German plant physiologist Huber proposed in 1932 that the heat pulse method first used heat pulse as a tracer of plant liquid flow and was first used in practical research. Huber uses a resistive wire as a thermal pulse source to sense the arrival time of the thermal pulse through a single thermocouple mounted below the resistance wire, which is the prototype of the stem flow meter. However, this method is difficult to explain the temperature rise and fall of the removal of the thermocouple. Later, Huber used a method of setting the thermocouple probes up and down in the heat source to effectively separate the motion of the heat pulse in the liquid flow conduction system from the thermal interference in the external environment. However, the thermal pulse conduction rate measured by Huber is significantly lower than the actual flow rate. Marshall improved Huber's design by inserting heating elements and temperature measuring nodes into the xylem of the plant. He assumed that there was no obstruction between the flow and the xylem during the movement of the heat pulse, and that heat could be exchanged freely between the sap and the sap. However, the flow rate calculated according to Marshall's theory is also lower than the actual rate and needs to be corrected. Swanson found that the real reason for the deviation between the heat pulse rate and the actual liquid flow in the previous calculations was: “injury effectâ€, thus denying Marshall's hypothesis about the homogeneity of xylem in the stem. He believes that when the thermal probe is installed, the callus is generated at the damaged part around the probe, which reduces the thermal conductivity of the area around the probe, resulting in a lower thermal pulse rate than the true flow rate. If the damage is corrected in the calculation, the applicability of the thermal pulse technique will be greatly improved. In 1981, Swanson gave a modified parameter table by numerical simulation model to simulate thermal convection and conduction phenomena. Swanson's research is a milestone in the development of thermal pulse technology. In order to obtain a more accurate flow rate through the trunk section in a unit time, Edwards et al. studied the flow changes at different depths below the formation layer, and obtained the relationship between the liquid flow rate and the depth. So far, the thermal pulse flow detection method and theoretical system have been completed.
On the basis of the thermal pulse rate method, Granier et al. improved the thermal pulse flow detector using the thermal pulse hysteresis effect to improve the thermal diffusion flow probe measurement device using the dual thermocouple detection dissipation principle. In contrast to the thermal pulse method, a prominent feature of the thermal diffusion probe is the ability to continuously exotherm the flow rate for continuous or arbitrary time intervals. The thermal diffusion method has higher accuracy and is being used more and more widely in the study of sap flow.
The heat pulse method is to insert two sensor probes longitudinally into the xylem or stem of the fabric to be tested, the insertion depth is the same, and a heating element is inserted between the probes. The heater is 50 mm from the upper temperature probe and 10 mm from the lower temperature probe. When the heater releases the heat pulse, the time is counted, and when the temperature of the upper and lower probes is the same, the time used is recorded, thereby determining the speed of the heat pulse. For the low-speed stem flow, the stem flow rate is slow, the probes at both ends reach the same temperature, the heat loss is large, and the measurement result is large. For the defect of the thermal pulse method, the new measurement method heat ratio (HRM) method places the heater at the midpoint of the two probes, and records the temperature of the probe at time t (t-1) as the heat pulse releases heat. ) The difference in temperature at all times. The advantage of the thermal ratio method over the thermal pulse method is that the heater is first at the midpoint of the two probes in the thermal ratio method, thus solving the probe position deviation problem. Secondly, because the thermal ratio method measures the difference between the temperature at the time of the probe t and the temperature at the time of (t-1), it is much less than the time required to wait for the temperature of the two probes to be the same, greatly reducing the heat pulse conduction process. Measurement error caused by heat loss.
Inserting the sensor probe into the xylem can cause mechanical damage to the plant, which not only interferes with the path of the stem flow, but also the function of the corresponding wound in the plant will create an invading body at the wound to block the flow channel of the stem, so the measurement results will be affected more than the actual The value is too small, and the finite difference mathematical model can be used to construct an algebraic equation to solve the wound correction problem. Usually measured is the velocity of the stem flow at discrete points. Only the sapwood transports the stem flow in the xylem. The area is the cross-sectional area of ​​the wood minus the area of ​​the heartwood. The flow rate of the stem flow is weighted according to the sapwood area of ​​the sample.
The heat pulse method is to insert two sensor probes longitudinally into the xylem or stem of the fabric to be tested, the insertion depth is the same, and a heating element is inserted between the probes. The heater is 50 mm from the upper temperature probe and 10 mm from the lower temperature probe. When the heater releases the heat pulse, the time is counted, and when the temperature of the upper and lower probes is the same, the time used is recorded, thereby determining the speed of the heat pulse. For the low-speed stem flow, the stem flow rate is slow, the probes at both ends reach the same temperature, the heat loss is large, and the measurement result is large. For the defect of the thermal pulse method, the new measurement method heat ratio (HRM) method places the heater at the midpoint of the two probes, and records the temperature of the probe at time t (t-1) as the heat pulse releases heat. ) The difference in temperature at all times. The advantage of the thermal ratio method over the thermal pulse method is that the heater is first at the midpoint of the two probes in the thermal ratio method, thus solving the probe position deviation problem. Secondly, because the thermal ratio method measures the difference between the temperature at the time of the probe t and the temperature at the time of (t-1), it is much less than the time required to wait for the temperature of the two probes to be the same, greatly reducing the heat pulse conduction process. Measurement error caused by heat loss.
Inserting the sensor probe into the xylem can cause mechanical damage to the plant, which not only interferes with the path of the stem flow, but also the function of the corresponding wound in the plant will create an invading body at the wound to block the flow channel of the stem, so the measurement results will be affected more than the actual The value is too small, and the finite difference mathematical model can be used to construct an algebraic equation to solve the wound correction problem. Usually measured is the velocity of the stem flow at discrete points. Only the sapwood transports the stem flow in the xylem. The area is the cross-sectional area of ​​the wood minus the area of ​​the heartwood. The flow rate of the stem flow is weighted according to the sapwood area of ​​the sample.
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