The device comprises two separable parts including a first part 54 having a power supply and processing electronics 52 and a second part comprising a display module 51 and sensing module 55. The display module 51 may comprise a disposable and irreversible thermochromic strip 57 (of the type in Fig. 2). The sensing module 55 contains an electrode arrangement 56 in the form of a capillary fill device (CFD) comprising a pair of spaced apart plates defining a known volume with electrodes therein. In use, an electrical potential is applied across the electrodes of the CFD resulting in a flow of current through the sample fluid proportional to the electrochemical activity. Electronic means processes the response to produce an electrical signal of magnitude indicative of the characteristic which is recorded on the display 57.