Electrical Resistance Meters for Soil Moisture Measurement

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Technical Note (National Research Council of Canada. Division of Building Research), 1961-03-01

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Electrical Resistance Meters for Soil Moisture Measurement Penner, E.

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Basis of the Method

ELECTRICAL RESISTANCE METERS FOR SOIL MOISTURE MEASUREMENT

Only hydrophilic porous dieleotrics are suitable for the construction of moisture meters. The electrical resistance of such materials is extremely sensitive to small variations in water content and forms the basis of this method of measuring soil moisture. In addition, there is also a response to physical and chemical changes, temperature, ion concentration and other factors.

N.B.H.

N(J.

323

ｾＮ

FOR INTERNAL USE

DATE March 1961 APPROVED By

NOTlE

CHECKED BY

DIVISION OF BUILDING RESEARCH

'fE C1HI N II CAlL

RILEM Seminar by Correspondence

-"Measurement of Moisture in Materialslt

(N. B. Hutcheon - correspondent)

NATIONAL RESEA'RCH COUNCIL OF CANADA

The moisture meter was introduced thirty years ago as a simple and practical tool for measuring soil moisture in irrigated fields (I). The accuracy of measurement required in irrigation practice was not sUfficiently great to challenge the method seriously. The results of detailed investigation have shown subsequently that it is not possible to use these meters 'for accurate soil moisture measurements.

Types of Meters

The simplest meter consists of two parallel electrodes embedded in a block of gypsum. Other electrode designs intro-duced were intended to confine the electrical field within the meter. Materials such as nylon and fibreglass, much more

resistant to ｾ･｡ｴｨ･ｲｩｮｧ than gypsum, have greatly increased field life of such devices. These materials have also extended the response range of the meter to more moist conditions.

Unfortunately, there aTe also disadvantages and among the more serious is the greater sensitivity to electrolytes.

SUBJECT

PREPARED BY E. Penner NOT FOR PUBLICATION

PREPARED FOR

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-Serious as this may seem, most ｯｾ these ･ｾｾ･｣ｴｳ can be avoided or accounted ｾｯｲ in practice; the greatest ､ｩｾｾｩ｣ｵｬｴｹ lies in the nature of the relationship that exists between meter

resistance and some ｾｵｮ｣ｴｩｯｮ ｯｾ the soil ｭｯｩｳｴｵｲ･ｾ

Direct Moisture Content Calibration

The function most frequently correlated with meter resistance is·the percentage moisture content of the ｳｵｲｲｯｵｮ､ｩｮｾ

ｳｯｩｬｾ In practice the technique is to bury the saturated meter in a,saturated mass of soil. The soil and block are then dried in stages. Between stages the moisture content of the soil is determined and at the same time the resistance of the meter is followed until it is constant. The relationship determined in this way applies only to the one s011. This method of calibra-tion excludes the use of the concept of moisture potential, or suction, which determines the behaviour and distribution of water in materials.

A Direct Soil Moisture-Suction Calibration Method

The Suction Princiale •. - At low water ｣ｯｮｾ･ｮｴｳ attraction between water molecules an any solid results from surface forces. The extent of adsorption is related to the exposed surface area.

With increasing water contents the water is held in the pores of the structure by surface tension forces. The

concave menisci formed have a lower vapour pressure than a flat water surface and the ratio ｯｾ these vapour pressurest the

humidity ratiot is frequently used to describe the potential at

which water is retained. In the moisture range where the meters are of particular value water retention is more conveniently related to some other index of moisture potential such as the radius of curvature of the air-water interface or to the

equivalent negative head of water (capillary rise) as indices of moisture potential. The latter, usually referred to as "moisture suction", is commonly used in soil science, and is often given in terms of Schofield's pF (2), which is the logarithm of the capillary rise expressed in centimeters of water.

Calibration Technique. - A porous block meter placed in contact

with

a moist soil will exchange moisture until moisture potentials in the soil and in the meter are equal. The moisture- contents of the soil and of the block may at this stage be quite different. The meter may thus be regarded as a device for measuring soil

moisture potential, or soil moisture suction, provided that it is SUitably calibrated. It might then be used to determine the moisture suction in any soil. Then if the moisture contents at

"

3

-various suctions are known for that soil, the moisture content may be found without the need to calibrate the meter in situ

for each particular soil. Croney et al. have used moisture meters in this way (4).

A number of soil moisture meters have been calibrated in the DBR laboratories as suction-measuring devices. The

calibration curve for the Bouyoucos moisture meter shown in Fig. 1 was determiIied oytlie ｾ､ｩｲ･ｯｴ suction method. In the same figure a suction-moisttireconterit curve is shown for Leda clay. In principle, when the partiCUlar soil and meter are in suction equilibrium the moisture cohtent of the soil can be estimated from the two curves. It may be noted that for both relationships there are two sets of curves, one for drying and one for wetting.

Evaluation of Moisture Meter. - An obvious problem with the electrical resistance type of soil moisture meter is created by the hysteresis effects between wetting and drying conditions. When using a meter simply to determine soil moisture suction

only one set of curves is involved, with the associated hysteresis of the meter. Conversion of suction to moisture content of the soil involves a further relationship complicated by hysteresis effects in the soil. It may be noted, see Fig. 1, that there is an unavoidable uncertainty in moisture determination unless it is known that near-equilibrium conditions have been estab-lished between the meter and the soil on either the wetting or the drying cycle. It will seldom be. possible to say with

certainty that the drying or the wetting curve applies, under naturally varying conditions of precipitation and evaporation.

Bouyoucos moisture meters have been found to be unresponsive below pF = 2.2 and fibreglass below pF

=

1.5. Nylon meters are responsive down to saturation but the diffi-culty in calibrating the meters below pF - 2 tends to introduce some uncertainty in this portion of the curve.

Nylon, fibreglass and ｂｯｵｹｯｵ｣ｯｾ moisture meters calibrated by the direct suction method were installed in the field and their resistances were carefully measured for two

growing seasons. Their evaluation is being based on precipitation measurements, moisture deficit calculations frbm weather records (3)

and moisture contents by direct sampling.

During calibration, from two to seven days were required for the meters to acquire a constant electrical resistance. It is believed that meter response lags appreciably behind soil moisture changes but this has not been investigated.

There are indications that these moisture meters are useful for following soil moisture trends in the field but since

4

-hysteresis effects are unavoidably present it is unrealistic to expect suoh meters to give aocurate and reliable pF or moisture content values no matter how they are calibrated.

Referenoes

1. Bouyoucos, G. J. and A. H•.Mick. An electrical resistance method for the continuous measurement of soil

moisture under field conditions. Teohnical Bulletin No. 112, Michigan State College, Agricultural

Experiment Station, 1940, 38p.

2. Sohofield, R. K. The pF of the water in soil. Transaotions of the Third International Congress of Soil Science, Vol. 11, 1935, p.31-48.

3. Bozozuk, M. and K. N. Burn. Vertical ground movements near elm trees. Geotechnique, X, No.1, Maroh 1960, p.19-330 40 Croney, D., J. D. Coleman and Bamela M. Bridge. The Suotion

of Moisture held in Soil and other Porous Materials. DSIR, Road Res. Tech. Baper No. 24, 1952.

5 o

Ｎｾ

o VS MOISTURE CONTENT FOR SOIL

LOG ELECTRICAL RESISTANCE

3 4

SOIL MOISTURE CONDITIONING

WITH ACID SOLUTIONS

WITH PRESSURE MEMBRANE ｾ

ａ

ＭＭＺＭｾＮＮ｣｣

ｾ

J

,\J

WITH SUCTION

! .

\\

PLATE II pF VS E L . ' o

If

RESISTANCE OF • BOUYOUCOS BLOCK O L - - - l . - . . L ._ _--L-L --I. .L.-...;..._ _ＮＮＮｌＮＮ｟ｾＮ｡ＮＮ｟｟ｊ

o

10 20 30 40 50 60

MOISTURE CONTENT % DRY WT. 6 lL." 2 Q, o 5

i=

FIGURE I

SUCTION - MOISTURE CONTENT ｒｅｌａｔｉｏｎｾｈｉｐ FOR SAMPLE OF

AIR-DRIED LEDA CLAY

a

A BOUYOUCOS MOISTURE METER

CALIBRATION CURVE