How Many Sensors Do I Need to Test for My Flooring Job?

ASTM F2170 is the most reliable and accurate standard for moisture testing concrete slabs because it’s based on decades of scientific research and testing. It tells you everything you need to know to be able to conduct the test, the sensor hole depth, detailed specifications of the sensors, and the number and, to a lesser extent, the placement of sensors.

When you’re moisture testing a slab in compliance with ASTM F2170, one of the first things you need to do is determine the number of sensors you need. The standard makes this pretty easy. Start with the question: how many square feet of concrete slab are you working with?

There are two sections of the standard that deal with sensor number and placement.

ASTM F2170 10.1.1 specifies how many sensors to use:

“Perform three tests for the first 1,000 square feet and at least one additional test for each additional 1,000 square feet.”

That’s pretty straightforward. It’s a simple calculation that gives a precise answer to the question, “How many?”

So for example, if you have 10,000 square feet of concrete to test, you’d need 3 sensors for the first 1,000. That leaves 9,000 square feet. You’d need one sensor for every 1,000 square feet there, so that’s an additional 9. So the total is 12 sensors.

That’s the minimum you need.

Section F2170 10.1.2 talks about where to place the sensors:

“Select test locations to provide information about moisture distribution across the entire concrete floor slab, especially areas of potential high moisture. Include a test location within 3 ft of each exterior wall.”

The first thing to notice is that you must include a sensor within 3 feet of each exterior wall. This is especially important for slabs on-grade because there might be higher moisture in these areas from groundwater intrusion.

The second thing to notice is that you need sensors in other areas of potential high moisture. These are the areas that will probably be the last to dry. This is up to your discretion, so you need to pay attention to areas like the following:

  • If there were multiple pours of concrete, the most recently poured areas are probably wetter than the others.
  • Different pours might be drier or wetter than others because of slightly different mixes and aggregates.
  • Areas that are exposed to less sunlight than others might be wetter and be slower to dry.
  • Areas closer to walls and structural beams might be wetter because the concrete walls and beams also contain moisture.
  • Areas that receive less airflow during the drying process might be wetter and be slower to dry.

Ed Wagner recommends you use the C555 Concrete Moisture Meter to quickly and easily pinpoint areas of higher moisture. By placing the C555 on the surface of the slab, you can identify potential moisture “hot spots.” By comparing these readings from several areas of the slab, you can determine which areas are suspect, and these are the areas in which to place sensors.

You might need a few more sensors than the recommended minimum to cover all of the suspect areas, but it’s a good idea to do so. Each sensor weighs about an ounce, and you know what they say about an ounce of prevention!

Introducing the Wagner Meters Rapid RH® L6 System

The Wagner Meters Rapid RH L6 system is the fastest, easiest, most cost-effective system for testing concrete slabs in compliance with ASTM F2170.

The Rapid RH L6 system uses single-use sensors for speed, economy, and ease of use. Once the L6 sensors are installed in the slab, they’re equilibrated and ready to take readings in only 24 hours. And because they’re single-use, there’s no need to move them from location to location and wait for them to equilibrate again. Repeat readings can be taken without additional equilibration time.

Unlike reusable probes, the L6 sensors never need calibration because they come calibrated from the factory and include a NIST-traceable certificate of calibration to keep with your records.

The Rapid RH L6 system’s Total Reader® reads and displays the temperature and relative humidity (RH) data on its LCD display. Each time you take a reading, the data is stored in the L6 sensor’s onboard memory. This keeps a complete chronological log of your readings stored in the sensor.

You can retrieve the entire log whenever you want by using the Total Reader and the DataMaster™ L6 app, which runs on your iOS or Android mobile device. The Total Reader reads the data from the sensor and transmits it via Bluetooth® to the DataMaster L6 app.

The DataMaster L6 app stores the data and allows you to display it, chart it, create PDF reports and email the reports from your device.

Backup copies of your data can be stored in the cloud and are stored in the sensors that are permanently installed in the slab. This all-digital path from the sensor to the final report, plus automatic backup ensures the highest data integrity and accuracy, and gives you complete peace of mind.

For more information about F2170, visit www.astm.org.
For more information about Rapid RH L6, visit www.rapidrh.com.

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