1、Designation: G 84 89 (Reapproved 2005)Standard Practice forMeasurement of Time-of-Wetness on Surfaces Exposed toWetting Conditions as in Atmospheric Corrosion Testing1This standard is issued under the fixed designation G 84; the number immediately following the designation indicates the year of orig
2、inaladoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscriptepsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers a technique for monitoring time-of-wetness
3、 (TOW) on surfaces exposed to cyclic atmosphericconditions which produce depositions of moisture.1.2 The practice is also applicable for detecting and moni-toring condensation within a wall or roof assembly and in testapparatus.1.3 Exposure site calibration or characterization can besignificantly en
4、hanced if TOW is measured for comparisonwith other sites, particularly if this data is used in conjunctionwith other site-specific instrumentation techniques.1.4 The values stated in SI units are to be regarded as thestandard.1.5 This standard does not purport to address all of thesafety concerns, i
5、f any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Summary of Practice2.1 This practice describes a technique for detecting andrecording
6、surface moisture conditions. The moisture serves asan electrolyte to generate a potential in a moisture sensingelement galvanic cell that consists of alternate electrodes ofcopper and gold, silver and platinum, or zinc and gold. Thespacing of the electrodes may be 100 to 200 m, the widthdimension is
7、 not considered critical (Fig. 1). However, whenzinc is used as an electrode material, the effects of thehygroscopic nature of the corrosion products on the perfor-mance of the sensor should be kept in mind. Also, the use ofcopper as a sensor material should be avoided in sulfurdioxide-laden atmosph
8、eres to avoid premature deterioration ofthe sensors copper substrate. The output (potential) from thiscell is fed through a signal conditioning circuit to an indicatingor recording device. The objective is to record the time thatmoisture is present on the sensing element during any givenperiod. The
9、fact that a potential is generated is critical to thistechnique.As pertains to this practice, the absolute value of thepotential generated is essentially of academic interest.2.2 This practice describes the moisture-sensing element,procedures for conditioning the elements to develop stablefilms on t
10、he electrodes and verifying the sensing-elementfunction, and use of the element to record TOW.3. Significance and Use3.1 This practice provides a methodology for measuring theduration of wetness on a sensing element mounted on a surfacein a location of interest. Experience has shown that the sensing
11、element reacts to factors that cause wetness in the same manneras the surface on which it is mounted.3.2 Surface moisture plays a critical role in the corrosion ofmetals and the deterioration of nonmetallics. The deposition ofmoisture on a surface can be caused by atmospheric or climaticphenomena su
12、ch as direct precipitation of rain or snow,condensation, the deliquescence (or at least the hygroscopicnature) of corrosion products or salt deposits on the surface,and others. A measure of atmospheric or climatic factorsresponsible for moisture deposition does not necessarily givean accurate indica
13、tion of the TOW. For example, the surfacetemperature of an object may be above or below both theambient and the dew point temperatures. As a result conden-sation will occur without an ambient meteorological indicationthat a surface has been subjected to a condensation cycle.3.3 Structural design fac
14、tors and orientation can be respon-sible for temperature differences and the consequent effect onTOW as discussed in 4.2. As a result, some surfaces may beshielded from rain or snow fall; drainage may be facilitated orprevented from given areas, and so forth. Therefore variouscomponents of a structu
15、re can be expected to perform differ-ently depending on mass, orientation, air flow patterns, and soforth. A knowledge of TOW at different points on largestructures can be useful in the interpretation of corrosion orother testing results.3.4 In order to improve comparison of data obtained fromtest l
16、ocations separated on a macrogeographical basis, auniform orientation of sensor elements boldly exposed in thedirection of the prevailing wind, at an angle of 30 above the1This practice is under the jurisdiction of ASTM Committee G01 on Corrosionof Metals and is the direct responsibility of Subcommi
17、ttee G01.04 on AtmosphericCorrosion.Current edition approved May 1, 2005. Published May 2005. Originallyapproved in 1981. Last previous edition approved in 1999 as G 84 89 (1999)e1.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.hori
18、zontal is recommended. Elevation of the sensor aboveground level should be recorded.3.5 Although this method does not develop relationshipsbetween TOW and levels of ambient relative humidity (RH),long term studies have been carried out to show that the TOWexperienced annually by panels exposed under
19、 standard con-ditions is equivalent to the cumulative time the RH is above agiven threshold value.2This time value varies with locationand with other factors. Probability curves have been developedfor top and bottom surfaces of a standard panel at one locationwhich show the probable times that a sur
20、face will be wet as apercentage of the cumulative time the relative humidity is atspecific levels.3If needed, it should be possible to developsimilar relationships to deal with other exposure conditions.4. Sensor Preparation, Conditioning, and Calibration4.1 The moisture sensing elements are manufac
21、tured byplating and selective etching of thin films of appropriate anodeor cathode material on a thin, nonconducting substrate. Theseelements may be procured from a commercial source.4Thinsensing elements are preferred in order to preclude influencingthe surface temperature to any extent. Although a
22、 sensorconstructed using a 1.5-mm thick glass reinforced polyesterbase has been found to be satisfactory on plastic surfaces(low-thermal conductivity, and where the temperature of thesensing element was measured as being within 60.5C of thesurface), this will not be the case with the same sensingele
23、ment on a metal surface with a high-thermal conductivity.For metal surfaces, the sensing element should be appreciablythinner. Commercial epoxy sensor backing products of thick-ness of 1.5 mm, or less, are suitable for this purpose.4.2 Checking the Moisture Sensing Elements:4.2.1 Check the moisture
24、sensing element for short circuit-ing due to low-resistance bridges between the electrodes orbreakdown in the dielectric properties of the base. The open-circuit resistance between the two sets of electrodes should bein excess of 100 MV when the sensing element is dry (roomcondition at 50 % relative
25、 humidity or lower).4.2.2 Check the action of the galvanic cell of the sensingelement and the adequacy of the potting at the connection toexternal leads by immersing the sensing element, including theconnection, for1hinanaqueous solution containing 10 mg/Lof sodium chloride (NaCl) and 1 % ethanol. U
26、nder thiscondition, the potential measured should be in excess of 0.03 Vfor copper-gold cells and should remain at this value. For thesensor consisting of a zinc-gold cell, the potential measuredunder this test should be in excess of 0.4 V. After immersion,rinse the sensor in distilled water and all
27、ow to dry.4.3 Conditioning of the Sensing Element:4.3.1 Activate sensors by spreading 1 drop of NaCl solution(10 mg/L of NaCl containing a wetting agent of 1 % ethanol or0.1 % polyoxyethylene isooctylphenol) on the electrode grid.4.3.2 Expose the activated sensor at 100 % relative humid-ity (in a de
28、siccator over water) for a week. The resultingcorrosion product film makes the activation more permanent.After being verified (see 4.4), store the sensor in a desiccatoruntil ready for use.4.3.2.1 WarningThe atmosphere in many laboratoriescan have contaminants that can affect the operation of the2Gu
29、ttman, H., “Effects of Atmospheric Factors on Corrosion of Rolled Zinc,”Metal Corrosion in the Atmosphere, ASTM STP 435. ASTM, 1968, pp. 223239.3Sereda, P. J., Cross, S. G., and Slade, H. F., “Measurement of Time-of-Wetnessby Moisture Sensors and Their Calibration,” Atmospheric Corrosion of Metals,A
30、STM STP 767, ASTM, 1982, pp. 267285.4The sole source of supply of the apparatus known to the committee at this timeis the Sereda Miniature Moisture Sensor, Model SMMS-01, available from EpitekElectronics, Ltd., a Division of Epitek International Inc., 100 Schneider Road,Kanata, Ontario, Canada K2K1Y
31、2. If you are aware of alternative suppliers, pleaseprovide this information to ASTM International Headquarters. Your comments willreceive careful consideration at a meeting of the responsible technical committee1,which you may attend.FIG. 1 Sensing ElementG 84 89 (2005)2sensors (that is, HCl and SO
32、2fumes, contact with fingers,organic nonwetting agents, and so forth). Since contaminationeffects have been observed, handle the sensors with care.4.3.3 Fig. 2 and Fig. 3 illustrate a design of a simpleconditioning chamber in which the sensing element can beexposed to 100 % relative humidity. To att
33、ain the desiredconditions, mount the apparatus in a thermally insulated boxlocated in a constant temperature room. It is desirable that thetemperature of the humidity source in the chamber be con-trolled to 60.2C.4.4 Verification of Sensing Element Functioning:4.4.1 At 100 % RH, the copper-gold sens
34、ors should gener-ate a potential in excess of 0.01 V and a potential in excess of0.1 V for zinc-gold sensors. (The potential is essentially thevoltage drop across a 10 MV resistance with the load andrecorder having an input impedance in excess of 1000 MV.)The potential measured will decrease with ti
35、me of measure-ment because of the depletion of available ions in the electro-lyte. Leave the sensor cells in an open circuit while they arebeing verified. This step can take as little as1hifthetemperatures are constant.5. Field Installation and Maintenance of Sensor5.1 Mount the sensing element in i
36、ntimate contact with thesurface to be monitored using suitable adhesive or a double-faced,34-in. (20-mm) wide tape taking care to avoid contami-nation of the sensor with the fingers.55.2 Clean the sensing elements at least annually in the caseof copper-gold sensors and every six months in the case o
37、fzinc-gold sensors. Cleaning is achieved by lightly brushing thegrid along its length. Deionized or distilled water and a soft,clean toothbrush are recommended.6. Signal Conditioning and Data Recording6.1 The high-impedance and low-signal voltage output ofthe moisture sensor requires that the signal
38、 be conditioned toallow it to be interfaced with a data-recording device. Such acircuit (Fig. 4) has been described by Sereda et al,3and isavailable as a field usable off-the-shelf commercial modularinterface unit.6When using the circuit in Fig. 4, noted that thereference voltage (Vref) value for th
39、e integrated circuit (IC1) isdetermined by the output voltage of the sensor, for example,0.01 V for copper-gold and 0.10 V for zinc-gold sensors. Thedesign of the circuit is such that there is a 4-W minimumrecorder load requirement which would make long-term bat-tery power supply operation of the in
40、terface inconvenient. Thecommercial interface unit offers a 5-V logic compatible output(CMDS, TTL, and so forth) or an amplified (503) analogsignal.Alow-power battery supply version of the circuit in Fig.4 has been developed7and is shown in Fig. 5. This circuit givesthe device true unattended field
41、operation capability. Therecording device can either be a relay-operated analog timingdevice or an integrated circuit-driven counter.87. Time-of-Wetness Report7.1 When potential is recorded by means of a recorder ordata-logging system, the potential readings can be processed asfrequency distribution
42、 giving the percent of time when variouslevels of potential are exceeded. This provides the TOW forany selected level of potential.35Scotch brand polyester film No. 75, manufactured by Minnesota Mining andManufacturing Co., St. Paul, MN, or equivalent is suitable. If you are aware ofalternative supp
43、liers, please provide this information to ASTM InternationalHeadquarters. Your comments will receive careful consideration at a meeting of theresponsible technical committee1, which you may attend.6The sole source of supply of the apparatus known to the committee at this timeis the Moisture Sensor I
44、nterface, 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 thisinformation to ASTM International Headquarters. Your comments will receivecarefu
45、l consideration at a meeting of the responsible technical committee1, whichyou may attend.7Centre de Recherche Noranda, 240 Boulevard Hymus, Pointe-Claire, QuebecH9R 1G5.8Veeder-Root Model 7998 Mini-LX Totalizer or other comparable commercialequivalent, available from Digital Systems Division, Hartf
46、ord, CT 06102 is suitable.If you are aware of alternative suppliers, please provide this information to ASTMInternational Headquarters. Your comments will receive careful consideration at ameeting of the responsible technical committee1, which you may attend.FIG. 2 Humidity Sensor Calibration Appara
47、tusG 84 89 (2005)37.2 Record the TOW and report as a percent of total time foreach month.8. Precision and Bias8.1 The actual TOW experienced by any surface in anatmospheric exposure is a complex function of a large numberof variables including weather, climate conditions, and localcircumstances. Com
48、parisons between sensors in any exposuretest will show both the actual variations in TOW together withrandom statistical variations that affect the instants when theTOW clock turns on and off.8.2 In actual atmospheric exposure tests with similar sen-sors in comparable locations, the variation in TOW
49、 readingswas strongly dependent upon whether subfreezing tempera-tures were encountered. A comparison between two similarsensors, either copper and gold or zinc and gold over a 1-monthinterval with no subfreezing temperatures yielded a standarddeviation about 4 % of the mean monthly TOW. A comparisonbetween the copper and gold and a zinc and gold sensor yieldeda standard deviation of about 8 % of the mean monthly TOWunder nonfreezing conditions.28.3 In circumstances when subfreezing conditions can existwhen moisture condensation occurs substantially greater varia-tions can b
