Here is how the DualAir_S_V1 measures dissolved air:

The darker lines marked with arrows illustrate the flow of process fluid either into the DualAir_S_V1, or bypassing the tester, depending upon the arrangement of the valves. The bypass valve is provided by the customer.

At the starting condition, the Fill and Bleed Valves are opened and the Bypass Valve and PIT Fill Valves are closed. The Compression Piston is moved almost fully into the Sample Tube.

When the Sample Tube is filled with fresh process fluid, the Bypass Valve is opened, the Fill Valve is closed, and the Bleed Valve is closed.

The PIT Fill Valve is then opened. The Compression Piston is moved fully into the Sample Cylinder, causing process sample to fill and overflow the PIT tube. The Compression Piston is then moved slowly out of the cylinder until the liquid level in the PIT Tube reaches the Upper Reference line. The PIT Fill Valve is then closed. The Compression Piston is further removed from the cylinder until the liquid level in the PIT Tube reaches the Lower Reference Line. This action places a known negative pressure on the trapped sample.

The number of turns of the Threaded Compression Shaft needed to move the liquid level to the Lower Reference Line is used to calculate the dissolved air. If the process fluid also contains entrained air, then the entrained air value previously determined is also used in the calculation of dissolved air.