From Chris Baylor,
Your Guide to Woodworking.
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I recently received the following question from a reader:

Since PT plywood has a high copper content will it effect a moisture level reading?

I have PT plywood on the subfloor down stairs and the same plywood upstairs without PT. I can't get the same moisture level content between floors. Is it the reading that's different and not the moisture level?

In my answer, I mentioned that because most wood moisture detectors utilize electric current conductance to determine the amount of moisture content in the wood, having a higher level of copper in the wood (from the ACQ-treating) would definitely throw off the readings when compared to non-treated material.

The U.S. Forestry service has a PDF document that explains how moisture meters can be affected by varying levels of conductance.

If the wood in the two floors has been in place for more than a few months, it is very likely that both floors have become acclimatized to your local environment, and will likely have very similar “true” moisture content. It’s just going to be very difficult to measure the accurately using a wood moisture detector.

How should one work around this problem? After a bit of time using your meter, you should be able to determine a "fudge factor" for pressure treated lumber. In other words, you'll likely figure out that ACQ stock will consistently test at a certain moisture percentage higher than untreated stock.

Once you have an idea of how your meter reacts to treated wood, you'll be able to test the moisture content of pressure treated wood more accurately.