Moisture Meter Technical Information
Abbreviations used in this document:
● DWP - Deep wall probes
● ERH - Equilibrium relative humidity
● HDF - High Density Fibre
● Laitance - Condition of the floor surface
● RF - Radio frequency search mode
● rh - Relative Humidity
● MC - Moisture content
● %WME - %Wood moisture equivalent
DEWPOINT - definitions
Dewpoint: The temperature at which dew (or condensation) forms on cooling a gas. Or, put another way, it is the temperature at which air becomes
saturated in equilibrium with water. Important measurement in condensation risk assessment in buildings. Surface temperatures need to be maintained
above the dew point otherwise moisture vapour in the air will condense on the surface (think misted up mirror in the bathroom after a shower, or
water droplets on a cold beer, just taken out of the fridge).
GEL-BRIDGE (very old moisture test method)
Gel-bridge Method: Why the instruments using the Gel-bridge method which gave a % of moisture in concrete are now obsolete.
Historically, concrete existed as a cement/sand only mixture. In the last few years, many new supplements are being added to the concrete mixture in
order to speed up the drying/curing rate. Some of these additives may contain conductive salts or alkalis that may artificially inflate any
gel-bridge conductivity reading. “Blue metal” added to the concrete mixture will react in a similar fashion, as it is conductive.
The gel-bridge method uses a resistance, or conductivity method. Because of the more widespread use of conductivity increasers as noted above,
readings taken using the gel-bridge method have the potential to become more erratic.
For this reason, Protimeter discontinued the gel-bridge instruments which used the gel-bridge and rh methods and instead are producing a rh method
only Hygromaster. The rh method does not rely on electrical conductance, but equilibrium relative humidity which may take a little longer to perform
but it is likely to give more accurate results.
Humidity and conversion of moisture content to humidity. This simply cannot be done as each mix of concrete will have different moisture/humidity
characteristics. It would be impossible to know what characteristics apply at any give site, hence the use of humidity as a measurement. This is
effectively a non contact measurement and does not rely on electrical characteristics of the mix.
MOISTURE EXPLAINED - HUMIDITY (rh & ERH)
When water evaporates as a vapour in air to the extent that the air becomes saturated (any more moisture will form droplets, and condense) then the
air, at a given temperature is said to have 100% relative humidity rh value.
Using a hygrometer technique the readings are taken at various locations on the subfloor. To gain a meaningful rh reading from a floor slab, it is
necessary to trap and enclose a volume of air over the subfloor, wait sufficient time for the air to absorb vapour from the concrete or screed. When
the air has absorbed all the water it can from the floor then it is in equilibrium with the moisture content in the concrete. This is known as
achieving ERH. This may take 24 hours or more depending on the condition of the slab and the prevailing site temperature etc.
MOISTURE EXPLAINED - MOISTURE CONTENT (MC)
Not to be confused with humidity! This is the amount of water by weight as a percentage of the weight of all other material in the slab (sand,
cement aggregate etc). It is not unusual for the MC value to be over 100% in some materials - green wood - for instance. The actual moisture
content value will vary not only with changes in the amount of water present but according to the density (weight per unit volume) of the material.
To measure moisture content, a volume sample of the floor must be taken and the moisture dried out and the sample weighed and then compared to the
weight of the remaining dry material. Not a very practical method but can be useful correlating with relative humidity measurements if the density
and moisture gradients of the in-situ materials are known.
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