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Reading the tea leaves

In the knowledge that many floorlayers, in wanting to measure the moisture that can effect flooring, can misunderstand simple measurements, Richard provides information on the most common methods of taking moisture readings.

IN recent articles we’ve looked at how there are five types of moisture in buildings, and we’ve considered how the amount of moisture in the air varies with temperature. It’s a surprise to many floorlayers that when we measure for moisture we don’t want to know how much moisture is in the subfloor, nor what the relative humidity of the air is.

Rather, we want to measure the moisture which could affect the flooring and this is where simple measurements can be misunderstood.

For example, some types of moisture meter are able to ‘read’ through a damp-proof membrane. The floor may be very wet, but the DPM will prevent this moisture from affecting the flooring. And some fast-setting admixes in screeds actually crystallise the free moisture so it won’t escape from the subfloor but still reads as if it’s present.

The most common methods of taking moisture readings are:
Hygrometer – a small box which is sealed to the subfloor surface so it traps a small volume of air.
Over a period of time, usually longer than 24 hours, the air reaches equilibrium with the moisture coming out of the subfloor and so the relative humidity of the airspace can be measured. Some hygrometers have built-in metres and others are designed for use with a proprietary handheld meter.

The advantages of the hygrometer are that it gives very accurate readings of the moisture being released by the subfloor, and this is what we want to know. The main disadvantage for the average floorlayer is it takes a long time to be sure of an accurate reading, and it can be a bit of a nuisance to the people who want to use the premises while the test is being run. However, this is the principle method set out in British Standards because it’s measuring the right thing. This method is just as accurate for calcium sulphate (anhydrite) screeds, but in all cases surface contamination or laitance must be removed before attaching the hygrometer.

Sleeve – a plastic sleeve which is sealed into a hole drilled in the subfloor (usually to the depth of 40% of the thickness of the slab). The air space in the hole reaches equilibrium faster than for a surface box as it’s smaller and readings can usually be taken after about four hours.

A sleeve also measures the moisture being released by the subfloor, but it must be used with care as if it’s drilled through a DPM then it’ll measure the moisture below the DPM and compromise the DPM itself. Anyone drilling into a subfloor must also be aware of the possibility of pipes or cables below the surface.

RF or impedance meter – a handheld device that emits a radio-frequency or electrical signal. The signal speeds up in wet materials and slows down in dry, so the device gives readings based on the resistance to the signal. The reading is very quick, which is why these devices are popular, but the units aren’t relative humidity readings and they’re based on what is in the subfloor, not what’s being released from it. Each manufacturer gives guidance in their technical instructions so the readings can be compared with RH values.

Readings from an RF meter are considered to be ‘indicative’ and are best used for guidance to know whether more accurate readings should be taken with a hygrometer and, if so, whereabouts on the floor the wettest areas are so the boxes can be positioned in the right places.

These types of meter can be affected by metal (eg, pipes and fixings in the slab) and may also give different readings on calcium sulphate (anhydrite) screeds, so the manufacturer’s instructions must be followed. Some have a separate scale for anhydrites.

For wood, the use of a pin meter is essential. This consists of two pins which are inserted into the wood and the meter reads the resistance to an electrical impulse passed between them. In wood this gives an accurate percentage of moisture by weight, and the more expensive devices will have the option to set the species of wood beforehand to give even greater accuracy. In other materials the devices will give useful readings which are known as ‘wood moisture equivalent (WME)’, but these may only be surface readings if the pins cannot be driven into the material.

Other methods of moisture measurement are the speedy tester which is also known as the carbide bomb which involves taking a sample from the subfloor and crushing it to a powder before placing it in a reaction vessel with a chemical which reacts with moisture in the sample to form a gas. The pressure of gas obtained gives an indication of the moisture in the subfloor. The moisture can also be measured using oven drying where the weight of a sample before and after drying in an oven is compared to give a percentage of how much water was dried off.

The latest edition of the CFA Guide to Contract Flooring, which can be downloaded at www.cfa.org.uk/Contract-Flooring-Association-Information-and-Downloads/ has some more detailed information and the Flooring Industry Training Association offers a one-day course on Moisture – preventing floor failures (www.fita.co.uk/Flooring-Training-Courses/).
Richard Renouf is an independent flooring consultant

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