Building Dynamics, LLC
1216 Ashton Road
Ashton MD 20861
[email protected]
(240) 899-6926_______________________________________________
Tracking Structural Drying- Water Activity vs. Moisture Content
Ed Light, CIH and Roger Gay
Building Dynamics, LLC May 6, 2014 (DRAFT)
BDL conducted a pilot test of prototype instrumentation under development by
Decagon Devices, Inc. for the field measurement of Water Activity (aw) in building
materials. Experiments were conducted simulating the drying of wet framing wood.
Side-by-side measurements compared the ProCheck meter attached to a VP-3 sensor
inside a rubber housing to Moisture Content (MC) read by a Delmhorst B-2100 pin
moisture meter.
OBJECTIVES
1. Consider the feasibility of measuring of aw in structural materials under field
conditions.
2. Make recommendations to Decagon on procedures for field measurement of aw.
3. Compare drying time based on MC to that based on aw.
INITIAL TRIALS
BDL evaluated water activity measurement systems provided by Decagon on wet
framing wood. The system was found to be simple and durable for field use, although
screws used to attach the sensor housing become stripped after several uses.
Temperature and relative humidity readings matched those from a Tasco thermopsychrometer.
Aw should be read when moisture in the air under the housing reaches
equilibrium. BDL tracked water activity readings over time and observed that equilibrium
was reached within the following time periods (approximate) under various conditions:
Material Status
Time to Equilibrium
Dry
Damp
Wet
10 minutes
1 hour
1 day
Another sampling strategy issue involves spatial variability. Because differential
drying across the wood surface results in varying aw readings, the wettest spot should
be monitored to ensure that drying is completed.
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Based on these observations, an experiment was designed to track drying with
the Decagon system side-by-side with a moisture meter measuring MC. Two methods of
aw measurement were also compared, one with the sensors remaining in a fixed
position and one where the sensors were detached after each reading, re-attached and
then read after one hour. Although the first approach enables continuous data-logging,
BDL was concerned that leaving the sensor housing in place might reduce drying and
produce aw readings which were not representative of surfaces exposed to air
circulation.
METHODOLOGY
1. Submerged a 30” 2X4 in water for 4 hours and started measurements after 1
hour.
2. Dried wood under ambient air conditions of 65 degrees F and 45% relative
humidity.
3. Periodically recorded moisture content with a Delmhorst meter based on the
peak measurement made from inserting the 5/16” pins at six locations.
4. Periodically recorded water aw from 4 Decagon sensors placed side-by-side in
the middle of the board. Two of the sensor housings were left in place for the
duration of the experiment.
5. The other two sensors were removed after each reading, leaving the surface
open to air drying, and then re-attached 1 hour before making the next reading.
6. The higher reading from each pair of sensors was used.
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Wet 2X4 with four VP-3 sensors attached. The two middle
sensors were removed after each measurement. Water activity
was read by plugging sensors into the ProCheck meter (in front).
Moisture content was measured with the orange meter (Delmhorst BD2100.
Criteria used to evaluate drying were as follows:
Minimum standard
used by practitioners
Conservative standard
to account for variability
Water Activity
0.70
Moisture Content
19%
0.60
15%
RESULTS
Water Activity
(fixed sensors)
Baseline (dry)
Drying Hours
2
5
8
16
41
66
Water Activity
(sensors re-attached)
.41
.88
.91
.94
.96
.76
.55
.88
.85
.77
.60
.45
.41
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Moisture Content
Moisture Content
8%
31%
26%
17%
12%
9%
8%
Figure 1 shows aw was still around .90 when MC was19%. It was not until MC
was reduced to 10% that aw had been reduced to the level precluding most mold
growth (.70). MC had to be further reduced, to 9% to achieve a water activity of .60.
Figure 2 indicates that wood under the sensor housing retained moisture longer
than surrounding material open to the air. aw in the two fixed sensors remained above
.90 for about 24 hours, while measurements from the sensors which were detached
between readings, allowing the sampling spot to air dry, steadily declined to .55 in 24
hours.
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PRELIMINARY CONCLUSIONS
Note: These are based on a limited number of data points tracking the drying of one
piece of wood under one condition.
1. The Decagon system can be used by technicians for measuring Water Activity of
building materials in the field.
2. The system appeared to be durable for field use, with the exception of the screws
used to attach the sensor housing, which may become stripped after a few uses.
3. Leaving the sensor in place to track drying for data-logging or periodic reading
may not produce readings representative of the wet material. Sensors left at
restoration sites may also be compromised.
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4. Equilibrium is achieved in newly attached sensors after about one hour under
damp conditions. Equilibrium is reached sooner when the substrate is dry and
can take much longer when the substrate is very wet.
5. It took approximately three times longer to dry wood to a water activity of .70
compared to that for meeting a moisture content of 19%. Water activity remained
at a level which may support mold growth after drying objectives based on
moisture content are met.
PRELIMINARY RECOMMENDATIONS
1. To ensure that all areas of the wet material are dry, the wettest area should be
identified and monitored. This can be located by measuring moisture content
across the wood and locating the water activity probe where the highest reading
is made.
2. Because water-activity measurements from sensors which remain fixed in place
do not produce representative readings, drying tracked over time should be
based on sensors which are detached between readings. Data-logging may not
be representative of overall drying.
3. Time to reach water activity equilibrium under the sensors is variable, depending
on substrate drying status. To track drying, accuracy when the material is
moderately wet to dry is most critical. Since equilibrium under these conditions is
achieved with one hour, this could be specified as the minimum time to make a
reading. The reading at one hour when the substrate is very wet will be slightly
low.
4. Additional screws should be provided to attach sensors.
5. Confirm that the sensor orientation does not affect the reading (i.e., horizontal vs.
vertical).
6. A follow-up pilot study should be conducted at a restoration project, tracking
water activity and moisture content of various construction materials while they
are drying.
7. Experiments should be conducted to determine whether mold actually grows
where a material dried to moisture content guidelines (i.e., <19%) but water
activity remains elevated (i.e., >.90) is sealed (i.e., covered with drywall or trim).
8. A more detailed research project should be conducted to evaluate field
measurement of water activity in structural materials under a range of conditions.
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