Campbell CS616 Manuel d'instructions télécharger pdf (Page 33)

CS616 and CS625 Water Content Reflectometers

7.3 Calibration Equations

TABLE 7-1 lists the calibration coefficients derived in the Campbell Scientific

soils laboratory. Both linear and quadratic forms are presented. The choice of

linear or quadratic forms depends on the expected range of water content and

accuracy requirements. These coefficients should provide accurate volumetric

water content in mineral soils with bulk electrical conductivity less than

0.5 dS m

-1

, bulk density less than 1.55 g cm

-3

, and clay content less than 30%.

TABLE 7-1. Standard calibration coefficients for linear and quadratic forms.

Linear Quadratic

C0 C1 C0 C1 C2

–0.4677 0.0283 –0.0663 –0.0063 0.0007

The linear equation is

VWC = -0.4677+0.0283*period .

The quadratic equation is

VWC = -0.0663 - 0.0063*period + 0.0007*period

Period is in microseconds. The result of both calibration equations is

volumetric water content on a fractional basis. Multiply by 100 to express in

percent volumetric water content.

FIGURE 7-3 shows the difference between the linear and quadratic calibration

forms over the typical range. A CS616/CS625 output period of 16

microseconds is about 1.2% VWC and 32 microseconds is 44.9%. The linear

calibration is within ± 2.7% VWC of the quadratic. The linear calibration

underestimates water content at the wet and dry ends of the range and

overestimates it by up to about 2.6 % VWC at about 20% VWC.

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Campbell CS616 Manuel d'instructions Page 33