First of details of the Cloud sensor can be found here ..... http://tech.groups.yahoo.com/group/sgl_observatory_automation/
In the Files area .... I am using the info from the SGL Files area.
On a clear day the MLX90614 reads the temperature of the upper atmosphere and will return a figure of ~ -23 or so. It also reads the ambient temperature. If clouds (even extremely fine stuff - it is that sensitive) move in the sky temp will fall. As the cloud thickens the sky temp will fall further. You can just watch the sky temp but the difference between ambient and sky temp can be used to create a traffic light system of cloud/part cloud/cloudy. In astroimaging the slight falls in sky temp are really useful to know about as that can be cloud or dust which can affect the seeing.
For example ......
It works extremely well - I just need to stop the dewing. The sensor has to be open to the sky. Any covering will mean the sensor is just reading the temp of that. As for the anti fogging stuff ..... I am not sure that would be a very long term solution.
It is impractical to use a fan. The box the sensor is mounted in is tiny. I was considering using a loop of nichrome attached inside the box around but not touching the sensor. The wire only needs to be warm. I am not talking lots of heat here. The amount of wire needed against the voltage is all greek to me though so all help is very welcome. 8-((