Campbell PC208 Instruction Manual download pdf (Page 5)

1. Background and Theory

1.1 The Origin of Heat-pulse

Heat-pulse methods date back some 60 years to the work of Huber (1932) who first

conceived the idea of using heat as a tracer of sap flow. In his early experiments on

tropical liana, Huber found such high rates of sap flow that when heat was applied for one

to two seconds it was still recognizable as a pulse at the junctions of a thermocouple

sensor some 30 cm downstream from the heater. The time for the first appearance of heat

at the sensor was assumed to be the same as the time taken for the sap to move this

distance. However in later work, at slower sap speeds, Huber recognized the importance

of distinguishing between the effect of convection by the moving sap and the transport of

heat by thermal conduction. To separate these two effects, Huber and Schmidt (1937)

developed an early version of the ‘compensation’ heat-pulse method in which one sensor

was downstream and the other sensor was upstream of the heater. The time of peak

warming of the upstream sensor compared to the downstream one, was used to

‘compensate’ for the effects of thermal conduction. In both cases the sensor and the heater

were external to or just under the bark on the surface of the sapwood and it was assumed

that the speed of the sap was identical with that of the heat pulse.

1.2 Idealized Heat-pulse Theory

Later work by Marshall (1958) developed a theoretical foundation for the heat-pulse

technique and showed that Huber’s assumptions were not well founded. From a

theoretical viewpoint, Marshall showed that the speed of sap is not the same as that of the

heat pulse. Rather, the heat pulse velocity is equal to the weighted average of the velocity

of the moving sap and the stationary woody matrix, the weighting factor being determined

largely by the physical properties of the woody matrix. Marshall (1958) also proposed a

new probe arrangement. He advocated the use of a line heater and temperature probe to

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Campbell PC208 Instruction Manual Page 5