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

植物液流测量是一种在受控环境和田间条件下测定植物蒸腾速率的方法。目前，测定草本植物蒸腾作用的方法主要有稳态热平衡法和热脉冲法。热脉冲法利用径向插入树干的两个热电偶之间的温差来估算树液流速。当使用两个数据时，关于这种方法准确性的一个主要问题与测量程序的适当使用有关：一个用于低对流热速度，另一个用于高对流热速度。V=（X_1-X_2）/（2 x t_0）（1）其中X_1和X_2分别是从加热器到加热器上方和下方的热电偶结的距离（mm）; t_0是从脉冲发射到初始温差的第一次重现所经过的时间（秒）。Cohen建议上述方程可用于准确估计低速或中等速度（0至0.17 mm/秒）。对于较高的液流速度，他利用树干的热扩散率给出了另一个不易精确测量的方程，科普上述情况，我们提出了多热电偶热脉冲法，使之能覆盖任何速度，用方程（1）测量树干液流。我们从理论上证明了三个温度传感器和热脉冲器足以通过将它们放置在适当的位置来测量任何液流速度。给出了确定热传感器最佳位置的理论方法。

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.