Wash and trim off both ends of the rhubarb. Using a mandoline cutter, slice the rhubarb as thin as you can, leaving on the rhubarb skin (please be careful and use the security attachment)
Bring the water and sugar to boil in a saucepan.
Transfer the rhubarb to a long shallow dish that will hold the rhubarb slices, depending on the length you wish to create for each chip. Pour over the hot syrup, ensuring that all the slices are coated. Cover the dish with cling film (plastic wrap) and leave to cool for about 15 minutes.
Preheat the oven to the lowest setting you can, no more than 90°C/70°C fan/200°F.
Dry the rhubarb slices on kitchen paper and place them apart on non-stick trays (slices touching will end up sticking together).
Dry in the oven for up to 2 hours. Keep an eye on them after an hour, to ensure they're not darkening. If they're small, they will require less time to dry.
Notes
Rhubarb chips work best if using large sticks of rhubarb.Storage: keep in an airtight container in a cool place - can keep for up to 2 months.