The main property of oxygen which helps in its
detection and measurement of its percentage in the
given sample of air is that of Para-magnetism.
Basically this means that oxygen gets attracted
towards a magnetic field.
As you can see in the sketch there are two
platinum resistance wires which are exposed to
two different chambers.
In the first chamber there
is the air in which the oxygen content needs to be
monitored plus there is an artificially created
magnetic field, while the second chamber there is the same air but there is no magnetic field
present in this chamber.
The wires form a part of the Wheatstone bridge circuit and if you
remember from your study of physics, the Wheatstone bridge is used to measure an unknown
resistance by obtaining a balance between two legs of the circuit.
The air whose content has to be
noted is supplied at one end and is filtered by passing it through a diffuser and goes on to the two
chambers as shown in the picture. Hence while one chamber of the meter attracts oxygen the
reference chamber attracts only air.
This causes a difference in the temperature of the two wire
sets because of the difference in thermal conductivity of oxygen with respect to air.
This causes
imbalance in the resistance of the bridge legs and the degree of this imbalance is in proportion to
the oxygen content in the sample to be measured.
Last but not least there is a meter which is calibrate to show this difference in resistance as a
percentage of the oxygen in the given sample, thus allowing the ship staff to note the oxygen
content in the given space.
*Calibration of oxygen analyser (two-point calibration):*
Normal calibration is performed using two different gases:
a zero gas of low oxygen concentration (0% oxygen) and a span gas of high oxygen
concentration ( 20.9% oxygen)
1. Apply pure N2 (99.9%) from calibration gas cylinders and see the display, if it is 0% OK. If
not, then adjust from settings to make it ZERO.
2. Apply clean and fresh atmospheric air (NOT from an air conditioned room or confined space)
and see the display it should be 20.9 or 21.0%, if not adjust SPAN setting to display it correctly.
3. Calibration is done and is ready for sampling enclosed space or any other space. - If needed
consult supplier to get it calibrated on test bench available with him.
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