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Published in Vadose Zone Journal 2:611-617 (2003)
© 2003 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

SPECIAL SECTION - ADVANCES IN MEASUREMENT AND MONITORING METHODS

A Handheld Device for Intrusive and Nonintrusive Field Measurements of Air Permeability

Marc Jalbert{dagger} and Jacob H. Dane*

Department of Agronomy and Soils, Auburn University, AL 36849-5412
* Corresponding author (danejac{at}auburn.edu).

{dagger} (deceased) Back


Received 7 February 2003.



   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS
 METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
Air permeability is an easy to measure soil parameter that is of direct importance in gas transport studies. Its values can also be used as indicators of soil hydraulic conductivity. Near the soil surface, both air and water permeability values are important for hydrological and agricultural studies involving, for example, soil aeration and water runoff during rainfall events. We provide a design of a rugged, lightweight, handheld, single-reading device allowing for fast measurements of air permeability near the soil surface. The device makes use of two interchangeable air probes. The contact probe, well documented in the petroleum engineering literature, is proposed as an addition to the traditional insertion probe. The advantages and drawbacks of each probe type are discussed briefly. Central to the in situ measurement of air permeability is the concept of the probe geometric factor. Empirical relationships are presented to make the application of this concept more amenable. Relative differences in air permeability values obtained with the two probes seem to be acceptable for permeability measurements. Even though in most cases contact probe air permeability values were higher than insertion probe values, no clear trend existed. The differences were attributed to differences in soil compaction, bypass flow, and different measurement volumes associated with the two probe types. For the flow rates and pressures encountered during the measurements, the flow rate behaved as a linear function of the pressure gradient. In other words, the assumed Darcy-type equation was applicable.

Abbreviations: ALH, sandy loam, high in organic matter • CS, coarse sand • FS, fine sand • MS, medium sand • SL, sandy loam • TNH, loam soil, high in organic matter • TNL, sandy loam, low organic matter • VCS, very coarse sand • VFS, very fine sand


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS
 METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
AIR PERMEABILITY has long been a parameter of interest to soil scientists. It is of direct importance when studying topics such as transport of volatile organic compounds in the subsurface (Moldrup et al., 1998; Poulsen et al., 1999) or soil–atmosphere gas exchange (Hutchinson and Livingston, 2002), as well as of interest to agriculture and the turf grass industry. Furthermore, extensive air permeability measurements can provide useful information about the hydraulic conductivity distribution at the field or catchment scale (Loll et al., 1999). Notably, the permeability of the soil surface is important to hydrologists and soil conservationists because it affects phenomena such as water runoff and soil erosion (Römkens et al., 2002). Parallel to efforts undertaken by the soil science community in the field of air permeametry (Ball and Schjønning, 2002), a great deal of research has been performed by petroleum scientists to assess the conductive properties of oil reservoirs to fluids such as brine, crude oil, and gas. Because it is more simple to use, air has traditionally been preferred, rather than water or oil, for estimating the intrinsic permeability of core samples. Quite recently, numerous studies in the petroleum literature (Eijpe and Weber, 1971; Goggin et al., 1988; for a review see Hurst and Goggin, 1995) encouraged the use of so-called minipermeameters (or probe permeameters) to study the air permeability structure of rock samples in the laboratory or of geologic outcrops in situ. The devices used in the field often comprise a source of pressurized N, together with electronic or mechanical flow meters and manometers (e.g., Chandler et al., 1989). Such devices are connected to a probe that simply comes in contact with the surface of the porous medium of interest. Measurements performed by such methods, although limited to near-surface sampling volumes, have the advantage of being rapid and nonintrusive. Even though probe permeameters have occasionally been used on unconsolidated porous media (e.g., Eijpe and Weber, 1971), none of the references provided in the exhaustive review of Hurst and Goggin (1995) came from the soil science literature. It is our intent to show how well-accepted concepts among the petroleum community can be useful to soil scientists interested in soil air permeability. Following the petrogeology literature, we provide a newly designed, small, battery-powered, rugged device adapted to rapid field determination of soil air permeability in the approximate range of 5 to 150 µm2. The applicability of contact probes to soil permeability studies is briefly discussed and compared with the traditional use of insertion probes.


   MATERIALS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS
METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
The air permeameter we designed consists of a box (25 by 19 by 8 cm) containing a 1.5-VDC battery-powered air (aquarium) pump (Model A-790, Rolf C. Hagen, Montreal, Canada), a low-pressure differential pressure transducer (Model 600D-012, 0-500 Pa, Cole-Parmer, Vernon Hills, IL; Auto Tran Inc., Eden Prairie, MN), and a 3.0-V commercial voltmeter. The schematic diagram (Fig. 1) also shows the 12-V rechargeable battery, which provides power to the aquarium pump, the pressure transducer and the voltmeter. Based on the requirements, the voltage was reduced and stabilized by transistor-based regulators. Tygon tubing connects the aquarium pump in the box to either an insertion probe (Fig. 2a) or a contact probe (Fig. 2b). A second Tygon tubing connects the probe to the pressure transducer. In an attempt to reduce the cost and complexity of the device relative to the design of Chandler et al. (1989), no flow rate control was included. This resulted in an easy to use, single-reading device, which is, however, more limited in range than the Chandler et al. (1989) design. It should be noted that the single-reading design allows for easy automation of the data acquisition process when using a voltmeter with storage capacity, possibly in conjunction with a GIS system. The air pump, obtained through an aquarium supply shop, has a nominal flow rate of about 1 L min-1. When the air permeameter is in use, the air pressure drop between the inside of the probe above the soil surface and the free atmosphere, due to friction forces in the soil, is measured with the 0- to 500-Pa differential pressure transducer, of which the electrical output is displayed by the voltmeter. Sintered glass and cotton plugs (15 mm long) were included as filters, and pressure fluctuation dampers in the air lines between the pressure transducer and the probe and between the pressure transducer and the atmosphere. The box weighs about 1700 g (excluding tubing and probe). It can be held by hand and oriented in any direction, in contrast with some other devices comprising mechanical parts affected by gravity, such as liquid manometers, mechanical flowmeters, or gravity-driven syringes (Davis et al., 1994).



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  		Fig. 1. Schematic diagram of permeameter.

 


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  		Fig. 2. Schematic and dimensions of (a) insertion probe (can probe: D = 95 mm, H = 25 mm), and (b) contact probe (D = 25 mm, D0 = 65 mm).

 
In this study, we used two types of probes. The first type, the insertion probe, has traditionally been used in soil science (e.g., Grover, 1955; Liang et al., 1996; Iversen et al., 2001). This probe consists of a metal cylinder, with inside diameter D, which is inserted into the soil to depth H (Fig. 2a). Our first prototype cylinder consisted of an upside-down tin can. This design was later changed to a modified golf course cutting cup sampler, which allowed for much easier insertion into the soil than the tin can. The bottom edge of the cutting cup sampler was made smooth and sharpened to ensure good sealing between probe and soil (Liang et al., 1996). This design also included an adjustable guide to allow for preset insertion depths. The second probe, the contact probe, corresponds to the nonintrusive surface probes used in the petroleum literature. In our case, the probe consists of a foam rubber annulus (Davis et al., 1994), of internal diameter D and external diameter D0, glued to a large rubber stopper to support the foam rubber annulus (Fig. 2b).

It is important to note that the pump's characteristics should not change with time. Because we noticed that toward the end of this research project the 1.5-VDC pump began to behave somewhat erratically, it was temporarily replaced by a 120-VAC aquarium air pump (Apollo 5, Apollo Enterprises, Inc., Oxnard, CA). This new pump was only used during the laboratory measurements to determine the linearity of air flow through porous media under conditions similar to those for the 1.5-VDC pump. It was consequently necessary to insert a capillary tube in the supply line to reduce the nominal flow rate to that of the 1.5-VDC air pump. All other equipment remained the same.


   METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS
 METHODS
RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
On the basis of Goggin et al. (1988), Grover (1955), Liang et al. (1995), and Iversen et al. (2001), we assumed and later verified that application of Darcy's Law was valid for our low-pressure, moderate-flow permeameter with either the contact or the insertion probe. Therefore, influences of gas slippage, gas compression, and inertial forces were ignored (for more details about high-pressure or high air velocity calculations, see Goggin et al., 1988). Assuming an isotropic, homogeneous soil, and taking the geometry of the probes into account, we state

[1]
where k is the permeability of the porous medium to air (L2), µ is the air dynamic viscosity (M L-1 T-1), Q is the air flow rate supplied by the pump (L3 T-1), D is the inside diameter of the probe (L), G is a geometric factor depending on the type and the dimensions of the probe used, and {Delta}P is the pressure difference between the air inside the probe, above the soil surface, and the free atmosphere (M L-1 T-2). It should be mentioned that the use of Eq. [1] tacitly assumes a uniform water content distribution over the measured soil volume. As the air permeability changes with water content, the latter should be reported as well. We will now discuss the parameters occurring in Eq. [1].

Dry air viscosity is mainly dependent on temperature. The approximation

[2]
where T is the temperature (°C), shows a deviation of <0.15% from the data published by Mason and Monchick (1965) in the range -10°C < T < 40°C. The relative influence of humidity on air viscosity is <1% for these temperatures and atmospheric pressures >77 kPa (Mason and Monchick, 1965; ASHRAE, 1989), and can thus be neglected for all practical purposes.

For contact probes, Tartakovsky et al. (2000) derived analytical and numerical solutions of the flow behavior in the vicinity of the probe. The rigorous geometric factor values they obtained for different D/D0 ratios, while neglecting inertial forces, are reported in Table 1 (Tartakovsky, personal communication, 2001; the reader should be aware of the factor 2 in our Table 1 compared with Fig. 7 of Tartakovsky et al. [2000], due to different notations). The value for G tends toward 2 when D/D0 -> 0; that is, the geometric factor becomes conceptually related to, for example, the electrical capacitance of a conducting disk (for a similar problem, see Carslaw and Jaeger, 1959, p. 215). The empirical relationship

[3]
where {eta} = 1 - D/D0 shows <0.2% deviation from values presented by Tartakovsky et al. (2000) for D/D0 <= 0.98 (Table 1). However, values of D/D0 > 0.8 are not recommended as this will result in limited measurement volumes prone to high variability (see Tartakovsky et al., 2000; Iversen et al., 2001).


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  		Table 1. Geometric factor for contact probes as a function of probe dimensions D/D0.

 
For insertion probes, G was numerically calculated by Liang et al. (1995). However, their analysis was affected by a limited simulation volume. Following the approach presented by these authors, we calculated G for D/H = 0.25, 0.5, 1, 2, 4, 6, 8, 10, using the finite element code Hydrus-2D (Simunek et al., 1996). The soil volume for which the simulations were performed had a diameter greater than 25 times D and 20 times H, and a depth greater than 15 times D and 10 times H. A no-flow condition was applied to any boundary below the simulated soil surface. The probe thickness was taken as D/20. Between 8000 and 10 000 mesh nodes were used for each calculation, and the triangular element density was increased near the bottom edge of the probe (Liang et al., 1995). We observed that as D/H tends to 0, and as the volume subjected to flow inside the probe becomes negligible compared with the volume subjected to, let's say, 99% of the flow in the soil outside the probe, the pressure at the bottom of the probe tends to atmospheric. Consequently, when D/H -> 0, the value for G tends to ({pi}/4)(D/H), which is the geometric factor for a simple soil column. This limit behavior was also observed by extrapolation of the quantity G/(D/H) to D/H = 0, and it agrees with the data review of Liang et al. (1995). The approximation

[4]
satisfies the limit behavior for G when D/H -> 0, and shows <1.5% relative deviation from our numerical results for D/H <= 10 (Fig. 3). These numerical results generally agree with the geometric factors obtained by Liang et al. (1995) using their code ANSYS B, even though their simulations used a limited volume around the probe. For the same reason as for contact probes, D/H ratios >10 are not recommended.



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  		Fig. 3. Geometric factor for insertion probes as a function of the probe proportion D/H. The solid circles represent Hydrus-2D simulations, while the solid line represents Eq. [4].

 
Because our air pumps were not compensated for the influence of back pressure, the volumetric flow rate Q decreased with increasing values of {Delta}P. However, the ratio Q/{Delta}P of interest for permeability measurements remained a monotonic function of the voltmeter reading U. The permeameter was thus calibrated by measuring Q/{Delta}P as a function of the voltmeter reading U. To accomplish this, we made use of a mechanical flow rate meter with a variable resistance, resembling flow through porous media with different air permeabilities. For a given air flow rate Q, a water manometer was used to measure the pressure difference {Delta}P between the inlet and outlet of the flow rate meter. The outlet was open to the atmosphere. The ratio Q/{Delta}P was subsequently calculated and related to U by simultaneously measuring the output of the pressure transducer, which was also connected to the bottom of the flow rate meter. One point of the permeameter characteristic or calibration curve Q/{Delta}P as a function of U was thus obtained. Additional points resulted from repeating the same procedure but using different values for the resistance to air flow. The calibration curve in the form of {Delta}P/Q as a function of U for the 1.5-VDC pump is presented in Fig. 4a, while the curve for the 120-VAC pump is displayed in Fig. 4b. If the pumps had delivered a constant flow rate throughout the applied pressure range, {Delta}P/Q as a function of U should have been a straight line. Limiting our measurements to values of U between 1.35 and 6.60 V, for a pressure transducer set at 1.00 V at atmospheric pressure and having an upper limit of 6.73 V (Fig. 4), the device is suitable for permeability measurements between 5 and 150 µm2 if equipped with a probe having G = 2 and D = 5 cm. Many naturally occurring dry surface soils lie in this range. However, a more advanced design could have widened the measurement range by allowing selection of values for the pump supply voltage other than 1.5 V. The measurement range can also be adjusted by varying the dimensions and geometric factor of the probe used. In our measurement range, the permeameter characteristic curves were fitted with quadratic expressions (Fig. 4) to simplify the conversion between voltmeter readings and {Delta}P/Q. The quadratic functions exhibit relative deviations of no greater than 10% from the calibration data.



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  		Fig. 4. Permeameter characteristic curve (a) for 1.5-VDC pump and (b) for 120-VAC pump.

 
Permeability values were obtained with both probe types in large containers filled with isotropic sands and dry surface soils. The containers were large enough to satisfy zero flow at the boundaries, as assumed during the determination of the geometric factors. For each porous medium, an air permeability measurement using the contact probe (D = 25 mm, D0 = 65 mm, G = 2.39, rubber foam thickness = 10 mm) was performed first. Next, the insertion probe (tin can; D = 95 mm) was pushed into each porous medium to a depth H = 25 mm, and a measurement was obtained (G = 1.50, Eq. [4]). Four well-graded sands (FS 12, 14, 55, 75; Flintshot Ottawa sand, F&S Abrasives, Birmingham, AL) referred to as CS, MS, FS, and VFS for coarse, medium, fine, and very fine sand, respectively, and three dry, natural top soils, referred to as TNH, TNL, and ALH. These natural soils varied between a sandy loam and a loamy sand with the TNH having the highest organic matter content and the TNL the lowest (Dane et al., 1997). The soils were passed through a 2-mm sieve before being placed into the containers. The air permeability value for the coarse sand, measured with the insertion probe (tin can), was calculated from direct application of the calibration data because the voltmeter reading was slightly outside the range of validity of the quadratic equation shown in Fig. 4a.

Additional measurements were obtained in situ on a very coarse sand and a medium sand on experimental plots located on the Auburn University Turf Grass Management Unit and on a bare sandy loam soil in an agricultural field. These in situ measurements were performed with insertion probes only (tin can and modified cup cutter).

The check for linear flow, a packed sand column 10 cm high and with a 4.9-cm i.d. was subjected to a series of known flow rates Q, while simultaneously measuring {Delta}P with a water manometer. For the Darcy equation to hold, a plot of Q vs. {Delta}P should result in a straight line relationship according to

[5]
where A (m2) is the cross-sectional area of the sample, {Delta}z (m) is the height of the sample, and all other variables are as defined above. To make sure that the conditions during field measurement were not different, similar measurements were obtained on an undisturbed sandy loam soil sample (height = 6.0 cm, diameter = 5.35 cm).

Finally, we were interested in comparing the (intrinsic) permeability values determined with air and water. The same undisturbed sample used to determine air permeability values was subsequently used to determine the saturated hydraulic conductivity K (m s-1) values according to

[6]
where {Delta}H (m) is the hydraulic head difference across the sample. The obtained K values were then converted to k values by

[7]
where {rho}w (kg m-3) is the density of water. To obtain complete saturation, the dry sample was first flushed with CO2 and then saturated from the bottom with a deaerated 0.005 M CaCl2 solution. During the upward flow a hydraulic head gradient of -1 was maintained. The sample was then subsequently dried and saturated again to obtain a second set of air and water permeabilities to check for repeatability.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS
 METHODS
 RESULTS AND DISCUSSION
CONCLUSION
 REFERENCES
 
Container Measurements
Contact probe measurements were sensitive to hand pressure applied to the probe to promote good contact with the porous medium. If this pressure is insufficient, bypass flow or even solid particle detachment may occur near the probe. Therefore we decided to obtain measurements while manually applying pressure to the contact probe, without inducing significant soil compaction, until the voltmeter reading reached its highest, constant value (lowest permeability). Figure 5 shows air permeability values obtained using the contact probe and the insertion probe (tin can) on the four sands and three top soils stored in the large containers. The relative deviation between values obtained with the contact and insertion probe seems to be acceptable for permeability measurements. Even though in most cases contact probe measurements resulted in higher air permeability values than insertion probe measurements, no clear trend existed. The differences were attributed to differences in soil compaction, bypass flow, and measurement volume associated with the two probe types. It is interesting to note that, although the soil texture for the three top soils was more or less the same, the air permeability decreased with decreasing organic matter content.



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  		Fig. 5. Comparison of experimental air permeability values obtained by application of the contact and insertion (can) probe. CS: coarse sand; MS: medium sand; FS: fine sand; VFS: very fine sand; TNH: loam soil, high in organic matter; ALH: sandy loam, high in organic matter; TNL: sandy loam, low organic matter.

 
Although above we stated that hydraulic conductivity values cannot be directly determined from air permeability values because of such factors as gas slippage, permeability values determined with air on dry soils should nevertheless be comparable with permeability values determined with water on saturated soils, assuming no water–solid phase interactions. In other words, depending on the conditions, the values should not be too different from the intrinsic permeability values of the different soils. Independent of this study, Dane et al. reported (1999) saturated hydraulic conductivity values for the CS, FS, and VFS and converted them to permeability values. The outcome was 132, 36.2, and 11.1 µm2, for CS, FS, and VFS, respectively. These values compare quite favorably with those reported in Fig. 5. Based on mean grain diameters, the permeability values for the sands also compared favorably with the intrinsic permeability values presented in Bear (1975)(see Fig. 5.5.1, p. 133).

Field Measurements
Air permeability values were obtained in situ in a sandy loam (SL) agricultural field and on experimental turfgrass management plots containing VCS and MS. The results are presented in Table 2. Compared with the data presented in Fig. 5, the values measured in situ are of the same magnitude. It should also be mentioned that the values obtained for the sandy loam with the tin can and the modified cup cutter compare very well, indicating the usefulness of the easier to use modified cup cutter probe.


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  		Table 2. Average air permeability values measured in situ with an insertion probe (tin can or modified cup cutter) on a medium sand (MS), a very coarse sand (VCS), and a sandy loam soil (SL). The numbers in parentheses refer to the number of measurements.

 
Linearity Measurements
The linearity of the flow, the basis of the use of Eq. [1], was checked for both air pumps used in our experiments. In the range of flow rates applied during our measurements, we obtained linear relationships with r2 values of 0.984 and 0.992 for the 1.5-VDC and the 120-VAC pumps, respectively (Fig. 6a and 6b).



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  		Fig. 6. The air flow rate Q as a function of the pressure difference {Delta}P across a packed sand column (a) using the 1.5-VDC air pump (r2 = 0.984) and (b) using the 120-VAC air pump (r2 = 0.992).

 
Measurements on Undisturbed Soil
Finally, to assure that air flow behaved similarly through undisturbed soil as through packed sand samples, we measured k values on an undisturbed sandy loam sample. The same sample was subjected to saturated hydraulic conductivity measurements to allow comparison of permeability values determined with air and water. The results are reported in Table 3. The first set of data (air-dry sample, {theta} = 0.046), was obtained without any measures to promote a uniform water distribution. Before obtaining the second set of data, however, we capped the sample at both ends and rotated it for 3 d to promote a uniform water distribution. The average value for the permeability only slightly increased from 30.9 to 33.1 µm2. The subsequent permeability determined with water yielded a value of 22.1 µm2. A repeat of the measurements resulted in an average value of 34.0 for air and a value of 23.6 µm2 for water. Hence, on average, the ratio of the air permeability/water permeability was 1.43. We speculate that the difference was in large part due to water–solid phase interactions, despite the precaution of using a salt solution. Other contributing factors that were ignored are gas slippage, gas compression, and inertial forces. It should be noted that for all air data sets, the permeability varied only slightly, again indicating linear flow for the existing conditions.


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  		Table 3. Air permeability values, ka, for different combinations of Q and {Delta}P, their average values, ka,av, and their corresponding water permeability values, kw.

 
Anisotropy
For a soil exhibiting local anisotropy with a vertical permeability kz and a horizontal permeability kr, permeability measurements performed with contact and insertion probes should be different. Tartakovsky et al. (2000) showed that for contact probes, the air permeability value, , obtained while assuming an isotropic soil, is the geometric average of the two permeability components; that is,

[8]

Using a change of variables similar to Tartakovsky et al. (2000), it can be shown that for insertion probes

[9]
where an approximation to the function G(D/H) is given by Eq. [4]. Theoretically, an assessment of the soil's level of anisotropy is possible by measuring , then excavating the probe and the soil contained in it to allow for a measurement of kz with the same permeameter and following the approach of Iversen et al. (2001). The probe thus becomes an open-ended column with geometric factor ({pi}/4)(D/H). Equation [9] can then be solved for kr. However, it should be kept in mind that the and kz measurements are not performed on the same sampling volume, and care should be taken in the interpretation of the results.

Because of compaction and particle movement problems associated with the contact probe, its use is not recommended for very compressible soils such as peat or freshly tilled soils. However, most field soils are expected to have some level of consolidation and resistance to compression. To prevent air bypass, the overburden pressure on the contact probe can be reduced by smoothing the soil surface. The contact probe, however, may be the only possibility to measure field air permeability of very consolidated soils in which an insertion probe cannot be used without significantly disturbing the soil. Finally, since most field soils dry rapidly near the surface, contact probe measurements would be less influenced by the subsurface moisture status than insertion probe measurements.


   CONCLUSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS
 METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
REFERENCES
 
On the basis of existing concepts and equipment presented in the petrogeology literature, we combined and extended the most desirable components to design a rugged, handheld, single-reading device allowing for fast measurements of soil surface air permeability. Empirical relationships involving geometric factors and voltage readings were presented to quickly convert the latter to air permeability values. The validity of using a contact probe was discussed briefly, and our experimental results are encouraging in the sense that air permeability values determined with the contact probe and the insertion probe were comparable for seven dry porous media. Air flow was shown to be linear under the conditions of our measurements. As expected, permeability values obtained with air were greater than those obtained with water.


   ACKNOWLEDGMENTS
 
We would like to thank D. Tartakovsky of the Los Alamos National Laboratory for sharing numerical values of the contact probe geometric factor. We also appreciate the help of C. Meadows of the Research Instrumentation Shop of the Alabama Agricultural Experimental Station with building our prototype. The junior author wishes to express his sorrow over the untimely death of Marc Jalbert. At the time of his death Marc was only a few months from completing his Ph.D. program. Auburn University awarded him a posthumous Ph.D. degree in December 2001.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS
 METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 




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