Computers for beginners

[tridacna]

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).

I too have a reef tank (600 gal) - been doing it for 20+ years. Wanna swap frags?

You're right about the differing units of measurement but scientists rarely use any of those. I suppose that you could also describe velocity in furlongs per fortnight but m/s is probably the unit which is more likely going to be used in a paper.

 

[MargaritaMike]

I have just read about 6 pages of CR@P. It started when an employee of a DC manufacturer made a comment saying he thought his brand was a good choice for a first computer. Whether he was advertising or not, it was pointed out that in the sense of transparency he should disclose his employment status. Then it started. Post after post which had almost NOTHING to do with the original question posed by Rob'o. I started reading this quote in hopes I might get some insight to DCs because my computer (of quite some time) died recently. I was actually caught on a couple of dives without one and used the rec diving tables (they still work). I did buy a computer after the trip and probably didn't do enough research, BUT, as it turns out, I bought a good one. Folks, on these questions asked by someone who really wants an answer to their question, let's try not to get into pissing contests. Thanks - M²

 

[Rob'o]

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..

Thanks for the sound advice

 

[Rob'o]

So I've ordered myself a Oceanic Geo2 white/blue unit price was £259 the data cable was £58

Thanks for all the help guys

 

[Centrals]

So I've ordered myself a Oceanic Geo2 white/blue unit price was £259 the data cable was £58

Would you consider another one, any brand, as a back up because computer failure under water is NOT rare?
Incidentally, I dive with two. The more liberal one is the back up.

Aqualung i300 Black / Blue Dive Computer
Only has Pelagic Z+ algorithm unlike Geo 2 which has Z+ and DSAT(more liberal). However, it is the latest model from Aqualung while Oceanic is obsolete.
GBP 152.00 is a reasonable price to pay.

 

[tcoen]

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)

Hi Dmaziuk,

You are correct that freshwater has a density of 1000 kg/m^3 at 4°C. However, this is the peak density of freshwater. At higher or lower temperatures the density is less. For example, at 20°C the density is about 998 kg/m^3.
Water - Density and Specific Weight

As you point out, dive computers measure the absolute pressure. Shearwater computers directly use this absolute pressure measurement in decompression calculations. Therefore the water type setting (a.k.a. density or salinity setting) does not impact the decompression calculations. The water type setting only impacts displayed depth.

I am not aware if all other manufacturers use pressure directly in the decompression calculations, or if some use the converted depth.

For those interested, to convert from measured absolute pressure to depth, two other values need to be known: the surface pressure and the water density. Surface pressure is sampled before the dive and must be remembered, since it cannot be measured again once underwater. The water density is typically assumed to be a constant value dependent upon the water type. As pointed out above, it does vary slightly with location and temperature, but these are minor factors not usually considered as their impact is negligible and likely below the accuracy of the pressure sensor.

The Wikipedia article gives a good summary of the formula to convert pressure to depth:
Pressure - Wikipedia

Note that the pressure in the Wikipedia formula is liquid pressure, thus it is Pliquid = Pabsolute - Psurface.

Finally, the water type setting actually does have a minor impact on decompression, since deco stops are given in depths, not pressures. So for example, a stop at 10 fsw is at a slightly higher ambient pressure than one at 10 ffw, which impacts on or off gassing rates. Also, Shearwater dive computers use depth rates for ascent and descent rates (e.g. m/min or ft/min).

Best regards,
Tyler Coen
Shearwater Research

 

[Rob'o]

Would you consider another one, any brand, as a back up because computer failure under water is NOT rare?
Incidentally, I dive with two. The more liberal one is the back up.

Aqualung i300 Black / Blue Dive Computer
Only has Pelagic Z+ algorithm unlike Geo 2 which has Z+ and DSAT(more liberal). However, it is the latest model from Aqualung while Oceanic is obsolete.
GBP 152.00 is a reasonable price to pay.

To be honest I'll be planning dives using the PADI table anyway but if my computer fails I can easily get a back up from the dive shop while I'm there

 

[Neilwood]

To be honest I'll be planning dives using the PADI table anyway but if my computer fails I can easily get a back up from the dive shop while I'm there

TBH unless you are doing square profile dives (down, along and straight up as you might on a wreck), tables will cut you short on a lot of dives. Prime example in the UK would be shore dives where you follow the bottom out and back. Square profiles generally take this as "bottom time" and therefore ongassing at "full speed" whereas the reality is that you will be ongassing at a lower rate for the majority of the time so have a far lower N2 loading.

My typical UK dive might be surface swim to a buoy descend at it to 5m, follow the bottom out to 15-20m, turn parallel to the shore for about 10mins, turn to the shore and swim up slowly either directly at the buoy or forming a square (where the last leg is effectively a safety stop @3-5m).

 

[GlennL]

I too have a reef tank (600 gal) - been doing it for 20+ years. Wanna swap frags?

I'm jealous. I only have a 240g.

 

[Jeff Loudan]

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

Thank you for your consideration of Cochran computers. Our dive computers have always employed automatic gas switching (in fact, we may have been the first). As for the OLED screen and configurable display, I have no response other than to say that several years ago Cochran Undersea Technology chose to focus all of their engineering and manufacturing skills on our military dive computers. There were several factors that led to this decision, not the least of which was that no other manufacturer could provide our Defenders with a dive computer that implemented their algorithms. This decision also impacted the cosmetic appearance of our products, since our military customers expect consistency. Many of our military customers also wanted passive display screens, rather than bright flashy displays. And while the external appearance of Cochran dive computers may not have changed over the past few years, I assure you that the 'brains' of the computer have been evolving and are quite capable. In fact, the Lifeguard system has been engineered to provide extremely accurate instrumentation on a CCR, to allow our users to perform in depth analysis of a variety of parameters. Good diving and be safe !!

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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.


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