Thinking Diver

Where divers think about stuff


Why a third computer ?

by Erik Dasque

People ask me why I use three computers on my rebreather. It’s a good question and frankly the decision is probably based on personal preference, careful planning, irrational fears and wrong assumptions.

The default set up for my Dive Rite O2ptima rebreather is two Hammerhead handsets. The primary is used to drive the Solenoid (which injects oxygen into the loop) and provides decompression information (Buhlman GF) as well as a read on the O2 Cells 1,2 and 3.IMG_0039 The secondary is used to drive the Diva HUD (Head Up display with vibrating capabilities) displaying information from the same cells there as well as on its display. A lot of O2ptima divers dive their rebreather this way with one handset on each arm and the HUD.

Some choose to get the optional deco capability on the secondary handset, a $500 pin code from Hammerhead and a reasonably good choice which gets you a backup decompression computer should the primary fail (and you find yourself manually running your rebreather, using SCR mode or bailing out on Open Circuit).

The O2ptima has a neat feature which allows you to install a fourth sensor that can be read from a third party computer connected via a fisher cable. That’s what I was doing with some degree of success and failure with my VR3. This gives you a second deco computer, possibly a different deco algorithm and a fourth cell to get O2 readings from. Unfortunately a fourth cell can fail or behave erratically and as such I didn’t trust the VR3 as much as I would have wanted to. It also always seemed to be reading .10 higher than cell 1,2,3 which bothered me. One could also use a Liquivision X1, Shearwater Pursuit or Delta-P VRX with this scheme, reading from one sensor.

Having switched to the Liquivision X1 from the VR3 and following closely the announcements for the PPO2 interface, IMG_0036the X-Link, I have been working on a solution to connect three cells to a Fisher cable. Kevin from HammerHead set my head up with a split harness and 5 wires Fisher cable so I could continue monitoring Sensors 1,2,3 with the HammerHead handsets in addition to Sensors 1,2,4 with my Liquivision X1/X-link. Of course the same could be done with a Shearwater Pursuit or Delta-p VRX.

So what do I like about this setup ?

  • O2 readings from 4 cells instead of 3 with voting logic on cells 1,2 and 4 for the external PPO2 reading
  • Different software and hardware. If the HH primary handset was to freeze up due to an odd software issue (as opposed to electrical) it’s entirely possible the same thing could happen on my secondary. With the Liquivision X1, I am on a different hardware and software platform, one altogether different bugs maybe but _different_ is the key. Redundancy through additional and different hardware/software.
  • VPM algorithm vs Bulhman GF. Always good to have two different profiles from two different algorithm
  • Different voting algorithm on a different group of cells
  • Possibility to fall back to 2 cell, one or constant PPO2 if I need to
  • Very easy to configure the bailout gases (not that the HH or VR3 were that hard just not very quick to configure)
  • Same deco computer, parameters, profile and as such deco plan as my usual dive buddy
  • the X1 is very bright which is great for dark water diving (hello Boston). Also, I can see my buddy’s PPO2 from 10 feet away
  • Same software for planning dives (V-planner) and diving those plans. Plan your dive, dive your plan

And while Fisher cables can be problematic, when well maintained they are reliable. IMG_0041Now, I still have and use my two HammerHead handsets. With the secondary tucked away in a belt pocket, the HUD and primary remain my main dive instruments.

The Liquivision is a very good backup and could be the primary if I had a different setup (MCCR or OC, for example) and could be substituted by a Shearwater Pursuit or a Delta-p VRX. In addition to tables, of course and a deco plan written on white electrical tape.




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