Sensing water content in microwave oven

Discussion in 'General Electronics Chat' started by kender, Dec 28, 2009.

  1. kender

    Thread Starter Senior Member

    Jan 17, 2007

    I need to estimate the water content in a laboratory microwave oven. The oven is used to vaporize the water out of the substrate. The goal is to turn off the generator when enough water have been removed through evaporation.

    My gut feeling is that power consumption of the generator would depend on the amount of water in the chamber, provided that geometry remains the same (e.g. beaker with 100cc of sand and 50cc of water vs. same beaker with 100cc of sand 25cc of water). Have anyone used this approach for moisture content sensing? What would be a good name/keyword for such approach? Could anyone post some references to books or application notes?

    Other/additional/alternative types of sensors can be used too. I can modify the oven as much as necessary (even tear it down and put together my own).

    Have a good New Year!

    - Nick

    P.S. At the moment the focus of this thread is the microwave approach rather than alternative sensing methods. I already know a couple of alternative approaches, and I will use some of them in addition to the one in question here.
    Last edited: Dec 29, 2009
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    You will need to move air through the chamber to remove the water vapor. Place a humidity sensor in the outlet stream to sense the moisture remaining.
  3. VoodooMojo

    Active Member

    Nov 28, 2009
    i was thinking of a standing wave meter and a waveguide and remembering the oscillation of water at 25000 (give or take a few) MHZ and then BeenThere's humidity sensor simply beautiful!!

    never overlook the obvious!
    Last edited: Dec 29, 2009
  4. thyristor

    Active Member

    Dec 27, 2009
    Just to clear up a popular misconception, there is no water absorption frequency around 2.4-2.5GHz. It's at 22Ghz or thereabouts.

    A microwave oven works by exploiting the unusually strong dipolar property of water. Although water is electrically neutral, the dipolar state of water allows molecules to behave as if they were composed of small negative and positive charges on either end of a rod. The microwave produces an alternating electro-magnetic field, effectively pushing, pulling, rotating and twisting the water molecules thus adding kinetic energy. This addition of the energy to the water molecules comes from the addition of kinetic energy into the molecule itself, not through an increase in the molecules (or any sub-components) excitation state as a result of RF absorbtion.

    If this were not so and the water were heated through absorption of microwave energy, then 2.4GHz WIFI systems and 2.4Ghz DECT phone systems etc would not work very well near people, who are effectively huge bags of water - although there are some nice looking bags occasionally :)