The units utilise the principle of UV absorption for measurement otherwise known as Dual or Split Beam Spectrometry.  Air is pumped via the sample inlet and internal dust filter into the measurement chamber where a UV light is absorbed strongly by any mercury vapour.  The UV source is driven by a unique inverter circuit which excites the lamp to produce a selective 253.7 millimicron wavelength output.  The measurement chamber contains a beam splitter which provides a reference and a sample UV beam.  The reference beam passes to the reference UV enhanced sensor to provide a reference signal.

The sample UV beam passes through the glass measurement chamber to the measurement sensor.  Any mercury vapour in the measurement chamber will be strongly absorbed by the UV light and so the signal to the measurement sensor is reduced.

The two signals are passed to the unit’s electronics where processing takes place to linearise the unit’s response, drive the LCD display, recorder output and audible alarm.

Consideration of certain problems associated with the measurement of mercury vapour are taken into account in the design of the units.

Mercury vapour molecules are relatively large molecules and tend to be adsorbed or stick to surfaces especially if these surfaces are dirty.  This will result in a slow and sluggish response and after exposure to high concentrations a long time is required for the instrument to return to a zero reading in fresh air.

The internal surface area and volumes are kept to a minimum to reduce this problem.  The measurement chamber is glass to reduce solarisation and this chamber is protected by a replaceable dust filter.  The sample pump is placed after the measurement chamber so any contamination build up will not be seen by the unit.  Also the pump provides a flow rate giving a very fast response time (3 seconds to see 90% of a change).

These considerations allow the units to have a very stable zero and sensitivity to detect mercury vapour levels as low as 1 µg/m3.