Articles
THE EFFECTS OF HEAT STORAGE DURING LOW FLOW RATES ON THE OUTPUT OF GRANIER-TYPE SAP-FLOW SENSORS
Article number
846_2
Pages
45 – 52
Language
English
Abstract
The Granier-method is a widely used method for measuring sap-flow in trees.
It is based on the detection of convective heat transport in the form of heat dissipation from a heated sensor.
Like all heat-transfer-based sap-flow methods the accuracy of the Granier-method relies on the fact that heat transport in the xylem occurs mainly by convection and that conductive transport can either be omitted or taken into account in calibration.
If this is not the case, sap-flow rate may either be under- or overestimated depending on the specific setup.
In this study we used a 3-dimensional sap-flow and heat transfer model to study the effects of heat conduction and storage during low flow rates on the sap-flow rates measured by the Granier-method.
Our results show that in well insulated stems time, lags between the real flow rate and the flow rate measured by Granier-method can be up to 2 hours and the lag depends on the length of the low flow period.
If the sapwood is well connected to the outer environment the time lags are considerably shorter and independent of the length of heat storage period.
This study emphasizes the importance of proper calibration and evaluation of the measurement conditions when any heat transfer-based methods are used for estimating water storage capacitance of trees or for studying sap-flow dynamics.
It is based on the detection of convective heat transport in the form of heat dissipation from a heated sensor.
Like all heat-transfer-based sap-flow methods the accuracy of the Granier-method relies on the fact that heat transport in the xylem occurs mainly by convection and that conductive transport can either be omitted or taken into account in calibration.
If this is not the case, sap-flow rate may either be under- or overestimated depending on the specific setup.
In this study we used a 3-dimensional sap-flow and heat transfer model to study the effects of heat conduction and storage during low flow rates on the sap-flow rates measured by the Granier-method.
Our results show that in well insulated stems time, lags between the real flow rate and the flow rate measured by Granier-method can be up to 2 hours and the lag depends on the length of the low flow period.
If the sapwood is well connected to the outer environment the time lags are considerably shorter and independent of the length of heat storage period.
This study emphasizes the importance of proper calibration and evaluation of the measurement conditions when any heat transfer-based methods are used for estimating water storage capacitance of trees or for studying sap-flow dynamics.
Publication
Authors
S. Sevanto, T. Hölttä, E. Nikinmaa
Keywords
conduction, convection, heat transfer, sap-flow, xylem
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