Sap Flow Measuring System through Heat Pulse Method by use of Thermal Multi Couples

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Sap flow measurement in plants shows promise as a method for determining transpiration rate in controlled environment and field condition. The two most common approaches (steady state heat balance and heat pulse method) have been used so far to measure the transpiration in herbaceous plants. The heat pulse method uses the temperature difference between two thermocouples radially inserted into the stem to estimate sap velocity.A major question concerning the accuracy of this method is related to the appropriate use of the measurement procedure when two data are used : one for low and the other for high convective heat velocities.Heat velocity (V,mm/sec) is estimated by equation (1)V= (X_1-X_2) / (2 x t_0) (1)where X_1 and X_2 are the distances (mm) from the heater to the thermocouple junctions above and below the heater, respectively ; t_0 is the time (sec) elapsed from pulse emission to the first re-occurrence of the initial temperature difference. Cohen suggested that above equation may be used for an accurate estimation of low or moderate velocities (0 to 0.17 mm/sec). For higher velocity, he gave another equation by use of the thermal diffusivity of the stem which is not easy to be measured accurately.To cope with the above situation, we proposed the multi thermo couples heat pulse method so that it can cover any velocity to measure sap flows in the stem by equation (1). We made it evident theoretically that three thermosensors and heat pulser are sufficient to measure any sap flow velocity by placing them at the appropriate locations. We gave the theoretical way to determine the optimum positions of thermo sensors.