Regarding the parameter opacity, an optical parameter, it likely will be difficult to obtain with conventional optical instrumentation. Unfortunately, your question is not very precisely formulated. In order to work out precise options, one would need to know more details. For example: what exactly is the environment of your problem (closed or open system, lab conditions or external env.)? What temperature do you mean by "hot"? How large is the volume under consideration in which the measurement is to take place? How large are the particles you are addressing? And finally, what do you really want to measure, the particles (size, number density, etc.) or the volume fraction of water vapor?

Hello René. Thank you for the points raised. I am talking about an open system in an external environment (materials yards). The temperature of the materials dumped is around 100-200°C. The volume of materials is small, 1 or 2 m3. They are wetted before the dumping operation to avoid dust emission, mainly total suspended particles (TSP) and PM10. I would like to measure the particles concentration or any parameter related to the particles emission. The opacity was the easiest way that we found to determine if there is or not PM emission, however, the vapor generated in the process interferes in the measurements.

A brute force approach would be to set up a continuous sampling system where a sample of the air is heated to make sure that there ar no water drops. Flow that heated sample into a heated optical cell of path length "L" and measure the opacity across the optical cell. You might need to experiment with finding the optimum temperature

In the presence of water vapor, it is necessary to condition the sample stream for example by heating it so that the relative humidity of the air stream to be measured does not exceed about 50% above which water will accrete on the particles growing them.