Endress+Hauser MMP20 SONO Mobil, Руководство по эксплуатации

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MMP20 SONO-DIS (option D)
Hauser Page 35
Potential problems or influencing factors when kiln-drying:
• When kiln-drying with a gas burner (blowtorch), make sure that no solid particles escape into the
air (or from the kiln vessel!) as the loss of weight would mean that the water content values
determined would be too high. Some users stir the fresh concrete when kiln-drying, while others
do not. This results in different kiln-dried values. If the fresh concrete is not stirred, there is the
danger that the water would be chemically bound in the concrete due to the longer drying time.
Such water cannot escape even at very high temperatures. The measured water content is
generally higher when the concrete is stirred vigorously than when it is not stirred, as too many
solid particles can escape to the air due to the stirring action.
• When drying with a microwave oven, it is important to select the drying time depending on the
power (800Watt or 1000Watt), and also pay attention to the volume of the kiln-dried sample
(e.g. 1.5kg to 2kg). With the same fresh concrete sample, deviations in the range
from +-3 to +-10 liters/m
3 are not unusual when drying with gas burners or microwave ovens.
The information sheet published by the German Society for Concrete and Construction
Technology (DBV) entitled "Special Fresh Concrete Checks" deals specifically with kiln-drying
procedures with microwave ovens. Note that the water can be chemically bound in the concrete
with drying times longer than 20 minutes. This can falsify the result as the water content would
then be too low. If the sampling volumes in the microwave are too large, there is also the danger
of the chemical binding of water, which would cause the reading for the measured water content
to be too low.
•
When taking concrete samples for kiln drying, considerable deviations can occur. If the concrete
has been left in the bucket for quite a while, it may segregate, with the result that the water
content of kiln-dried samples of concrete taken at the surface would be far too high. This is
particularly true of concrete with flow class F5 and F6.
• When weighing the kiln-dried sample, pay attention to the temperature of the kiln-dried sample.
When weighing a very hot kiln-dried sample, the upward lift of air streams can produce a
considerable error on the scales. For example, when weighing a 4kg weight, the hot air stream
can produce a difference of 30 grammes, depending on the weighing unit. With a weight of 4kg,
this would correspond to a moisture difference of +0.75%. In the worst case scenario, the
+0.75% moisture then corresponds to an
error of +17 liters of water per cubic meter !
• Additives in the concrete behave like water during the kiln-drying process, i.e. they enter into
the measurement of the kiln-dry water content and the measurement with SONO-WZ.
• There are additives that bind water chemically in such a way that water is crystalline bound
relatively quickly and therefore cannot fully escape during the kiln-drying process (particularly
if kiln-drying with a microwave without stirring).
If the measurement performed with the SONO-WZ does not match a correct kiln-dried sample which
is measured in parallel, it is possible to set the SONO-DIS to a correct water measured value using
the "G-Set" parameter in the "CHANGES" menu. The
"Construction Site Test" form provided in the
appendix is recommended for comparing kiln-dry measurements with SONO-WZ.
The importance of having a representative sample for kiln-drying is
illustrated using the example of this 32mm piece of gravel. Given a
sampling volume of 1.5kg for kiln-drying with a microwave, this
single piece of gravel represents a value of 5.3 liters of water/m
3 !
If 5kg of sample is kiln-dried, the piece of gravel still represents
1.5 liters/m
3 . Therefore, one piece of gravel more or less can produce
considerable errors, depending on the dry-kilning method and
sampling.