With 7-metre high swells this weekend, I’ve had time to put a few words together about Nitrox Analyser Mark 2. When unable to source a PM128A panel meter for my first Nitrox analyser, I adapted another smaller LED panel meter. For this analyser I sourced a CX101A panel meter with LCD display. Here’s a quick explanation for anyone wishing to build the same.
The meter sits inside a plastic case, needing a wider project box that the LED meter. The small plastic case also holds the circuit card against the contacts to drive the display. They’re not soldered, so if you use this meter take note of the orientation of the parts and make sure that the circuit card is centered when you put it back together.
The CX101A has the same chipset but on a different circuit card to the CX102A (LED). However, the modification is similar and the first step is to find and unsolder a 910 ohm resistor from the circuit card. An advantage of this LCD panel meter is that the contacts that you need to connect to are the risers that protrude from the circuit card through the case and, by using connector plugs, the soldering can be performed on plugs instead of the circuit card.
A number of the risers / connector leads must be joined, including to set the decimal point, as per the diagram that comes with the meter (for 9v independent ground). Hook-up wires are then connected for power, the sensor and the reference voltage. The reference voltage is set using the same resistors and trimmer circuit used with the LED analyser (see previous blog).
On this analyser I added a press button switch and resistor, which bypasses the on/off switch, as a battery test button. If pressing the button shows a reduced reading compared to turning the unit on, then the voltage is starting to drop and it’s time for a new battery.
The next step is to test the analyser with air and adjust the internal trimmer pot so that the display reads around 21, before closing the case. The large pot, with the knob on the front of the box, is used to adjust to 20.9. I also tested using a cylinder of oxygen (reading 99.4% which is close, given 1 bar of air before filling).
So if you want to build your own economical Nitrox Analyser, here's an option with an LCD display. The sensor was again the biggest cost and it does take a couple of hours of cutting out and drilling holes, unsoldering, soldering and calibrating. If you do build a Nitrox analyser like this, please post to our facebook page - good luck!