I believe the Cochran algorithm is better. Also being able to upload my dives instantaneously to my iPhone was pretty cool.

Hmmm... That's odd as I did 24 dives in 13 days so far this year (March and August dive trips) most at 60-75 minute duration. I also had accidentally left the Perdix AI on high screen brightness (versus auto) on 1/2 of them and I bluetoothed every dive to my iPhone after I got back on the boat/back to shore. My Perdix AI 3.6 V Saft is still reading 3.44 V. My non-AI Perdix backup worn on all dives (also left on high brightness) is still showing 3.49 V on it's 3.6V Saft batery. So, to me, the battery life seems good. Changing a battery is easy too - pinching 2 O-rings is major user error (even pinching 1 would be), and not even sure how one could do that unless they were very careless.

WRT algorithms, I dove Nitrox on about 1/2 of the dives and was only NDL limited on 1 dive where we had spent significant time below 100 feet on the earlier dive - so the algorithm worked great and I never found it limiting (buddies Zoops were beeping). Why do you feel the Cochran algorthm is better?

 

[GlennL]

I don't remember seeing it in this thread, but I seem to recall seeing it in other threads where it was stated that the Cochran computer you or I can buy does not have the exact same algorithm that the Navy uses.

So, is it more like two computers that have different Gradient Factors settings? Or is it more like two computers where one runs Buhlmann ZHL-16C w/Gradient Factors and the other runs a proprietary algorithm "based on" the Buhlmann algorithm?

Interesting.....

Also on the topic of computers, and I feel it fits the "basic" category, what is the big difference in fsw vs ffw? One of the claims of the Cochran computers is that it detects the difference. How much difference is there really? Please explain.

 

[stuartv]

Interesting.....

Also on the topic of computers, and I feel it fits the "basic" category, what is the big difference in fsw vs ffw? One of the claims of the Cochran computers is that it detects the difference. How much difference is there really? Please explain.

33 feet of seawater results in 1 atm of pressure. 34 feet of fresh water does the same thing. Both of those and especially the seawater number are approximate, as seawater density varies based mostly on salinity. Salinity is pretty different in different parts of the oceans. Fresh water is generally not 100% pure, so its density can also vary a tiny bit.

The only difference it makes to your dive computer is in what depth it shows you. Deco calculations are based on ambient pressure, not depth. Depth is simply calculated, based on ambient pressure (and salinity setting). So, if you're 33 feet deep in seawater, but your computer is set to freshwater, it might say you are 34 feet deep. But, the on-gassing and off-gassing calculations will be the same regardless (i.e. however many minutes at 2 ATA).

 

[dmaziuk]

According to shearwater:

• Fresh Water = 1000kg/m³
• EN13319 = 1020 kg/m³
• Salt Water = 1025 to 1035 kg/m³

(which isn't 1000% true as for fresh water the number is for really fresh water at 4 degrees centigrade; in many places the real number will be in 1005-1015 range)

As @stuartv said it's irrelevant for the algorithm, the model works from pressure and so does the computer. So that "adapt to salt water" thing sounds like a marketing feature rather than an actual one. On similar note, I'm not sure what the latest wisdom on the temperature is except I think exertion factors heavily into it -- and it don't sound like cochran attempts to track that. So, meh.

 

[GlennL]

Those are my thoughts. I never change my computer from fresh to salt or vice versa. Shame on me.

 

[tridacna]

Salinity is pretty different in different parts of the oceans

Actually it's quite consistent. As scientists, we describe salinity in seawater as a ratio of dissolved salts to liters of water. It's typically about 35 grams/liter. Normal range varies between 33-37 (expressed as 33‰ - 37‰).

A dive op in Bermuda used to BS their tourist divers that they had EXTREMELY salty water and that to compensate, they should take an additional 2-3 lbs to 'compensate'. They were just sick of floaty divers.

 

[Jeff Loudan]

According to shearwater:

• Fresh Water = 1000kg/m³
• EN13319 = 1020 kg/m³
• Salt Water = 1025 to 1035 kg/m³

(which isn't 1000% true as for fresh water the number is for really fresh water at 4 degrees centigrade; in many places the real number will be in 1005-1015 range)

As @stuartv said it's irrelevant for the algorithm, the model works from pressure and so does the computer. So that "adapt to salt water" thing sounds like a marketing feature rather than an actual one. On similar note, I'm not sure what the latest wisdom on the temperature is except I think exertion factors heavily into it -- and it don't sound like cochran attempts to track that. So, meh.

All Cochran dive computers that are air integrated track the workload (breathing rate) as well as the temperature. The user has the option of enabling these environmental factors to affect the decompression calculations.

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Cochran Undersea Technology & Cochran Military designs, manufactures, and markets state-of-the-art diving computers for recreational, technical, CCR and commercial divers from our facility located in Richardson, Texas. Cochran has been designing dive computers for itself and others for over twenty five years. Cochran is the exclusive provider to the U.S. Navy and its dive computers are compliant with the "Authorization for Navy Use (ANU) Program" . Its dive computers are also being used by other international Navies and have NATO part numbers.


Come visit Cochran Undersea Technology & Cochran Military at Booth 2363 at the DEMA SHOW, Orange County Convention Center
Orlando, Florida Nov 1-4, 2017.

 

[Diver0001]

Hi all so im about to start my DM internship and need to purchase my first dive computer been looking at the mares smart and quad can't quite decide. If anyone has experience with these or have any recommendations in the same sort of price range £300 max would be appreciated

Yeah.... I'm going to have to break with the herd here. For a DM who assists in training divers the computer isn't going to make a damned bit of difference. Literally any computer you buy will be good enough. In fact, I still sometimes give my computer to a student and dive using the tables during training, especially at the OW level.

Features like how easy they are to read and/or navigate the menus are more important to no-stop diving than the algorithm. In that sense it's hard to find a computer these days that's easier to navigate than a Mares Puck and frankly if I were buying a computer today to "work" with then I would probably buy a puck.

As for the comment that someone made above that he would be miffed if he had to end a dive "early" because the DM had to surface.... this point is moot if the plan is right. The DM plans the navigation of the dive and may choose (or need) to stay shallower than some of the divers in the group if he/she has been diving a lot lately. The only reason a client may feel they need to follow the DM's computer is if they don't have one of their own, which is pretty much taboo these days, although I know it still happens.

The point here is that even with the most liberal computer on the planet a dive guide will sometimes be in a position where their NDL is limited by previous activities. It's a fact of life and any working DM will plan around that regardless of how liberal or conservative their computer is.

For technical diving, we have another kettle of fish entirely and then things like algorithms and certain functionality is important. For guiding dives or assisting with training, however, it's really quite irrelevant, imo.

R..

 

[GlennL]

Actually it's quite consistent. As scientists, we describe salinity in seawater as a ratio of dissolved salts to liters of water. It's typically about 35 grams/liter. Normal range varies between 33-37 (expressed as 33‰ - 37‰).

A dive op in Bermuda used to BS their tourist divers that they had EXTREMELY salty water and that to compensate, they should take an additional 2-3 lbs to 'compensate'. They were just sick of floaty divers.

I ain't no scientist, but In my reef tank where I grow coral I measure salinity in ppt(for my probes/calibration ) and specific gravity (for my refractometers).

 

[runsongas]

my instructor dives cochran computers but i decided not to get one

the way i saw it was that it wasn't a bad computer, but a used shearwater petrel made more sense at roughly the same pricepoint

if the cochran didn't have automatic gas switches, a monochrome oled screen, and a configurable display, i probably would have given it more of a chance