Dryfix News

Electronic Moisture Meters

June 11th, 2012

All posts, Damp / Rising Damp / Damp Proofing /, Surveying

Electrical moisture meters are invaluable tools which are used to aid surveyors in damp diagnosis. Moisture meters are a hand held device used to profile the distribution or pattern of dampness throughout a building and determine the quantity of moisture present in materials.

Electrical moisture meters are the ideal tool to use in pre-purchase surveys or where damage limitation is required as they are clean and non destructive.

Because building materials vary and are complex in their make up most electrical meters are resistance meters and calibrated only for timber, meaning when used on wood the results shown are quantitive and reflect the actual amount of moisture present. Because the makeup of timber varies very little between different types, one meter calibrated for timber can take quantitive readings from different types of wood.

When taking readings from timber using electrical moisture meters the results are read as percentage of Wood Moisture Content (WMC %). Electronic meters however, become extremely sensitive when timber nears saturation point at 30% WMC. When timber nears saturation point, the cells of the timber are swollen with free moisture and the timber will no longer expand or contract, at this point electrical meters become sensitive and readings taken in excess of 30% are no longer representative to the amount of excess moisture present, the wood is simply known to be saturated. At 30% however timber is at risk of wet rot decay and the excess level of moisture becomes irrelevant.

Virtually all materials even when deemed dry will have a percentage of moisture within them, this is known as the materials hygroscopic moisture content. Timber for example, when used internally and classed as dry (or having an acceptable level of moisture) will contain a moisture level between 8 – 16% Wood Moisture Content (WMC). Timber like most materials is hygroscopic and derives this moisture from the atmosphere, when atmospheric moisture levels increase the moisture content of the timber will increase also, In any environment timber will find equilibrium with the surrounding atmosphere (see chart). The moisture content of a material can also increase for other reasons such as; ingress of water, rising damp, penetrating damp and salt damp, it is the role of the surveyor to profile and analyse the moisture content of materials to determine the cause/source of dampness.

If using resistance meters on other materials such as masonry readings taken are not quantitive, they are only an interpretation of the moisture present in the material comparative to timber. In this circumstance readings are qualitative and results are based on an arbitrary scale usually between 0 – 1000 and interpreted as Wood Moisture Equivalent (WME).  Using meters on a material which they are not calibrated for will not achieve quantitive results of how wet the materials is, it simply provides a scale which can be used to distinguish readings taken from one area to another – used for moisture profiling.

So why use meters that do not display quantitive results?

Unlike timber which changes little between its various forms, masonry materials vary dramatically. In this instance surveyors would need a different meter calibrated for each different material. Considering buildings are constructed using a mixture of materials such as; bricks (varying), mortar (cement & lime), backing plaster (cement, lime & gypsum) and a variety of finish plasters, we would need several different meters all calibrated specifically to record from each material – a very expensive tool box would be required. This is why generally most surveyors will use one meter calibrated for timber providing quantitive results, which is also capable of providing qualitative results on other materials.

Over the past few years however more meters have become available specifically calibrated to masonry materials such as concrete, which are used to determine the moisture content of concrete floors and structures such as the Tramex range of moisture meters – http://www.tramex.ie/

Sophisticated moisture meters such as the MMS Pro from Protimeter also have the ability to record atmospheric conditions such as; relative humidity, specific humidly, air temperature, dew point temperature and take surface temperature readings using a probe.

Electronic moisture meters can however be influenced by any material which conducts electricity resulting in exaggerated readings, care should therefore taken near foil backed wall papers and salts to avoid misleading results. Most masonry materials however are free from conductive materials which affect resistance when in a dry condition. Therefore, in over 99.5% of cases changes in electrical resistance when using a moisture meter is likely to be the result of moisture and/or soluble salts.

Moisture meters should not be relied upon for diagnosis based on the readings taken from a material. They should be used to map a profile of moisture and the distribution of salts through the material or structure to assist the surveyor in their inspection and reaching a conclusion.

How does an electrical moisture meter work?

Electrical currents are easily conducted through a moist or wet material. Electronic moisture meters act as a signal generator which output a small voltage supplied by a battery through a material. The meter measures either resistance or conductivity of the electrical current which will then display results based either on the amount or resistance or conduction achieved. The more conduction or less resistance the greater the amount of moisture likely to be present.

Moisture meters vary between manufacturers and some have multiple functions to test for the presence and quantity of moisture. Resistance probes and radio frequency are probably the most common modes provided on most electrical moisture meters.  There are primarily three different modes:

1: Resistance probes

Resistance probes work by measuring the resistance of an electrical current between two metal pins. The internal circuitry of the meter acts much like an ohmmeter, the ohmmeter reads the amount of resistance that an electrical circuit may employ in conducting electricity and displays a reading. Readings are usually shown as colored LED lights or numerically, some meters are even audible. Dependent upon the calibration of the meter and material tested results are interpreted as Wood Moisture Content (WMC)when used on timber or Wood Moisture Equivalent (WME) when used on materials other than timber.







2: Capacitance Meter

A capacitance meter works by an electronic field created by two opposing electrically charged pads. When the pad is placed upon a walls surface the electromagnetic field passes through the wall, where moisture is present within the material it will deflect the field providing a reading based on the degree of deflection. These meters are good for reading deep within walls or investigating potential moisture through surface coverings. The results from this type of meter are usually shown by a needle passing over a scale and read as ‘Full Scale Deflection’ (FSD).






3: Radio Frequency

Radio Frequency works by sending radio waves beneath the surface of a wall through a sensor. Again moisture present within the material will deflect radio waves providing a reading based on the degree of deflection. These meters are also good for reading beneath the surface of a material or surface coverings. The results from this type of meter are usually shown as colored LED lights or numerically, some meters are even audible. Dependant upon the calibration of the meter and material tested, results are interpreted as Wood Moisture Equivalent (WME).






As you can see electronic moisture meters can be complex to understand and interpret that’s why in the hands of the untrained miss diagnosis can occur. Provided they are used by a trained and qualified surveyor who knows how to use them properly they do provide a quick, non destructive investigation tool for moisture profiling and determining quantitive moisture levels in timber (and masonry dependant upon the meter).

Our damp and timber reports are only undertaken by experienced and fully qualified surveyors who hold the National Qualification for remedial surveying C.S.R.T. awarded by the BWPDA / PCA, our industry body. This proves that our surveyors have been independently examined and tested, proving their extensive knowledge and expertise.

All of our surveys are carried out in accordance with The British Standards Code of Practice.

Our surveyors carry all the necessary diagnostic and inspection equipment onsite, including thermal imaging and fibre-optic cameras to examine hidden cavities, highlighting blockages and bridged damp proof courses. Salt analysis testing and equipment for in-depth moisture analysis of masonry walls and plaster is also available. Photographic evidence of our findings is recorded and included within our report, so you can be sure of correct diagnosis.

Our reports contain comprehensive advice of any problems identified and include our recommendations, specifications and costs for repairs, if necessary. Edited pictures along with category damage ratings are also included to assist your understanding about the urgency of any repairs needed.

Our reports are available in both printed media and/or converted into secure electronic Portable Documents (PDF) for email.

Suspect a Problem?

If you suspect a problem and require a survey, or need advice please Contact us.





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