I'm closely comparing my InstroMet and Blake-Larsen sun recorders, especially through this period of long days of sunshine, both of which work in different ways.

The Thresholds for the Blakes-Larsen are in some way closely based on a Cambell-Stokes device, which burns a trace using a magnifying glass onto a peice of paper.

Currently I am finding that the Blakes-Larsen misses the first 30 to 45 minutes of sun at dawn and similarily at dusk. As I have never seen a Cambell-Stokes recorder in operation I was wondering on a clear sunny day how long after sun rise (or before sunset) is it before it starts burning a trace.

The location of my sun recording instruments means in the morning it sees the sun within about 10 minutes of sun rise.

An example of the differences from the last 4 days are as follows:

18th
BL 12.71 hours
Instro 13.42

19th
BL 12.53 hours
Many thanks
Intro 13.51

20th
BL 10.03 hours
Instro 11.57

21st
BL 10.29 hours
Insro 11.76

The 18th and 19th had pretty much unbroken sunshine, so are the best comparisons for this exersise.

Keith (Southend)