ASTM G84-89(2005)
(Practice)Standard Practice for Measurement of Time-of-Wetness on Surfaces Exposed to Wetting Conditions as in Atmospheric Corrosion Testing
Standard Practice for Measurement of Time-of-Wetness on Surfaces Exposed to Wetting Conditions as in Atmospheric Corrosion Testing
SCOPE
1.1 This practice covers a technique for monitoring time-of-wetness (TOW) on surfaces exposed to cyclic atmospheric conditions which produce depositions of moisture.
1.2 The practice is also applicable for detecting and monitoring condensation within a wall or roof assembly and in test apparatus.
1.3 Exposure site calibration or characterization can be significantly enhanced if TOW is measured for comparison with other sites, particularly if this data is used in conjunction with other site-specific instrumentation techniques.
1.4 The values stated in SI units are to be regarded as the standard.
This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:G84–89(Reapproved2005)
Standard Practice for
Measurement of Time-of-Wetness on Surfaces Exposed to
Wetting Conditions as in Atmospheric Corrosion Testing
ThisstandardisissuedunderthefixeddesignationG84;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope moisture is present on the sensing element during any given
period. The fact that a potential is generated is critical to this
1.1 This practice covers a technique for monitoring time-
technique.As pertains to this practice, the absolute value of the
of-wetness (TOW) on surfaces exposed to cyclic atmospheric
potential generated is essentially of academic interest.
conditions which produce depositions of moisture.
2.2 This practice describes the moisture-sensing element,
1.2 The practice is also applicable for detecting and moni-
procedures for conditioning the elements to develop stable
toring condensation within a wall or roof assembly and in test
films on the electrodes and verifying the sensing-element
apparatus.
function, and use of the element to record TOW.
1.3 Exposure site calibration or characterization can be
significantly enhanced if TOW is measured for comparison
3. Significance and Use
with other sites, particularly if this data is used in conjunction
3.1 This practice provides a methodology for measuring the
with other site-specific instrumentation techniques.
duration of wetness on a sensing element mounted on a surface
1.4 The values stated in SI units are to be regarded as the
in a location of interest. Experience has shown that the sensing
standard.
elementreactstofactorsthatcausewetnessinthesamemanner
1.5 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
prevented from given areas, and so forth. Therefore various
the sensor’s copper substrate. The output (potential) from this
components of a structure can be expected to perform differ-
cell is fed through a signal conditioning circuit to an indicating
ently depending on mass, orientation, air flow patterns, and so
or recording device. The objective is to record the time that
forth. A knowledge of TOW at different points on large
structures can be useful in the interpretation of corrosion or
This practice is under the jurisdiction of ASTM Committee G01 on Corrosion
other testing results.
of Metals and is the direct responsibility of Subcommittee G01.04 on Atmospheric
3.4 In order to improve comparison of data obtained from
Corrosion.
test locations separated on a macrogeographical basis, a
Current edition approved May 1, 2005. Published May 2005. Originally
´1
approved in 1981. Last previous edition approved in 1999 as G84 – 89 (1999) . uniform orientation of sensor elements boldly exposed in the
DOI: 10.1520/G0084-89R05.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
G84–89 (2005)
FIG. 1 Sensing Element
direction of the prevailing wind, at an angle of 30° above the base has been found to be satisfactory on plastic surfaces
horizontal is recommended. Elevation of the sensor above (low-thermal conductivity, and where the temperature of the
ground level should be recorded. sensing element was measured as being within 60.5°C of the
3.5 Although this method does not develop relationships
surface), this will not be the case with the same sensing
between TOW and levels of ambient relative humidity (RH), element on a metal surface with a high-thermal conductivity.
long term studies have been carried out to show that the TOW
For metal surfaces, the sensing element should be appreciably
experienced annually by panels exposed under standard con-
thinner. Commercial epoxy sensor backing products of thick-
ditions is equivalent to the cumulative time the RH is above a
ness of 1.5 mm, or less, are suitable for this purpose.
given threshold value. This time value varies with location
4.2 Checking the Moisture Sensing Elements:
and with other factors. Probability curves have been developed
4.2.1 Check the moisture sensing element for short circuit-
for top and bottom surfaces of a standard panel at one location
ing due to low-resistance bridges between the electrodes or
which show the probable times that a surface will be wet as a
breakdown in the dielectric properties of the base. The open-
percentage of the cumulative time the relative humidity is at
circuit resistance between the two sets of electrodes should be
specific levels. If needed, it should be possible to develop
in excess of 100 MV when the sensing element is dry (room
similar relationships to deal with other exposure conditions.
condition at 50 % relative humidity or lower).
4.2.2 Check the action of the galvanic cell of the sensing
4. Sensor Preparation, Conditioning, and Calibration
element and the adequacy of the potting at the connection to
4.1 The moisture sensing elements are manufactured by
external leads by immersing the sensing element, including the
plating and selective etching of thin films of appropriate anode
connection, for1hinan aqueous solution containing 10 mg/L
or cathode material on a thin, nonconducting substrate. These
4 of sodium chloride (NaCl) and 1 % ethanol. Under this
elements may be procured from a commercial source. Thin
condition,thepotentialmeasuredshouldbeinexcessof0.03V
sensing elements are preferred in order to preclude influencing
for copper-gold cells and should remain at this value. For the
the surface temperature to any extent. Although a sensor
sensor consisting of a zinc-gold cell, the potential measured
constructed using a 1.5-mm thick glass reinforced polyester
under this test should be in excess of 0.4 V. After immersion,
rinse the sensor in distilled water and allow to dry.
Guttman, H., “Effects of Atmospheric Factors on Corrosion of Rolled Zinc,”
4.3 Conditioning of the Sensing Element:
Metal Corrosion in the Atmosphere, ASTM STP 435. ASTM, 1968, pp. 223–239.
4.3.1 Activate sensors by spreading 1 drop of NaCl solution
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,
(10 mg/Lof NaCl containing a wetting agent of 1 % ethanol or
ASTM STP 767, ASTM, 1982, pp. 267–285.
0.1 % polyoxyethylene isooctylphenol) on the electrode grid.
The sole source of supply of the apparatus known to the committee at this time
4.3.2 Expose the activated sensor at 100 % relative humid-
is the Sereda Miniature Moisture Sensor, Model SMMS-01, available from Epitek
Electronics, Ltd., a Division of Epitek International Inc., 100 Schneider Road,
ity (in a desiccator over water) for a week. The resulting
Kanata, Ontario, Canada K2K1Y2. If you are aware of alternative suppliers, please
corrosion product film makes the activation more permanent.
provide this information toASTM International Headquarters. Your comments will
After being verified (see 4.4), store the sensor in a desiccator
receive careful consideration at a meeting of the responsible technical committee ,
which you may attend. until ready for use.
G84–89 (2005)
4.3.2.1 Warning—The atmosphere in many laboratories The potential measured will decrease with time of measure-
can have contaminants that can affect the operation of the ment because of the depletion of available ions in the electro-
sensors (that is, HCl and SO fumes, contact with fingers, lyte. Leave the sensor cells in an open circuit while they are
organic nonwetting agents, and so forth). Since contamination being verified. This step can take as little as1hifthe
effects have been observed, handle the sensors with care. temperatures are constant.
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
exposed to 100 % relative humidity. To attain the desired 5.1 Mount the sensing element in intimate contact with the
conditions, mount the apparatus in a thermally insulated box surface to be monitored using suitable adhesive or a double-
located in a constant temperature room. It is desirable that the faced, ⁄4-in. (20-mm) wide tape taking care to avoid contami-
temperature of the humidity source in the chamber be con- nation of the sensor with the fingers.
trolled to 60.2°C. 5.2 Clean the sensing elements at least annually in the case
4.4 Verification of Sensing Element Functioning: of copper-gold sensors and every six months in the case of
4.4.1 At 100 % RH, the copper-gold sensors should gener- zinc-gold sensors. Cleaning is achieved by lightly brushing the
ate a potential in excess of 0.01 V and a potential in excess of grid along its length. Deionized or distilled water and a soft,
0.1 V for zinc-gold sensors. (The potential is essentially the
clean toothbrush are recommended.
voltage drop across a 10 MV resistance with the load and
6. Signal Conditioning and Data Recording
recorder having an input impedance in excess of 1000 MV.)
6.1 The high-impedance and low-signal voltage output of
the moisture sensor requires that the signal be conditioned to
allow it to be interfaced with a data-recording device. Such a
circuit (Fig. 4) has been described by Sereda et al, and is
available as a field usable off-the-shelf commercial modular
interface unit. When using the circuit in Fig. 4, noted that the
reference voltage (Vref) value for the integrated circuit (IC1) is
determined by the output voltage of the sensor, for example,
0.01 V for copper-gold and 0.10 V for zinc-gold sensors. The
design of the circuit is such that there is a 4-W minimum
recorder load requirement which would make long-term bat-
tery power supply operation of the interface inconvenient. The
commercial interface unit offers a 5-V logic compatible output
(CMDS, TTL, and so forth) or an amplified (503) analog
signal.Alow-powerbatterysupplyversionofthecircuitinFig.
4hasbeendeveloped andisshowninFig.5.Thiscircuitgives
the device true unattended field operation capability. The
recording device can either be a relay-operated analog timing
device or an integrated circuit-driven counter.
7. Time-of-Wetness Report
7.1 When potential is recorded by means of a recorder or
data-loggingsystem,thepotentialreadingscanbeprocessedas
frequency distribution giving the percent of time when various
Scotch brand polyester film No. 75, manufactured by Minnesota Mining and
Manufacturing Co., St. Paul, MN, or equivalent is suitable. If you are aware of
alternative suppliers, please provide this information to ASTM International
Headquarters.Your comments will receive careful consideration at a meeting of the
responsible technical committee , which you may attend.
The sole source of supply of the apparatus known to the committee at this time
is the Moisture Sensor Interface, Model WSI-01, available from Epitek Electronics,
Ltd., a Division of Epitek International, Inc., 100 Schneider Road, Kanata, Ontario,
Canada, K2K1Y2. If you are aware of alternative suppliers, please provide this
information to ASTM International Headquarters. Your comments will receive
careful consideration at a meeting of the responsible technical committee , which
you may attend.
Centre de Recherche Noranda, 240 Boulevard Hymus, Pointe-Claire, Quebec
H9R 1G5.
Veeder-Root Model 7998 Mini-LX Totalizer or other comparable commercial
equivalent,availablefromDigitalSystemsDivision,Hartford,CT06102issuitable.
If you are aware of alternative suppliers, please provide this information to ASTM
International Headquarters. Your comments will receive careful consideration at a
FIG. 2 Humidity Sensor Calibration Apparatus meeting of the responsible technical committee , which you may attend.
G84–89 (2005)
FIG. 3 Humidity Sensor Calibration Apparatus
VOLTAGE REGULATOR LOAD ACTIVATING CIRCUI
—Receives voltage from power line transformer and provides a regulated D.C. —Comparator IC1 output is fed to optoisolator IC2 which provides triggering pulses to
voltage to the interface circuit. triac T1.
INTERFACE CIRCUIT —Triac T1 permits current to flow through the load (running time meter or alarm).
—Reference voltage (0.010 or 0.10V) is derived from potentiometer VR1. PARTS LIST
Reference voltage can be adjusted at test point 1 (TP1). IC1 CA 314OE OP-AMP
—Operational amplifier IC1 compares reference voltage (Pin 3) and sensor IC2 MOC 301 Triac Driver
voltage (Pin 2), and activates the relay circuit when sensor voltage is greater
...
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