I have always disliked mounting sensors inside CL heads, and the Aristo VCL4500 is both quite hot and very bright. So I mounted the sensor inside a piece of 0.5inch standard aluminium square tubing in the side of which there is a 0.375inch hole which corresponds to a similar hole in the side of the Aristo unit. The piece of tubing is screwed to the outside of the unit and enough light gets to the sensor. The sensor is held parallel to the mounting surface by a long thin rectangle of glass slipped into the tube. This also acts as a heat protector although it is probably not necessary. I finished the unit in a hurry so that I could use it, and it's not very pretty. I have attached a photo.
The setup is excellent. The VCL4500 unit that I have is the basic one, in which the green tube is on all the time without being able to be varied (except off) and the blue is variable from zero to bright enough to swamp the green almost entirely. The sensor with the Vario unit gives a "first order" adjustment to time if the contrast is altered, so calibration is not as important as I first thought. I am using up lots of random old papers at the moment and when I settle on one or two papers that are readily available here at the end of the Earth I will do a full calibration. At present it's trial and error but not nearly as much error as I expected.
An advantage of having the sensor outside is that it reduces the very powerful light to get values that fit conveniently with the timer (although that is adjustable if necessary)
Last edited by john_s; 04-06-2013 at 05:24 PM. Click to view previous post history.