How does wallboard product temperature affect a moisture measurement ?
The relative dielectric of water is a temperature affected property. The relationship is well documented as shown in the table below:
|INFLUENCE OF TEMPERATURE ON RELATIVE DIELECTRIC OF WATER|
|TEMPERATURE (°C)||DIELECTRIC (Ɛr)|
|dƐr/dT = -0.4/°C|
At first glance this may be considered a problem or, at least something to be compensated for. Both ST-2200 and ST-3300 are equipped with temperature compensation inputs. In practice, in an on-line application, process conditions tend not to vary too significantly at any fixed point in the process. Inside the dryer at all moisture measurement points, the internal board temperature should be approximately 100°C. After the dryer, any free moisture will be located within the central core of the board where the temperature will tend to decline quite slowly. At the takeoff location, the measurement by the IMPS-4400 sensor array or by large area sensors (ST-3300 & ST-2200), the inner core temperature may be only slightly below 100°C given the short time, typically a few minutes after exiting the dryer.
An interesting issue is the effect of wallboard product temperature in relation to the operation of a hand-held instrument. Two things will influence this measurement; first is the moisture distribution and second is the effect of dielectric temperature coefficient.
Moisture distribution is quite dynamic as the board exits the dryer. Figure 1 represents the moisture distribution having a cross-sectional bell-curve profile. As the board cools, moisture migrates toward each surface as a process of equilibration occurs. Moisture migration and board cooling will both contribute to higher moisture meter readings.
Figure 2 illustrates the dynamics of board drying. Under ideal circumstances the migration and evaporation of the free water results in a constant surface temperature of 100°C. Drying at a rate exceeding the moisture migration rate will result in the outermost board region (sub-surface) having no free moisture with an associated temperature rise and possible sub-surface calcination. Calcining will almost always occur to some degree no matter how good a job is done in the dryer but, so long as it is not excessive, further migration of free moisture may recombine with the calcined product, effectively repairing the surface of the board.
Based on the recombination of free moisture into chemically bound water of crystallization will likely cause a reduction in overall product dielectric resulting in a lower handheld meter reading.
Taking all the above factors into consideration, it is clear that the hand-held reading should not be relied on as an absolute reference. In reality, it is a reasonable “Relative” quality indicator but absolute numbers will be different for hot boards fresh out the dryer and those having cooled in the warehouse. Incorporating product temperature compensation in a hand-held moisture instrument is difficult since a pyrometer or contact RTD would only indicate surface temperature and the cost and added complexity would significantly add to the cost of the instrument.
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