The
OxiQuant S (that's the small one, at the bottom) used to be available from ProAct.
Here is the sensor (cell) ... which is (was*) also available from ProAct.
Replacement cell cost has gone from £72 to £167!
That quote must be for the complete OxiCheck, rather than just the cell.
*
It was listed in their 2011 catalogue.
