Theory of Time Response of Tensiometers

Towner, G. D.

doi:10.1111/j.1365-2389.1980.tb02108.x

Cited by 24 publications

(5 citation statements)

References 6 publications

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“…It should be noted that while the fluid‐selective microsensors used in this work are used under positive pressures, their function is fundamentally the same as that of conventional tensiometers. Tensiometer response has been widely studied, and the first order response described by has been observed for more than 60 years [e.g., Richards , 1949; Klute and Gardner , 1962; Towner , 1980]. The constant k in is expected to be a function of both the porous medium and the sensor, although for media with sufficiently high permeabilities, sensor capillary barrier permeability dominates sensor response [e.g., Klute and Gardner , 1962; Towner , 1980; Selker et al , 1992].…”

Section: Methodsmentioning

confidence: 99%

Dynamic capillary effects in a small‐volume unsaturated porous medium: Implications of sensor response and gas pressure gradients for understanding system dependencies

Hou

Chen

Kibbey

2012

Water Resources Research

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.[1] Rate dependencies in system properties observed during nonsteady state unsaturated and multiphase flow are often referred to as dynamic capillary effects. One widely studied dynamic capillary effect is the apparent dependence of measured capillary pressure on the rate of saturation change. While this phenomenon has been observed for over four decades, a clear picture of the source of the phenomenon and its true magnitude remains elusive. Furthermore, reported dependencies on system properties and state variables have been contradictory. The focus of this work was on quantifying the relationship between measured capillary pressure and rate of saturation change using a small volume system with highly characterized fluid-selective microsensors. Experimental measurements in three systems were used to calculate the dynamic capillary coefficient as a function of saturation during drainage. Corrections for sensor response and flow-induced gas pressure gradients were applied to explore how these potential artifacts would impact measured values. Significant differences in values were observed in uncorrected measurement between the three systems, but corrected values were very similar in all cases. Corrected values were found to be on the order of 10 3 Pa s or less-one to two orders of magnitude lower than the uncorrected values, and two or more orders of magnitude lower than most published values for similar porous medium/fluid combinations. Because of the small size of the experimental system used, results suggest that at the representative elementary volume (REV) scale, the dependence of measured capillary pressure on the rate of saturation change may not be as significant as previously thought for unsaturated systems. It is hypothesized that the larger magnitude of some previously reported values may result at least in part from porous medium packing microheterogeneities that influence flow and pressure gradients in larger systems.Citation: Hou, L., L. Chen, and T. C. G. Kibbey (2012), Dynamic capillary effects in a small-volume unsaturated porous medium: Implications of sensor response and gas pressure gradients for understanding system dependencies, Water Resour.

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Section: Methodsmentioning

confidence: 99%

Dynamic capillary effects in a small‐volume unsaturated porous medium: Implications of sensor response and gas pressure gradients for understanding system dependencies

Hou

Chen

Kibbey

2012

Water Resources Research

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“…1999 ). The product SK (Sensitivity × Conductance) is best fitted by SK = 0.02 s −1 corresponding to a response time τ of approximately 50 s. Using the measured value for the conductance K (see ), we obtain a value of S = 3.42 × 10 12 Pa m −3 which is of the order of values given in Klute (1986) and Towner (1980) for several pressure transducers.…”

Section: Dynamic Ot Experimentsmentioning

confidence: 64%

A prototype osmotic tensiometer with polymeric gel grains

Biesheuvel

Loon

Raangs

et al. 2000

European J Soil Science

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Summary An accurate field method that directly measures the entire range of relevant soil water potentials for periods of months to years is not yet available. The osmotic tensiometer (OT) has the potential to fulfil these demands but is still in an early stage of development. We have designed a novel OT that should overcome several drawbacks of former designs by the use of cross‐linked polymeric gel grains instead of an aqueous solution of linear chain polymers inside the porous ceramic cup. For this reason, the cup can have large pores without polymer leakage. This ensures that water exchanges rapidly with the soil, resulting in a negligible pressure lag with changing temperatures, which is the most important source of a systematic measurement error of an OT. A second reason for better performance might be the inherent elasticity of the grains which should inhibit formation of a polymer cake at the filter. To study several of the key variables of the new sensor, transient and stationary experiments were performed in an aqueous solution the temperature, salt type and salt concentration of which were varied. Under similar conditions the swelling of the polymer grains was studied in more detail. The OT that we developed responds rapidly to changes in water potential, but is very susceptible to the influence of salts. To study and minimize salt influence, swelling tests of unconfined grains are proposed in combination with tests of the OT itself.

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“…Among these equations, y = 0.009x2 + 0.7786x + 7.0652 had the best correlation by having the lowest Square values (R 2 ) 0.929, which was the highest among the equations, albeit the values do not show any strong correlation. In general, the difference in soil moisture % measured by tensiometer against gravimetric technique is considerable [6] . As a result, there is considerable reason to believe that the soil moisture characteristic curve used to calculate soil moisture is erroneous.…”

Section: Development Of the Methods Of Correctionmentioning

confidence: 99%

Development Of Equation Of Curve To Estimate Volumetric Soil Moisture Content In CaseOf Tensiometer To Overcome The Moisture Measurement Error Due To Time Lag Issues

Majhi,

Biswas

2023

SFS

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A field experiment was carried out at the Instructional Farm of Bidhan Chandra krishi viswavidyalaya, located in Mohanpur, Nadia, West Bengal, in order to explore the variations in estimation of soil moisture due to time lag issues in case of tensiometer. In this work, the gravimetric technique and tensiometers were used to calculate the volumetric soil moisture content. With regard to the gravimetric approach, which was considered as the reference method, the variation of volumetric soil moisture content derived from tensiometer was analysed. For the tensiometer, it was calculated that the coefficient of variation (CV) and standard deviation (STD) values were 0.213 and 6.95 respectively. The tensiometer data was processed to compare with that volumetric moisture data which was obtained via the gravimetric approach. The objective of this study is to generate an equation of curve which would estimate the actual (error-free) volumetric soil moisture content for tensiometer from the measured volumetric soil moisture content. The equation of the curve, y = 0.009x2 + 0.7786x + 7.0652, was generated by plotting volumetric soil moisture content under tensiometer against volumetric soil moisture measured by gravimetric method (observed data). As tensiometer is not capable of producingcorrect moisture values from known tension values using soil moisture characteristics curves, due to time lag issues, soin determining the correct soil moisture status, the method of development of the equation is necessary and was proved to be very much fruitful whenever this instrument was used in the field.

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Theory of Time Response of Tensiometers

Cited by 24 publications

References 6 publications

Dynamic capillary effects in a small‐volume unsaturated porous medium: Implications of sensor response and gas pressure gradients for understanding system dependencies

Dynamic capillary effects in a small‐volume unsaturated porous medium: Implications of sensor response and gas pressure gradients for understanding system dependencies

A prototype osmotic tensiometer with polymeric gel grains

Development Of Equation Of Curve To Estimate Volumetric Soil Moisture Content In CaseOf Tensiometer To Overcome The Moisture Measurement Error Due To Time Lag Issues

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