Can Calcium Chloride Pounds Be Converted to Relative Humidity Percent?

When determining if a concrete slab is ready for finishing, flooring, or occupancy, testing the relative humidity (RH) and moisture vapor emission rate (MVER) are two commonly used methods.

However, many professionals wonder if there is a correlation between RH and MVER.

In this article, we will explore the differences between these two tests and shed light on their merits.

By understanding the nuances of RH and MVER testing, contractors and industry experts can make informed decisions and avoid potential flooring problems.

Join us as we delve into concrete moisture testing and discover the best methods for ensuring successful flooring installations.

RH and MVER Tests: Is There a Correlation?

When looking to determine if a concrete slab is dry enough to proceed with a finish, flooring or occupancy, there are several methods commonly specified for testing the relative humidity (or moisture condition) of the slab.

A dry slab is never at 0% humidity, but determining the level of moisture still held in the concrete can be the difference between a successful flooring installation and a problem-prone floor system.

The two most frequent test methods specified in the industry today are Moisture Vapor Emission Rate (MVER) testing (with results expressed as pounds/1000 square feet) and relative humidity (RH) testing with in situ probes (with results expressed as a percentage).

When faced with the two options, contractors, and other industry professionals often ask, “What is the correlation between RH and MVER?”

Simply stated…there isn’t one.

While it might seem logical that there would be some relationship between the two, the reality is that there is nothing more than an imprecise use of the term “moisture test” that links the two test methods.

Surface Similarities

Beginning around the 1940s, moisture levels were tested by placing an enclosed amount of desiccant on a slab’s surface for a period of time. Calcium chloride, or CaCl, was the most common desiccant used for this type of moisture testing and it is often referred to as the anhydrous calcium chloride test.

Any subsequent change to the weight of the desiccant was thought to indicate the amount of water vapor that had left the slab to be absorbed by the desiccant. That weight was represented as a ratio of the total moisture content within the slab, and is referred to as MVER, expressed in terms of “pounds/1000 square feet.”

Calcium Chloride Test

Extensive research done by the CTLGroup in the 1990s showed several problems with the CaCl test:

  • MVER test kits cannot be calibrated.
  • The test measures, at most, the top ½ – ¾ inch of the slab, but not deeper.
  • Surface treatment, including trowelling practices, curing agents, ambient conditions and more, can skew MVER test results. In fact, calcium chloride testing has been specifically disallowed for lightweight concrete applications because the lightweight aggregate can impact results for false high or low results.
  • The limits set for the test (i.e. 3 lbs/1000 sq ft) were somewhat arbitrarily chosen and published.
  • There is no scientific backing for the test method as either a qualitative or a quantitative measure of concrete moisture. (1)

The real difficulty of MVER or CaCl testing lies in the fact that it primarily tests only the surface conditions of the slab. Drying concrete tends to have a gradient effect – moisture levels are higher deeper in the slab.

As moisture evaporates from the surface of the drying slab, it then allows additional moisture to rise through the natural capillaries of the concrete in a progressive cycle until the moisture content in the slab reaches a balance with conditions around it. MVER is incapable of providing accurate readings of those internal levels.

A traditional “moisture test,” MVER is still regulated by ASTM F1869. Ultimately, though, it has proven to be an unscientific and problematic test method, plagued by subsequent moisture-related flooring problems.

Deep Down Differences

RH testing for concrete, on the other hand, measures internal moisture levels of a concrete slab by placing sensors, or in situ probes, within the concrete slab itself.

Testing that had begun in Sweden and elsewhere in the 1990s demonstrated that for slabs that dried from one side, placing the probes at 40% of the slab depth would give a reading that would reflect accurate moisture conditions of the slab if it were sealed (i.e. a floor covering installed) at that point in time. (For slabs drying from two sides, the correct depth is 20% of the slab depth.)

Based on the realities of moisture vapor’s distribution in drying concrete, RH testing can accurately determine the internal moisture levels, or RH, of the concrete.

Free Download – Which Rapid RH Sensor is Right For You?

Understandably, the difference between MVER and RH testing has had a significant impact on the flooring industry, allowing concrete and flooring installers to make informed decisions when choosing products that will tolerate the actual RH levels of the slab, or allowing them to make remediation choices before elevated moisture levels result in flooring problems.

As concrete science changes with the additions of admixtures, new aggregates, and a variety of finish options in demand, RH testing continues to provide accurate concrete moisture measurement for industry professionals.

The Rapid RH®

With the solid research in RH testing as its basis, Wagner Meters’ Rapid RH® products stand at the lead in innovative, field-tested RH testing. The Rapid RH® Smart Sensors and companion Total Reader make the Rapid RH® among the fastest, most accurate and cost-effective RH test systems on the market today.

Its award-winning innovation and practical design offer flooring experts and concrete professionals the technology to keep fast, experienced and accurate data at their fingertips for all of their business, reporting and scheduling decisions when installing floors that will last a lifetime.

The Rapid RH® is the latest in field-tested, scientifically-backed, industry-proven RH testing for concrete and flooring professionals.

You can learn more about RH testing and the Rapid RH® here, or take a free webinar on RH testing for concrete floors by clicking here.

Howard Kanare, Concrete Floor Moisture Tests, August 2007.

Last updated on November 7th, 2023


  1. Jeff Unger says:

    Do you have a suggestion on how to test a post Tension concrete slab after it has been poured? we must install VCT, LVT and direct glue concrete.
    Thank you,

    • Jason Spangler says:


      Thanks for the question. The overarching answer is with great caution. My first recommendation, especially in this situation, would be to push for third-party testing utilizing a company that has experience with post-tension slabs. The bottom line, someone needs to come in with ground-penetrating radar or some other comparable technology that can identify those cables prior to drilling. The last thing anyone wants is to hit one of those cables. Good luck.

  2. D. Grimes says:

    ASTM needs to stop recognizing calcium chloride tests. It is by any measure inaccurate, ineffective and a poorly constructed method of identifying vapor emissions.

  3. Tom Nellman says:

    Hey Jason,

    Thanks for post on the contrast between MVER and RH, gained a better understanding. Reading it reminds me that we can get more exact about some of the concrete work we do. Going to definitely make RH testing part of our operations in 2020.


    Grand Prairie, TX

  4. Mr. Spangler, may I ask you one question ?

    Is there any stable correlation between RH (%) and mass water content (%) of the “green” concrete slab ?
    Many Russian resin flooring contractors prefer to determine the humidity / water content of the concrete slabs using “electronic meters”.

    • Jason Spangler says:

      Thanks for the question. As far as I know, there isn’t a correlation and/or conversion calculation. Again, as with other forms of concrete moisture testing, electronic meters are only testing the top part of the slab and there could be vast differences when comparing those conditions to the conditions deeper in the slab. I would also wager that as time goes on, more and more of these resin/coating companies will start to recognize and publish concrete moisture testing (RH%) thresholds.

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