requires temperature sensors and a heater element which are cheap and easy to be
implemented (Davis et al. 2012). In order to perform sap flow measurements, there
are three main methodologies: heat ratio method (HRM) (Burgess et al. 2001), heat
balance (Sakurtani 1981), and thermal dissipation method (TDM) (Granier 1985).
TDM consists on using continuous heating in the stem and measuring the
thermal dissipation capability when the sensor and stem are in steady state or
thermal equilibrium (Allen and Grime 1995; Granier 1985). The variations in the
heat dissipation that occurs during day are in function of sap flow. First, a heater
element with an attached thermocouple is introduced in the stem by drilling a few
millimeters of depth and the stem is heated electrically at a constant ratio and
another thermocouple is introduced a few centimeters below the heater probe
position in order to measure the amount of heat taken by the sap flow stream at
xylem (Allen and Grime 1995; Davis et al. 2012). As observed at Fig. 13.8.
The HRM consists on sending short heat pulses of 1–2 s of duration to the plant
stem. Consequently, the mass flow of sap is obtained from the speed measurement
of the heat pulses moving along the stem where two thermocouples are introduced
at equal distance but one below the heater and the other one at an upper distance
from the heater (Marshall 1958; Swanson and Whitfield 1981; Davis et al. 2012).
The main difference between TDM and HRM in terms of hardware is that HRM do
not use a thermocouple attached to the heater but uses two thermocouples to
measure the heat movement across sap stream. Furthermore, both temperature
sensors return to same reading after 60 s of sending the heat pulse (Sakurtani 1981;
Baker and van Bavel 1987).
13.8 Plant Morphological Sensors
As was aforementioned in the sap flow section, plants have a sap flow system
where the xylem transport water, phloem transport sugars, and other nutrients and
transpiration is a water loss mechanism which occurs during CO 2 exchange process during photosynthesis. As a consequence of water and nutrient flow across the
plant, stems, and trunks produce diameter variations which are completely related
to sap flow. Therefore, during day hours occurs a steam diameter decrease while it
increases during nighttime (Berger and Selles 1993; Garnier and Berger 1986).
Consequently, this stem diameter information result very useful for plant physiology research purposes and plant production systems where drought and other
water-related stress conditions need to be monitored with accuracy (Morandi et al.
2007).
There are two main types of sensing elements which are utilized for morphological sensors: strain gauges and linear-variable-displacement-transducer (LVDT)
(Morandi et al. 2007). Strain gauges consist in a variable resistor which appears in
different shapes, this devices produces electrical resistance variations due to
mechanical displacement. Strain gauge-based sensors are widely utilized as stem/
trunk diameter sensors and fruit growth sensors (Ortuño et al. 2010). The theory of
13 Instrumentation and Control to Improve the Crop Yield
379
implemented (Davis et al. 2012). In order to perform sap flow measurements, there
are three main methodologies: heat ratio method (HRM) (Burgess et al. 2001), heat
balance (Sakurtani 1981), and thermal dissipation method (TDM) (Granier 1985).
TDM consists on using continuous heating in the stem and measuring the
thermal dissipation capability when the sensor and stem are in steady state or
thermal equilibrium (Allen and Grime 1995; Granier 1985). The variations in the
heat dissipation that occurs during day are in function of sap flow. First, a heater
element with an attached thermocouple is introduced in the stem by drilling a few
millimeters of depth and the stem is heated electrically at a constant ratio and
another thermocouple is introduced a few centimeters below the heater probe
position in order to measure the amount of heat taken by the sap flow stream at
xylem (Allen and Grime 1995; Davis et al. 2012). As observed at Fig. 13.8.
The HRM consists on sending short heat pulses of 1–2 s of duration to the plant
stem. Consequently, the mass flow of sap is obtained from the speed measurement
of the heat pulses moving along the stem where two thermocouples are introduced
at equal distance but one below the heater and the other one at an upper distance
from the heater (Marshall 1958; Swanson and Whitfield 1981; Davis et al. 2012).
The main difference between TDM and HRM in terms of hardware is that HRM do
not use a thermocouple attached to the heater but uses two thermocouples to
measure the heat movement across sap stream. Furthermore, both temperature
sensors return to same reading after 60 s of sending the heat pulse (Sakurtani 1981;
Baker and van Bavel 1987).
13.8 Plant Morphological Sensors
As was aforementioned in the sap flow section, plants have a sap flow system
where the xylem transport water, phloem transport sugars, and other nutrients and
transpiration is a water loss mechanism which occurs during CO 2 exchange process during photosynthesis. As a consequence of water and nutrient flow across the
plant, stems, and trunks produce diameter variations which are completely related
to sap flow. Therefore, during day hours occurs a steam diameter decrease while it
increases during nighttime (Berger and Selles 1993; Garnier and Berger 1986).
Consequently, this stem diameter information result very useful for plant physiology research purposes and plant production systems where drought and other
water-related stress conditions need to be monitored with accuracy (Morandi et al.
2007).
There are two main types of sensing elements which are utilized for morphological sensors: strain gauges and linear-variable-displacement-transducer (LVDT)
(Morandi et al. 2007). Strain gauges consist in a variable resistor which appears in
different shapes, this devices produces electrical resistance variations due to
mechanical displacement. Strain gauge-based sensors are widely utilized as stem/
trunk diameter sensors and fruit growth sensors (Ortuño et al. 2010). The theory of
13 Instrumentation and Control to Improve the Crop Yield
379