Thanks guys - with 2 mins searching on the net I found Mr Deratz, now enjoying his retirement on the Sunshine Coast north of Brisbane. He was very kind but couldn't really remember how it worked, since he designed and built it about 43 years ago! He did say he probably built about 100 of these - which makes me proud to own one even if I can't figure it out. I understood about half of what you suggest Bill, but will think on it. Not sure what Delta black/white readings are but will google that. The VU meter at the bottom has three sets of figures, corresponding to the speed choices above it - 5,10 or 20 seconds, so you read the amounts based on one of the three lines...which would appear to indicate seconds. He said it uses a cadmium sulphide sensor in the paddle, so it has some memory. Any suggestions on the sequence of how you use it would be appreciated - and yes, I do make test strips, but as Bill says, the fun is in the journey. I just can't help being curious. Thanks for your help so far.

PB