Review of the Narked@90 Cell Checker
So .... how do you know if the Oxygen cells you are using in your rebreather are actually accurate in measuring the pO2 of your loop?
You could do an O2 flush at 3 meters / 10ft to see if you can get to 1.3 and one at 6 meters / 20ft to see if your cells will get to 1.6. I have found this almost impossible to achieve, while sitting at 6 meters I had to dump all gas and inject oxygen for 5 or 6 times before I got readings of 1.5 or higher on my Inspiration Vision.
A more scientific approach would be doing the checks using Narked@90's cell checker. The mini-check has already been reviewed, but the big brother of it hasn't, so I like to share my experience with that one.
I have an X1/Xlink attached to my vision, but the Xlink is the hardwired version, so that excluded the mini-check for me, I went for the last cell checker John had in store.
When the package was delivered, I found a sturdy case inside, ofcourse in the lastest diver fashion colour: BLACK.
A closer look at the top panel:
For each of the 3 cells you are measuring you will get a mV reading. In the center is the pressure gauge that shows how many bar/psi overpressure of O2 there is in the pressure pot. It will go from 0 to 2.5 bar.
In the top right you will find an inflator stem to which you can attach the manual inflator hose from the oxygen tank.
There are 3 types of lids for the pressure pot, depending on the type of cells you will be testing. I ordered mine with the SMB coax lid for inspiration vision cells.
The cells are attached to the lid:
First step is to connect the lid with the attached cells to the cable of the cell checker. This way you can take the mV readings in air as shown below
Then you fasten the lid to the pressure pot and flush it with oxygen until the mV readings and the pressure gauge stabilize. After letting the overpressure drop to 0, the knob for venting is switched to 'hold' and you can take the mV readings for a pO2 of 1.0.
The push button for adding oxygen is pretty big, but also quite sensitive, on my first attempt I added 0.3 bar of oxygen. But you will soon get the feeling for it, with slight pushes you can add oxygen in 0.1 bar increments.
The analog pressure gauge is nice, but a digital one would have even been nicer here, if you overshoot you know you are taking a reading at for example 1.53 bar instead of 1.50.
Anyhow, during the measurements the display looks like this:
The complete testing setup in my garage:
and the device for logging all this data like this:
To the inside of the case of the cell checker, there is a chart with expected mV values at 0.6, 0.7 and 0.8 bar overpressure for each starting value in air.
My cell nr. 3 stared with 11.3 mV in air, so it should reach 53.5 mV at a pO2 of 1.0 and 85.6 mV at a pO2 of 1.6
Too bad my cell reached only 76.2 mV at a pO2 of 1.6, so there is a strong indication it is not linear.
Together with the cell checker you will receive an Excel spreadsheet in which you can enter all the measurements. It will calculate the expected mV values for you and show for each measurement whether the cell passed or failed the test at that pO2. And it will also show a nice graph of how your cells behaved.
In my case all 3 cells show bad behaviour, which could result in severe hyperoxia. My cell nr1 for example measures 11.3 mV in air and 54.1 in pure oxygen at atmospheric pressure (which was 1007 mBar when I did the test)
Based on these settings a pO2 of 1.3 should be reached when this cells is at 70.8 mV (see line 5, blue column in the image above). But this cell reaches 70.8 mV when the pO2 is close to 1.5 (measured 71.6 mV at 1.5, see line 7 white column).
So... the controller thinks it is serving me a pO2 of 1.3 while I'm actually breathing 1.5. Even worse .... 1.4 on the controller would actually be 1.7 in the loop.
Without being dramatic, I think this cell checker might have saved my life. For shallow diving at low setpoint there's no problem, but I have some 'big' dives coming up in the 50 - 70 meter range doing 1.5 to 2 hours deco. Breathing a pO2 of 1.7 or higher for 1.5 hours might not have been a very helathy thing.
I also learned the lesson of doing better bookkeeping of my cells, keeping track of what cell has been replaced at what date gives better insight in its age.
Last edited by Beanie; 14th June 2010 at 12:03.