Have a look at page 77/78 of the document called "Student Manual" available for download on this page:

http://www.ohlearning.com/training/training-materials/w505-control-of-hazardous-substances.aspx

There are suggested capture and trasnport velocities for various contaminants.

The supplier of the LEV system should have supplied you with the operating pararmeters of the system when it was installed.

You will struggle to check internal duct velocities with a vane anemometer.

I would normally use a pitot-static tube with a differential manometer for duct velocities.

On my P601/602 we were taught only to use an anemometer for capture hood face velocities, and once you have done some LEV testing in the field you will realise why.

Duct air velocity only indicates whether the system is working to the set value. It does not necessarily indicate whether the system is adequately removing the contamination. I see many systems where the 'performance' is as specified but the 'effectiveness' is inadequate. A simple initial indication can be gained by the use of a smoke tube. The visible movement of the 'smoke' will indicae whether the air movement is such that it could be properly removing the contamination. As my mentor on LEV many years ago (Frank Gill) once said to me: "Any fool can move a lot of air. It is how you design the capture system that is critical."

Incidentally, if you look at the penultimate page of Health and Safety Matters Oct/Nov issue, in the interview with Phil Roberts  a HM specialist inspector, he includes smoke tubes among his three favourite pieces of equipment.

Chris