Greisinger gmh 3531 ph orp rh temperature meter greisinger gmh 3531 the b gmh 3531 b is a handheld meter that is designed to carry out measurements of b ph b levels b redox b orp b rh b levels and b temperature b the meter carries out standard or mv sub h sub measurements in orp mode that allow for automatic conversion to hydrogen system electrodes calculation of the rh value is by means of an orp measurement and by manual entry of ph value or use of previously record