ASTM G84-89(2012)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: G84 − 89 (Reapproved 2012)
Standard Practice for
Measurement of Time-of-Wetness on Surfaces Exposed to
Wetting Conditions as in Atmospheric Corrosion Testing
ThisstandardisissuedunderthefixeddesignationG84;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoptionor,inthecaseofrevision,theyearoflastrevision.Anumberinparenthesesindicatestheyearoflastreapproval.Asuperscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope cellisfedthroughasignalconditioningcircuittoanindicating
or recording device. The objective is to record the time that
1.1 This practice covers a technique for monitoring time-
moisture is present on the sensing element during any given
of-wetness (TOW) on surfaces exposed to cyclic atmospheric
period. The fact that a potential is generated is critical to this
conditions which produce depositions of moisture.
technique.Aspertainstothispractice,theabsolutevalueofthe
1.2 The practice is also applicable for detecting and moni-
potential generated is essentially of academic interest.
toring condensation within a wall or roof assembly and in test
2.2 This practice describes the moisture-sensing element,
apparatus.
procedures for conditioning the elements to develop stable
1.3 Exposure site calibration or characterization can be
films on the electrodes and verifying the sensing-element
significantly enhanced if TOW is measured for comparison
function, and use of the element to record TOW.
with other sites, particularly if this data is used in conjunction
3. Significance and Use
with other site-specific instrumentation techniques.
3.1 This practice provides a methodology for measuring the
1.4 The values stated in SI units are to be regarded as
durationofwetnessonasensingelementmountedonasurface
standard. No other units of measurement are included in this
inalocationofinterest.Experiencehasshownthatthesensing
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,
condensation, the deliquescence (or at least the hygroscopic
2. Summary of Practice
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
sationwilloccurwithoutanambientmeteorologicalindication
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-
ently depending on mass, orientation, air flow patterns, and so
forth. A knowledge of TOW at different points on large
This practice is under the jurisdiction ofASTM Committee G01 on Corrosion
structures can be useful in the interpretation of corrosion or
of Metals and is the direct responsibility of Subcommittee G01.04 on Atmospheric
Corrosion.
other testing results.
Current edition approved Jan. 1, 2012. Published March 2012. Originally
3.4 In order to improve comparison of data obtained from
approved in 1981. Last previous edition approved in 2005 as G84–89(2005). DOI:
10.1520/G0084-89R05. test locations separated on a macrogeographical basis, a
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G84 − 89 (2012)
FIG. 1 Sensing Element
uniform orientation of sensor elements boldly exposed in the the surface temperature to any extent. Although a sensor
direction of the prevailing wind, at an angle of 30° above the constructed using a 1.5-mm thick glass reinforced polyester
horizontal is recommended. Elevation of the sensor above base has been found to be satisfactory on plastic surfaces
ground level should be recorded. (low-thermal conductivity, and where the temperature of the
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
andwithotherfactors.Probabilitycurveshavebeendeveloped
4.2 Checking the Moisture Sensing Elements:
for top and bottom surfaces of a standard panel at one location
4.2.1 Check the moisture sensing element for short circuit-
which show the probable times that a surface will be wet as a
ing due to low-resistance bridges between the electrodes or
percentage of the cumulative time the relative humidity is at
breakdown in the dielectric properties of the base. The open-
specific levels. If needed, it should be possible to develop
circuit resistance between the two sets of electrodes should be
similar relationships to deal with other exposure conditions.
in excess of 100 MΩ when the sensing element is dry (room
condition at 50% relative humidity or lower).
4. Sensor Preparation, Conditioning, and Calibration
4.2.2 Check the action of the galvanic cell of the sensing
4.1 The moisture sensing elements are manufactured by
element and the adequacy of the potting at the connection to
plating and selective etching of thin films of appropriate anode
externalleadsbyimmersingthesensingelement,includingthe
or cathode material on a thin, nonconducting substrate. These
connection, for1hinan aqueous solution containing 10 mg/L
elements may be procured from a commercial source. Thin
of sodium chloride (NaCl) and 1% ethanol. Under this
sensing elements are preferred in order to preclude influencing
condition,thepotentialmeasuredshouldbeinexcessof0.03V
for copper-gold cells and should remain at this value. For the
sensor consisting of a zinc-gold cell, the potential measured
Guttman, H., “Effects of Atmospheric Factors on Corrosion of Rolled Zinc,”
Metal Corrosion in the Atmosphere, ASTM STP 435. ASTM, 1968, pp. 223–239.
under this test should be in excess of 0.4 V. After immersion,
Sereda, P. J., Cross, S. G., and Slade, H. F., “Measurement ofTime-of-Wetness
rinse the sensor in distilled water and allow to dry.
by Moisture Sensors and Their Calibration,” Atmospheric Corrosion of Metals,
ASTM STP 767, ASTM, 1982, pp. 267–285.
4.3 Conditioning of the Sensing Element:
The sole source of supply of the apparatus known to the committee at this time
4.3.1 Activatesensorsbyspreading1dropofNaClsolution
is the Sereda Miniature Moisture Sensor, Model SMMS-01, available from Epitek
Electronics, Ltd., a Division of Epitek International Inc., 100 Schneider Road, (10mg/LofNaClcontainingawettingagentof1%ethanolor
Kanata, Ontario, Canada K2K1Y2. If you are aware of alternative suppliers, please
0.1% polyoxyethylene isooctylphenol) on the electrode grid.
provide this information toASTM International Headquarters.Your comments will
1 4.3.2 Expose the activated sensor at 100% relative humid-
receive careful consideration at a meeting of the responsible technical committee,
which you may attend. ity (in a desiccator over water) for a week. The resulting
G84 − 89 (2012)
corrosion product film makes the activation more permanent. 4.4.1 At 100% RH, the copper-gold sensors should gener-
After being verified (see 4.4), store the sensor in a desiccator ate a potential in excess of 0.01 V and a potential in excess of
until ready for use. 0.1 V for zinc-gold sensors. (The potential is essentially the
4.3.2.1 Warning—The atmosphere in many laboratories voltage drop across a 10 MΩ resistance with the load and
can have contaminants that can affect the operation of the recorder having an input impedance in excess of 1000 MΩ.)
sensors (that is, HCl and SO fumes, contact with fingers, The potential measured will decrease with time of measure-
organic nonwetting agents, and so forth). Since contamination ment because of the depletion of available ions in the electro-
effects have been observed, handle the sensors with care. lyte. Leave the sensor cells in an open circuit while they are
4.3.3 Fig. 2 and Fig. 3 illustrate a design of a simple being verified. This step can take as little as1hifthe
conditioning chamber in which the sensing element can be temperatures are constant.
exposed to 100% relative humidity. To attain the desired
conditions, mount the apparatus in a thermally insulated box 5. Field Installation and Maintenance of Sensor
located in a constant temperature room. It is desirable that the
5.1 Mount the sensing element in intimate contact with the
temperature of the humidity source in the chamber be con-
surface to be monitored using suitable adhesive or a double-
trolled to 60.2°C.
faced, ⁄4-in. (20-mm) wide tape taking care to avoid contami-
4.4 Verification of Sensing Element Functioning:
nation of the sensor with the fingers.
5.2 Clean the sensing elements at least annually in the case
of copper-gold sensors and every six months in the case of
zinc-goldsensors.Cleaningisachievedbylightlybrushingthe
grid along its length. Deionized or distilled water and a soft,
clean toothbrush are recommended.
6. Signal Conditioning and Data Recording
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
referencevoltage(Vref)valuefortheintegratedcircuit(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-Vlogic compatible output
(CMDS, TTL, and so forth) or an amplified (50×) analog
signal.Alow-powerbatterysupplyversionofthecircuitinFig.
4hasbeendeveloped andisshowninFig.5.Thiscircuitgives
the device true unattended field operation capability. The
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, ModelWSI-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
FIG. 2 Humidity Sensor Calibration Apparatus H9R 1G5.
G84 − 89 (2012)
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 IC3 LM-340T-12 Voltage Regulator
than reference voltage (Pin 6). RCA T2850B
Triac
FIG. 4 Line Powered Wetness Detector
recording device can either be a relay-operated analog timing frequency distribution giving the percent of time when various
device or an integrated circuit-driven counter. levels of potential are exceeded. This provides the TOW for
any selected level of potential.
7. Time-of-Wetness Report
7.2 RecordtheTOWandreportasapercentoftotaltimefor
7.1 When potential is recorded by means of a recorder or
each month.
data-loggingsystem,thepotentialreadingscanbeprocessedas
8. Precision and Bias
Veeder-Root Model 7998 Mini-LX Totalizer or other comparable commercial
8.1 The actual TOW experienced by any surface in an
equivalent,availablefromDigitalSy
...