Gauge pressure is measured relative to the ambient atmospheric pressure. Thus, changes in the atmospheric pressure due to weather conditions or altitude directly influence the output of a gauge pressure sensor. A gauge pressure higher than the ambient pressure is referred to as positive pressure. If the measured pressure is below the atmospheric pressure, it is called negative or vacuum gauge pressure.Although most gauge pressure sensors technically are differential pressure sensors — measuring the difference between the media and atmospheric pressure — true differential pressure sensors are used to identify the difference between two separate physical areas. PCE-P50 features two pressure ports and a differential pressure function for measuring true differential pressure.