Archive for the ‘Oxygen analysers’ Category

Oxygen analysers.

These are instruments for measuring the percentage of oxygen in a sample of gas.
There are two main types, the fuel cell kind and the non fuel cell kind.
The fuel cell type.
The cell is composed of a permeable plastic membrane of Teflon, mounted on the front of a gold-plated sintered steel disk cathode.
The membrane and the disc close the end of the cell which contains the electrolyte: potassium hydroxide, and a copper anode.
Oxygen passes through the plastic membrane and reacts with the electrolyte, generating an electric current.
The sensor has been designed to generate a signal which is directly proportional to the oxygen concentration passing over the membrane.
There are two types of this fuel cell, C1 and C2 cells.
The C1 cell is for measuring oxygen in air, and the C2 cell for measuring oxygen in nitrous oxide.
The C1 cell should not be used in anaesthetics.
C2 cells have a shorter life than the C1, roughly about half as long. This life depends upon the concentration of oxygen that the cell is subjected to, the longer and higher the concentration then the shorter the life.
The cells may be damaged in the presence of Carbon Dioxide at concentrations higher than normal expired air for long periods.
If the cells are sterilised in Ethylene Oxide the membrane will blister and perish.
Some fuel cells might read high on installation, allow at least 30 minutes before calibration and 24 hours before complete baseline stability.
C2 cells are designed for best service in N2O gas mixtures and when not in use the cells should be removed from the instrument and left exposed to room air, until further oxygen analysis is required. This will extend the working life of the cell.
This information relates to the Teledyne micro fuel cell, but probably applies in much the same way to other makes of cell.
They come with a shorting clip, this provides a current loop, assuring that the cell is ready for immediate use when placed in the analyser.
If the clip has been off the cell for sometime, placing the cell into service will result in the reading being off the scale.
This is due to oxygen defusing into the cell until the electrolyte has become saturated.
The user can either remove the cell from the instrument and short its output with the clip, or leave the cell in the instrument, in either case it will be necessary for the cell to equilibrate in an unsaturated state before being placed in to use.
For every hour that the cell has been un-shortened, approximately one hour is required for stabilisation.

Fault finding.
This applies to Teleydyne fuel cells, others may vary.

1. Erratic readings.
check that the cell is properly installed in the holder and all contacts, cell, leads, plugs and sockets are clean and soldered joints are good. Check for physical damage to cell, check cell age.

2. High or low outputs.
check cell has been at a proper temperature for 8 hours, install shorting clip or leave in holder for 8 hours.
Check the temperature compensating circuit.
Impedance of the circuit at ambient temperature should be 200 to 250 ohms. High impedance caused by dry joints will give apparent high output and high meter readings.
If the cell is wet allow to dry out with the shorting clip on or in its holder for 8 hours.
3. Output drifts towards zero.
check the age of the fuel cell, install shorting clip for 8 hours.
Another type of fuel cell is the Polarographic Electro chemical oxygen sensor.
This is also disposable and has about the same working life as the micro cell, but the difference is that the micro cell starts to degrade as soon as it is made, but the P.E.C sensor only starts to degrade when it is put in to use.
The technical difference between the two cells is that the micro-cell generates its own current, and the P.E.C. has a current applied to it.
Maintenance of these analysers is fairly easy, the main problem being worn out fuel cells.
When checking a fuel cell, check it on 100 %, put it in a pure oxygen atmosphere and adjust the calibrating knob to that value, it should be right in room air and read 21%.
The trouble with these types of instruments is that the cells will only last for 9 months or so and then have to be replaced at some expense for the cells are not cheap, and of course if you are working somewhere away from easy supply then your problems are all the greater.
Some newer cells will last a couple of years, technology is improving them day by day.
Another type of oxygen analyser is the Magneto-Dynamic kind, this is by far the best, expensive to buy in the beginning but it does not use degradable fuel cells.
The principle of operation is as follows, it measures the para-magnetic susceptibility of the sample gas by means of a measuring cell.
This cell consists of a nitrogen filled dumbbell suspended on a platinum wire in a strong, non uniform magnetic field.
A change in the oxygen content of the sample gas surrounding the dumb bell results in a change in the torque developed on the dumb bell suspension which causes the dumbbell to rotate.
The image of a beam of light reflected from the mirror attached to the dumb bell falls upon a differential photo cell arrangement which senses any displacement of the dumbbell.
The signal is then amplified by an all solid state integrated amplifier with current output.
This current flows through the single turn feed back winding on the dumb bell to maintain the dumbbell in its original position.
Since this current is proportional to the oxygen content of the gas sample, it is used to develop the meter reading on the analyser.
This current feedback force balance design is resistant to mechanical shock and has an outstanding accuracy and linearity.
The para-magnetic susceptibility of oxygen varies inversely with the square of the absolute temperature, thus a temperature sensitive element in contact with the magnet/measuring cell assembly is included in the feedback current circuit to provide compensation for changes in analyser temperature.
The sample gas is brought in to thermal equilibrium with the measuring system by means of a heat exchanger tempering coil also in thermal contact with the magnet structure.
It sounds rather complicated but they are very reliable and so with luck, you should not have to become involved with the insides, still it’s nice to know how it works in case anyone should ask you.
If you do get one of these there is a very good service book with it, to try to go into fault finding would take far to long, and because of their reliability probably a waste of time.
The instrument that I am referring to is made by the English firm of SERVOMEX in Crowborough, Sussex.
As far as I know, they are the leading firm in this field.

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