Shearwater computer settings for a CCR dive

I’ve had my rEvo rebreather for about 3 years now and I’ve been working on-and-off to dial-in my preferred settings on my multiple Shearwater computers. After some trial-and-error, I thought I would post my current settings for others.

To be very clear, I’m not advocating a specific gradient factor or CCR setpoint, etc.

What I find useful is how to use different gradient factors on different computers and different settings for NDL display, last deco stop depth, etc.

Overall computer setup

First, it would help if I described my current overall computer configuration before diving into the specific settings for each one. I have three Shearwater computers I use on my rebreather during dives.

  1. I have a Petrel2 “controller” that is attached via a DiveCan bus to three O2 cells. This is the “main” computer on my rebreather.
  2. I have a “backup” NERD2 that is hardwired via a wet-mate analog connection to two additional O2 cells. These two systems are therefore completely independent of each other and each calculates decompression obligation, each based on actual PPO2.
  3. I have a Teric as an “offboard” computer that is not connected to the rebreather in any way whatsoever. It is set to an “internal” setpoint and calculates my decompression obligation assuming that I (or my computer) is maintaining a specific PPO2 level.

Petrel

This is the main computer and the “controller” for the rebreather. This is the computer I use for my deco profile during a normal dive and hence has the most conservative settings. Here are various settings I have configured and a short explanation for some:

  • Low SP : 0.5 – Most people have 0.7. I prefer to use 0.5 for my low set-point. That is a different topic altogether.
  • High SP : 1.3 – This can vary for a specific dive, but by default I use 1.3. Most people use this as well.
  • Auto SP : Off – For many reasons, I prefer to manually control when to change the SP from low->high (on descent) or high->low (on ascent). Again, a topic for a deeper discussion another day.
  • NDL Display : Delta+5 – Since my Petrel is the main computer and this is generally the deco profile I’m going to use, I like to know how much additional time-to-surface is going to be incurred if I spend an extra 5 minutes at my current depth.
  • Clear Counter: On – This is a relatively new feature which enables a “count-up” timer to start after clearing the final decompression stop. I often will spend an extra 3-10 minutes in the water after my deco clears depending upon dive duration, conditions, etc.
  • GF: 45/80 – I’m a relatively conservative diver and usually have my GF Low at 45 for most dives. My general approach is that I follow my Petrel2 computer deco profile unless something has gone wrong.
  • Last Stop : 20 feet – Most of my diving is in the ocean which can have “ugly” conditions. I prefer to run my last deco stop at 20 feet. It means a bit more time in the water but I’m more comfortable at 20 feet on a “normal” dive.

NERD2

This is essentially a “backup” computer in case two or more of the cells connected to the Petrel aren’t working or the Petrel battery dies (you did check the battery voltage before diving, right?) or something else goes wrong. Note that I also use the Air Integration (AI) feature of the NERD2 and have sensors connected to my O2 and Diluent tanks.

Because it generally means something is wrong if I’m using it for a decompression profile, it has more aggressive settings.

  • GF: 70/85 – This is the “normal” setting I have for my gradient factors when I have something wrong and might want to get out of the water quicker. NOTE: I will often configure this GF for whatever my buddy is using on their computer so I have an idea of what their deco obligation / profile is.
  • Center Row: PP02, PPO2, AI Mini – I have two independent O2 sensors connected via “hardwire” to the NERD2. The third sensor position displays pressures on both tanks instead of a PPO2 reading.
  • AI Mode: T1+T2 – This allows me to see “mini” versions of the pressure on both tanks at the same time
  • Last Stop: 10 feet – This allows me to conduct my last stop anywhere up to 10 feet without triggering alarms. If I need to get out quicker, I will generally conduct my last stop above 20 feet but below 10 feet. Unfortunately, you cannot change your last stop setting during a dive. I’ve filed a feature request with Shearwater.
  • NDL Display: SurfGF – This is a relatively new feature which shows what the gradient factor would be if you were to surface immediately. Useful to know if you are having issues and need to get to the surface faster. It can be thought of a “risk indicator” when surfacing before completing all of the decompression stops.
  • Clear Counter: On – Same reason as the Petrel.

Teric

This is the “oh shit” computer in case my rebreather completely fails. The general guideline on a rebreather is to stay on the loop as long as possible since your bailout gas supply is likely very limited.

It also means that if I’m using it for my decompression profile, I’m definitely in a bailout situation and the rebreather has completely failed and you want to get out of the water as quickly as safely possible.

One of the very nice features of the Teric is the ability to have different “home” screens for bailout vs. CCR operation. We’ll explore below:

  • GF: 80/85 – This is a pretty aggressive gradient factor. Again, it assumes that the rebreather is not working at all and that you are on open circuit bailout with a limited gas supply.
  • Last stop: 10 feet – Get out of the water sooner with a last stop at 10 feet vs 20 feet. In reality, in this situation you could just ignore the stop depths as detailed below.
  • Auto SP: Off – I don’t want to mess with changing my set point on my backup when I’m trying to get down onto a wreck quickly. I find that assuming the set point is always 1.3 even when it is lower during the descent does NOT result in any meaningful difference in deco obligation.
  • CCR Home Screen Middle Row – This is what would be displayed during “normal” operation
    • Left: Gas – Diluent gas currently in use
    • Middle: Set Point – Internal set point configured. Usually 1.3.
    • Right: T1+T2 – Backup to the NERD2, shows tank pressure for Diluent and O2 tanks.
  • Bailout Home Screen Middle Row – In this case, I’m in an “oh shit” bailout situation and so I don’t care about set points, tank pressures, etc. What I want to know is everything about where I am on the decompression profile
    • Left: Gas – Shows the current open circuit gas selected. Obviously important as I’m likely to be switching gases at least once.
    • Middle: SurfGF – What would my gradient factor be if I ran out of gas and had to surface?
    • Right: GF99 – This essentially shows your current gradient factor. This allows you to run your own decompression profile while assessing the risk. You could alternately use the CEIL setting and then follow that all the way up.

Notes / to-do

Note: One thing I always do is make sure to configure all of my bailout gasses in my closed circuit gasses on all computers BUT turn them off. The reason for this is that I might want to “plumb in” an off-board gas to my rebreather and this just makes that easier to select that gas as a diluent.

To Do: Get pictures of different screens to illustrate what they look like under different conditions.

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