ASTM G84-89(1999)e1

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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e1
Designation:G84–89(Reapproved1999)
Standard Practice for
Measurement of Time-of-Wetness on Surfaces Exposed to
Wetting Conditions as in Atmospheric Corrosion Testing
ThisstandardisissuedunderthefixeddesignationG 84;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscript
epsilon (e) indicates an editorial change since the last revision or reapproval.
e NOTE—Section 9 was editorially deleted in October 1999.
1. Scope technique.As pertains to this practice, the absolute value of the
potential generated is essentially of academic interest.
1.1 This practice covers a technique for monitoring time-
2.2 This practice describes the moisture-sensing element,
of-wetness (TOW) on surfaces exposed to cyclic atmospheric
procedures for conditioning the elements to develop stable
conditions which produce depositions of moisture.
films on the electrodes and verifying the sensing-element
1.2 The practice is also applicable for detecting and moni-
function, and use of the element to record TOW.
toring condensation within a wall or roof assembly and in test
apparatus.
3. Significance and Use
1.3 Exposure site calibration or characterization can be
3.1 This practice provides a methodology for measuring the
significantly enhanced if TOW is measured for comparison
duration of wetness on a sensing element mounted on a surface
with other sites, particularly if this data is used in conjunction
in a location of interest. Experience has shown that the sensing
with other site-specific instrumentation techniques.
elementreactstofactorsthatcausewetnessinthesamemanner
1.4 This standard does not purport to address all of the
as the surface on which it is mounted.
safety concerns, if any, associated with its use. It is the
3.2 Surface moisture plays a critical role in the corrosion of
responsibility of the user of this standard to establish appro-
metals and the deterioration of nonmetallics. The deposition of
priate safety and health practices and determine the applica-
moistureonasurfacecanbecausedbyatmosphericorclimatic
bility of regulatory limitations prior to use.
phenomena such as direct precipitation of rain or snow,
2. Summary of Practice condensation, the deliquescence (or at least the hygroscopic
nature) of corrosion products or salt deposits on the surface,
2.1 This practice describes a technique for detecting and
and others. A measure of atmospheric or climatic factors
recording surface moisture conditions. The moisture serves as
responsible for moisture deposition does not necessarily give
an electrolyte to generate a potential in a moisture sensing
an accurate indication of the TOW. For example, the surface
element galvanic cell that consists of alternate electrodes of
temperature of an object may be above or below both the
copper and gold, silver and platinum, or zinc and gold. The
ambient and the dew point temperatures. As a result conden-
spacing of the electrodes may be 100 to 200 µm, the width
sation will occur without an ambient meteorological indication
dimension is not considered critical (Fig. 1). However, when
that a surface has been subjected to a condensation cycle.
zinc is used as an electrode material, the effects of the
3.3 Structural design factors and orientation can be respon-
hygroscopic nature of the corrosion products on the perfor-
sible for temperature differences and the consequent effect on
mance of the sensor should be kept in mind. Also, the use of
TOW as discussed in 4.2. As a result, some surfaces may be
copper as a sensor material should be avoided in sulfur
shielded from rain or snow fall; drainage may be facilitated or
dioxide-laden atmospheres to avoid premature deterioration of
preventedfromgivenareas,etc.Thereforevariouscomponents
the sensor’s copper substrate. The output (potential) from this
of a structure can be expected to perform differently depending
cell is fed through a signal conditioning circuit to an indicating
on mass, orientation, air flow patterns, etc. A knowledge of
or recording device. The objective is to record the time that
TOWat different points on large structures can be useful in the
moisture is present on the sensing element during any given
interpretation of corrosion or other testing results.
period. The fact that a potential is generated is critical to this
3.4 In order to improve comparison of data obtained from
test locations separated on a macrogeographical basis, a
This practice is under the jurisdiction of ASTM Committee G-1 on Corrosion uniform orientation of sensor elements boldly exposed in the
of Metals and is the direct responsibility of Subcommittee G01.04 on Atmospheric
direction of the prevailing wind, at an angle of 30° above the
Corrosion.
Current edition approved Feb. 24, 1989. Published May 1989.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
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G84–89 (1999)
FIG. 1 Sensing Element
horizontal is recommended. Elevation of the sensor above element on a metal surface with a high-thermal conductivity.
ground level should be recorded. For metal surfaces, the sensing element should be appreciably
3.5 Although this method does not develop relationships thinner. Commercial epoxy sensor backing products of thick-
between TOW and levels of ambient relative humidity (RH), ness of 1.5 mm, or less, are suitable for this purpose.
long term studies have been carried out to show that the TOW
4.2 Checking the Moisture Sensing Elements:
experienced annually by panels exposed under standard con-
4.2.1 Check the moisture sensing element for short circuit-
ditions is equivalent to the cumulative time the RH is above a
ing due to low-resistance bridges between the electrodes or
given threshold value. This time value varies with location
breakdown in the dielectric properties of the base. The open-
and with other factors. Probability curves have been developed
circuit resistance between the two sets of electrodes should be
for top and bottom surfaces of a standard panel at one location
in excess of 100 MV when the sensing element is dry (room
which show the probable times that a surface will be wet as a
condition at 50 % relative humidity or lower).
percentage of the cumulative time the relative humidity is at
4.2.2 Check the action of the galvanic cell of the sensing
specific levels. If needed, it should be possible to develop
element and the adequacy of the potting at the connection to
similar relationships to deal with other exposure conditions.
external leads by immersing the sensing element, including the
connection, for1hinan aqueous solution containing 10 mg/L
4. Sensor Preparation, Conditioning, and Calibration
of sodium chloride (NaCl) and 1 % ethanol. Under this
4.1 The moisture sensing elements are manufactured by
condition,thepotentialmeasuredshouldbeinexcessof0.03V
plating and selective etching of thin films of appropriate anode
for copper-gold cells and should remain at this value. For the
or cathode material on a thin nonconductive substrate. These
sensor consisting of a zinc-gold cell, the potential measured
elements may be procured from a commercial source. Thin
under this test should be in excess of 0.4 V. After immersion,
sensing elements are preferred in order to preclude influencing
rinse the sensor in distilled water and allow to dry.
the surface temperature to any extent. Although a sensor
4.3 Conditioning of the Sensing Element:
constructed using a 1.5-mm thick glass reinforced polyester
4.3.1 Activate sensors by spreading 1 drop of NaCl solution
base has been found to be satisfactory on plastic surfaces
(10 mg/Lof NaCl containing a wetting agent of 1 % ethanol or
(low-thermal conductivity, and where the temperature of the
0.1 % polyoxyethylene isooctylphenol) on the electrode grid.
sensing element was measured as being within6 0.5°C of the
4.3.2 Expose the activated sensor at 100 % relative humid-
surface), this will not be the case with the same sensing
ity (in a desiccator over water) for a week. The resulting
corrosion product film makes the activation more permanent.
After being verified (see 4.4), store the sensor in a desiccator
Guttman, H., “Effects of Atmospheric Factors on Corrosion of Rolled Zinc,”
until ready for use.
Metal Corrosion in the Atmosphere, ASTM STP 435. ASTM, 1968, pp. 223–239.
4.3.2.1 Warning—The atmosphere in many laboratories
Sereda, P. J., Cross, S. G., and Slade, H. F., “Measurement of Time-of-Wetness
by Moisture Sensors and Their Calibration,” Atmospheric Corrosion of Metals,
can have contaminants that can affect the operation of the
ASTM STP 767, ASTM, 1982, pp. 267–285.
sensors (that is, HCl and SO fumes, contact with fingers,
4 2
Sereda Miniature Moisture Sensor, Model SMMS-01, available from Epitek
organic nonwetting agents, etc.). Since contamination effects
Electronics, Ltd., a Division of Epitek International Inc., 100 Schneider Road,
Kanata, Ontario, Canada K2K1Y2 has been found suitable. have been observed, handle the sensors with care.
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G84–89 (1999)
4.3.3 Fig. 2 and Fig. 3 illustrate a design of a simple 5. Field Installation and Maintenance of Sensor
conditioning chamber in which the sensing element can be
5.1 Mount the sensing element in intimate contact with the
exposed to 100 % relative humidity. To attain the desired
surface to be monitored using suitable adhesive or a double-
conditions, mount the apparatus in a thermally insulated box 3
faced, ⁄4-in. (20-mm) wide tape taking care to avoid contami-
located in a constant temperature room. It is desirable that the 5
nation of the sensor with the fingers.
temperature of the humidity source in the chamber be con-
5.2 Clean the sensing elements at least annually in the case
trolled to 60.2°C.
of copper-gold sensors and every six months in the case of
4.4 Verification of Sensing Element Functioning:
zinc-gold sensors. Cleaning is achieved by lightly brushing the
4.4.1 At 100 % RH, the copper-gold sensors should gener-
grid along its length. Deionized or distilled water and a soft,
ate a potential in excess of 0.01 V and a potential in excess of
clean toothbrush are recommended.
0.1 V for zinc-gold sensors. (The potential is essentially the
voltage drop across a 10 MV resistance with the load and 6. Signal Conditioning and Data Recording
recorder having an input impedence in excess of 1000 MV.)
6.1 The high-impedance and low-signal voltage output of
The potential measured will decrease with time of measure-
the moisture sensor requires that the signal be conditioned to
ment because of the depletion of available ions in the electro-
allow it to be interfaced with a data-recording device. Such a
lyte. Leave the sensor cells in an open circuit while they are
circuit (Fig. 4) has been described by Sereda et al, and is
being verified. This step can take as little as1hifthe
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