After opening the packages I got from Patrick, I let them sit overnight. To keep from going stir crazy from inaction, I down loaded the specifications from the manufacturers website. The next morning, I checked the output of each of the sensors with a digital multi meter. They were checked with a 10 K Ohm resistor across the output of the sensor, in accordance with manufacturers specifications. In the air of my lab, each of the sensors had an output of about 10 millivolts (MV). I held my breath to lower the oxygen in my lungs and I blew my oxygen low air into the sensor opening through a straw. The output went down. I vented some oxygen from a tank into each of the sensors and the outputs went to around 50 MV. So far so good. Now how was I going to make those displays show the real PPO?

My first approach was to use an operational amplifier circuit that I whipped up. It worked great, but it was going to require another battery and a watertight place in my system. It actually was a dead end.

Then something clicked in the back of my brain...in the Oxygen Hacker and on Patrick Duffys site there is a technique to modify a Liquid Crystal Display (LCD) display to read in PPO for testing tank air, EAN, and Trimix. I had not really thought out what that modification was. Then it hit me as I thought about how a display works and what some really clever fellow had done. Again we stand on the backs of others.

There are four functional parts of a display, which is really a 200 MV Digital Volt Meter. The signal comes into the display into an OP AMP that raises the input voltage to a higher voltage for input into an Analog Digital Converter. Most AD converters are 0-5 volts full range. The digital output from the AD is read by a Microprocessor (PIC) that drives the Display be it LED or LCD.

This is the back of my LED display. Two resistors are identified. One is the trimpot used to make small adjustments display. The second is the main gain resistor. They are in series and together make up the gain or feedback resistor that controls the gain or amplification of the OP AMP. Both of these resistors are removed. The trimpot is clipped off and the parts sticking to the board are unsoldered and removed. Next the main resistor is desoldered. I found that the easiest way to remove the surface mount main resistor was to lay the tip of my iron along its length. When the solder melts the resistor can be flipped off the board.

Next I soldered some leads to the pads

And attached the wiper terminal and one end terminal of the new pot to the leads. The other end terminal is folded over and clipped off, after testing. The 10K pot has replaced the gain resistor of the OP Amp on the display. The display can be adjusted to the desired value.

From previous experience I knew that care must be taken to attach the battery leads to the right pads. For my display, this is the right connection.

The decimal point was set by making a connection across these two pads.

I adjusted the display with the sensor exposed to regular air in the lab.

And with pure oxygen from one of my small tanks gave it a test. A perfect 1.00. Everything works so far.

Next I shortened the leads to the POT and attached the POT the the front of the display. I connected the sensor and adjusted the display to read ambient air oxygen of .209. I checked again with pure oxygen and got 1.00. Just about as perfect as you can get.

The next step is the watertight box for the displays. I chose the double clear Otterbox that I ordered from my local dive shop. The idea was to be able to look at either display by flipping the box over. If the displays do not agree, I bail out. At my age with thousands of great dives behind me and spending from 30 to 100 hours underwater a year, no one dive is that important. Why take a chance and miss all of those great dives in the future?

Making It Water Tight

Back to Start of Sensor Section