When you have a dampness (excess water) problem the first stage is to get a basic understanding of the problem which is excess/or free water. The electrical moisture meters will help with this.
Moisture readings in timber are what electrical meters were designed for, and they give a factual reading via a % Wood Moisture Equivalent. The default timber calibration is offtern pine.
The two main types of electrical moisture meters are:
Conductivity meter uses two pins and the meter measure the conductivity between the pins on the surface to be tested, this gives wood moisture content % in timber which is quantitative.
Capacitance/impedance which uses radio waves to measure 10-19mm below the surface and often called search mode which allows a rapid assessment, but it can be very misleading by its range calibration in the meter and are not suitable for moisture diagnosis by itself.
|Timber %WME||Relative Readings|
|Protimeter Surveymaster||5% to 99%||70 to 999|
|Flir MR77||0% to 99%||0 to 100|
|Tramex ME5||5% to 30%||0 to 100|
In all other building materials, the pin readings are relative and don’t measure the moisture content of the material, it is the visual damage and pattern of meter readings which helps understand the damp issue. Therefore, the results are qualitative especial in pre-purchase surveys as the owner does not allow or want pinholes from the capacitance meter therefor the inaccurate search/scan mode is used and will cause a misdiagnosis. Both methods are used together to indicate dampness.
There may be salts in the building structure which will cause high reading in an electrical moisture meter as they greatly increase the electrical conductivity of material, even when the material is free of excess moisture, but they indicate that there may have been a problem in the past.
A report should tell you what type of moisture meter was used, and the method used. An educated eye for understanding the pattern of damage caused by damp may be helped by using an electrical moisture meter. In masonry the results must be taken with extreme caution especially if scan mode only is used and why you must get a qualified person to carry out the inspection.
|Ineffective damp proof course||>25%||95||90||90||85*||10||0||0|
|Plaster bridging or condensation behind skirting||>20%||75||0||0||0||0||0||0|
Depth of moisture meter pins
The pins of the meter must only be inserted 2-3mm into the surface to be measured for a correct and calibrated result on the digital or analogue display. By pushing the meter pins deeper, you will get a higher reading but this maybe an error in relation the meters calibration in moisture in timber (WME). When taking readings on timber the pins must be inserted parallel to the grain to get a correct moisture reading.
Using Deep hammer probes
The standard pins in a meter give a reading of the surface of the timber which is ok in small dimension timber. but to understand what is happening in hard timbers i.e. large joists and beams you should use a deep hammer probes give you the moisture reading deeper inside, and this can guide any remedial measures by comparing the surface reading with the reading at depth.
One method of assessing at timber floor is to find the joist below the floorboard, then take a surface reading with the moisture meter, then using the hammer probe take moisture readings at different depths.
Deep probes into masonry
By drilling two 5mm holes 20 to 40mm apart into the masonry you can insert the probes which will then give a relative reading at depths and will help in diagnosing the form of dampness.
Calibration of the electrical moisture meter
For a conductivity meter there is a testing wire which you place on the pins and will confirm is the devices is working correctly.
For the scan mode thigs are not as simple. For your Protimeter Surveymaster you select a suitable wall in your office and take a reference reading if this changes recalibration may be necessary. For instance, the Flir MR77 has a built-in recalibration function.
The manufactures handbook for you meter should advise the best method of calibration and meter settings for different types of timber