Electrical resistance meters

[edit] Introduction

When identifying rising damp in buildings, architects, surveyors and other consultants may use moisture meters to confirm or obtain a diagnosis. The most common types of moisture meters are carbide meters and electrical resistance meters.

Carbide meters give an accurate measure of moisture content, although they require the removal of a sample - which could be considered destructive. As a result, this method of testing may only be advisable after an electronic resistance meter or some other method has discovered the presence of moisture.

Electrical resistance meters measure dampness either on the surface, or within the building fabric itself. Generally, if the meter reading indicates that the fabric is dry, then it is dry.

[edit] Wood Moisture Equivalent

Electrical resistance meters were developed for use in timber, and if the reading indicates the fabric is wet, this does not necessarily mean that it is wet, as the presence of other substances such as soluble salts will give a similar reading, and in older walls, salts may be present even where damp is not. When using these meters on materials other than wood, the readings will only indicate the presence of moisture as it relates to the moisture content of timber.

The Wood Moisture Equivalent (WME) is the theoretical moisture content - or moisture profile - that the substrate would contain if it were wood.

[edit] How do electrical resistance meters work?

Moisture meters generate readings based on how easily a small electric current can pass through a designated test area. If the current flows easily, this theoretically indicates the presence of moisture. The wetter the material, the more readily the current will flow.

There are two main types of electrical meters that are used for this process: pinned and pinless:

[edit] Pinned meters

Pinned meters take readings by measuring moisture levels recorded from two to four small probes fixed to the meter. These pins are inserted into the test substance, and then an electrical current is sent through the sample area to record electrical resistance or resistivity. This type of meter takes readings for a very specific area and will require multiple measurements which may be considered destructive (since numerous small holes will be created for each reading).

Taking readings with a pinned meter can generally be broken down into four steps:

1. The meter should be set to correspond to the type of material being tested.
2. The pins of the meter should be perpendicular to the surface and then pushed as far as safely possible into the sample area.
3. The reading should be recorded as either a percentage or reference point.
4. The process should be repeated in different areas with a concentration of samples in areas, such as around windows, that may be more prone to moisture.

[edit] Pinless meters

Pinless meters use a sensor pad to send an electromagnetic signal to a designated area to assess whether or not moisture is present in a wider area. The sensor’s wave movement data registers the moisture level when contact is made. This method can read depths up to 1.5 inches and does not require an invasive sample.

Taking readings with a pinless meter can also be broken down into four general steps:

1. The surface should be cleared of any debris or standing water before a measurement is taken.
2. The meter should be positioned and then pressed against the sample area and held until the reading stabilises.
3. The reading of moisture content or percentage value should be recorded.
4. The meter can be repositioned in other areas of the surface to determine the extent - if any - of moisture present.

[edit] False readings

There is a possibility that electrical resistance meters could deliver inaccurate readings that may be artificially elevated. These can be caused by a number of factors, including:

• Free water in materials caused by condensation, leaks, penetrating damp and other factors.
• Salts that contaminate materials, such as plaster, mortar, brickwork, wallpaper paste and chimneys.
• Materials with metal content, such as foil backed wallpaper, lead based paints and some timber preservatives that contain heavy metals.
• Materials such as clinker blocks and black ash mortar, used in the construction of older properties in certain parts of the country.

All electrical resistance meters should be calibrated before measurements are taken.

For more information see: Assessing moisture in porous building materials.

[edit] Related articles on Designing Buildings Wiki

• Assessing moisture in porous building materials.
• Carbide meter.
• Condensation.
• Damp.
• Damp proofing.
• Moisture.
• Rising damp.
• Rising damp in walls - diagnosis and treatment (DG 245).
• Wall insulation and moisture risk.