Stack Opacity Affected By Rain

[PSP113]

Hello. I work in a coal-burning power plant. We have noticed that during a couple heavy downpours recently, our stack opacity has gone up significantly for short periods and then returned to normal. The opacity meter is situated about half way up our stack. Can anyone offer suggestions why heavy rain would cause these erroneous readings?

 

[rmw]

Well, let's see. You have heavy rain falling straight down (I have to assume straight down) through a dust and/or moisture laden atmosphere being monitored by a device that looks through whatever is in between the sensors.

Could it be that the addition of the presence and density of the rain droplets, dirty or not are being detected by the opacity monitor and registered?

rmw