GAUGE SENSITIVITY OPTIMIZATION IN AIR POCKET TENSIOMETRY

Abstract

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 m-depth 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 water-filled length, F, for air pocket tensiometers. It is shown that the critical ratio governing the nature of S*-optimization is approximated by (II* - II{sub 0})/z* > 1, S* is optimized when F {yields} 1. However, when (II* - II{sub 0})/z* < 1, S* is optimized as F {yields} 0. The central role of (II* - II{sub 0})/z* arises from the fact that S* = P{sub a}/V{sub g}, where P{sub a} refers to the absolute pressure of all tensiometer headspace gases excluding water vapor, and V{sub g} refers to the volume of the gas phase within the tensiometer headspace. When (II* - II{sub 0}) is less than z*, S* goes to zeromore » because the absolute pressure in the tensiometer headspace approaches the vapor pressure of the tensiometer water (P{sub 0}) when attempts are made to fill the tensiometer column with liquid water. In the more familiar case of II* - II{sub 0} being larger than z*, the dominance of P{sub a} over P{sub 0} 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* - II{sub 0}) and F sufficient to provide three orders of magnitude variation in S*. 17 refs., 5 figs.« less