Measurement of water potential using thermocouple hygrometers.
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Date
1982
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Abstract
Theory predicts that the time dependent voltage curve of a thermocouple
psychrometer where there is no change in output voltage with
time during the evaporation cycle defines the wet bulb temperature T[w]
corresponding to the water potential. In practice, a change in
voltage with time does occur and it is convenient to define the
voltage corresponding to the water potential as the maximum point of-
inflection voltage.
A predictive model based on calibration data at a few tempertures
is used to obtain the psychrometer calibration slope at any temperature.
Use of this model indicates that psychrometers differ from
each other and therefore must be individually calibrated if accuracy
better than ±5 % in the measurement of water potential is required.
Dewpoint hygrometers are shown to be less temperature sensitive than
psychrometers and have the added advantage of a voltage sensitivity
nearly twice that of psychrometers, typically -7,0 x 10¯³ μV/kPa compared
to -3,7 x 10¯³ μV/kPa at 25 °C.
The accurate temperature correction of hygrometer calibration curve
slopes is a necessity if field measurements are undertaken using either
psychrometric or dewpoint techniques. In the case of thermocouple psychrometers,
two temperature correction models are proposed, each
based on measurement of the thermojunction radius and calculation of
the theoretical voltage sensitivity to changes in water potential.
The first model relies on calibration at a single temperature and the second at two temperatures. Both these models were more accurate
than the temperature correction models currently in use for four leaf
psychrometers calibrated over a range of temperatures (15 to 38°C).
The model based on calibration at two temperatures is superior to
that based on only one calibration. The model proposed for dewpoint
hygrometers is similar to that for psychrometers. It is based on the
theoretical voltage sensitivity to changes in water potential. Comparison
with empirical data from three dewpoint hygrometers calibrated
at four different temperatures indicates that these instruments
need only be calibrated at, say 25°C, if the calibration slopes are
corrected for temperature.
A model is presented for the calculation of the error in measured
thermocouple hygrometric water potential for individual hygrometers
used in the dewpoint or psychrometric mode. The model is based on
calculation of the relative standard error in measured thermocouple
psychrometric water potential as a function of temperature. Sources
of error in the psychrometric mode were in calibration of the instrument
as a function of water potential and temperature and in voltage
(due to electronic noise and zero offsets) and temperature measurement
in the field. Total error increased as temperature decreased,
approaching a value usually determined by the shape of the thermocouple
junction, electronic noise (at low voltages less than 1 μV)
and errors in temperature measurement. At higher temperatures,
error was a combination of calibration errors, electronic noise and
zero offset voltage. Field calibration data for a number of leaf
psychrometers contained total errors that ranged between 6 (at a °C)
and 2 %(at 45 °C) for the better psychrometers and between 11 (at
0° C) and 5 % (at 45 C) for the worst assuming that the zero offset was
0,5 μV. Zero offset values were less than 0,7 μV at all times. The
dewpoint errors arose from calibration of the dewpoint hygrometer as
a function of water potential, extrapolation of the calibration slope
to other temperatures, setting the dewpoint coefficient and errors in
voltage and temperature measurement. The total error also increased
as temperature decreased, because of the differences in temperature
sensitivity between dewpoint and psychrometric calibration constants.
Consequently, the major source of error in the dewpoint mode arose
from the difficulty in determining the dewpoint coefficient. This
error, which is temperature dependent, contains three subcomponent errors; the temperature dependence, random variation associated with
determining the temperature dependence and error in setting the
correct value. Calibration and extrapolation errors were smaller
than those of the psychrometric technique. Typically, the error in
a dewpoint measurement varied between about 6 and 2 % for the best
hygrometer and between 10 and 3 % for the worst for temperatures
between 0 and 45 °C respectively. At low temperatures, the dewpoint
technique often has no advantage over the psychrometric technique,
in terms of measurement errors.
In a comparative laboratory study, leaf water potentials were
measured using the Scholander pressure chamber, psychrometers and hydraulic
press. Newly mature trifoliates cut from field grown soybean
(Glycine max (L) Merr. cv. Dribi) were turgidified and, after
different degrees of dehydration, leaf water potential measured.
One leaflet from the trifoliate was used for the thermocouple
psychrometer and another for the press while the central leaflet
with its petiolule was retained for use in the pressure chamber.
Significant correlations between measurements using these instruments
were obtained but the slopes for hydraulic press vs psychrometer
measurement curve and hydraulic press vs pressure chamber were 0,742
and 0,775 respectively. Plots of pressure-volume curves indicate
that the point of incipient plasmolysis was the same (statistically)
for the thermocouple psychrometer and the pressure chamber, but
much larger for the hydnaulic press. The above-mentioned differences
between the three instruments emphasize the need for calib rating
the endpoint defined us i ng the press against one or more of the standard
techniques, and, limi ting the use of the press to one person.
Cuticular resistance to water vapour diffusion between the substomatal
cavity and the sensing psychrometer junction is a problem
unique to leaf psychrometry and dewpoint hygrometry; this resistance
is not encountered in soil or solution psychrometry. The cuticular
resistance may introduce error in the leaf water potential measurement.
The effect of abraiding the cuticle of Citrus jambhiri to
reduce its resistance, on the measured leaf water potential was
investigated. Psychrometric measurements of leaf water potential
were compared with simultaneous measurements on nearby leaves using
the Scholander pressure chamber, in a field situation. Leaf surface
damage, due to abrasion, was investigated using scanning electron
microscopy. Thermocouple psychrometers are the only instruments which can
measure the in situ water potential of intact leaves, and which may be
suitable for continuous, non-destructive monitoring of water potential.
Unfortunately, their usefulness is limited by a number of difficulties,
among them fluctuating temperatures and temperature gradients within
the psychrometer, sealing of the psychrometer chamber to the leaf,
shading of the leaf by the psychrometer and resistance to water
vapour diffusion by the cuticle when the stomates are closed. Using
Citrus jambhiri, several psychrometer designs and operational
modifications were tested. In situ psychrometric measurements compared
favourably with simultaneous Scholander pressure chamber
measurements on neighbouring leaves, corrected for the osmotic
potential and the apparent effect of "xylem tension relaxation"
following petiole excision.
It is generally assumed that enclosure of a leaf by an in situ
thermocouple psychrometer substantially modifies the leaf environment,
possibly altering leaf water potential, the quantity to be
measured. Furthermore, the time response of leaf psychrometers to
sudden leaf water potential changes has not been tested under field
conditions. In a laboratory investigation, we found good linear
correlation between in situ leaf psychrometer (sealed over abraided
area) and Scholander pressure chamber measurements (using adjacent
leaves) of leaf water potential, 2 to 200 minutes after excision
of citrus leaves. A field investigation involved psychrometric
measurement prior to petiole excision, and 1 min after excision,
simultaneous pressure chamber measurements on adjacent citrus
leaves immediately prior to the time of excision and then on the psychrometer
leaf about 2 min after excision. Statistical comparisons
indicated that within the first two minutes after excision, psychrometer
measurements compared favourably with pressure chamber measurements.
There was no evidence for a psychrometer leaf water potential
time lag. For the high evaporative demand conditions, water
potential decreased after excision by as much as 700 kPa in the
first minute. Psychrometer field measurements indicated that within
the first 5 min of leaf petiole excision, the decrease in leaf water
potential with time was linear but that within the first 15 s, there
was a temporary increase of the order of a few tens of kilopascal. The thermocouple psychrometer can be used to measure dynamic changes
in leaf water potential non-destructively, with an accuracy that
compares favourably with that of the pressure chamber.
Using in situ thermocouple leaf hygrometers (dewpoint and
psychrometric techniques employed) attached to Citrus jambhiri
leaves, an increase in measured water potential immediately following
petiole excision was observed. The increase ranged between
20 to 80 kPa and occurred 30 s after petiole excision and 100 s
after midrib excisions. No relationship between the actual leaf
water potential and the increase in water potential due to excision,
was found.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1982.
Keywords
Plant-soil relationships., Plant-water relationships., Hygrometers., Theses--Agrometeorology.