MediaSensor™ P51 sensors offer three different pressure measurement types: Absolute, Gauge, and Sealed Gauge. Absolute sensors (or PSIA) measure pressure relative to a sealed chamber at a perfect vacuum, which remains unchanged regardless of ambient pressure conditions. In other words, if atmospheric pressure changes are expected, such as if installed at sea level vs. high in the mountains, transducer accuracy will be affected. This would be a problem only with lower pressure ranges where large changes in atmospheric pressure will be more noticeable. Conversely, for high pressure applications, even large changes in atmospheric pressure will be barely, if at all, noticeable. For example, if the application requires a 0 to 3 psi pressure range, and there is a 2 psi ambient pressure change, the transducer could be off by 67% if an absolute transducer was used. The absolute transducer does not compensate for this change in atmospheric pressure. That same 2 psi ambient change would not be noticeable in an application requiring a 500 psi transducer, even if an absolute transducer was used. Continuing with the example, a 2 psi change in ambient pressure would not be noticed if a gage (PSIG) sensor was used, no matter what the application pressure range. Gage sensors are different in that they measure the application pressure relative to ambient pressure rather than a chamber sealed at a perfect vacuum. Ambient pressure gains access to the back side of the sensing element through a vent tube that is vented (open) to the atmosphere. Therefore, gage sensors compensate for changes in atmospheric pressure. For low pressure measurement applications, a gauge pressure sensor will be required because any small change in atmospheric pressure (such as that caused by a change in elevation) can cause an accuracy shift if not compensated for. Sealed gage (or PSIS) sensors measure pressure relative to a sealed chamber at 0 psig. Gage sensors are used for lower pressure applications, and Sealed Gage sensors are used in higher pressure gage applications.  In both cases, measurement in a vacuum is not required, nor provided. Absolute sensors are used where vacuum, or both vacuum and positive pressure measurement is needed in the application.