The in-situ probe test is now the main test method specified under AS 1884-2012.

It's sometimes called an 'invasive RH test', because test holes need to be drilled into the slab.

Sensor probes are inserted into the holes, enabling them to take RH readings well below the surface of the slab.

The location of the holes will depend on the size and shape of the slab.

You can look for areas that might have high RH levels by using a capacitance meter.

Although the capacitance meter can't be relied on as a final test in a concrete subfloor, it can help you to decide where to place the probes.

For more details on capacitance meters, see: Other moisture testing methods.

The depth of the holes will also vary depending on whether the slab is drying from one side only or from both sides.

On-ground and below ground slabs (also called 'on grade' and 'below grade') can only dry from one side.

Suspended (or 'above grade') slabs are able to dry from two sides when the underside is exposed to the air.

General procedure

1. For slabs drying from one side only, drill a hole to a depth of 40% of the slab thickness. (That is, if the slab is 100 mm thick, drill a 40 mm deep hole.)

For slabs drying from both sides, drill a hole to 20% of the slab thickness. (That is, 20 mm in a 100 mm thick slab.)

Make sure you measure the depth of the hole to ensure it's accurate, because this is critical to achieving a precise RH reading.

2. Clean out the concrete dust carefully with a wire bristle brush and vacuum out the hole. The manufacturer of the instrument will probably have included a brush and vacuum attachment in the kit for this purpose.

3. Insert a plastic liner into the hole and check that it goes right to the bottom. Seal the hole with the cap supplied by the manufacturer.

4. Allow the test hole to equilibrate with the slab for at least 72 hours.

5. Insert a probe into the sleeve and wait an hour or two for it to acclimatise.

Note that the probe will need to reach the same temperature as the concrete before it will give an accurate reading - even a small difference in temperature can result in a significant error in the RH measurement.

6. Connect a meter to the probe and take an RH reading.

Check that the reading doesn't drift by more than 1% RH over a 5 minute period.

Equilibration may take from several hours to several days, depending on the type of meter being used and the initial temperature difference between the probe and the concrete.

Repeat this process for each of the test holes you have drilled.

When you've finished taking the readings, remove the probes and the top lip of the sleeve.

The holes can then be filled with a cement-based patching compound and smoothed off.

An alternative version to the in-situ probe is the Rapid RH 4.0 EX.

It has a 'smart sensor' that's inserted into the lined test hole and left to equilibrate for 72 hours.

After this time the 'Easy Reader' is inserted into the hole until it makes contact with the sensor, and the RH measurements are taken almost immediately.

Although these probes are not reusable, they can be left in the slab for up to six years, allowing the RH levels to be monitored throughout that time.

Cautions

Some floors you come across may have had a silicate-based moisture barrier installed, such as Protect Crete Moisture Fix 1, Aquron 2000 or Deflecta. We'll talk about the process of applying these moisture barriers in the unit: Subfloor coatings and toppings.

These sorts of floors are not suitable for an in-situ probe test, because if the moisture barrier has been applied correctly, it will give you an incorrect reading.

One of the tell-tale signs that this type of moisture barrier is in place is a high RH reading from the in-situ probe and a pH reading of between 7 and 9.

If a moisture barrier has been installed, you should use the insulated hood test method.