Objective: To design a reliable, automated, fast, and inexpensive procedure for removing dissolved gas from acoustical media used in acoustical measurements.

Methods: Acoustical characterization of ultrasound hyperthermia equipment often includes measurements where large quantities of coupling media are required. Unfortunately, the most accessible resource - tap water - contains about 12 ppm of free dissolved gas, which acts as randomly distributed scaterrers prohibiting consistent device characterization. Various methods have been proposed for temporary removal of free gas from water, which are generally called as “water degassing”. Two of the most common approaches are boiling and exposure to decreased atmospheric pressure (EDP). The former method takes relatively long time and therefore needs to be done well in advance before the ultrasound measurement to prepare and store required amounts of media. The latter approach is much faster and can be used even during the measurement to keep the media quality at constant level. The EDP however requires more equipment of which operation needs to be synchronized. Here we propose a fully automated, programmable, and inexpensive system based on the EDP. A custom made chamber is filled with tap water using a pump, then water inlet and outlet valves are closed and a vacuum pump forces the air out of the chamber. Pressure drop inside the chamber (Δp= -85 kPa) results in a decrease of gas partial pressure in air surrounding the media and in an increase of dissolved gas bubbles diameter supporting its release through diffusion and Archimedes' principle, respectively. Media is mixed to maximize the effect. Finally, air vent and water outlet valve are opened and water is pumped out of chamber. The operation is fully synchronized with an 8-bit microcontroller. To determine efficiency of the device, oxygen partial pressure was measured before and after the procedure.

Results: The amount of dissolved oxygen in volumes of 4 L of regular tap water can be reduced to 3.3 ppm in 8 minutes using the procedure.

Conclusions: Quality of acoustical media during measurements of ultrasound hyperthermia equipment can be reliably and inexpensively sustained using proposed system.