Introduction | Radiation Force Balances | Measurement | Additional Information | Further Reading

Water

Degassing

During ultrasonic power measurements, the presence of gas or vapour-filled bubbles in the medium will scatter the ultrasound from the transducer under test, by 'shielding' the target, or by coalescing and gathering under the output face of the transducer under test, so scattering the incident beam. Bubbles can cause instabilities and underestimates of the true power. Bubbles of sufficient size can be pre-existing, or can develop from initial microbubbles under the influence of the acoustic field - a process known as cavitation.

The likelihood that cavitation will arise in any experimental set-up depends on pressures generated in the acoustic field, propagation distances, the force balance configuration used and the quality of the water. However, it will generally be most prevalent at the lower frequencies (below 2 MHz) and high powers. To minimise the likelihood of bubbles affecting the accuracy of power measurements, it is recommended in the applicable standards that the medium in which the measurements are being carried out be degassed beforehand. The two principle methods for degassing water are:

• Reduced Pressure : reducing the overpressure of the water (which should be initially distilled) to around 104 bar for at least 3 hours such that the dissolved gas is effectively pumped off. This can be carried out by setting up a fine spray of water into an evacuated chamber.

• Boiling : sustained boiling of the water (15 minutes at 100 °C) will also slowly de-gas it.

With both of these methods, as soon as the degassing process has been completed, the medium will start to regas if there is an air-water interface in the storage vessel. For example, a 1 litre laboratory beaker filled with degassed water and left to stand in free air will regas to tap water levels in around 5 hours. Water surface agitation will substantially decrease the regassing time. Degassed water should thus be kept in a sealed container until required for use. Determining the extent to which water is degassed should be done by measuring the dissolved oxygen content using a laboratory meter. As a guide, the dissolved oxygen level should be 4 mg/l (ppm) or less to minimise the likelihood of cavitation affecting measurements. In comparison, portable tap water has a dissolved oxygen level of around 8 mg/l.

Chemical additives

The need for degassed water may represent a significant problem when implementing power measurements in non-laboratory environments. An alternative approach is to use the chemical additive sodium sulphite, Na₂SO₃. This is fully soluble in water and acts as an oxygen scavenger, removing the dissolved oxygen from solution. A 4 g/l solution will reduce the dissolved oxygen content to < 0.1 mg/l. It is important that the solution is made using deionised water and high-purity grade sodium sulphite and when made in this way the negligible oxygen level is maintained for at least two to three days. The chemical is inert and the only precaution which needs to be taken is to rinse out the target and water tank following use to remove the possibility of the solid product being deposited on drying.

Helium

Another method of efficiently degassing water in the hospital or clinic environment is to bubble helium gas through the water. The inert gas displaces other, more soluble, gases such as oxygen. Helium gas is bubbled into water through a sparger, similar to the process used to oxygenate fish tanks. Gas flow rates of 500 to 1000 cc/min remove 95-98% of dissolved oxygen from one litre of water in a few minutes. This is thus a very efficient method for degassing relatively small amounts of water, but requires proportionately more time and helium to degas greater volumes. As with the vacuum and boiling techniques described above, water degassed using helium will reabsorb gas from the atmosphere over a period of a few hours.