1、Designation: G84 89 (Reapproved 2012)Standard Practice forMeasurement of Time-of-Wetness on Surfaces Exposed toWetting Conditions as in Atmospheric Corrosion Testing1This standard is issued under the fixed designation G84; the number immediately following the designation indicates the year of origin
2、aladoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscriptepsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers a technique for monitoring time-of-wetness (T
3、OW) 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 enhan
4、ced 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 asstandard. No other units of measurement are included in thisstandard.1.5 This standard doe
5、s not purport to address all of thesafety concerns, if 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 pract
6、ice describes a technique for detecting andrecording 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
7、electrodes may be 100 to 200 m, the widthdimension is 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 mate
8、rial should be avoided in sulfurdioxide-laden atmospheres 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 presen
9、t on the sensing element during any givenperiod. The 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
10、conditioning the elements to develop stablefilms on the 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 locati
11、on of interest. Experience has shown that the sensingelement 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
12、 can be caused by atmospheric or climaticphenomena such 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 d
13、eposition does not necessarily givean accurate indication 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 subjec
14、ted to a condensation cycle.3.3 Structural design factors 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, a
15、nd so forth. Therefore variouscomponents of a structure 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 or
16、der to improve comparison of data obtained fromtest locations separated on a macrogeographical basis, auniform orientation of sensor elements boldly exposed in the1This practice is under the jurisdiction of ASTM Committee G01 on Corrosionof Metals and is the direct responsibility of Subcommittee G01
17、.04 on AtmosphericCorrosion.Current edition approved Jan. 1, 2012. Published March 2012. Originallyapproved in 1981. Last previous edition approved in 2005 as G8489(2005). DOI:10.1520/G0084-89R05.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, Uni
18、ted States.direction of the prevailing wind, at an angle of 30 above thehorizontal 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
19、carried out to show that the TOWexperienced annually by panels exposed under 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
20、 of a standard panel at one locationwhich show the probable times that a surface 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
21、, Conditioning, and Calibration4.1 The moisture sensing elements are manufactured 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 or
22、der to preclude influencingthe surface temperature to any extent. Although a 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 wit
23、hin 60.5C of thesurface), this will not be the case with the same sensingelement 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
24、purpose.4.2 Checking the Moisture Sensing Elements:4.2.1 Check the moisture 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 exce
25、ss of 100 MV when the sensing element is dry (roomcondition at 50 % relative 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, for1hinanaqu
26、eous solution containing 10 mg/Lof sodium chloride (NaCl) and 1 % ethanol. Under 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
27、excess of 0.4 V. After immersion,rinse the sensor in distilled water and allow 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
28、 grid.4.3.2 Expose the activated sensor at 100 % relative humid-ity (in a desiccator 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.2Guttman, H., “Effects of Atmospher
29、ic 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,ASTM STP 767, ASTM, 1982, pp. 267
30、285.4The 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 K2K1Y2. If you are aware of alternati
31、ve suppliers, pleaseprovide this information to ASTM International Headquarters. Your comments willreceive careful consideration at a meeting of the responsible technical committee,1which you may attend.FIG. 1 Sensing ElementG84 89 (2012)24.3.2.1 WarningThe atmosphere in many laboratoriescan have co
32、ntaminants that can affect the operation of thesensors (that is, HCl and SO2fumes, 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
33、 which the sensing element can beexposed to 100 % relative humidity. To attain 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 Verifica
34、tion of Sensing Element Functioning:4.4.1 At 100 % RH, the copper-gold sensors 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 i
35、mpedance in excess of 1000 MV.)The potential measured will decrease with time 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. Fie
36、ld Installation and Maintenance of Sensor5.1 Mount the sensing element in intimate 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 an
37、nually in the caseof copper-gold sensors and every six months in the case ofzinc-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 a
38、nd low-signal voltage output ofthe moisture sensor requires that the signal 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
39、 the circuit in Fig. 4, noted that thereference voltage (Vref) value for the 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 requi
40、rement which would make long-term bat-tery power supply operation of the interface 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 developed7
41、and is shown in Fig. 5. This circuit givesthe device true unattended field 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-loggi
42、ng system, the potential readings can be processed asfrequency distribution giving the percent of time when various5Scotch brand polyester film No. 75, manufactured by Minnesota Mining andManufacturing Co., St. Paul, MN, or equivalent is suitable. If you are aware ofalternative suppliers, please pro
43、vide this information to ASTM InternationalHeadquarters. Your comments will receive careful consideration at a meeting of theresponsible technical committee,1which you may attend.6The sole source of supply of the apparatus known to the committee at this timeis the Moisture Sensor Interface, Model WS
44、I-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 receivecareful consideration at
45、 a meeting of the responsible technical committee,1whichyou 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, Hartford, CT 06102 is su
46、itable.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 committee,1which you may attend.FIG. 2 Humidity Sensor Calibration ApparatusG84 89 (2012)3lev
47、els of potential are exceeded. This provides the TOW forany selected level of potential.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 varia
48、bles including weather, climate conditions, and localcircumstances. Comparisons 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 te
49、sts with similar sen-sors in comparable locations, the variation in TOW 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 existwh
