Calibration and field performance of electrical resistance moisture blocks

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Calibration and field performance of electrical resistance moisture

blocks

NATIONAL RESEARCH COUNCIL CANADA

DIVISION OF BUILDING RESEARCH

CALIBRATION AND FIELD PERFORMANCE OF ELECTRICAL RESISTANCE MOISTURE BLOCKS

by E. Penner

Internal Report No. 231 of the

ANAL YZED

Division of Building Research

OTTAWA September 1961

PREFACE

The measurement in situ of moisture content in building materials, including soils, is one of the challer..ging but frustrating

problems facing building science workers. It is important because

moisture is either an important constituent. as in the case of clay soils. or is an agent affecting the stability and durability of materials. One of the possible methods which has received attention by many workers employs electrical resistance elements responding to

moisture content. Work done in the calibration and field trials of

several available elements of this type for use in soils work is now

reported. This is an interim report since the studies will not be

completed until the condition of the gauges has been determined. The author, a soil scientist engaged in studies of moisture effects and frost heaving in soils, is a research officer with the Soil Mechanics Section of the Division.

Ottawa

September 1961

N. B. Hutcheon. As sistant Director.

-"

CALIBRATION AND FIELD PERFORMANCE OF E LECTRICAL RESISTANCE MOISTURE BLOCKS

by E. Penner

A reliable in situ method of determining the rnois tu r e status of soil would have many advantages over direct sampling and oven drying. This has long been an accepted fact in soils work and in non-destructive moisture d et e r m in at iori of all moisture adsorbing materials. Consequently a great many devices and techniques have been proposed, but after

compr ehensi ve evaluation all have proved to be of limited value. The

age-old method of direct sampling and oven drying of soils, is still in use.

This paper is concerned with the electrical resistance moisture block method that has been used extensively in agriculture since 1940. A direct laboratory method of calibration was used in this study and a

small field installation operated for two growing seasons. Results of

both laboratory and field work serve as a basis for critical evaluation of the method.

ORIGIN AND BASIS OF METHOD

Bouyoucos and Mick (1) introduced the gypsum block method as a simple and practicable tool for measuring field moisture content. It was first used in agriculture as an indicator of water deficiency in

irrigated fields. Since the accuracy of rn e a su r ern e.nt required in irrigation practice was not critical, it was not questioned seriously at first.

Difficulties inherent in the method became evident, however. when attempts were made to use the meters as precise indicators of moisture content.

Since the early work with gypsum blocks a number of materials

and electrode designs have been introduced. The most notable among

these are nylon and fi b r e gl as s used as the porous dielectric, and concentric and four electrode systems to replace the original parallel electrode design.

Onl y hydrophilic porous dielectrics ar e suitable for the

construction of moisture meters. The electrical resistance of such mat e rial s is extremely sensitive to small variations in water content and forms the

ud

2

-a response to physic-al -and chemic-al ch-anges, temper-ature, ion

concentration and other factor s , Serious as this may seem, mo st of

these effects can be partly avoided or accounted for in practice; the greatest difficulty lies in the nature of the relationship that exists between meter resistance and some function of the soil moisture.

THE SUCTION PRINCIPLE

At low water contents attraction between water rnolecules

and any solid results from surface forces. The extent of adsorption

is related to the exposed surface area.

As water contents increase, 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; the ratio of these vapourpressures, the humidity ratio, 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 r elated to some other index of rnoistur e potential such as the radius of curvature of the air -water interface or to the equivalent negative head of water (capillary rise) as indice s of moi stur e potential. The latter , usually r eferr ed to as "moisture suction, II is commonly used in soil science, and is often

given in terms of Schofield's pF (2), the logarithm of the capillary rise expressed in centim.etres 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 the block may be

quite different at this stage. If suitably calibrated the meter may be regarded as a device for measuring soil moisture potential, or soil

moisture suction. It might then be used to determine the moisture

suction in any soil. If the moisture contents at various suctions were

known for that soil, the moisture content could be found without the need to calibrate the meter in situ for each particular soil.

The three types of porous blocks used in this study were nylon,

fibreglass and Bouyoucos blocks. The electrical resistance of blocks

was measured after reaching suction equilibrium with a porous plate of

---'

3

-of a pressure membrane apparatus for the high pF range (Fig. 2). The electrical Ie a d s were taken out through the wa.lls of the apparatus

using rubber grommets. The equipment was kept in a constant temperature

room held at 20

.:!:.

1/IO°C.

The porous plate was conditioned at various suctions

either by pressurizing the inside of the apparatus or by placing the water

in tension with VaCCUlTI. On successive days resistances were measured

with a Bouyoucos bridge until no significant change occurred between two consecutive measurements. At high values of pF', this s o rn etirn e s

required up to seven days.

Nylon and fibreglass blocks without flat surfaces, as required for calibration, were cast in plastic in rectangular rnou l d s . After plaster-cast nylon blocks became available commercially, some of these were

included in the calibration. Finally, a few commercially available

r e s inc irnp r e gnat e d Bouyoucos blocks were included to evaluate the

influence of this treatment on field durability.

Figure 3 shows a typical calibration curve for Bouyoucos

blocks. Suction was the controlled variable and the resistance was

measured until successive readings indicated near -equilibrium conditions.

In the same figure is shown a pF -moisture content curve for a sample of

Leda clay initially air -dry. The figure illustrates how moisture content may be estimated from the electrical resistance reading of the block. provided both the soil and the block ar e in suction equilibrium.

Figures 4 and 5 are typical pF-calibration curves for the

best of the nylon and fibreglass blocks. In both cases the portion from

pF = 0 to 3 was repeated and it may be seen that reproducibility was not

good.

CALIBRATION DIFFICULTIES

The resistance at saturation decreased in most blocks after

several wetting and drying cycles. It continued to decrease even after

field installation, particularly in blocks installed at the 3 - and 5 -ft levels. This tended to influence seriously the accuracy at the wet end where small resistance changes are relatively inportant.

In general the Bouyoucos blocks responded satisfactorily to

the method of calibration used and s rno ot.h curves were obtained for all

blocks of this type. By contrast, some nylon and fibreglass blocks were

discarded on the basis of inconsistencies in the resistance readings during calibr at iori ,

d

4

-FIELD INSTALLATION

The field installation was in Leda clay, a postvgla ci a.l

marine depo sit, on the Montr eal Road site

0:

the National Re sear ch Council.

The blocks were placed in batches at 1-, 3- and 5-ft depths in the side

wall of a freshly dug trench. The details of the installation are given

in Fig. 6. Three Bouyoucos blocks, two fibreglass and two nylon blocks

were placed at the 1- and 3-ft levels; three Bouyoucos blocks, one nylon

and one fibreglass were placed at the 5-ft level. The method used to place

the blocks was designed to prevent changes due to moisture migration along the electrical leads to the blocks as would be pos sible in vertical borehole installations.

FIELD RESULTS

Electrical resistances were measured on the same day each week during the summer months of 1956 and 1957.

Figure 7 presents the results from the best performing block of each type at each level, as based on the drying portion of the calibration curve.

Shown also in this figure is the daily rainfall, the moisture contents of weekly auger samples, and rnoistu r e deficiency curves calculated from weather records according to a modification of the Thornthwaite method (3).

No attempt was made to estimate quantitatively the moisture content or moisture content changes as pF -moisture content curves of

the soil would be required for each level. Present plans call for recovery

of the blocks in the summer of 1961. At the same time undisturbed soil

samples will be taken at each level. It should be stressed that the pF

curve shown in Fig. 3 represents a sample of Leda clay that had been

previously air -dried and is therefore meaningful only for illustrative purposes.

The change in suction indicated by the blocks corresponds

with the moisture deficiency calculations (Fig. 7). There is of course

a response to rainfall, but this is less evident.

The rnoistu r e content of the soil as determined by auger sam.pling and oven drying shows a great deal of variation, although the sampling area was Ii rnite d and the auger holes were carefully backfilled

5

-textural changes over short di st an c e s , auger s a.rnp linp is not considered a particularly reliable rn cth o d for obtaining soil sa.rripl e s for moisture content determinations.

A comparison of the field performance of several types of blocks shows that at low values of pF the Bouyoucos blocks are the least

satisfactory and the nylon blocks are the best. The nylon blocks operate

at all conditions less than saturation, the fibreglass fr o m pF = 1. 5 and the Bouyoucos block resistances show no change until a pF equalling 2.2

is exceeded. For the nylon blocks, although apparently responsive at

high moisture contents, the calibration curves were considered unreliable

in this range and therefore the low pF values are not particularly meaningful.

In general, the different kinds of blocks b ch av e d sirnil a r ly

at higher values of pF. All followed the trend of the deficiency curves

r e m a.r kab ly well. In the summer of 1956 the rn a xirriurn moisture deficiency was slightly greater than 4 in. of water . Only the blocks at the I-ft

level showed unsatur ated conditions. In the s urn rn c r of 1957 the maximum moisture deficiency was nearly 8 in. of water. At the 1 -ft level the nylon block indicated a pF of 4. 1 on 1 August. the fibreglass block 4.25 and

the Bouyoucos block 4.2. At 3 It the highest pF occurred about 5 September

for all blocks and all showed a p F near 3.75. At the 5 -ft depth, the highest pF occurr ed about 15 Septernber; the Bouyoucos block r e gi stered a pF

of 2.65 and the nylon block about 2.4; the fibregla.ss block did not respond.

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 sirnp ly to deterrnine 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 from Fig. 1, that there is an unavoidable uncertainty in

the moisture det e r rnin at ion unless it can be established that the meter

and the soil are either on the wetting or the drying cycle. If wetting occurs befor e complete drying has taken place or alternati vel y if drying occur s before the blocks and soil are c ornp l et e l v saturated, values lying on scanning

curves between the wetting and drying curve shown will apply. Frorn this

it is obvious that hysteresis is a major difficulty in establishing precise moisture content values from resistance readings.

6

-Bouyoucos blocks have an advantage over other blocks

because the electrolyte content in the pore water of these blocks is rno r e

or less constant due to the buffering action of the gypsum. Casting the

nylon and fibreglass blocks in plaster -of-paris not only rna de possible the method of direct calibration but also provided these blocks with this

same advantage. Nevertheless a gradual reduction in electrical resistance

at saturation occurred in the blocks below the water table for long periods

of time. This would tend to invalidate the laboratory calibration curves

at low values of pF'.

The influence of the electrical field extending outside the Bouyoucos blocks was noted during calibration but this effect was not evaluated.

Both fibreglass and Bouyoucos blocks were relatively unresponsive

in the low suction region. The difficulty in reproducing the curves in

this region of the pF curve placed nylon blocks more or less in the same clas s ,

There are indications that these rnoisture meters are useful in the field for following soil rnoisture trends. Since hysteresis effects are unavoidably present and because of other difficulties discussed, however, it is unrealistic to expect such meters to give accurate and reliable pF or moisture content values no matter how they are calibrated.

ACKNOWLEDGEMENT

The author appreciates the assistance of his colleagues in

carrying out these studies and for supplying the moisture deficiency results.

REFERENCES

1. Bouyoucos, G. J. and A. H. Mi ck , An electrical resistance method

for the continuous measurement of soil moisture under field

conditions. Michigan State College, Agricultural Experiment

Station, Technical Bulletin No. 172, 1940, 38 p.

2. Schofield, R. K. The pF of the water in soil. Transactions, Third

International Congress of Soil Science, Vol. 11, 1935, p. 37 -48.

3. Bozozuk, M. and K. N. Burn. Vertical ground movements near

1

SEALED CHAMBER BURET - MOISTURE BLOCKS POROUS PLATE

b

SEALING COMPOUND -SPIRAL GROOVE

a

1",11" ,,,,0

1

PIPET

POROUS PLATE HOLDER

--LEGEND:

a

l

WATER OUTLET

bi

AIR INLET

FIGURE I

C

I

BURET

SPIRAL GROOVE

PRESSURE

CH AMBER

POROUS PLATE HOLDER

..

POROUS PLATE

C

MOISTURE BLOCK

a

SEALING COMPOUND

-

PIPET---LEGEND:

a)

WATER

OUTLET

b)

AIR

INLET

NOTE: PRESSURE MEMBRANE

INSTALLED FROM C TO

C'

FIGURE

2

50

10

01..-_ _

. . . 1 - - 1 - ..1.-..1...- --1- ... ----''''---_ _'''---...

o

60

LOG ELECTRICAL RESISTANCE

7

2

3

4

5

0

\,

SOIL MOISTURE

6

ｾ

CONDITIONINGWITH ACID SOLUTIONS

ｾｯ

z

₅

t-0 VS MOISTURE CONTENT

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FOR SOIL u ::::> (f)

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FIGURE

3

SUCTION - MOISTURE

CONTENT RELATIONSHIP FOR SAMPLE OF

AIR-DRIED LEDA CLAY

a

A BOUYOUCOS

MOISTURE

METER

4 I i i i ｩ ｩ ｬ ［ ｾ - - - CYCLE I - - - - CYCLE 2

3

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SOIL MOISTURE CONDITIONING

WITH PRESSURE MEMBRANE

WITH SUCTION PLATE

3

LOG RESISTANCE 4

5

;' 1i

fi'tj

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6

L---FIGURE

4

4.

1

I

I

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_I

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/

SOIL MOISTURE CONDITIONING

...

WITH PRESSURE MEMBRANE

_s,

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,

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7

., CYCLE I

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CYCLE 2

II--

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:I

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,

I

0

1

t

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LOG RESISTANCE

FIGURE

5

pF

VS

ELECTRICAL

_RESISTANCE

_FOR

_FIBREGLASS

_BLOCK

6

i

==8

8'-0"

ｾｦＭＭ

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FIGURE

6

o r--,----r---r----,--r--".--,---,---,---r---,--,.--...,----,-.---.--,.---,--,,---,

\ \\

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I \ ,

｜ＮＺｾｾｅｾｉｔｈ

I

\ ...1 \ \ 3' DEPTH \ \ ｜｟ｾ pF-O ! ｒｅｓｉｓｔａｎｾｃｅ I i I pF-2'2 1956-1957 ｗｉｎｾｅｒ ｐｅｒｉｾ I I I I

FIBREGLASS BLOCKS SHOW NO CHANGE IN RESISTANCE UNTIL pF'I'5 IS EXCEEDED

I

i I

BOUYOUCOS BLOCKS SHOW NO CHANGE IN UNTIL pF'2'2 IS EXCEEDED

I

I" " " "

11-Y

NYLON BLOCK SHOW RESISTANCE CHANGE I

DOWN TO SATURATION p F - 0 i f---+-,----rt---rl-....,---+---+---IpF -1·5 2 2 4 4 pF OF FIBREGLASS BLOCKS pF OF NYLON BLOCKS 2 p F OF BOUYOUCOS BLOCKS 4 0 MOISTURE DEPLETION INCHES OF 4 WATER B 50 MOISTURE 40 CONTENT % DRY WT 30 20 2 RAINFALL INCHES

JUNE • JULY • AUG SEPT • OCT NOV • DEC • JAN • FEB MAR • APR • MAY • JUNE • JULY • AUG • SEPT • OCT •

FIGURE 7

PERFORMANCE OF BOUYOUCOS FIBREGLASS 8 NYLON BLOCKS COMPARED WITH MOISTURE DEPLETION, MOISTURE CONTENT 8 RAINFALL