Instrumentation & Measurement Magazine 23-2 - 40

Fig. 2. (a) Laboratory pressure probe setup (adapted from [6] with permission
© American Society of Plant Biologists, 1999). (b) Prototype of an implantable
microtensiometer (adapted from [13], with permission, © The Royal Society of
Chemistry, 2014,).

xylem sap through the leaf's petiole (detected visually) is considered to equal the water potential of the leaf placed inside
the chamber.
Albeit simple, and often used in practice, this method features major limitations. It is destructive and can be performed
exclusively on leaves. Its absolute accuracy is limited to ±0.1
MPa. It is affected by subjective visual readout of the equilibrium pressure, sample-to-sample differences in speed of
propagation of the pressure gradient through the leaf in the
chamber, and by the transpiration rate (the method is more
accurate in non-transpiring leaves). Precision is severely affected by the influence of environmental conditions (sun,
shade, humidity, time of day, etc.), and by variability of measurements on individual leaves. Each leaf requires manual
preparation, making the method laborious, slow, and without
any prospects for automation. Nevertheless, long-term field
studies report that even the application of such a crude, manual method of water-potential measurement may reduce the
40	

irrigation water-supply by 46% and simultaneously increase
grapevine quality [1].
A non-invasive, automatized alternative for indirect xylem water potential monitoring is a method based on vapor
pressure measurement on the stem-surface. It relies on the
fact that water potential is proportional to transpiration rate.
Transpiration can be quantified by a vapor pressure in the
air surrounding the transpiring elements (i.e., leaf, stem). Increase in transpiration rate results in higher vapor pressure,
indicating higher absolute value (i.e., more negative) water
potential. Vapor pressure can be determined by confining a tissue sample in an air/water tight, thermodynamically isolated
chamber. At equilibrium, water potential of the sample in the
chamber is directly related to the relative humidity, measured
with a thermocouple [14].
Based on the underlying measurement principle, two
groups of techniques are candidates for field operation: nonequilibrium psychrometric techniques (wet-bulb temperature
depression) and hygrometric techniques (dew-point temperature depression). Two types of non-equilibrium (wet-bulb)
psychrometric techniques are most commonly used: Richardstype and the Spanner-type (i.e., Peltier) psychrometers. The
temperature measuring junction of the Richards-type needs to
be kept wetted by a drop of distilled water. On the other hand,
Spanner-type psychrometers are automatically wetted by
cooling the measuring junction below the dew-point temperature before each measurement, by applying the Peltier effect
thermoelectric principle. Thus, only Spanner-type can be used
in the field [14].
Dew-point hygrometric methods detect dew-point temperature depression of water vapor inside the sample chamber.
The detection principle can be either thermal-using a pair of
thermocouples-or optical. Optical hygrometers cool a mirror using the Peltier effect and detect the dew point with a
photodetector.
Their accuracy is around ±0.1 MPa [13], and the measurement range depends on design: -200 to -300 MPa for
Richards-type TCP, -7 to -8 MPa for Spanner-type TCP, -40 MPa
for thermal hygrometers, and -300 MPa for optical. All of these
surpass greatly the range of pressure probes, however at the
cost of linearity in near-zero region of the range [14].
Due to proven reliability in controlled laboratory conditions, and good correlation with pressure bomb readings,
vapor pressure techniques are used as one of the referent
xylem water potential measurement techniques. As an advantage, they enable non-destructive measurement. However,
applicability of non-destructive thermocouple psychrometry in the field is limited by delicate installation, cumbersome
mounting hardware, long time required for equilibration and
isolation from environmental parameters such as temperature
and evaporation (Fig. 3).

Thermal Sensors of Xylem Sap Flow
The most widely used automated field-method of xylem flow
measurement is thermal anemometry (Fig. 4). It involves a
nail-heater and a set of needle/nail-like temperature probes

IEEE Instrumentation & Measurement Magazine	

April 2020



Instrumentation & Measurement Magazine 23-2

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