A generator for producing pure air at high pressure includes a compressor, electrical means for driving the compressor and means for purifying compressed air generated by the compressor. A first sensor is provided for detecting the pressure of the compressed air and a second sensor for detecting the absolute temperature of that air. Means is provided for transferring an electrical signal from the first sensor to an electronic controller. Means is also provided for transferring an electrical signal from the second sensor to the controller. The controller processes these electrical signals and varies the power supplied to the means for driving the compressor in such a manner that the ratio of pressure of the compressed air to the absolute temperature of the compressed air remains substantially constant.