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History of the development of the trunk heat balance method in last forty years

Kučera, J., Urban, J.
Acta horticulturae 2012 no.951 pp. 87-94
electric current, electrodes, electronics, energy, heat, heat treatment, phloem, sap flow, temperature
The aim of this contribution is to describe the step-wise improvement of the trunk heat balance method (THB) from the first idea to the actual sophisticated instrumentation. Both technical and theoretical aspects of the development of the method are considered. The original idea of heating trunk tissues with an electrical current passing between inserted plate electrodes has been accepted as an optimal arrangement despite the misleading original idea of energy distribution. The first important improvement eliminated the significant influence of highly electrically conductive phloem on the accuracy of the measurement in trunks with thick phloem tissues. Following fundamental physical analysis of both power field and heat field showed false conception of the homogeneous energy distribution between two parallel plate electrodes in an electrically conductive environment. Besides, the physical analysis confirmed the presumption that the stability of the flow measurement based on heat balance increases with the size of heated volume. The actual measuring layout uses three stainless plates for direct heating of plant tissues and the needle temperature sensors are inserted in slots of those plates. This way uses the averaging effect of metal plates and reduces an error caused by a radial temperature gradient that makes a lot of troubles especially by point-shaped temperature sensors. The electronics maintains stable temperature difference between heated and non-heated parts of the measuring points. This arrangement substantially improves the response of the measuring system to fast sap flow changes and last but not least greatly reduces the power consumption.