Cheshunt Compound Shelf Life ?

Thats right Jim.

The first time I took a measurement I was so horrified by the result that I didn't notice anything else. It was only later, when repeating the test, that I noticed the blackening of one of the probes. This suggested electrolytic action of some sort, which set me off on a trail of trying to find out how the garden pH meters worked. I did try cleaning the probes, but it still gave the same result. Each time, it took 5 seconds or more for the needle to move from 7 to 1.5. I assumed that this was in the nature of the instrument to have a small time delay, but now I realise it was that the voltage was probably genuinely changing as one probe got coated. Its nice when all the little details get explained.