An amount of a substance contained within blood circulating within the human body tissue is evaluated. An application region of an apparatus makes skin contact at the position of the tissue. The apparatus includes electrodes arranged on a dielectric substrate to present an active surface at the application region. Force data is derived from a detector, that is compared against a first predetermined level 1401. In response to this comparing step, capacitive coupling procedures are performed or these procedures are inhibited. Thus, procedures are inhibited if a sufficient force has not been applied. If a sufficient force has been applied, capacitive coupling procedures capacitively couple selected pairs of the electrodes by producing electric fields that penetrate the tissue, to produce monitored output data, from which the evaluation is made.
