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DMM600 Duff Moisture Meter

4. Discussion of Water Content Measurement with

DMM600

4.1 Discussion of Gravimetric and Volumetric Water Content

The DMM600 is sensitive to water content on a volume basis, i.e. volumetric

water content. Volumetric water content is the volume of water in a given

volume of material. Gravimetric water content is a weight-basis parameter and

is the ratio of the mass of water to the mass of the material with no water.

Volumetric and gravimetric water content are related by the bulk density of the

material. Multiplying gravimetric water content by material bulk density

yields volumetric water content.

The choice of water content description, whether gravimetric or volumetric, is

largely traditional in different professions. It is important to understand the

difference and how to convert between them.

The DMM600 presents water content measurement results in volumetric terms

because that is what the DMM600 measures. Conversion to gravimetric

requires knowledge of the material bulk density. Bulk density of duff tends to

be similar for many forest types. If the bulk density of a particular material is

known or measured, the displayed volumetric water content value is easily

converted to gravimetric by adjusting the calibration coefficients.

As an example, consider that the bulk density of a lodgepole pine was

determined to be 0.25 g/cm

. The calibration equation to convert circuit output

frequency to volumetric water content is

Volum etricWaterContent freq freq=+ −5288 5905 0142

..* .*

To convert to gravimetric water content, each calibration equation coefficient

is divided by the bulk density. Applying for this example gives

GravimetricWaterContent freq freq=+ −21152 23620 0568

..*.*

This is not a general calibration equation but an example.

PCDMM can be used to load the modified calibration equation. When

downloaded, the DMM600 will alternately display volumetric water content

using standard calibration and gravimetric water content using the modified

equation.

NOTE

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Campbell DMM600 Manuale delle Istruzioni Pagina 9