OXYGEN ANALYSER

Typical Number in Hospital: 20

Cost Bands: 2,3,4

References: 6

There are a number of situations in anaesthesia and intensive care where it is important to measure the oxygen concentration. In addition, oxygen analysers are important tools in the checking of oxygen therapy and anaesthetic apparatus for correct performance.

There are three main types found in hospitals:

1. Paramagnetic analysers. Oxygen is unique among gases in that it is paramagnetic (moves towards stronger magnetic fields) whereas most other gases are diamagnetic (move away). In the paramagnetic analyser (see separate entry) a small glass bulb of nitrogen experiences a force acting towards a weaker part of a magnetic field when the gas surrounding it is diamagnetic. Such devices can be made fast acting, highly accurate and durable.

2. Fuel cell analysers. Oxygen diffuses through a permeable membrane (usually PTFE - Teflon) at a rate dependent on the concentration and this oxygen feeds a chemical reaction leading to the production of a small electrical current. These take 30 seconds or so to reach equilibrium and the cells normally only last for a few months.

3. Polarographic electrodes. This uses the Clark electrode in which oxygen diffuses through a PTFE membrane into a solution containing a platinum cathode and a silver-silver chloride reference electrode. Oxygen dissociates in the presence of water and a small potential between the electrodes (usually 0.6 V). This produces a very small current between the electrodes which is amplified and displayed. The electrodes do not last indefinitely.