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AEM offer high quality brass PSIa sensors in 30(2BAR), 50 (3.5BAR) and 75 (5BAR). PSIa is referenced to absolute zero. Absolute zero is the pressure measurement when all the pressure exerted by the atmosphere has been removed. The most common use for absolute pressure sensors is reading manifold pressure. PSIa sensors are required of you wish to measure vacuum. A PSIa sensor will read about 14.7 PSI when the sensing element is exposed to atmospheric pressure at sea level however they can be recalibrated to read zero at atmospheric and a negative number at values less than atmospheric pressure.