Campbell HydroSense II Uživatelská příručka stáhnout pdf (Strana 10)

HydroSense II Soil Moisture Measurement System

In simple terms, soil consists of three main constituents – mineral particles

(sand, loam, clay), water and air. The air and water occupy the spaces or pores

formed between the mineral particles. Typically in agricultural soils these pore

spaces make up approximately 50% of the soil by volume, with water and air

together making up the remaining 50% in varying quantities. As a result, water

content normally ranges from 0% to 50%. In some conditions, the sensor is

unable to determine the soil water content and in these cases, an out of range

value is displayed (“---”).

N.B. even when the sensor cannot determine the VWC, the signal period

measurement will always be displayed. In some special media this value can be

used along with a soil specific calibration to estimate water content.

5.3 Water Deficit Display

The purpose of the HydroSense II Water Deficit display is to help with water

management decisions for irrigators.

In Water Deficit mode, the HydroSense II stores “wet” and “dry” values for up

to 10 different soil types and compares the current measurement to those

reference values. It then shows relative water content (RWC) on a scale where

0% represents the “dry” value stored for that soil and 100% represents the “wet”

value. While any two volumetric water content measurements can be stored as

“wet” and “dry” references, it is recommended to store the wilting point of your

crop as the “dry” value and field capacity as “wet”. The relative water content is

calculated as follows:

100x

VWCVWC

RWC

drywet

dry

In this equation, VWC is the current measurement of volumetric water content,

VWC

dry

is the volumetric water content of the “dry” reference and VWC

wet

is the

volumetric water content of the wet reference point.

For example, a particular clay-loam soil may have an 18% wilting point and

35% field capacity. Using these values a VWC measurement of 24.5% would

display as a relative water content of 38.2%. Similarly a VWC measurement of

30% would show an RWC of 70.6%.

An RWC value greater than 100% indicates that the soil has a water content

value greater than the stored “wet” value and a negative value reflects that the

water content is below the “dry” point.

Along with the relative water content, the HydroSense II calculates and displays

the amount of water required (in mm of rain) to return the soil to the “wet” level.

A negative deficit value indicates that that the water content is above the “wet”

value for that soil type.

Since the CS658 and CS659 measure average water content along the length of

their rods (20cm and 12cm respectively), their sample volumes are quite

different and this affects the water deficit value. For this reason it is important

that the current measurement and reference values are all made with the same

probe type. Some customers prefer to use different probes with different rod

lengths for different soils, however if the probe length used to set the reference

values is different to that used later to evaluate relative water content, the

HydroSense II may give erroneous readings.

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