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AVALIAÇÃO DE TENSIÔMETRO COM CÂMARA DE AR USANDO LEITURA DIRETA E TENSÍMETRO
Abstract
The present work was carried out in Agricultural Engineer Departament of Escola Superior de Agriculturade Mossoró - ESAM, to verify the efficiency of the air-pocket direct reading and a pockettensimeter for measuring soilwater tension in air-pocket tensiometers installed at 15cm deep in 32 pots with bermuda grass, having each pot an air-pocket tensiometer and one withmercury manometer (control). Measurements were performed during 28 consecutivedays. It was verified that the direct readings through the air-pocket height were statistically similar to the mercurymanometer tension readings for measurements made in the morning. However, in the afternoon, it provided highertension values readings. On the other hand, the pocket tensimeter readings were lower than those of the mercurymanometer tensiometer in both periods. The direct reading air-pocket tensiometer was efficient for measuring the soilmatric potential, but with low accuracy for research purposes. Similar characteristics were observed in the pockettensimeter, except that it is not influenced by the time of reading.
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Potential applications of air pocket type tensiometers in measuring hydraulic head profiles in deep vadose zones are discussed. Advantages of this method include (i) the ability to obtain tensiometer measurements far beyond the approximately 9 mdepth often associated with the limit of conventional tensiometry, (ii) ease of regular gauge calibration, and (iii) low cost. Advantages relative to buried, dedicated pressure transducer tensiometers are gained at the expense of substantial losses in gauge sensitivity, S*. In view of this compromise, an analysis was performed to determine the optimal fractional waterfilled length, F, for air pocket tensiometers. It is shown that the critical ratio governing the nature of S*-optimization is approximated by (II* - IIo)z*, where II* represents the absolute matric head, IIo is the vapor pressure of water expressed in head units, and z* is the depth of the tensiometer tip. When (II* - IIo)/z* > 1, S* is optimized when F->1. However, when (II* - IIo)/z* < 1, S* is optimized as F->0. The central role of (II* - IIo)/z* arises from the fact that S* = Pa/Vg, where Pa refers to the absolute pressure of all tensiometer headspace gases excluding water vapor, and Vg refers to the volume of the gas phase within the tensiometer headspace. When (II* - IIo) is less than z*, S* goes to zero because the absolute pressure in the tensiometer headspace approaches the vapor pressure of the tensiometer water (Po) when attempts are made to fill the tensiometer column with liquid water. In the more familiar case of II* - IIo being larger than z*, the dominance of Pa over Po assures that S* increases as the instrument is filled. To test the predicted nature of S*, laboratory experiments were performed on 1.11-6.36-, and 11.91-m long tensiometers over a range of values of (II* - IIo) and F sufficient to provide three orders of magnitude variation in S*. Measured S* agreed well with predicted values, and supports the conclusion that response times are minimized with F->0, in situations where (II* - IIo)/z* < 1.
(C) Williams & Wilkins 1994. All Rights Reserved.
Field‐installed tensiometers are generally equipped with pressure gauges or mercury manometers. These read‐out devices require much maintenance, while the cost of gauges is high when used in large quantities. This paper describes a pressure transducer system for field‐installed tensiometers. The system consists of a pressure transducer with attached syringe needle and digital read‐out. The needle is inserted through the septum stopper which closes the upper end of the tensiometer. The pressure in the air below the septum stopper, in equilibrium with the water pressure, is read on the digital read‐out, which is calibrated in millibars. Upon withdrawal of the needle from the tensiometer, the septum stopper seals the tensiometer. The pressure transducer system is easy to use and requires little maintenance. Possible errors with the measurement system are discussed.
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