A sensor for deposit build-up in plant, for example a paper-making installation, comprising a probe (30) mounted with respect to a mounting body (31) and including at least one strain gauge for measuring the flexural strain in a flexurally deformable body strained by imbalance in the probe in response to the combined mass of the probe and the deposit build-up thereon. The process of the invention envisages using the resulting signal for controlling the application of a deposit-controlling composition. An optional further sensor (88) enables detection of the nature of the deposition whose quantity is sensed by the strain gauge.