DIY Oxygen Analyser

Updated 5-1-2007

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Notes on accuracy
Parts list
The Printed Circuit Board
Assembly

The analyser this page describes, was initially devised way back in 2001 when there were only a few commercially available units to purchase and these were VERY expensive. Before you contemplate building your own have you made sure you can't buy a ready-made, working device for less? after all, five years on, there are many more nitrox divers and also many more suppliers. For example, go to Conrad Electronics website and search for "oxygen"; at the time of writing (Jan 07), they can supply a complete analyser for 90 (a GREISINGER GOX100). You'll still have to devise a means of plumbing it (as you do with my DIY version) but it's all digital and ready to go.


O2 analyser

In the past many people have considered building their own oxygen analysers but have been discouraged by the high prices manufacturers charge for the sensor element. This page describes the construction of an oxygen analyser suitable for pre-dive confirmation of Nitrox mixes for less than £80 (including the sensor but excluding the "plumbing").

Not until I'd done the initial design and bought most of the parts did I discover that there were several internet sites already devoted to this very subject, thanks to everyone who sent me details of them, the addresses of a couple are listed below.

http://www.peardrop.co.uk

http://www.airspeedpress.com

These offer an easy solution to someone with a steady hand and confidence in their electronic workshop skills.
However, the common feature of the designs on the above pages is that having bought a digital panel meter (DPM) on which to display the results, one has to modify its circuit board to increase the sensitivity.
As these are fairly small units, often utilising surface mount components, the potential for permanently damaging the DPM is high, especially if the necessary soldering iron is wielded by someone who's more at home with a 2lb hammer.
In addition, all electronic semiconductors can be damaged by static discharges so normal anti-static precautions should be observed.

If you're not sure which way to go, leave your options open by buying a DPM first (perhaps one of the ones mentioned on the above sites because the Lascar DPM I used cannot be modified in this way), have a good look at it and if you feel confident enough to "hack" the board by all means do so and save yourself the hassle of the pcb manufacture involved in this design.

If you decide not to hack the DPM, then as long as it will run from a 9 volt supply (+/-4.5V), can measure a voltage referenced to a split supply and has a full scale reading of 100 - 200mV, it can be used in this design.


unit in use

Principles:
The principles are very straightforward: fuel cell sensors produce an output voltage directly proportional to the partial pressure of oxygen in the sampled gas - this voltage can then be scaled and measured on a suitable instrument.
This voltage is typically in the range of 7 - 17mV for air, this implies that 100% O2 would produce approximately 35 - 85mV. These figures are the expected output over the life of the unit.
A digital voltmeter therefore, with a full scale range of 100 to 200mV will adequately display the results.
An amplifier with a variable gain of between 1.2 and 3.0 is used to give the correct scaling i.e 1mV/%.
A ten turn potentiometer is used to adjust the gain. A single turn 'pot would be much cheaper but won't have the resolution for accurate setting. Two potentiometers (a coarse and fine adjustment) would work, however, the coarse setting would always be a source of instability.

The unit as it stands will work with the Siemens ST-11, the Teledyne R17/R22 types and the IT (Dr Gambert Gmbh) M-03/M-25. If the Maxtec (formerly Ceramatec) MAX/CAG250E is to be used, the value of R6 must be increased to at least 1M.
There are minor differences in specifications of the above sensors, for more information about the errors involved, I've produced some information about the accuracy of the instrument.


For coupling to the cylinder, the major problem is that the business end of the sensors have a M16 x 1 thread.
As those mechanically minded out there will realise, this is quite a fine thread and I've so far come up with only a couple of things that will mate with it (a fixing nut for an electrical switch and a retaining nut for a screwlock type DIN socket, neither of which is easily adaptable.
The Siemens ST-11 is supplied with a flow diverter which has a female M16 thread for the sensor and simply pushes into a 15mm bore (it's meant to fit a "T" piece on medical equipment), I'm not sure if Teledyne's are supplied with them but IT's and Maxtec's aren't.
The easy option is to buy a kit of adapters from Vandagraph (address below), this is what they supply with their own analysers. The "DINKIT" includes everything needed to couple the sensor to DIN valves and costs £44 (VAT included) +£2.00 P&P.
In addition, to couple the DINKIT to standard pillar valves you'll need a "VANYC" adapter, they cost an extra £25 (VAT included) +£1.50 P&P.

SubAqua Products (address on the Parts List page) also sell a Sampling Adapter for £22.30, stock no. 5910. However, I'm not familiar with this item and Keith Sabine informs me that he had to modify his Maxtec sensor to make it fit. SubAqua Products may be able to confirm its compatibility with other sensors).

Being mean, I've adapted a plastic "T"-piece (Durapipe air fitting) together with some bits from electrical glands, some flexible tubing and a quantity of epoxy resin (Araldite). The annulus for mating with the pillar valve was cast with epoxy - I used a A-clamp sealing plug as a mould after removing the O-ring.
An alternative method might be to trim one of the sides of a 20mm grommet and glue that into the "T"-piece, this would mate with the face of the pillar valve (not the O-ring).
I glued a circular 16mm nut from an old switch - liberated from the scrap box at work - into a gland adapter to hold the sensor. Again, an alternative might be to use a trimmed 20mm grommet glued into the "T"-piece, this would enable the sensor to be push-fitted instead of screwed into the "T"-piece.

There are images of my adapter on the assembly page.
There are also some pictures of an adapter made by Stewart Alan Ball of Hartlepool Divers.

He used a 20mm plastic water pipe T-piece with compression fittings from B&Q. The centre leg was sawn off leaving a flat flange. To this he epoxy'd and screwed a DIN fitting dust cap with a small hole drilled in the centre - obviously for DIN cylinder valves but this could be modified to fit an "A" clamp.
On one end of the "T" he epoxy'd into the screw-down fitting a gable gland nut and plastic hose similar to mine, on the other end he epoxy'd in a nut of the same thread as the sensor. This makes a professional looking piece of kit, the "T" piece cost £3.50 and the DIN dust protector £4.50 from Denneys of Redcar.

The gas flow is intended to fill the "T"-piece, enter the tubing near the sensor (where there should be plenty of turbulence to ensure a fast response) and slowly vent to the atmosphere along the tube. The small pinhole is to (hopefully) help draw any air from "dead spaces" in the "T"-piece by a venturi action.

Everything is glued together with epoxy and a loop of shock cord holds it against the pillar valve (this is retained by being fed through the fixing holes of a conduit saddle clamp which is silicone-glued to the "T"-piece).
A small plastic ball on the shock cord locates in the "dimple" on the pillar valve.

It's certainly not pretty, but it works well and cost next to nothing.

Another starting point might be with a washing machine tap adapter...... I'm sure other people could produce something more elegant, the important thing is to ensure there are no leaks at any point around the "T"-piece as air could be drawn into the "T"-piece and give erroneous readings.


The circuit:
For the amplifier, I initially decided to use an OP-77 low offset, precision op-amp as this is readily obtainable (at least it was at the time of writing), since then, some suppliers have stopped stocking this but it can be replaced with an OP-177 (or a Linear 1077 if necessary).

Power is applied by pressing a momentary switch with a time delay to automatically disconnect the supply (a 9V PP3 alkaline battery), this should greatly extend battery life for those of us who're a little forgetful.

The schematic is shown below; the prototypes have been built and everything (except the sensor) fits neatly into a 100mm x 75mm x 40mm box.

I've produced a printed circuit board artwork which can be reproduced on a LASERjet printer or copied by hand and then etched.

Everything appears to work correctly with both a Teledyne sensor and an IT sensor.

O2 analyser schematic

Circuit description:
Electronically minded folks won't need this, but for those less technical I've briefly described what all the bits do.
Under normal conditions, Q1 will be non-conducting, no current will be drawn from the battery, the voltage across pins 4 & 7 of U1 will be zero and the DPM will be off.
When the switch is momentarily closed C1 charges to 9V, this causes Q1 to conduct, its drain (pin 4 of U1) will effectively be shorted to the negative side of the battery, the DPM will come to life and the junction of R1 & R2 will settle at about half the battery voltage.

When the switch is released, C1 will very slowly discharge into R7 (R5 has no effect as Q2 will not be conducting at this point.
After about three minutes (depending upon the values of C1 & R7), the voltage across C1 will have fallen to a level such that Q1 begins to turn off, its drain voltage will start to rise which in turn causes Q2 to start conducting, this speeds up the discharge of C1 through R5 so that everything shuts down rapidly from this point onwards.
If you want a longer delay, simply increase the value of C1.

Diodes D1 & D2 are for electrostatic protection, they ensure that the input of U1 can never exceed the supply rails by more than 600mV. They can be omitted if you take care not to handle the exposed connections on the jack plug.
C2 & C3 are for decoupling the supplies to minimise electrical noise etc.
The amplifier output voltage will always drive to a voltage level that results in the voltages at pins 2 and 3 being equal. The voltage at pin 2 is attenuated by the network R8, R9 and P2 and it is the combinations of these that determine the gain of the amplifier.
The lowest gain is given by: (R8+R9+P)/(R8+P), whilst the highest gain is: (R8+R9+P)/R8.

If you decide to optimise the amplifier gain to suit one particular sensor, the following should prove useful.
The starting point will always be the value of potentiomer, P2, as the available values are limited, typically 10K, 20K 50K or 100K. You'll then need to establish the maximum and minimum gains, AH & AL; i.e. AH = 20.9/Vmin and AL = 20.9/Vmax where Vmin & Vmax are the limits of the sensor output quoted on the datasheet.
You can now insert these figures into the formulae below:
R8=(AL*P2)/(AH-AL)
R9=(P2+R8)*(AL-1)
Using the extremes for all sensors, these give theoretical values for R8 & R9 of 33K & 15K respectively.
To be really practical, you should aim for a slightly smaller minimum gain and a slightly higher maximum gain.

Initial calibration:
When the unit is "on" without the sensor connected, the offset potentiometer P1 should be adjusted until the DPM displays zero (0.0) in this state.

With the sensor connected and in air, the output of the sensor will be somewhere in the range of 7 to 17mV, the Calibration potentiometer P2 is adjusted until the DPM reading is 20.9mV (1mV = 1%).


Calibration:
The following should enable fairly accurate offset nulling.

For those with access to 100% oxygen only:
1. The initial calibration procedure above should be performed.
2. Calibration should then be performed with 100% O2 - adjusting P2 for a reading of 100.0
3. Then, monitoring air, tweak the Offset potentiometer P1 until 20.9 is displayed.
These last two steps should be repeated until no further adjustment is necessary.

For those with access to 100% nitrogen only:
1. The initial calibration procedure needn't be performed.
2. Using 100% nitrogen, adjust P1 for a reading of 0.0.
3. Then, monitoring air, adjust P2 until 20.9 is displayed.
These steps should be repeated as necessary.
Subsequent normal calibration can be carried out in air, P1 should only need adjusting when a new sensor is fitted.

The best results should be gained by combining both the above techniques. i.e. using nitrogen for zeroing a new sensor and 100% O2 for routine full-scale calibration before use.


The sensor:
A suitable Teledyne model is the R-17VAN, this costs £65 including VAT plus P&P of £1.50 (this requires a 3.5mm mono jack plug to connect it - confirm this when ordering), they can be got from:

Vandagraph Ltd, 15 Station Road, Keighley, W. Yorks. BD20 7DT
Tel: 01535 634900

An equivalent to the above is the ST-11 from BPR Medical, these have the longest claimed life of any, cost around £39 plus £6.20 P&P plus VAT and are available from:

BPR Medical, Innovation Centre, Oakham Business Park, Mansfield NG18 5BR.
Tel: 01623 628281
They also have an e-commerce site at: www.carnet-withoutfrontiers.com where individuals can buy on-line.

Maxtec's CAG250E's cost £30.42 plus £2.50 P&P plus VAT and are available from:

Envin Scientific, Unit 1, Stockwell Est. Aylburton, Glos. GL15 6ST
Tel: 01594 844707

(These require a 2.5mm mini power plug - see below).

The really good news is that IT (International Technologies) M-25 sensors look similar to Teledyne's, have similar specifications (apart from the Offset errors), they respond in half the time but only cost £26.16 including P&P and VAT (in 2000).
They can be got from:

Omni Components, 4 Percy Street, Coventry CV1 3BY.
Tel: 024 7622 5757

The drawback is that they will only accept cheques in advance (no credit cards), and assembly/delivery takes about three weeks.
The standard M-25 requires a 2.5mm a mini power plug, but if you ask them nicely, they can fit a 3.5mm jack socket so it's interchangeable with Teledyne's (probably the best idea !). I ended up with an M-03 which has the highest output of the lot but is unfortunately quite sluggish, so if it's speed you want, be sure to insist upon the M-25.

If you want to stay with the 2.5mm mini power connector, Maplin do two types, a long version (HH63T) and a short version (HH62S) both cost 42p (+VAT) but I'm not sure which type is required.


Total Costs:
Buying the individual parts is certainly cheaper than the cost of a ready-made instrument, but bear in mind that the bought unit includes everything you need and requires no extra work with "plumbing".

Scanning the advertisements in electronic hobbyist magazines for cheaper components may well be beneficial - Maplin "do" a cheap digital multimeter for about £10 and it shouldn't be too difficult for the adventurous to modify one of these - saving the cost of the box and battery holder.

If you not sure of what bits you'll need to construct the instrument, I've compiled a parts list from the Maplin catalogue.

I estimate the price (excluding "plumbing") to be a about £117 with a Teledyne sensor, but with IT's this falls to about £76 !
Buying a DINKIT will add £46 and a VANYC adapter will add a further £26.50, although with a little innovation and effort, the handy DIYer can probably build his/her version for less than a fiver.

To cut the costs further, the battery holder can be dispensed with and by modifying the layout, the battery can still be housed inside, suitable PP3 connectors are Maplin's HF28F and cost 17p (+VAT).


Assembly
The Printed Circuit Board
Parts list
Notes on accuracy

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Contact: Dave Cordes.

www.dcordes.freeuk.com/analyser.htm