chamber may be changed by simply turning the dial to a different setting electronic comparator 30 compares the selected pressure with the actual pressure within chamber 10 as transmitted through amplifier 28 comparator 30 produces an output signal that is forwarded to control electronics 34 that controls an air pump 36 and a selectively open vent valve 38 the control electronics 34 in response to the comparator signal either turns on electric air pump 36 or opens electric vent valve 38 to add or remove air from chamber 10 in a preferred embodiment pump 36 is a diaphragm pump control electronics 34 are conventional as are air pump 36 and vent valve 38 in typical operation a user selects a desired air pressure within chamber 10 by adjusting the dial on the pressure select control 32 assuming an initially deflated chamber electric air pump 36 is activated to pump air into chamber 10 until pressure sensor 26 senses the pressure within the chamber is substantially equal to the desired selected pressure if the air pressure within chamber 10 should increase or decrease due for example to a change in temperature or atmospheric pressure of the ambient air within the room where the mattress is placed the change in pressure will be sensed and either air pump 36 turned on to force air into chamber 10 or vent valve 38 opened to bleed air from the chamber in this manner the preselected pressure is maintained when a person lies upon the mattress the pressure within chamber 10 is increased substantially this increase in pressure is sensed by.