United StatesDepartment ofAgriculture

Forest ServiceForestProductsLaboratory

MadisonWisconsin

Forest ServiceResearch NoteFPL-0106March 1980

Effectsof WoodPreservativeson ElectricMoisture-MeterReadings

Summary

Describes the effects that some commonly usedwood preservatives have on readings of electricmoisture meters, when the meters are used to deter-mine moisture content of preservative-treated wood.

NOTICE

Mention of a chemical in this publication does not constitute arecommendation; only those chemicals registered by the U.S.Environmental Protection Agency may be recommended, andthen only for uses as prescribed in the registration and in themanner and at the concentration prescribed. The list of regis-tered chemicals varies from time to time; prospective users,therefore, should get current information on registration statusfrom the Environmental Protection Agency, 401 M Street, NW,Washington, DC 20024.

EFFECTS OF WOOD PRESERVATIVES ON

ELECTRIC MOISTURE-METER READINGS1–

By

WILLIAM L. JAMES, Physicist

Forest Products Laboratory,2– Forest ServiceU.S. Department of Agriculture

- - - -

Many wood preservatives contain inorganic salts that may act as electrolytesand thus change the electrical properties of the wood. The Forest ProductsLaboratory, in cooperation with the American Lumber and Treating Company,has investigated the effects of preservative chemicals in pressure-treatedwoods on the readings of electric moisture meters.

The measurement of the moisture content of wood by the use of electricmoisture meters has a distinct advantage over other methods because their con-venience and speed reduce the time required to determine the moisture contentof any piece of wood to a few seconds. They also represent the only practicalmethod of determining the moisture content of finished woodwork in place with-out seriously damaging the wood. For these reasons the use of electric moisturemeters has become widespread.

The readings of electric moisture meters for wood are based on the relation-ship between moisture content and the electrical properties of wood. Theseelectrical properties, however, depend to some extent on factors other thanmoisture content. Some of these factors can be determined readily, and the meterreadings can be corrected for their effect. Others cannot readily be determined,so these factors introduce some degree of uncertainty into the meter readings.Factors for which corrections are easily made are species and temperatureand, to a limited extent, wood density. Factors such as uneven moisture distri-bution, abnormal wood structure or composition, and extraneous chemical

1–This Note is a revision of Forest Products Laboratory Report No. 1682, originally published

unde r t he same t i t l e i n 1947 .2– M a i n t a i n e d a t M a d i s o n , W i s . , i n c o o p e r a t i o n w i t h t h e U n i v e r s i t y o f W i s c o n s i n .

FPL-0106

content introduce errors for which correction is difficult. This report describesthe effects of some commonly used preservatives on the readings of electricmoisture meters when used on preservative-treated wood.

Apparatus

Readings were made using ordinary commercial moisture meters, one of theresistance type and one of the radio-frequency power-loss type. Control or“actual” moisture content values were determined using a standard oven andtriple beam balance. For a more detailed description of the basic principles ofelectric moisture meters, see ‘Electric Moisture Meters for Wood.”3–

Specimen Preparation

The species selected for this study were southern yellow pine, Douglas-fir,and western hemlock. Specimen material was obtained as 16-foot boards, all2 by 6 inch except half of the Douglas-fir was 1 by 6. The available materialprovided 15 boards of southern pine sapwood and 5 boards of southern pineheartwood; 30 boards of Douglas-fir heartwood (fifteen 1 by 6 and fifteen 2 by 6);and 3 boards of western hemlock heartwood.

The boards were cut into five sections as shown in figure 3. Each of the 3-foot9-inch sections was treated with a different preservative, and the center l-footsection was kept as an untreated control.

The four preservatives used were (1) Fluor chrome arsenate phenol-Type A(Wolman salt FCAP) containing 37.5 percent sodium chromate, 25 percentsodium fluoride, 25 percent disodium hydrogen arsenate, and 12.5 percentdinitrophenol; (2) chromated copper arsenate-Type A (Wolman salt CCA) con-taming 55.6 percent potassium dichromate, 33.3 percent copper sulfate and11.1 percent arsenic pentoxide; (3) chromated zinc chloride containing at least17.5 percent sodium dichromate and 77.5 percent zinc chloride; and (4) creosote,grade 1, conforming to American Wood-Preservers’ Association standard 4f(Current designation, P1-64).

The specimens were pressure-treated with the preservatives in a smallpilot plant, with the methods and retentions duplicating as nearly as possible

3– James, William L. Electric Moisture Meters for Wood. U.S. Forest Service Research NoteF P L - 0 8 . 1 9 6 5 . F o r e s t P r o d u c t s L a b o r a t o r y , M a d i s o n , W i s .

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commercial standards. After the 3-foot 9-inch specimens were treated, theends were trimmed leaving the 1-foot center section free from the effects ofend penetration. The retentions obtained are listed in table 1.

All specimens were end coated after preparation to prevent end checkingduring subsequent moisture conditioning.

Experimental Procedure

Specimens were exposed to a series of four controlled temperature andhumidity conditions. Table 2 shows the relative humidities and temperaturesused, the corresponding moisture content to which untreated wood will equalize,and the length of time the specimens were stored in each condition.

The specimens were first exposed to 90 percent relative humidity and 80° F.and were weighed periodically until they showed no further change in weight,thus indicating that they had reached a moisture content in equilibrium withthese conditions. Moisture content readings were then made for each specimenwith both types of electric moisture meter. The readings were made near thecenter of one face of each specimen. Each specimen also was weighed at thetime the readings were made.

After these readings were made the specimens were conditioned, in order,at 65 percent humidity at 80° F., 30 percent humidity at 80° F., and 90 percenthumidity at 40° F. After equilibrium had been reached at each condition, themoisture meter readings and specimen weights were again recorded. Equilib-rium with the 90 percent-40° F. condition was speeded by spraying the speci-mens with water once a month for the first 9 months of exposure.

When all moisture meter readings had been made, and specimens were inequilibrium with 90 percent humidity at 40° F., the moisture content of eachspecimen was obtained by oven-drying a section cut from near its center. Plotswere then made of the readings of electric moisture meters against the equilib-rium relative humidity. The actual moisture content of the untreated controls,obtained by oven-drying, was also included in these plots. The actual moisturecontent of the controls at 30 and 65 percent humidity was computed from theweights at these conditions and the weights and actual moisture content valuesat 90 percent humidity and 40° F.

The “actual” moisture content of the treated specimens, as determined byoven-drying, was not used in these plots because they are subject to some

FPL-0106 -3-

errors; using equilibrium humidity gives simpler plots with a more fundamentalcommon basis. Moisture determinations by oven-drying made on treated speci-mens were tabulated for comparison with the untreated controls.

Specific gravity determinations were also made on all specimens, based onthe weight and volume when green (soaked).

Results and Discussion

Figures 4 through 8 show the average moisture content of each group ofspecimens as determined by the electric moisture meters when the specimenswere in equilibrium with three relative humidities; they also show the actualmoisture content of the untreated controls as determined by the oven-dryingmethod. Readings of the resistance-type meter on the untreated controls,labeled “control” in these plots, agree well with the oven-drying tests, butcorresponding readings of the power-loss meter were somewhat below theoven data. Readings of the moisture meters on preservative-treated specimens,however, did not agree generally with the actual moisture content of the match-ing controls. The amount and direction of the disagreement depended on thepreservative and meter type involved.

When using the resistance meter, readings made on preservative-treatedmaterial did not disagree seriously with control values when the material wasin equilibrium with 30 percent relative humidity. At the higher humidities,however, the readings on material treated with soluble salt-type preservativeswere progressively greater than the control values, and readings on creosote-treated materials were somewhat less than control values. The chromated zincchloride caused the greatest increase in the readings, FCAP considerably lessthan this but still substantial, and CCA had only slightly less effect than FCAP.

Readings of the power-loss meter on the controls at all humidities, and onthe preservative-treated material at 30 percent humidity, tended to be low; thissuggests that the meter may have been out of calibration. It would not affectthe conclusions regarding the effect of preservatives on these readings, however.The effect of preservatives on the power-loss meter paralleled the effect onresistance meters, but was generally greater.

The oven-drying determinations of the moisture contents of preservative-treated material (table 3) indicated higher moisture content in most preservative-treated material than in the untreated controls. With creosote-treated material,this result is almost certainly erroneous and due to the evaporation of volatiles

FPL-0106 -4-

other than water from the preservative during oven-drying. The most reasonableexplanation for the low readings of electric moisture meters on creosote-treated wood is that the actual equilibrium moisture content of the wood wasslightly depressed by the presence of the creosote. It is unlikely, but notimpossible, that the creosote could reduce the electric conductivity and power-loss factor of wood at a given moisture content.

On the other hand, the presence of nonvolatile salts in wood cause ovendeterminations of moisture to be low; therefore the data in table 3 that indicatethe moisture content of salt-treated material to be greater than that of untreatedmaterial are significant. This is reasonable because of the generally hygro-scopic nature of metallic salts. Thus, at least part of the observed effect ofsoluble salt-type preservatives on the readings of electric moisture meters isthat the moisture content of the salt-treated material was actually greater thanthe assumed value. It is clear from table 3 and figures 4 through 8 that the higherequilibrium moisture content of salt-treated material does not, in itself,account for the observed effect of soluble salt-type preservatives on the read-ings of electric moisture meters, except perhaps at the lowest relative humidityused

Tables 4 and 5 indicate the range and variability of the data obtained. Thevariability in actual moisture content of the controls was small, indicating thatmoisture equilibrium was attained at all conditions. The variability in the read-ings of the resistance-type meter on the untreated controls was only slightlygreater than that of the actual moisture data. Corresponding readings onpreservative-treated wood showed generally greater variability, although thereadings on creosote-treated specimens were only slightly more variable thanthose on the controls. Readings on material treated with chromated zinc chloridewere the most erratic.

Readings taken with the power-loss type of meter were considerably morevariable than corresponding readings using a resistance meter. This is usualbehavior, and is a natural consequence of the greater sensitivity of the power-loss meter to variables other than wood moisture. An example of such a variableis the density of the specimen material. Table 7 gives data showing the vari-ation in readings of a power-loss meter due to variation in density within agiven species. These data are generally based on the four highest and the fourlowest density specimens in each group; the exception was for southern pineheartwood, which was based on the two highest and two lowest density specimens.

FPL-0106 -5-

Conclusions

The preservatives used in this experiment affect the hygroscopicity or theelectrical properties, or both, of wood. Therefore, the preservatives affect thereadings of electric moisture meters. Of the preservatives used, chromatedzinc chloride had the strongest effect; FCAP had a substantial effect, but con-siderably less than chromated zinc chloride, and CCA had slightly less effectthan did FCAP. All of these preservatives resulted in readings that were toohigh. Creosote, by contrast, resulted in readings that were lower than should beexpected at the equilibrium moisture content conditions used.

The effect of all preservatives on the readings of electric moisture meterswas practically negligible at low moisture levels, but the effect of the solublesalt-type preservatives increased rapidly as the moisture content increased.The usefulness of electric moisture meters, therefore, is severely limited bywood preservatives that change the electrical properties of the wood; only atmoisture levels no greater than 8 or 9 percent are the readings on such treatedwood reliable.

Readings of the power-loss meter were affected by the presence of preserva-tives to a greater degree than were corresponding readings of the resistance-type meter, but neither type of meter would give reliable results on woodtreated with soluble salts.

Any attempt to develop tables of corrections for the use of electric moisturemeters on preservative-treated wood should be based on accurate estimates ofthe retention and distribution of each specific preservative in the specimenmaterial. Such corrections would be limited in applicabilityto the same species,preservatives, specimen dimensions, and method of treatment as used for devel-oping the correction. Finally, because of the large and erratic effect of manypreservatives, even the corrected readings would be subject to considerableuncertainty.

FPL-0106 -6- 1.-22

FPL-0106 -7-

Tabl

e 3.

--Ca

lcul

ated

mo

istu

re co

nten

t va

lues

of

wo

od tr

eate

d wi

th fo

ur pr

eser

vati

ves

and

brou

ght

toeq

uili

briu

m wi

th th

ree

rela

tive

hu

midi

ty co

ndit

ions

; va

lues

ba

sed

on ov

endr

y we

ight

::

::

Fluo

r ch

rome

ar

sena

te:Chromated copper

Humi

dity

:C

ontr

ol :C

hrom

ated

zi

nc ch

lori

de:

Creo

sote

:ph

enol

(F

CAP)

: arsenate (CCA)

cond

itio

ns:

mois

ture

:---

----

----

----

----

----

:---

----

----

----

----

-:--

----

----

----

----

----

-:--

----

----

----

----

-:content

:Moisture

:Co

mpar

ison

:M

oist

ure:

Comp

aris

on :

Mois

ture

:

Comp

aris

on:Mo

istu

re:Co

mpar

ison

::

cont

ent

:wi

th:c

onte

nt :

with

: content :

with

:con

tent

:

with

::

:control :

: control :

:control :

: control

----

----

--:-

----

----

:---

----

---:

----

----

----

-:--

----

--:-

----

----

--:-

----

----

-:--

----

----

--:-

----

---:

----

----

--:P

erce

nt :

Perc

ent

:Pe

rcen

t:P

erce

nt :

Perc

ent

: Percent

:Pe

rcen

t:Percent :

Percent

7.7

:+0.3

:8.0 : +0.6

13.7

:+

.3:

13.9 : + .5

23.8

:+1.6

:23

.5: +1.3

7.2

12.5

20.9 7.7

13.2

22.2 7.1

12.4

20.8

:-

.l:

- .2

:+

.2

:+

.4:

+ .5

:+1

.9

:+

.4:

+ .7

:+

.8

:7.0 :

- .3

:12.2 :

- .5

:20.6 :

- .l

:7.7 : + .4

:13.3 : + .6

:22

.2: +1.9

:7.

1:

+.4

:12

.4:

+.7

:20

.5:

+05

Perc

ent

Sout

hern

ye

llow

pi

ne sa

pwoo

d

:+2

.2:

9.3

:+1.9

::

+2.4

: 14.3 : + .9 :

:+4

.1: 21.3 : - .9 :

Sout

hern

ye

llow

pi

ne he

artw

ood

30:

7.4

65:

13.4

90:

22.2

:

9.6

:15

.8:

26.3

6.9

:...

....

....

:7.5 : + .6

12.5

:+.2

:13

.3: +1.0

21.6

:+2.1

:22.4 : +2.9

30:

7.3

:8.

765

: 12

.7:

14.2

90:

20.7

:23

.5

+1.4

: 14.2 :

+6.9

+1.5

: 18.7 :

+6.0

+2.8

: 24.8 :

+4.1

: : : : : : : : : : : :

Doug

las-

fir

2 by

6

30:

7.3

:8.

765

: 12

.7:

14.3

90:

20.3

:24

.4

+1.4

: 10.8 :

+3.5

+1.6

: 15.7 :

+3.0

+4.1

: 22.4 :

+2.1

Doug

las-

fir

1 by

6

30: 6.7

:8.

765

: 11

.7:

14.2

90:

20.0

:23

.6

+2.0

: 16.0 :

+9.3

+2.5

: 20.6 :

+8.9

+3.6

: 27.6 :

+7.6

West

ern

heml

ock

hear

twoo

d

30: 6.9

:9.

465

: 12

.3: 15.4

90:

19.5

: 25.6

+2.5

: 10.4 :

+3.5

+3.1

: 15.5 :

+3.2

+6.1

: 22.5 :

+3.0

Figure 2.--Radio-frequency power-loss type of electric moisture meter.

ZM 118 516

FPL-0106

Fig

ure

3

. --

Me

tho

d

us

ed

to

c

ut

wo

od

in

to

tes

t s

pe

cim

en

s.

A,

dia

gra

m

for

cu

ttin

g

16

-fo

ot

bo

ard

s 2

by

6 i

nc

he

s i

n c

ros

s s

ec

tio

n i

nto

te

st

pie

ce

s b

efo

re t

rea

tme

nt

wit

h p

res

erv

-a

tiv

es

; B

, d

iag

ram

fo

r c

utt

ing

te

st

sp

ec

ime

n

fro

m

tre

ate

d

pie

ce

; C

, te

st

sp

ec

ime

n,

an

dc

oa

ted

w

ith

a

lum

inu

m

pa

int.

FPL-0I06

F igure 4 . - -Average mo is tu re con ten t o f p ressure- t rea ted and un t rea ted 2 by 6D o u g l a s - f i r a s d e t e r m i n e d w i t h , A , a r e s i s t a n c e t y p e a n d , B , a r a d i o -f requency power - loss type o f mo is tu re meter . Curves Nos . I , FCAP- t rea tedwood; curves Nos. 2, CCA-treated wood; curves Nos. 3, chromated zincchloride-treated wood; curves Nos. 4, creosote-treated wood. All specimensi n e q u i l i b r i u m w i t h r e l a t i v e h u m i d i t y s h o w n a t 8 0 ° F .

M 74143

F igure 5 . - -Average mo is tu re con ten t o f p ressure- t rea ted and un t rea ted I by 6D o u g l a s - f i r a s d e t e r m i n e d w i t h , A , a r e s i s t a n c e t y p e a n d , B , a r a d i o -f requency power - loss type o f mo is tu re meter . Curves Nos . I , FCAP- t rea tedwood; curves Nos. 2, CCA-treated wood; curves Nos. 3, chromated zinc

FPL-0I06ch lo r ide- t rea ted wood; curves Nos . 4 , c reoso te - t rea ted wood. Al l specimensi n e q u i l i b r i u m w i t h r e l a t i v e h u m i d i t y s h o w n a t 8 0 ° F .

M 74144

F igure 6 . - -Average mo is tu re con ten t o f p ressure- t rea ted and un t rea ted 2 by 6southern ye l low p ine sapwood as de te rmined w i th , A, a res is tance type and, B,a rad io - f requency power - loss t ype o f mo is tu re mete r . Curves Nos. I , FCAP-t rea ted wood; curves Nos . 2 , CCA- t rea ted wood; curves Nos . 3 , ch romated z inc

FPL-0I06 ch lo r ide- t rea ted wood; curves Nos . 4 , c reoso ted- t rea ted wood. Al l specimensi n e q u i l i b r i u m w i t h r e l a t i v e h u m i d i t y s h o w n a t 8 0 ° F .

FPL-0I06

M 74145

F i g u r e 7 . - - Average mo is tu re con ten t o f p ressure- t rea ted and un t rea ted 2 by 6southern ye l low p ine hear twood as de te rmined w i th , A, a res is tance type and,B, a radio-frequency power- loss type of moisture meter. Curves Nos. Curves Nos. I , FCAP-treated wood; curves Nos. 2, CCA-treated wood; curves Nos. 3, chromated zincchIoride-treated wood; curves Nos. 4, creosote-treated wood. All specimenss h o w n i n e q u i l i b r i u m w i t h r e l a t i v e h u m i d i t y s h o w n a t 8 0 ° F .

M 74146

F igure 8 . - -Average mo is tu re con ten t o f p ressure- t rea ted and un t rea ted 2 by 6wes te rn hemlock hear twood as de te rmined w i th , A, a res is tance type and, B, arad io - f requency power - loss type o f mo is tu re meter . Curves Nos . I , FCAP- t rea tedwood; curves Nos . 2 , CCA- t rea ted wood; curves Nos . 3 , ch romated z inc ch lo r ide-t r e a t e d w o o d ; c u r v e s N o s . 4 , c r e o s o t e - t r e a t e d w o o d . A l l s p e c i m e n s i n e q u i l i b r i u m

F P L - 0 I 0 6 w i t h r e l a t i v e h u m i d i t y s h o w n a t 8 0 ° F .

U.S. GOVERNMENT PRINTING OFFICE: 1980-653-146