Re: [compost_tea] measuring aeration

From: <soilfoodweb_at_aol.com>
Date: Sun, 12 Oct 2003 03:47:43 -0400

Hi Dave -

You asked about how we do our samples. We suspend the probe in the water and measure oxygen using the probe. Typically we take samples from several places in the tea brewer, usually focusing on places where the mixing might not be the best, such as below aerators, in corners, and so forth.

Removing samples from the tea before measuring the oxygen concentration means you are allowing changes to occur to the tea before taking a true "in the tea brewer" reading.

While I am not an engineer, in my experience in doing oxygen measurements, the instant that air bubbles are released into the liquid, gases begin to diffuse and equilbrate. If oxygen concentration in the liquid is less than oxygen concentraiton in the gas bubble, oxygen begins to diffuse from the air bubble into the liquid. Very rapidly, within micrometers distance, the gases have equilibrated. Thus, you need to know gas concentration in the atmosphere in order to know the maximum possible oxygen concentration in the liquid.

If the microorganisms are growing, the can use up the oxygen diffusing into the liquid faster than you can supply oxygen.
So, the oxygen measurement obtained from an oxygen probe is a sum of all these processes. Oxygen probes measure oxygen in the liquid and gas in front of the membrane and since oxygen concentration equilibrates rapidly, there should be no difference in oxygen concentration between gas and the liquid once you are a few micrometers from the aerator.

Given that the water in the tea brewer is well-mixed, then all the water should be at the same concentration. So, mixing is an important factor so no areas of poor mixing occur.

It is critical to be beyond the rapid growth phase of the microbes in the tea brewing process before you take the tea out of the tea maker, or the tea can go anaerobic very quickly. IF YOU ARE BEYOND the rapid growth phase period, then you can have 5 to 6 hours before anaeorbic conditions occur. If you are in rapid growth phase, the tea may only take a minute or so to be into anaerobic conditions.

If course, it takes a few hours to accumulate enough anaerobic metabolites, which we smell as odor, to be able to smell that the brew is anaerobic.

Hope this helps.

Elaine Ingham
President, Soil Foodweb Inc.
SFI Corvallis, OR
SFI Port Jefferson, NY
SFI Lismore, NSW, Australia
SFI Hilversum, The Netherlands
SFI Cambridge, New Zealand
www.soilfoodweb.com

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Received on Sun Oct 12 2003 - 04:12:44 EDT

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