Portable Density and Moistures Gauges (PDMG), also known as Nuclear Density Gauges (NDG), measure the density of a layer by emitting gamma radiation from the source in the end of the rod and detecting it in the body of the gauge.

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As the radiation travels through the material in the layer, some of it is absorbed and some is deflected or scattered. The denser the material you’re testing, the less radiation is detected.

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The gauge then calculates the wet density based on a few things, but mainly how much radiation was detected and how far away the source is from the detector (obviously dependent on the depth you’re testing at).

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Have a look at the density counts you write down. For measurements at the same depth, the lower the density count, the higher the wet density figure will be.

 

A few things to keep in mind:

 

Counts must be at least one minute. The shorter the count time, the less accurate the measurement.

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The gauge must be seated on the layer properly, if you have gaps between the bottom of the gauge and the layer it’s sitting on, this empty space will be counted as voids in your material, and will result in a lower wet density reading.

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Background radiation may affect the readings. Standard counts on the reference block in the box must be done at each new lot. The software in the gauge then takes any background radiation into account in the calculations.

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While the gauges can give an indication of moisture content, it’s not very reliable. Accuracy of readings depends on the moisture content of the soil and the depth you’re testing at. As a guide, reliable measurements of moisture can only be made to a depth of about 150mm.