Video: Spraying with a HVLP gun with excessive air pressure

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This shows a procedure called video-visualisation. An operator (wearing an air-fed visor) is shown spraying a vehicle. At the same time the concentration of isocyanates surrounding the sprayers visor is measured and recorded from second to second using specialist equipment from the Health and Safety Laboratory.

These measurements are then superimposed on a graph showing isocyanate concentration on the Y axis and time across the X axis. There is also a small box above the graph that shows the numerical concentration from second to second. The video plays in a screen above the graph and it is possible to show what activities cause the highest and lowest exposures.

When used correctly, an HVLP spray gun has a higher transfer efficiency than a conventional gun. In this case, however, the air pressure on the gun has been increased above that recommended by the manufacturer. This makes the gun operate more like a conventional spray gun. The graph of the spraying activity again shows a series of peaks and troughs. Although not shown in this clip, some of these peaks now go beyond the top of the graph (exceeding 7,500). When averaged over the spray job, the concentration in operator's breathing zone is now 1387 microgram/cubic metre compared to 1067 for a properly set up HVLP gun and 1708 for a conventional gun.

In other words, interfering with the settings on an HVLP gun can cancel out the improvements in transfer efficiency and lead to unnecessarily high concentrations of isocyanate in air (as well as clogging up the extract filters more quickly).