Campbell HydroSense II Manuel d'instructions télécharger pdf (Page 23)

Appendix B. How Many Soil Water Content Measurements Are Enough

B-3

N is the number of water content measurements and L is the acceptable range

as defined by the user. For example, the acceptable water content range might

be ±2.5% so L would be 5.0.

Equation 4 relies on a good estimate of the population standard deviation. A

useful approach using these statistical methods is to establish an estimate of the

standard deviation by taking several water content measurements, then use that

standard deviation estimate and the associated t value in [4]. The range is

specified by the user and the calculated N value defines the number of

measurements. The same s and t values are used, but the number of

measurements can be determined for several different ranges.

⋅⋅

4 4 2 201 174

235

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The calculation of N using the sample standard deviation shows that 3

measurements are required to give a 90% probability that the sample mean

gives the desired ±2.5%.

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Campbell HydroSense II Manuel d'instructions Page 23