Here's a description of a simple O2 analyser I made a while back. It is pretty easy to construct for anyone with a reasonable grasp of electronics and some skills in drilling holes etc... I've checked it against commercial sensors and it is always within about 0.5% of their readings, so I'm reasonably confident of it these days.

First the parts list. The sensor comes from Ceramtec in the USA. As far as I'm aware they do not have a distributor in the UK, the one I have was ordered as part of a batch for people on the scuba-uk maiing list by Anke Otto (thanks Anke!). It cost about £50. The rest of the bits with the exception of the sampling adaptor shouldn't cost more than £20.
 
 

Part description	Part Number	Supplier
Oxygen sensor	CAG-250 Internal (B)	Ceramtec Inc (www.ceramtec.com)
Digital Panel Meter (DPM)	GW01B	Maplin Electronics
Lithium PP3 battery	FC35Q	Maplin Electronics
PP3 Battery Clip	HF28F	Maplin Electronics
MB2 plastic box	LH21X	Maplin Electronics
SPST subminiature toggle switch	FH00A	Maplin Electronics
Subminiature waterproof toggle switch cover	JR97L	Maplin Electronics
Variable Resistor 250 ohms	N/A	Maplin Electronics
1/4 watt resistor 120 ohm	N/A	Maplin Electronics
Knob	FK38R	Maplin Electronics
Analyser sampling adapter	5910	Sub-Aqua Products
Threaded spacers	LR72P	Maplin Electronics
6BA c/sunk bolts	LR56L	Maplin Electronics
6BA domed head bolts	BF05F	Maplin Electronics

You will also need a small piece of sheet perspex (I cut a bit from an old music centre) and wire, solder, tools etc.

1. First modify the DPM. You should get a circuit diagram with the DPM, on it (Fig 6-1) there is a potential divider formed by R2 (500 ohms), VR1 (200 ohms) and R1 (15.5K ohms). We need ro remove the preset VR1 from the PCB (carefully!) and replace with 3 flying leads that will connect to our 250 ohm variable resistor. We also need to remove R2, the 500 ohm resistor, and replace it with a 120 ohm one. Looking at the PCB below, R2 is the top resistor in the group of three on the left.

Finally we need to short jumper link DP1 on the board so we get a max display of 199.9 (OK, so we can only really get 100.0% O2, but the next range down at 19.99 isn't going to be much use is it?)

2. Next solder on a lead to an outer pin of the toggle switch from the battery +ve pad on the PCB, and the red wire from the PP3 connector to the toggle switch central pin. Connect the black lead from the PP3 connector to the battery -ve pad on the PCB. Connect the 3 wires from the PCB to the variable resistor (in the photo, the green lead connects to R2 on the PCB and the left pin on the variable resistor, the black wire goes to the chip C+REF input and to the center pin of the variable resistor, and the blue lead to R1 on the PCB and the right pin on the variable resistor).

3. Solder the red lead of the O2 sensor to the input +ve pad on the PCB, and the black lead to the -ve pad.

4. Cut holes in the case for the panel meter (approx 48mm x 19mm high) and mark out the position of the holes for the panel meter fixing screws. Cut a piece of perspex 68mm x 50mm high and drill holes such that they line up with the box holes and the panel meter holes. Then countersink the perspex holes. Now put the 6BA c/s bolts through the perspex and the front panel, with the threaded spacers behind. Mount the DPM on the threaded spacers using the domed head 6BA bolts.

5. Cut a hole (~15mm) for the O2 sensor in the bottom of the box. I glued mine in with superglue - when the sensor goes, I'll replace the box base too. Also drill holes for the toggle switch and variable resistor. The battery was just wedged in with a couple of pieces of foam plastic.

6. Finally check everything before turning on. You should be able to get a reading of 20.9 with the variable resistor somewhere reasonably central, if not R2 might need adjusting a bit. If you have a nitrox tank or O2 kit handy, try exposing the sensor to mixes other than air to check you are getting sensible readings. There's nothing really to calibrate - the sensor delivers about 11.5mV in air (20.9%), and is linear with O2 percentage i.e. 23mV is about 44% O2 etc.

I used a sampling adapter from Sub-Aqua Products, 63-65 Twyford Road, Eastleigh, Hants SO50 4ZG (tel 023 8061 2144). The only downside is that it didn't exactly fit the sensor, so I very carefully modified the sensor until it did fit - you have to be brave to do this! However, you don't really need the adapter if you have DIN tank valves - just hold the sensor against the tank valve and turn on the valve very gently.