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  –

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3 Responses to Ultrasonics – Degassing Using a Pulse Degas Cycle

  1. Francis Chan says:

    Hi John,

    Thanks a lot for your response.

    The flow rate I needed for my experiment is 1 litre/min (which translates to about a quarter of gallon per minute), so the water will have a residence time of about 2 minutes in average.

    I also intend to place the transducer vertically on the side of the tank, so the the wave plane will also be vertical (inside that very limited tank space) to avoid the bubbles trapped by the standing wave. If this works, I can apply the ultrasonic power continuously.

    The purpose of this experiment is to provide clear degassed water to a fish tank in our bio-monitoring project. Before degassing, the bubble sticking on the glass interferes with the image capturing.

    The water is fed from a 1.0 bar source already saturated with air at 60 deg.F, warmed up to 90 deg.F, then released to the small tank at atmospheric pressure (in which I will install ultra-sound transducer) by a throttling valve which limits the flow rate to the above said value. Thus the water when reaching this tank is at super-saturation, stay for 2 minutes and degassed somewhat, then flow to the 2 gallon fish tank with 5 zebra-fishes and monitored by software. In this case, I shall not over degas the water to suffocate the fishes.

    I am really delighted to know you also used 40 kHz in your video, this information is really helpful to me.


  2. John Fuchs says:

    The frequency used in the video was 40kHz. Sixty watts seems a little low for power but that, of course, will also depend on your flow rate. You may first want to conduct some experiments to determine what residence time you need to achieve the desired effect. – FJF

  3. fhpchan says:

    Hi John,

    It’s very interesting to see your video, it is very very useful.

    I wish to try something similar, with water flowing continuously into and out of the tank to attend inline degassing effect. Could you tell what frequency of ultrasonic you did use in the video?

    I think I will try 40kHz at the power of 60W with a small 1/2 gallon tank.


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