Pressure sensors come in 3 basic configurations:
1. Absolute pressure. This is a pressure sensor where the low pressure port has been evacuated to as near a perfect vacuum as possible and sealed. The active port will respond to positive pressure changes and can be calibrated in various engineering units (inches of mercury for barometer use) and would typically display 14.7 PSIA at sea level if scaled for PSIA readout. This is the type of pressure sensor used to measure elevation and elevation also requires the ablility to mechanically or electronically trim the calibration to compensate for local changes in barometric pressure due to weather conditions. The high top range is dependent on the specific sensor design.
2. Gauge pressure. This is a pressure sensor where the low pressure port is left vented internally and unavailable to the user. The active port will respond to pressure changes but because the low port is vented this will always measure 0 PSIG if the active port is vented to atmosphere no matter what changes there are to local barometric pressure or how high in elevation the sensor is moved to. The top of range depends on the specific sensor specs.
3. Differential pressure. This is a two port pressure sensor that only measures the difference between the high and low user ports and is not affected by local barometric or elevation changes, just the difference between the two ports. Some can read both negitive and positive differential pressure by and have a "center zero", others only read in one direction but the port tubing can always be swapped to cause a positive reading. The top of range depends on the specific sensor specs. This is the type of sensor used to measure flows, speed, liquid levels, pressure drop across some item, etc.
People tend to not add the A or G when using the PSI units and sometimes that can be misleading or misunderstood as it assumes the reader knows what kind of pressure reading is being performed and the type of sensor being used.
In summary all pressure sensors are two port sensors, it just depends if the second port is avalible to the user and if not, if it's evacuated and sealed or just vented internal to the sensor.
Lefty