I have been interested in building an O2 Cell Checker for a while now, not only to test if my rebreather cells are current limited but also to check the linearity of them. That is when I remembered seeing an older thread on RebreatherWorld.com aptly named “The Under $50 Cell Checker” A bunch of people on that thread had some really cool designs. I merely copied what other people were doing and can’t take any credit for this idea.
So the question is can you build a cell checker for under 50 bucks? You betcha!
NOTE: I just want to throw this out there that I don’t advocate using out-of-date cells but I figured this is a nice little tool to check your cells before a big dive or dive trip. It also doesn’t guarantee they won’t fail on the dive itself or shortly after. I still rotate cells every 90-120 days and toss / chuck out any other cells older than a year.
I assume if you’re reading this blog post you probably already know what rebreather cells do. In a nutshell O2 cells are like tiny batteries. They produce a small voltage that is proportional to the amount of oxygen they are exposed to. Typically for rebreather cells they may read something like ~11mV in air and ~52mV in pure oxygen. This value should be linear and can be graphed on a straight line and you can essentially calculate the error rate of your cells to see if they are within specifications.
If your cell reads 11mV in air then you should be able to calculate the expected mV in oxygen and other expected values on a linear scale.
Example: 11mv / .209 = ~52mV. 52mv / .99 = ~52mV
I’m not an electrical engineer so I’m going to quote Wikipedia for a few things.
Galvanic O2 cells used in most rebreathers are “an electrchemical device which consumes a fuel to produce an electrical output by a chemical reaction.” They produce a “current proportional to the rate of oxygen consumption when the cathode and anode are electrically connected through a resistor.”
When new, a sensor can produce a linear output for over 4 bar partial pressure of oxygen, and as the anode is consumed the linear output range drops, eventually to below the range of partial pressures which may be expected in service, at which stage it is no longer fit to control the system. The maximum output current eventually drops below the amount needed to indicate the full range of partial pressures expected in operation. This state is called current-limited.
Non-linear cells do not perform in the expected manner across the required range of oxygen partial pressures. (Source: https://en.wikipedia.org/wiki/Electro-galvanic_oxygen_sensor)
O2 Cell Checker Parts List
|*Costs may vary depending on cable and display method used|
|1/4 NPT Male, Bottom Connection, 2-1/2″ Dial, 0 to 60 PSI||$11.69||https://www.mcmaster.com/#4089K64|
|Plastic Submersible Cord Grip, PG Threads, for 0.12″-0.26″ Cord OD,||$2.43||https://www.mcmaster.com/#69915K47|
|Fast-Acting Pressure-Relief Valve for Air, Silicone Seal, 1/4 NPT, 45 PSI Set Pressure||$5.26||https://www.mcmaster.com/#48435K72|
|Pentek 158117 1/4″ #10 Slim Line Clear Filter Housing||$12.84||https://www.amazon.com/gp/product/B003VT79VA/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1|
|1/4″ NPT Full Port Ball Valve||$4.99||https://www.harborfreight.com/14-in-full-port-ball-valve-68254.html|
|1/4″ NPT Male = BC Inflator QD Post||$4.00||https://www.divegearexpress.com/adapter-1-4-inch-npt-male-bc-inflator-qd-post|
|1/4″ NPT Female Brass Pipe T-Connector||$1.97||https://www.harborfreight.com/14-in-female-brass-pipe-t-connector-68197.html|
|1/4″ NPT Male Brass Pipe T-Connector||$1.97||https://www.harborfreight.com/14-in-female-brass-pipe-t-connector-68197.html|
|Molex KK 2695 Series .100 connector (part 22-01-3037) x 3||$0.57||https://www.digikey.com/product-detail/en/molex-llc/22-01-3037/WM2001-ND/26433|
|Molex Non-Gendered Contact Gold 22-30 AWG Crimp x 9||$3.06||https://www.digikey.com/product-detail/en/molex-connector-corporation/08-56-0110/WM1129-ND/417329|
|Broken Fischer Cable with Unterminated End||$0.00||Already owned but you can buy them new from Narced at 90 or other places.|
Putting It All Together
It actually wasn’t that difficult and only took about an hour. Maybe a little more time was spent fiddling with the tiny Molex connectors because I purchased a crappy $20 Chinese crimping tool instead of the official Molex tool.
The main component here is the Pentek 158117 1/4″ #10 Slim Line Clear Filter Housing. It serves as the pressure pot and has 1/4″ NPT fittings. It has a pressure rating of 125psi.
All the pieces are essentially 1/4″ NPT. You can get the majority of tee fittings, couplings and ball valves at Harbor Freight, Home Depot, or Lowes. You’ll need to order a 1/4 NPT” BC Inflator QD Post from a place like DiveGearExpress.
You’ll also want to add an Over-Pressure Valve (OPV) which you can pick up at McMaster-Carr. I choose a 45psi one because I didn’t have much need to go over 100ft. Realistically you probably don’t need to test the cells over 2 ATA but I also wanted to use the cell checker as pressure pot for testing battery changes in dive computers.
A Quick Review of Atmospheric Pressure
If you remember from your basic SCUBA course..1 ATA equals 14.69psi. This is the approximate pressure we’re experiencing at sea level. Every 10m/33ft the pressure increases an additional 14.7psi. So why did I pick a 45psi gauge? Don’t forget..Most gauges are calibrated against gauge pressure as opposed to absolute pressure. 45psi on my pressure gauge is actually ~58.8psi absolute pressure (taking into account atmospheric pressure).
- Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.
- Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted.
Wiring Yours Cells
For wiring I opted to use a broken fischer cable so that I could use my existing Shearwater Petrel 1 as a display source since it already shows 3 cells along with their ppO2 and millivolt readings. The fischer cable was already a water-blocked cable and I was able to seal it with a cable gland on to the 1/4 NPT fitting. Once you figure out the pinout it was just a matter of soldering some extra wires and crimping some Molex connectors onto them.
Narced at 90 has a good guide for the Fischer cable and Shearwater pinout on their website. It is pretty easy to follow. All three cells share a common ground.
If you don’t have a fischer cable that you can use then the easiest way to wire your cells would be to hook them up to a multimeter or purchase some cheap mV LCD displays like here.
You’ll have to experiment with the best way to make your cable air tight. A bunch of people recommended using some epoxy on the end of the cable near the wires to seal the other end. It may take a little trial and error but it should work.
Before I even tried using it as a cell checker I filled the canister with water and used it as a DIY pressure pot. I was able to get to about ~94ft or 41psi gauge pressure before the OPV activated. Good enough for me! It’s also probably a good idea to fill the canister partly with water anyway to simulate some humidity in the cell chamber and to minimize the explosive force if the container were to fail (water is not really compressible..)
Visualizing The Results
I recorded the mV readings and their respective ppO2 values in pure oxygen. I graphed these values from 1.0 to 2.0 in Excel. It’s actually kind of cool. You can see the linearity of the cells and get really fancy and calculate the error rate or expected mV value vs. actual mV value. I tested with some old cells that were from 2016 to see how they would perform. They had less than 3-5% error rate. My Shearwater Petrel appears to stop displaying ppO2 values over 1.99 but if you switch to the millivolt readings you can see that they are still increasing as expected. Narced at 90 also distributes a better Excel spreadsheet that does all the math for you.