Unlike the absolute pressure loggers the differential pressure logger does not require a reference logger to compensate for the air pressure. The differential pressure logger has a two sided pressure sensor of which one measures always the air pressure and the other can measure water pressure. A differential pressure logger can be used at points where there is no problem leading an air tube to the logger or where the logger is placed outside the water. Since one side of the sensor must be in contact with the water it is not very suitable for measuring water levels in open channels, the tube leading from the logger to the air would be to vulnerable to survey during floods. However since a differential measuring method is more accurate than measuring with two absolute loggers it is a perfect device to measure small difference for instance the water pressure in a tank that catches the rain water. The air pressure side of a differential sensor should never be in contact with the water or any other fluids, this would destroy the sensor. So generally speaking a differential sensor is more vulnerable than an absolute pressure sensor.

The external differential logger is based on the same principle but its sensor is connected by a cable to the logger and it has an air tube inside the cable to obtain the air pressure. This external differential pressure logger is ideal for measurements in wells and bore holes since its sensor is only 17.5 mm in diameter it will fit in very small holes. Care should be taken when using a sensor with a vented cable, one must make sure that the vented cable remains free of water of water damp. The sensor will give unreliable results if water starts condensing inside the vented cable, the air pressure is not accurately obtained by the pressure sensor.