Trimix Computers

[Tomeck]

I dove with my friend who has a Shearwater Pursuit, his dive computer indicated 68 meters (223 feet), but my D9 indicated 70 meters (230 feet). I think my D9 is more precise, because I know this dive spot. Is it normal that Shearwater Pursuit indicate the depth shallower?

 

[DA Aquamaster]

The fine print put out by most companies states the accuracy of a computer is usually on the order of +/- 1% to +/- 2%.

A 2% error at 230 feet is 4.6 feet. If computer A reads 230 ft and compouter B reads 223, feet the 7ft difference is most likely split bewteen them.

Two computers both accurate to +/- 2% could have an extreme spread of 9.2 feet at 230 ft if each is at the extreme edge of their respective acceptable accuracy ranges.

cross checking three computers does not really solve your problem either as it would be possible to have 2 of them that are both accurate to -2% while the third is dead on and the 3rd computer would appear to be "wrong". Democracy is not always a good thing when the sample size is that small.

Knowing a dive spot helps, but not if your only depth reference is your computer. It could be a case of your computer having the error with the result that from your perception everyone else's computer is "wrong" and you'd have no way of knowing it until you saw a lot of computers that seemed to agree around an average depth.

 

[Scared Silly]

Now that the word accuracy and precise (precision) have come up lets use the correct terms:

http://www.scubaboard.com/forums/co...g-spg-inaccuracy-post3641033.html#post3641033

 

[Meng_Tze]

I dove with my friend who has a Shearwater Pursuit, his dive computer indicated 68 meters (223 feet), but my D9 indicated 70 meters (230 feet). I think my D9 is more precise, because I know this dive spot. Is it normal that Shearwater Pursuit indicate the depth shallower?

Hmm, it is a matter of reference. If you use one measurement as a baseline to reference the rest by, yes the D9 could seem more accurate, or maybe the other instrument is. Maybe both are and are just 3% apart?


7ft difference at 230ft max is 3.043 % .................I am not sure if that is going to be the difference between a DCS hit or not. Between a correct/incorrect ascend.......


One other thing at play here could be a seawater vs. fresh water setting?

Dive profiles are not THAT precise.

 

[Tomeck]

Hmm, it is a matter of reference. If you use one measurement as a baseline to reference the rest by, yes the D9 could seem more accurate, or maybe the other instrument is. Maybe both are and are just 3% apart?


7ft difference at 230ft max is 3.043 % .................I am not sure if that is going to be the difference between a DCS hit or not. Between a correct/incorrect ascend.......


One other thing at play here could be a seawater vs. fresh water setting?

Dive profiles are not THAT precise.

I dove in fresh water, but my D9 is always setting to seawater. If my D9 reads 230 feet in fresh water, the real depth is about 237 feet. There is not fresh water setting for D9.

I am from Switzerland where there are many watches, I love the precision.

 

[Meng_Tze]

I...
I am from Switzerland where there are many watches, I love the precision.

I have a few myself, but what does living in Switzerland have to do with anything?

 

[DA Aquamaster]

I dove in fresh water, but my D9 is always setting to seawater. If my D9 reads 230 feet in fresh water, the real depth is about 237 feet. There is not fresh water setting for D9.

I am from Switzerland where there are many watches, I love the precision.

Its the pressure that counts not the actual linear distance so as long as the dive tables are in feet sea water it makes no sense to even have a fresh water setting.

The other side of the coin is that a fresh water reading will always be deeper than a sea water equivalent so if anything it adds a bit of conservatism.

 

[sunkmail]

I dove with my friend who has a Shearwater Pursuit, his dive computer indicated 68 meters (223 feet), but my D9 indicated 70 meters (230 feet). I think my D9 is more precise, because I know this dive spot. Is it normal that Shearwater Pursuit indicate the depth shallower?

In my case, my VR3 reads 1 foot shallower than the Pursuit at around 100 feet.



Oh, and in regards to the tape
- Duct tape leaves more residual than the electrical tape.
- As long as it's easy to read, I can't see the colour making much difference

 

[Robert Thompson]

You don't buy your new dive computers and go to 300'. Like any new piece of equipment, you work your way down to depth over time. You adjust the conservatism based on sub-clinical DCS symptoms like exhaustion, so by the time you are diving to any depth that may be deep enough to matter you have already adjusted for the difference in calibration.

P.S. I am a big fan of computer generated deco schedules, especially for team diving scenarios.

 

[SlowRain]

For whatever it is worth. I use a VR3, however I will religiously cut tables bracketing target depth and times that are entered onto wetnotes or a slate as a backup.