Petrel reading depth wrong?

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Armymutt25A

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I took my Petrel out this weekend and noticed that it is reading shallower than my Oceanic Pro Plus 2.1. The Oceanic matches analog gauges, so I don't think it is wrong. When the Oceanic was indicating 120ft, the Petrel was indicating 113. When Oceanic was 108, Petrel was 102.4. I have it set for saltwater. It also said I had a deco obligation when the Oceanic said I had 7 minutes of no deco time left.

Thoughts and solutions?
 
As long as the computer is set correctly for salt/fresh I am not sure why the depth would read wrong. Possibly a restriction caused by salt crystals in the hole for the depth sensor? You could try soaking in warm water overnight and see if that corrects the issue. Failing that I would contact Shearwater and have them take a look at it, their customer service is legendary. As far as the deco obligations go, my petrel set on GF's of 30/85 is slightly more conservative then my oceanic VT4 with conservative set to off.
 
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Did you activate/switch on your Petrel before your descend? Are you diving at altitude?
 
First of all, most computers i dive with err within 10-50 cm. i've given up trying to figure out which one is the most correct one. Secondly, petrel's depth gauge gets its reading through the hole in the bottom, depending on how you wear the computer the reading may be affected by it. I put it on bare skin in warm waters, it got sucked against my skin and kept showing 4m even at 30m. I let shearwater know and in a couple of weeks time they announced that the dealers can fix it when you take the computer to maintenance. Probably by putting something under the computer to maintain some gap between the hand and the computer.
 
Or maybe machining some grooves into the derlin around the hole.
 
Check your salinity setpoint (page 48 in the manual)
 
This is mere speculation.

My preditor has a presure adjustent. You set in the presure of the atmosphere where you are diving. if yours is set for say 700 vice 1000 +/- then you are at a much higher elevation and doing so could alter your depth reading. The petral may not have such a setting. The error,,as you describe it,, is not constant in that it suggests that some where around 50 ft the reading will be corect. what does shearwater have to say?
 
Petrel is at "Auto" Altitude setting by default, but it can be changed to something else.
 
Regarding depth the depth this is from the petrel manual

"Salinity Water type (salinity) affects how the measured pressure is converted todepth� Settings:
• Fresh
• EN13319

• Salt
Fresh and Salt water differ by about 3% Salt water, being denser, willdisplay a shallower depth for the same measured pressure versus theFresh water setting
The EN13319 value is between Fresh and Salt It is from the EuropeanCE standard for dive computers, and is the Petrel’s default value"

regarding the deco, the standard GF setting for the petrel is 30/70. This can be adjusted in the settings to what you want to look your deco curve to look like. There are many documents on gradient factors. The oceanic is going to be running a straight buhlmann model without GF, meaning the deco model is a 100/100 model, putting you right on the M value line and having the highest first stop possible.
 
The oceanic is going to be running a straight buhlmann model without GF, meaning the deco model is a 100/100 model, putting you right on the M value line and having the highest first stop possible.

Do you know this for a fact, or are you assuming that its running straight buhlmann? I have no idea (so I'm not disputing this), but haven't seen it documented anywhere.

Mark
 
https://www.shearwater.com/products/perdix-ai/

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