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Fig. 3. Commercial psychrometric xylem water potential sensors (available online: [15], used with permission © ICT Intl. Ltd.): (a) Pressure-clamped sensing
chamber; (b) Psychrometer with a data-logger; and (c) Heat-shielding with reflective foil.

used to estimate the flow from the measured temperatures
[16]. We give an overview of three main groups of methods:
heat deformation (HD) methods, heat balance (HB), and heat
pulse velocity (HPV) [17]. HD and HB rely on continuous heating, while HPV features pulsed heating.

Heat Deformation Methods
In heat deformation (HD) methods, heat is continuously supplied to the sapwood through the nail-heater [17]. Sap flux
density is then estimated from the changes of the heat field
around the heater because of the heat-carrying (convection)
sap. Two main implementations of the HD method that are
used most often are the thermal dissipation probe (TDP) and
the heat field deformation (HFD) method.
The original Granier's thermal dissipation probe (TDP) design features two probes. The downstream probe is constantly
heated with approximately 0.2 W, and the other unheated
probe is positioned upstream to the sap flow. Both probes are
equipped with copper-constantan thermocouples to measure
their temperature difference (ΔT). Under conditions of thermal

stability in the heating element, in the wood structure and in
the sap, the constant heat input is equal to the heat dissipated
by convection and conduction at the wall of the probe. When
there is no sap flow, heat is not dissipated by convection but
only by conduction; hence, the measured temperature difference ΔT reaches its maximum, ΔTmax. Naturally, this typically
occurs at night. When sap flow increases, ΔT reduces proportionally to the increase of the convective heat dissipation. From
ΔT, sap flux density is calculated by the empirical formula [17].
The method is limited to measurement of downstream flows.
Key challenge with the TDP method is sensitivity to
changes in environmental temperature, especially at low sap
flows (at night). Temperature differences can originate either from static spatial natural temperature gradients across
the plant's stem, from dynamic diurnal temperature variations, and from fast transient changes in temperature caused
by wind. One of the proposed solutions [17] is slow pulsed
heating with an optimized on/off duty-cycle (in the order of
minutes). Also, diurnal fluctuations in stem water content induce inaccuracy in Granier-type TDP measurements of sap

Fig. 4. Thermal xylem sap flow sensor designs. Continuous-heating: a) Heat field deformation-HFD [18]; (b) Heat-balance-HB (available online: [21], used with
permission © EMS Brno, 2019), Pulsed-heating; c) Heat-ratio-HRM (available online: [15], used with permission © ICT Intl. Ltd.).
April 2020	

IEEE Instrumentation & Measurement Magazine	41
Instrumentation & Measurement Magazine 23-2

Table of Contents for the Digital Edition of Instrumentation & Measurement Magazine 23-2
