Adding a PP02 Sensor

The addition of a PP02 sensor to a rebreather is a chore that's been a requirement for many builders of home-shop manufactured rigs, as well as people modifying existing rebreathers. Many different methods have been used, including making the entire display from scratch and then mounting sensors someplace in the system. I wanted to add a means to measure PP02 in my IDA-64 rig, as part of an experiment I am doing to investigate the feasibility of converting it to fully closed circuit nitrox use from the pure 02 system that it was designed for. Without going into the details of that modification, suffice it to say that I needed to be able to measure the PP02 and did not wish to make a large project out of manufacturing a PP02 display myself. I decided to use the commercially available Draeger Oxyguage as a quick solution to the need. The Oxyguage is a single sensor PP02 display system with some nice features like back-lighting and audible low and high PP02 alarms. The unit is calibrated in air, so no 02 is wasted when calibrating. The units sold in the USA come with a very conservative audible alarm preset for high PP02 at the surprisingly low PP02 of 1.2, however I found out that by opening the case and shorting a set of contacts in the correct order that this setpoint can be altered. I set mine to 1.6, and then proceeded to install it into the counterlung of the IDA-64. It also can be installed into any other fabric-counterlung. Here's how I did it:  

	The first order of business is to obtain a Draeger counterlung that contains the plastic snap-in port that is used to hold the Oxyguage in the Atlantis or Dolphin rebreather. This part is also available outside the USA as a discrete part number, but in the USA I needed to order the entire counterlung. I cut the counterlung apart to remove the desired port. I found that the port was impossible to unscrew from the backing plate, and  at first I was worried that it might have been glued in place. I then cut the counterlung and ripped the material from the port/plate assembly and then the two pieces unscrewed with ease. Here is the port, showing the threads, the backing plate, and the scrap of old counterlung laying on the bench.
	Here I've laid the scrap of the cut-up counterlung where I wanted to install the Oxyguage sensor. I chose an area that would be in the gas-path of the auto/manual addition valve so that I can blast the sensor with diluent to check the sensor accuracy, or blow-away condensation. Here I'm getting ready to trace the hole using the counterlung scrap as a template.
	CAREFULLY trace the hole to be cut, making sure that there will be space for the sensor holder in the internal void of the counterlung. Check this twice, as there may be internally routed lines and hoses inside the counterlung.
	Here's the scary part: Take a SHARP pair of curved manicure scissors and cut the hole out of the material. The size is critical, and you aren't going to get a second chance! You can see that by peering into the hole that is made that the auto/manual addition valve is visible. This, again, is to allow the sensor face to be blasted by diluent for checking the accuracy of the system during a dive.
Above: The plug on the left side of the photo is the Draeger   blanking plug that is installed when the Oxyguage is removed.	Once that is done, the rest is easy. Pass the nut of the port through the hole (yes, it fits!) and then simply tighten the port in place. That's it! It makes a dandy rinse and drying port as well. Above: The port holds the sensor with the metal clips shown. These lock into a notch of either the blanking plug, or the sensor holder. The one on the left is shown in the latched position, the one on the right has been opened for removal of the sensor or the blanking plug by pushing the latching button. This is a very secure method.
	Here is the sensor holder for the Oxyguage. This plugs into the port in the same way as the blanking plug shown above, and holds one Teledyne sensor.
	To remove the sensor, you merely release the end of the 'nose' of the unit and the sensor drops out. This makes it easy to store in a film-can when the unit is not being used.
	Here's the whole system installed and ready to go diving. The whole project took a whopping 10 minutes and the result is that I have a good system for reliably measuring the PP02 of the mix.

This was an easy and well-worthwhile short project. I am very pleased with the system, and will use it as a tertiary PP02 measurement system when I get the rest of the modifications done.

This type of modification is applicable to rebreathers including the entire range of Russian devices, the FGG-III and FGT-1D rigs from Draeger, the LAR-V (Yes, you -can- make the LAR-V into a CCR if you try hard enough!), and into just about any other rig with a fabric type counterlung. In a rigid type scrubber (home-brew PVC or similar) the port can be bonded in with any sort of epoxy. This is a simple solution to the need to measure PP02 and uses a well tried device. The only drawback of this system is the fairly high cost of the Oxyguage, but I believe the end result is worth it if you are not willing or able to build your own display system.

On that last subject, if you DO want to build your own system, you could do no better than to contact Mark Monro, who has made up PC boards for doing just this. He sells, for a very reasonable cost, all of the parts to build PP02 sensors. You can see all of this on his PP02 Display page. The next project in the shop here is to complete one of his 3-sensor kits and to make a "Donkey Dick" to place three sensors into the Draeger counterlung port. Stay tuned!

Dave Sutton

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