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The SD-6M Sensor enables to monitor variations of trunk diameter in micron range. Two phenomena affect diurnal behavior of trunk diameter: growth and water balance of a plant. A growth component depends on a vegetation stage (see Fig.1 below). Two features of the daily trunk variation curve are to be noted: daily contraction amplitude and daily maximums trend (see Fig. 2 below). Usual daily transpiration cycle produces a temporary reduction of water content in plants, which is manifested in the proportional reduction of trunk diameter. In case of developing water deficit, the daily contraction amplitude increases. At the same time, a considerable water deficit may prevent a total replenishment of water reserves in plants during the night, resulting in downtrend of stem diameter maximums. Thus, the SD-6M sensor allows investigating intimate effects of environmental factors on water balance and growth of plants.



Fig.1. Grapevine: a typical seasonal trunk growth curve of two plants. Five stages can be easily distinguished: dormancy (1), active vegetative growth (2), deceleration (3), declining (4) and stabilization (5). Daily variations are almost indistinguishable in this large scale. 
Fig.2. Apple tree: a drought attack of a week duration caused both increase of daily contraction amplitude and even a temporary downtrend of the trunk diameter. Air vapor pressure deficit (VPD) reached 5 kPa whereas the normal daytime level is about 1-2 kPa.


The SD-6M Stem Microvariation Sensor includes a LVDT-based sensor mounted in special clip and a dc powered LVDT signal conditioner with optional power adapter for unipolar power supply. 


Linear Measurement Range: 0-5 mm
Diameter Range of Suitable Stems: 20 to 70 mm 
Output: 0-2 V or 4-20 mA (optional)
Output Temperature Coefficient: <200 ppm/?C FS
Supply Voltage: 15 Vdc 15%
Power adapter operates on 10 to 30 Vdc
Dimensions: 140 x 70 x 30 mm (sensor approx.)
120 x 70 x 37 mm (signal conditioner)