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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Designation:G84–89(Reapproved2005)
Standard Practice for
Measurement of Time-of-Wetness on Surfaces Exposed to
1
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
1
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.
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