| E. Ciências Agrárias - 2. Engenharia Agrícola - 4. Engenharia de Água e Solo |
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| DIHEDRAL SENSOR TO MEASURE WATER TENSION AND ACTIVITY OF SOIL AND PLANT SAMPLES |
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Adonai Gimenez Calbo 1
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1. Embrapa Instrumentação Agropecuária
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| INTRODUÇÃO: |
| Water potential is a reversible thermodynamic variable with dimension of pressure. Plant water transport, however, is irreversibly governed by water potential component fractions effectively transduced into pressures: the osmotic pressure, height dependent pressure, the cell turgor pressure and the negative pressure, water tension, in the walls and cavities of the apoplasm/xylem. Both in plants and soils, pressure components, can be measured with suitable pressure probe instruments, while the water potential and water activity, which are highly dependent on temperature, can be better evaluated using sensitive hydrometers. A dihedral sensor and its coupling methods to actuate as a pressure probe or as a hygrometer is described in this work. For soil water tension measurements, the sensor coupled to a porous element, which is easily read with a caliper between 0 and 100 kPa. Under a microscope micrometer, with the sensor vertices pressed against the organ, plant water tensions, 0 to 3.0 kPa, can be measured according to the air/water meniscus position. For the hygrometer function the sensor, without microscope aid, is operated with a reference saline solution, with which it measures water activities from 0.9 to 1.0, with a response time in the order of hours. |
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| METODOLOGIA: |
| Sensors made with flat hydrophilic plates fixed as a dihedron of small angle can be used to measure water tension and water potential as a function of the distance L from the meniscus to the edge, at equilibrium. Material, finishing and dimensions of the dihedral sensor are defined according to the application. The sensor frontal plate is transparent, while the second plate can be transparent, black or micro porous. For soil water tension, this sensor is cemented on top of porous elements with suitable bubbling pressure and water conductivity. Cemented over a porous thin plate it measures surface soil water tension, while cemented on top of a cylindrical porous element, partially covered by a water impermeable layer, it measures at a defined soil depth. For plants, illuminated by transmission under the microscope, water tension is measured when the sensor plates near the edge make direct water transference while pressed against the organ. For plant water potential the sensor is slightly different and has its edge inside a mini depression between the two lapidated glass plates, which pressed against the organ allow only a water exchange through the gaseous phase. For slower water activity measurements using a caliper, a reference salt solution is added between the sensor plates. |
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| RESULTADOS: |
| The separation between plates at a meniscus line is given by the product of L and tg(a), where L the distance between the meniscus and the edge and a is the dihedral angle. If pure water is contained inside the sensor, then the water tension (T) and the water potential (Y) are equal and given by Y=T=-2s/[L tg(a)], where s is the water surface tension. In practice, however, water tension is measured when sample and sensor are effectively coupled by liquid water pressure transferring joints, while water potential, which is a logarithmic thermodynamic function of the water activity, is measured when water exchange between sample and sensor occurs in the gaseous phase. Under the microscope a water exchange of a few nanograms is easily read and occurs in a few minutes, enabling water tension and water potential reading between 0 and 3.0 kPa. Without microscope and with hydraulic coupling done by a porous element the dihedral sensor read with a caliper can easily measure water tensions from 0 to 100 kPa, with a response time of one hour or less. Water activity measurements between 0.9 and 1.0, on the other hand, require the addition of a reference saline solution, between the plates, and reading is made in longer equilibration periods of a few hours. |
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| CONCLUSÃO: |
| 1) Made with two flat plates, the dihedral sensor edge contain paths for the water exchange involved in the variations of the position, represented by the distance L, variable which is used to calculate water tension, water potential and water activity in the dihedral sensor;
2) For measurements of soil water tension the dihedral sensor needs to be coupled to a suitable porous element, which acts as filter and allows rapid water exchange between soil and sensor;
3) Application of a suitable impermeable layer over par of the porous element surface enables t enable soil water tension measurements at specified depths with the dihedral sensor;
4) The micro chamber between the sensor lapidated glass plates and the organ helps in obtaining rapid temperature equilibrium and rapid water exchange for water potential measurements;
5) Without the microscope and using a reference NaCl solution, water activity between 0.9 and 1.0, can be measured in the dihedral sensor with a caliper.
6) The water exchange for measurements soil water tension and water activity, without the microscope, is in the order of micrograms, while for plant measurements of water tension and water potential, under the microscope, smaller volumes in the order of nanograms are exchanged in the measurements.
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| Palavras-chave: water activity, water potential, water tension. |