The hygrometer probe test is the most accurate of the concrete moisture testing methods. It measures the internal relative humidity of a concrete slab (40 percent of the slab’s thickness) at the apex of the concrete’s drying curve. By taking the measurement at 40 percent of the slab’s thickness, it is easier to predict the effect moisture will have on the material placed over the concrete surface. To look at the process we must examine the steps involved, which are as follows:

These are some of the different types of internal relative humidity moisture testing devices available (Photo 1). Each has the ability to give accurate internal relative humidity readings. The key to achieving these accurate readings is the proper use of this equipment.

The drill must be set to the length of the sleeve to be installed (Photo 2). Here we have a sleeve that is going to be inserted at 40 percent of the thickness of the slab. For example, a 4-inch slab would be about 1-3/4". The stop on the hammer drill is set to the proper depth of the hole to avoid drilling too deeply.

The hammer drill's stop allows the drill to stop at a preset depth. Note the sleeve in the foreground, the hole drilled is the size prescribed by the testing device manufacture, which will allow for an airtight seal preventing any outside humidity from affecting the internal relative humidity (Photo 3).

Once drilled, the hole must be thoroughly cleaned to remove any dust or contaminates (Photo 4). This is an important step as any dust or dirt will affect the accuracy sensors' reading. Many manufacturers recommend the use of a wire brush to clean the sides of the drilled hole.

Place the sleeve into the hole (Photo 5). You will notice it will require effort to get the sleeve into place as the flanges fit snug on the walls of the drilled hole. It is important to have this snug fit to keep any outside humidity from impacting the outcome of the test. If you have difficulty getting the probe into position, you can tap it into place with a rubber mallet or a tapping block. Be careful not to damage the sleeve.

After the sleeve has been in place for 72 hours, the hole is acclimated and ready for testing. Remove the protective cap on the sleeve and place the probe into the sleeve (Photo 6). Allow the probe to acclimate to the slab temperature to achieve the optimum reading this can take as long as one hour depending upon the temperature of the probes and the slab. When completed, be sure to record all the data. This includes the slab temperature, the ambient room temperature, and ambient relative humidity, the time elapsed from the start to the first test, the date, and the time.

By following these simple rules, you can get a very accurate picture of the internal moisture of a slab and how it will receive new moisture sensitive flooring.

Editor’s note: This article will be continued in the next issue of Floor Covering Installer, which will cover the ASTM F-1869 Calcium Chloride Test.