Ultrasonics – Degassing Using a Pulse Degas Cycle

Using a slow rate of pulsed ultrasonics is an excellent way to speed the degassing of liquids in preparation for use in ultrasonic cleaning and rinsing applications.  The blog

Ultrasonics – How to Degas Liquids

explained the mechanism by which dissolved gasses are extracted from liquids by ultrasonic cavitation.  Although this degassing mechanism is very effective at removing gas, there is a little “hitch.”  With constant ultrasonic excitation, the gas bubbles that are released coalesce to form larger bubbles that, although sufficient in size to be significantly buoyant, become “trapped” at the nodes of the ultrasonic waves in the liquid.  These suspended gas bubbles hinder further progress of the degassing process.  Turning off the ultrasonic energy periodically, allows the trapped buoyant bubbles to float to the surface of the liquid so that when ultrasonic energy is re-activated new bubbles can form.

The following video shows the fish tank described in the blog

The Ultrasonic Fish Tank in the Lab

At the beginning of the video, the non-ultrasonically activated tank has just been filled with room temperature De-Ionized water from a pressurized De-Ionized water supply (approximately 100 psi).  It is easy to see that there are a multitude of bubbles adhering to the glass front and back walls of the fish tank.  These bubbles have formed spontaneously within a few minutes after tank was filled due to the reduction of pressure created as the water left the pressurized supply.  If you look closely, you will also see a few small bubbles floating to the surface of the liquid.  These too, are bubbles formed due to the release of pressure and are exactly the same as those seen when pouring a carbonated beverage from a recently opened bottle or can into a drinking glass.

Video Showing Liquid Degassing Using Pulsed Ultrasonic Degas Cycle

When the ultrasonic energy is turned on, degassing of the liquid is immediately accelerated.  But, the “trapped” bubbles referred to above soon form as smaller gas bubbles released by each cavitation event group together or “coalesce.”  When the ultrasonic energy is turned off, these “trapped” bubbles float to the surface.  As the cycle is repeated, more bubbles are formed, trapped, and then released when the ultrasonic energy is again turned off.  The slow pulsing of ultrasonic energy as described in the preceding blog is beneficial to the degassing process and is included as a feature on many more advanced ultrasonic cleaning systems.  The pulse rate and duty cycle should be set so that released bubbles have time to float to the surface or at least rise significantly toward the surface before ultrasonic energy is re-instated during each pulse cycle.

Other benefits of pulse will be described in upcoming blogs.

–  FJF  –