It can be difficult to make a qualitative evaluation of the effectiveness of LEV hoods because the contaminants are either invisible (in the case of most gases and vapours) or difficult to see (with fine dusts). Two main techniques can be used to overcome this problem:
- smoke tests
- dust lamps
Smoke released in the vicinity of a hood will move with the air in the same way as gases and vapours and can, therefore, give a good indication of how these contaminants behave and can help us judge how well they are being controlled by the LEV hood.
Using smoke to visualise air movement into a fume cupboard
Smoke testing is a good way of evaluating the effectiveness of LEV hoods intended to control gases, vapours and fume (fine particles) which will usually behave in a similar way to the smoke. However, dusts, due to their mass, may behave differently. Using smoke is still useful – if the smoke isn’t captured effectively there is little chance that the dust will be controlled – but other methods will probably need to be used for a proper assessment of control. In such cases using a dust lamp will normally be useful.
Very fine, respirable, particles cannot be seen with the naked eye, even where they are present at a high airborne concentration. When an intense parallel beam of light passes through the dust cloud, light scattering occurs that makes the particles visible to an observer looking along the beam towards the lamp. This effect was investigated by John Tyndall in the 19th century. Consequently it is it is often referred to as the Tyndall effect.
This method can help us to visualise the movement of the dust cloud, and is described in the HSE publication MDHS 82 The dust lamp – A simple tool for observing the presence of airborne particles. The following diagram, taken from the document, illustrates the principle.
Using a dust lamp to visualise fine dust (Source: HSE)
Note that it is forward scattered light that shows up the particles – that is where the light beam originates from the opposite side of the source to the observer. So the observer must stand on the opposite side of the dust cloud to the lamp, looking along the beam (taking care not to stare directly at the light source), as shown in the diagram.
Dust cloud revealed using a Tyndall beam (Source: HSE)
Often it’s enough to use these qualitative methods to decide whether an LEV system is controlling the contaminants.