Performing a carbide measurement

Measuring the residual moisture content of the subfloor is an essential step in ensuring a good end result. Moisture can prevent the adhesive from bonding properly or cause the (anhydrite) floor to rot.
Is the subfloor dry enough? Assess the substrate with a carbide measurement in four steps.

What is a carbide measurement?

A carbide measurement is used to determine the residual moisture content of the substrate. The residual moisture content is measured by the reaction of calcium carbide with the moisture in the sample. When calcium carbide and water react, a gas (acetylene gas) is produced, which increases the pressure in the carbide meter. This pressure can be read on a pressure gauge and, in combination with the weight of the sample, converted into the moisture content of the substrate.

What is a carbide measurement used for?

Rising damp is not always visible. Rising damp can occur when floor covering is laid on solid ground without a crawl space, but also with old tiled floors. Pressurised moisture occurs mainly in old houses where there is no foil under the floor. In these floors without moisture-proof foil, rising ground moisture has the opportunity to work its way upwards. To ensure a good end result, it is necessary to understand the moisture content of the substrate.

Step 1: Sample the substrate

1.1 Ensure that the substrate is clean before taking a sample. Take a sample at a depth of at least 5 cm using a hammer and chisel.

1.2 Collect the sample material using the appropriate tools.

Step 2: Crushing the sample

2.1 The next step is to crush the sample taken at a depth of at least 5 cm. First remove the large pieces from the sample material.

2.2 Then place the sample in a crushing bowl and use a hammer to crush the sample material.

Step 3: Weigh the sample

3.1 Depending on the type of screed, the measurement is carried out with 20, 50 or 100 grams.

3.2 Place the correct amount of sample material together with an ampoule and the balls in the carbide meter.

Anhydrite screed: 100 grams (this gives a more accurate measurement)
Cement-bound screed: 50 grams is sufficient

Step 4: Shake the carbide meter

4.1 Shaking the carbide meter.

0-2 minutes
Shake the carbide meter for 2 minutes by vigorously moving the pressure bottle back and forth in a circular motion.
2-5 minutes
Place the pressure cylinder in the shade and wait for 3 minutes.
5-6 minutes
Shake again for 1 minute using the same technique.
6-10 minutes
Leave the pressure bottle to rest in the shade again for 4 minutes.
10 minutes
Shake the pressure bottle again briefly for approximately 10 seconds after exactly 10 minutes.

After 10 minutes, you can read the pressure on the nanometer. In combination with the weight, the moisture percentage can be read.

The moisture content must be read on the scale of the weighed weight (on this meter: red 100 grams, green 50 grams, blue 20 grams).

Type of surface

The permissible limits for moisture content depend on the type of substrate.

The residual moisture content of a cement screed is a maximum of 2% CM and 1.8% CM with underfloor heating.
An anhydrite screed is less resistant to moisture; the maximum residual moisture content for anhydrite is 0.5% CM and 0.3% CM with underfloor heating.