The test that I recall from my chemical engineering classes required a sample to have an additive to capture the disolved oxygen. The sample was sealed and then heated on a bunson burner for a period of time. I can't remember the additive but it basically absorbed all of the available oxygen in the sample. From there we weighed out the results. I'm not sure if there is a simple test like a PH test or not.

Another issue with the DO levels is the additional gasses that are dissolved in solution. Since this water is under several atmospheres of pressure under the dam some of these gasses will be immediately given off as soon as they pressure is released as it comes through the turbines. Some other gasses like the sulpher gasses will take some additional agitation before they can be released. The issue with the additional gasses is that they take up available space that could be taken by O2.

I'm sure Rick has forgotten more about this than I'll ever know, but what I know about the process is that it is a little more complicated than a simple sample.

John