Common Questions Asked About Moisture Testing

Q - What happens if I run the calcium chloride test in less than 60 hours?

A - If the building has a highly controlled interior atmosphere, the test can be run as short as 48 hours. However, the purpose of running the test between 60 and 72 hours is to get a good sampling of the moisture emission volume as a function of time. It is advised always, to conduct tests according to industry norms.

Q - What happens if I run the calcium chloride test over 72 hours?

A - Depending upon the moisture emission volume, the test can stop recording meaningful information when the dish of desiccant becomes totally saturated. If the test runs over 72 hours (but not more than 84) and the crystals are not totally liquefied, then the results are acceptable. However, it is best to take every precaution that the 60 to 72 hour window is followed as it reflects the specifications set forth by manufacturers regarding moisture sensitivity.

Q - I forgot to pre-weigh my crystals before exposing them!

A - The test will lose some of its value, but you can often use our own quality control weights to get some evaluation of the moisture vapor emission. We maintain between 30.0 and 30.5 grams in the tape-sealed container prior to exposure. If the test results come out low, it is advised that you run them again to be sure. If the test results come out very high (over 8 pounds for example) then the variability on the start weight will not really influence the fact that the slab is not producing the 3 pound limit most manufacturers specify anyway.

Q - What are the options to high moisture tests?

A - The problem of high slab emission is that long-term performance of the floor covering is in jeopardy. The owner of the building must make a decision regarding the floor covering. The history of the problem shows there are basically 5 choices:

  1. Don't install a floor at all. The floor system will just fail.
  2. Replace the concrete if old and permeable, or wait for it to dry longer if it is a new slab.
  3. Find an alternative floor system, if any.
  4. Have the slab emission/alkalinity condition properly treated.
  5. Risk the failure of the floor covering and install it anyway.

None of these decisions are easy to make, but the nature of moisture emission can be incompatible with floor systems. Generally, calling in professional help has been far less expensive than experiencing a floor failure.

Q - Why can't tests be conducted without proper climate control?

A - Moisture emission is contingent upon vapor pressure differences between the substrate and room interior. Temperature and humidity together creates a specific static vapor pressure. Concrete slabs often have a much higher static vapor pressure than a room interior. Therefore, in order to accurately measure the potential for a floor system failure, it is necessary to test for vapor emission in respect to the operational-climate of the building.

Q - What causes high moisture vapor reading?

A - The concrete itself is a moisture-dependent mechanism. The porosity of the slab, as a function of its design and placement, as well as the presence and integrity of the vapor-retarder under the slab, all has an effect on topical moisture. In addition, the surface profile (texture) and the building environment also play a role in the moisture vapor emission.

Q - What is the relationship between Pounds & Percentage?

A - Pounds refers to the equivalent weight of water that has escaped from 1,000 square feet of concrete area, in 24 hours. It is a measure of emission, just like your breathing is a vapor emission. Percentage refers to the weight of the water inside the slab, in relationship to the overall weight of the slab. Pounds is a measurement of dynamic volume whereas Percentage is a measure of static content. So far, there is no established relationship between the two measures. Many studies have been done to find that correlation, but it isn't well defined. That is probably because other variables are at work, such as the porosity of the concrete, surface finish profile and climatic variance between the slab and building atmosphere. Many European floor products specify a certain Percentage of moisture acceptability. Nearly all American products specify tolerance in Pounds. This is perhaps due to differences in construction technics of concrete slabs. When confronted by these specifications, it is wise to use the Pounds determination, as that is more relevant to American slab design.

Q - Why the concern for testing near cracks, joints and walls?

A - There has always been controversy about testing near cracks, joints and wall perimeters as they can be obvious sources of elevated moisture emission. Generally, we recommend that you collect test data away from these features, in order to maintain a better sample of the overall slab emission. In some cases however, like small closets or occupied homes, the perimeter is all you can have to work with. Keep in mind that moisture levels near these features can be elevated, and not indicative of the overall slab condition. Certainly the best information to be obtained is that from both areas on the slab, where the contribution of cracks, joints and wall perimeters can be better defined.

Q - How do I work the formula for calculation published by ASTM?

A - The American Society for Testing and Materials has recently published the document F-1869 which must be purchased from ASTM and can be accessed by clicking here. The formula for calculation of test results is as follows:

MVER = 24,000 X Gain / 453.612 X Area X Time

MVER = Moisture Vapor Emission Rate
Gain = Weight gain of calcium chloride crystals in grams
*Area = Area of dome contact to slab, less area of dish contact in square feet.
Time = Exposure time in hours

*NOTE: There is some controversy as to whether or not the dish in contact with the slab has any effect on moisture retardation, and therefore whether or not it should be deducted from the area under the dome. Vaprecision had traditionally NOT deducted this area in our calculations, however, in order to comply with the newly published protocol, we will be adjusting the calculation. The net result is a minor amount of variance in the overall test result.

The area of the Vaprecision dome is 70 square inches.
The area of the calcium chloride dish is 5.94 square inches.

The ASTM formulation calls for the Area, to be in square feet. There is 144 square inches in 1 square foot.

Area (sqft) = ( 70 - 5.94 ) / 144 = 0.44486 square feet

Now, lets calculate results using a 3.0 gram gain over 72 hours:

MVER = 24,000 X Gain / 453.612 X Area X Time
MVER = 24,000 X 3.0 / 453.612 X 0.44486 X 72
MVER = 72,000 / 14,529.155
MVER = 4.95 pounds

Doing the same thing, this time without deducting the area of the dish under the dome ( 70 / 144 = 0.486 ):

MVER = 24,000 X Gain / 453.612 X Area X Time
MVER = 24,000 X 3.0 / 453.612 X 0.486 X 72
MVER = 72,000 / 15,872.79
MVER = 4.54 pounds

As you see, there is a slight difference between calculations of results when deducting the area of the dish under the dome.

The Vaprecision formula is essentially simplified, as we have fixed sized dishes and domes. Traditionally, without deducting the area of the dish, the calculation works as follows:

MVER = 108.846 X Gain / Hours
MVER = 108.846 X 3.0 / 72
MVER = 4.54 pounds

The NEW calculation Vaprecision will publish will comply with the new ASTM protocol. For computation, we will carry out the decimal places as far as they can go:

MVER = 24,000 X Gain / 453.612 X Area X Time
MVER = 24,000 X Gain / 453.612 X 0.4448639 X Time
MVER = 24,000 X Gain / 201.7956 X Time

Taking the 24,000 and dividing by the 201.7956, we arrive at a figure of 118.932 as the NEW constant. Using the same Gain and Time examples as before:

MVER = 118.932 X Gain / Time
MVER = 118.932 X 3.0 / 72
MVER = 4.95 pounds

This new constant calculates results per ASTM's formula, except we simplify it to be a little bit more user-friendly!