A pressure relief mattress comprising: a plurality of inflatable bladders; a pressure sensor pad arranged to detect pressure exerted on a subject, the pressure sensor pad comprising a dielectric substrate, plural rows of flexible conducting strips disposed on a surface of the dielectric substrate and plural columns of flexible conducting strips disposed on an opposite surface of the dielectric substrate, the plural rows and columns of flexible conducting strips overlapping at a plurality of intersections to form a matrix of pressure sensor cells, a pressure-sensing electronic unit electrically connected with the plurality of pressure sensor cells, the pressure-sensing electronic unit being configured to selectively drive one or more of the pressure sensor cells for generating a pressure measurement result; and a pressure-control electronic unit communicatively connected with the pressure-sensing electronic unit, the pressure-control electronic unit being configured to control operation of one or more of the inflatable bladders according to the pressure measurement results; characterized in that the pressure-sensing electronic unit is configured to eliminate interference from stray electrical fields from neighbouring said pressure sensor cells of the one or more selected said pressure sensor cells, the pressure-sensing electronic unit being configured to selectively drive an active said conducting strip on the surface of the dielectric substrate with an alternating voltage with reference to a corresponding sensing said conducting strip on the opposite surface of the dielectric substrate, and to measure a capacitive current from the sensing said conducting strip for generating the pressure measurement result, the pressure-sensing electronic unit being adapted to control neighbouring said conducting strips of the active said conducting strip on the surface of the dielectric substrate and the sensing said conducting strip on the opposite surface of the dielectric substrate at a same voltage level. a plurality of inflatable bladders; a pressure sensor pad arranged to detect pressure exerted on a subject, the pressure sensor pad comprising a dielectric substrate, plural rows of flexible conducting strips disposed on a surface of the dielectric substrate and plural columns of flexible conducting strips disposed on an opposite surface of the dielectric substrate, the plural rows and columns of flexible conducting strips overlapping at a plurality of intersections to form a matrix of pressure sensor cells, a pressure-sensing electronic unit electrically connected with the plurality of pressure sensor cells, the pressure-sensing electronic unit being configured to selectively drive one or more of the pressure sensor cells for generating a pressure measurement result; and a pressure-control electronic unit communicatively connected with the pressure-sensing electronic unit, the pressure-control electronic unit being configured to control operation of one or more of the inflatable bladders according to the pressure measurement results; characterized in that the pressure-sensing electronic unit is configured to eliminate interference from stray electrical fields from neighbouring said pressure sensor cells of the one or more selected said pressure sensor cells, the pressure-sensing electronic unit being configured to selectively drive an active said conducting strip on the surface of the dielectric substrate with an alternating voltage with reference to a corresponding sensing said conducting strip on the opposite surface of the dielectric substrate, and to measure a capacitive current from the sensing said conducting strip for generating the pressure measurement result, the pressure-sensing electronic unit being adapted to control neighbouring said conducting strips of the active said conducting strip on the surface of the dielectric substrate and the sensing said conducting strip on the opposite surface of the dielectric substrate at a same voltage level.