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  Why should we use the RH Probe test method over other methods of moisture testing? The concrete slab at the time of the pour is at 100 percent moisture content (red line).  Once the slab begins to dry, it dries from the top to the bottom (yellow line), leaving the surface of the concrete much drier than the bottom.  This is why the methods of surface moisture testing measurement are questionable. Once a flooring material is installed and the room environment stabilized the slab goes into equilibrium (green line).  Note how the Relative Humidity probe’s moisture measurement is at the apex of the drying curve where it intersects with the equilibrium line.    

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How to conduct a RH probe test

Test equipment available (Photo 1)

  As illustrated, there are a wide variety of RH probe kits available on the market.  These meters start at about $1,000 and consist of a digital meter with a cylindrical probe to measure slab temperature and internal slab relative humidity.  The probes should have NIST-traceable calibration certificate stating the range of calibration and accuracy.  These probes need to be cleaned and recalibrated on an annual basis to insure accuracy.  

Multiple-use (disposable) RH Testing equipment (Photo 2)

The newest type of measuring equipment is the disposable moisture sleeve, which contains sensor that is placed into the concrete just like the sleeves used with the RH meters.  The difference is the sleeve contains microchip sensor which allows a person doing the testing to take multiple tests.  The advantage of this device is it allows for instant reading after the drilled hole and sleeve to reach equilibrium. ASTM F2170 currently recommends 72 hours.   

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Selection of test areas

The selection of test sites is done the same as all moisture testing 3 tests for the first 1,000 square feet and one for each 1,000 square feet.  It is recommended that no sleeves be set within one meter (3 feet) from existing exterior walls.  

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Drill and types of drills available (Photo 3)

There are several types of drills available on the market.  I have found a person needs a good, heavy duty hammer drill.  If you are going to make an investment in one, a battery operated works best as an electrical power supply is not always available on commercial job sites and carrying a lot of extension cords is a hassle.   There are a lot of different sizes of drills used.  Be sure you are using the proper size bit for the sleeve to be installed.  Some bits are metric and others are in inches.  

Setting the depth (Photo 4)

The depth of the hole can vary depending on the drying condition of the slab.  A slab drying from the top would require the hole to be placed at 40% the thickness of the slab (Example: a 4-inch (100-mm) slab would require a hole drilled at 1.5-inches (40-mm).  A slab drying from both sides, such as an elevated structural slab, would require a hole at 20% (Example: a 4-inch (100mm) slab would require a hole at 0.75-inch (20-mm) deep.  

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Cleaning the hole (Photo 5)

After the hole is drilled to depth the excess dirt and dust created by the drilling process must be removed.  Use a wire brush and clean the walls of the hole.  This removes the dust forced into the pores by the concrete drilling process, which tend to clog the pores.  

Vacuuming the hole (Photo 6)

After wire brushing the sides of drilled hole remove all of fine dust. Use a vacuum with a fine dust filter to vacuum out the sides and bottom of the drilled hole. Fine dust left in the hole can have an adverse effect on the external probes over time   

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Placing the sleeve or sleeve/sensor (Photo 7)

Most of the sleeves have an insertion tool used to insert the sleeve.  This tool prevents the side of the sleeve from being damaged making the external probe difficult to slide into place.  The sleeve illustrated is the sleeve/sensor combination.  

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Taking the reading (Photo 8)

Whither you are using a meter with an external probe or one with the small insert reader you must place them into the sleeve and allow them to acclimate.  The best way to tell is to watch the digital readout and allow it to stabilize.  The external readers will take longer because the sensor has to equilibrate to the internal conditions of the sleeve.  The sleeve/sensor is much quicker to equilibrate because the sensor is in the sleeve and will be ready to be read.

Finally are the items that need to be documented.  The ASTM F217 recommends the following items to be documented:

•Name and address of structure

•Identify floor to be installed

•Test location  (use room numbers or building grid)

•Depth of drilled hole from the surface

•Relative Humidity in concrete %

•Temperature of concrete ºF

•Ambient air temperature  ºF

•Ambient relative humidity %


•Instrument used

•Make, model, serial number of meter and/or sensors

•Last calibration date/ by whom

•Tests performed by


•Name of person doing the tests

•Company name, address and   phone number