Yesterday I made a batch of jam using Mr Miot's method. It's based on his standard method which is different from the method I've used myself previously.
First you freeze the lemon and the chopped and cleaned rhubarb. Freezing and defrosting the lemon should ease the extraction of pectin for setting the jam. Freezing and defrosting the rhubarb should extract water juices from it, keep just 200 ml.
Heat the sugar, juice from the lemon and the lemon along with the rhubarb juice up to boiling point (121°C). Once it's rolling along add the chopped rhubarb and return to the boiling point. Boil hard for a further 15 minutes (give or take) and then add the ginger. After removing any scum and a a few more minutes it should be ready to pot.
I jammed 2.2 Kg of rhubarb with 1.76 Kg sugar, two small lemons and 0.5 Kg of chopped crystalised ginger. Tasted okay on the night, but rhubarb and ginger takes a few days to reach full flavour.