Dehumidification and Temperature Control During SurfacePreparation, Application, and Curing for Coatings/Linings of Steel
Tanks, Vessels, and Other Enclosed Spaces
This NACE International (NACE)/Society for Protective Coatings (SSPC) report represents a consensus of those individualmembers who have reviewed this document, its scope, and provisions. Its acceptance does not in any respect precludeanyone, whether he has adopted the report or not, from manufacturing, marketing, purchasing, or using products, processes,or procedures not in conformance with this report. Nothing contained in this NACE/SSPC report is to be construed as grantingany right, by implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or productcovered by Letters Patent, or as indemnifying or protecting anyone against liability for infringement of Letters Patent. Thisreport should in no way be interpreted as a restriction on the use of better procedures or materials. Neither is this reportintended to apply in all cases relating to the subject. Unpredictable circumstances may negate the usefulness of this report inspecific instances. NACE and SSPC assume no responsibility for the interpretation or use of this report by other parties.
Users of this NACE/SSPC report are responsible for reviewing appropriate health, safety, environmental, and regulatorydocuments and for determining their applicability in relation to this report prior to its use. This NACE/SSPC report may notnecessarily address all potential health and safety problems or environmental hazards associated with the use of materials,equipment, and/or operations detailed or referred to within this report. Users of this NACE/SSPC report are also responsiblefor establishing appropriate health, safety, and environmental protection practices, in consultation with appropriate regulatoryauthorities if necessary, to achieve compliance with any existing applicable regulatory requirements prior to the use of thisreport.
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NOTICE TO THE READER: The NACE and SSPC releases of this publication contain identical wording in the samesequence. Publication format may differ.
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NACE 6A192/SSPC-TR 3
Foreword
The use of dehumidification and temperature control hasbecome more common during coating/lining operationsand much has been learned about ways to optimize itsuse to achieve maximum benefits at minimum cost. Thistechnical committee report presents current informationabout why and how dehumidification and temperaturecontrol are being used to achieve higher-quality coating/lining projects. It is intended to be a resource forengineers and coating consultants who write specifi-cations for coating projects involving tanks or enclosedspaces.
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This report was originally prepared by NACE Task GroupT-6A-60 on The Need for Dehumidification Equipment inthe Application of Linings. This revision was prepared byNACE Task Group 003 on Dehumidification. This TaskGroup is administered by NACE Specific TechnologyGroup (STG) 80 on Intersociety Joint Coatings Activities,and is sponsored by STG 03 on Protective Coatings andLinings – Immersion/Buried. The Task Group also hasrepresentation from SSPC Group Committee C.2 onSurface Preparation. This report is published by NACEInternational under the auspices of STG 80, and bySSPC.
Introduction
The use of dehumidification and temperature controlduring surface preparation and coating/lining applicationcan be beneficial in a variety of ambient conditions.When used properly, dehumidification (DH) provides airdew points well below the surface temperature andreduces the relative humidity (RH) at the surface.Reducing the RH at the surface can retard rust bloom.
The health and safety of personnel is also a factor in thedesign of a dehumidification system. Dehumidificationequipment that is properly sized for a given spaceprovides air flow for safe working conditions. The lower
explosive limits (LELs), toxicity levels, and oxygen levelsare all evaluated at each stage of the project.
The volume of coating to be sprayed per hour and thepercentage of solvent and solids to be added is calculatedusing manufacturers’ data sheets. The formulas for thesecalculations can be found in NFPA(1) 33.1 Theappropriate air-flow rate of the dehumidified and of theexhausted air through the enclosure and the properinstrumentation to be used for monitoring during bothstages of the project are also determined.
Glossary of Terms
Terms used in this report are widely used in severalengineering disciplines. Precise definitions are containedin other references, notably the ASHRAE(2) Handbook ofFundamentals.2 The explanatory definitions containedhere are sufficient for this report but are not as preciseand detailed as the ASHRAE definitions.
Absorbent: A desiccant material that holds water vaporthrough a hydration reaction that is reversible when thematerial is heated. Sodium chloride (table salt) andlithium chloride are examples of absorbent desiccants.
Adsorbent: A desiccant material that holds water vaporon its surface without a change in the chemical orphysical structure of the material. Silica gel and thenaturally occurring zeolites used for pet-waste granulesare examples of adsorbent desiccants.
Dehumidification: The removal of moisture from the air.
Desiccant: A material commonly used to absorbmoisture from the air; a solid or liquid material that has
the ability to collect moisture from the air and laterrelease the water vapor when the material is heated. Adesiccant used for dehumidification has a vapor pressurebelow that of the air to be dehumidified is in its active,dehydrated state.
Dew Point: The temperature of the air at which themoisture it contains condenses on nearby surfaces orsuspended dust particles. At constant pressure, eachdew point temperature represents a single value of airmoisture content. As a result, air dew point is often usedto describe air moisture content in absolute terms ratherthan relative humidity, which does not define the absoluteamount of moisture in the air unless the air temperatureis also known.
Flash Rusting: (1) Rusting that occurs on metal withinminutes to a few hours after cleaning is complete. Thespeed with which flash rusting occurs may be indicativeof salt contamination on the surface, high humidity, orboth; (2) Appearance of rust spots on the surface ofnewly applied water-borne film during the drying phase.
(1) National Fire Protection Association (NFPA), P.O. Box 9101, Quincy, MA 02269-9101.(2) American Society of Heating, Refrigeration, and Air-Conditioning Engineers, Inc. (ASHRAE), 1791 Tullie Circle NE, Atlanta, GA 30329-2305.
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Humidity Ratio: The amount of moisture in the air,expressed as the weight of the water vapor compared tothe weight of the air if it were perfectly dry. This results ina small decimal fraction. For example, air at 70°F (21°C)with 50% relative humidity has a humidity ratio of 0.0079.In the SI measurement system, this ratio is expressed asg water vapor/kg dry air. In the U.S. customary system ofmeasurement, the weight of water vapor is converted to awhole number by multiplying the humidity ratio by 7,000(the number of grains of water vapor in 1 lb). Therefore,air at 70°F (21°C) with 50% relative humidity has ahumidity ratio of 55 grains/lb (7.9 g/kg) of dry air.
Process Air: Dry air produced with a dehumidifier.
Reactivation Air: Air used to remove moisture from adesiccant material.
Relative Humidity (RH): The ratio, expressed as apercentage, of the amount of water vapor present in agiven volume of air at a given temperature to the amountrequired to saturate the air at that temperature.
Rust Bloom: Discoloration of steel surface indicating thebeginning of rusting.
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Temperature: A measure of hotness or coldness usuallyrecorded with a thermometer on either the Fahrenheit orCelsius scale; the dry-bulb temperature of the air, whichis the temperature of the air as measured by athermometer with a dry-sensing bulb properly shieldedfrom heat radiation sources such as sun or electricheaters.
Vapor Pressure: The pressure exerted by watermolecules, either suspended in an air sample or at thesurface of a solid material. A desiccant material attractswater vapor because the vapor pressure at its surface islower than the vapor pressure exerted by moleculessuspended in the air. In an effort to equalize thispressure differential, water molecules move from the airto the desiccant surface.
Wet-Bulb Temperature: The temperature of the airflowing across a thermometer with its sensing bulbsurrounded by a wetted wick. The water evaporatingfrom the wick cools the sensing bulb in proportion to theamount of evaporation. The evaporation effect (thereforea cooling effect) is greater when the air is drier. Bymeasuring the wet- and dry-bulb temperatures andplotting the values on a psychometric chart, the amountof moisture in the air can be determined.
Methods of Dehumidification
Dehumidification can be accomplished by compression,refrigeration, desiccation (liquid sorption, solid sorption),or a combination of these systems. While compressionand liquid sorption are common methods of dehumidifi-cation, their use is not generally applicable to fieldconditions. Therefore, only the refrigerant-based anddesiccant solid-sorption techniques are discussed indetail in this report.
Refrigeration
The cooling of air to below its dew point is an economicalmethod of dehumidification. This method is commonlyused at ambient temperatures of approximately 85°F(29°C) and high humidity. Ambient air is circulated overa system of refrigeration coils. The surface temperatureof the coils is set at temperatures considerably lower thanthe temperature of the incoming ambient air. As the aircools, it reaches saturation, and condensation forms.This condensation is collected and removed from thesystem. The air exits the cooling-coil section of thedehumidifier at a reduced temperature, dew point, andabsolute humidity. This refrigeration-based dehumidifi-cation system is illustrated in Figure 1. The cooler air,which has a lower dew point, can then be reheated tolower the relative humidity.
Refrigeration is often used to pre-cool and dehumidifyinbound air before it reaches a desiccant system in order
to obtain lower dew points after desiccation. The air canbe re-cooled, if necessary, by refrigeration.
Desiccant
Solid-sorption dehumidification systems utilize eithergranular beds or fixed desiccant structures. Thesestructures are contained within machines through whichan air stream is passed. The desiccant used is in anactive, dehydrated state and has a vapor pressure belowthat of the air to be dehumidified. The most commonlyused desiccants are silica gel and lithium chloride. Air ispassed through beds or layers of the desiccant, whichabsorb moisture from the air stream, producing ahydrated salt. Regeneration of the hydrated salt isaccomplished with heated air, which drives off the waterof hydration, returning the sorbent to its dehydrated state.The previously sorbed moisture is diverted to a separateair stream.
The exothermic hydration reaction typically raises thetemperature of the exiting air stream by 10 to 15°F (6 to8°C). Therefore, in hot climates, refrigeration-typedehumidifiers are frequently used in combination withdesiccant equipment to cool the air entering the space. Atypical desiccant dehumidification system is illustrated inFigure 2. Because this type of system absorbs moistureas vapor, it is commonly used at all temperatures andlevels of humidity.
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Compressor raises thepressure and temperatureof the refrigerant gas.
Compressor RefrigerateCondenser
RefrigerantExpansion Valve
Liquid RefrigerateStorage
RefrigerateEvaporator
ElectricReheat
Refrigerant iscondensed back toa liquid, releasingits heat to the airpassing through thecondenser coil.
Air Flow
Refrigerate expands inside thecoil, removing heat from theair passing through the line.
To Enclosure
Figure 1: Example of Refrigeration Type DehumidifierFIGURE 1: A Refrigeration-Type Dehumidifier
FIGURE 2: Desiccant Wheel
RefrigerantCondenser
Liquid RefrigerantStorage
Refrigerant expands inside the coil,removing heat from the air passingthrough the line.
RefrigerantEvaporator
Coil
Co
il
Sizing Equipment
The size of dehumidification equipment is typicallydetermined by considering the balance between airextraction from the space and the dehumidificationdesired to accomplish the specified dew point depressionfrom the surface temperature. If the capacity of thedehumidification equipment becomes marginal throughunexpected weather changes, its efficiency can beimproved by reducing the amount of air being extractedfor dust control.
The appropriate air-change rate for maintaining aprepared surface during blasting and between shifts whilemaintaining a large differential between dew point andsurface temperature for an extended period of time isdependent on air-space volume, equipment, geographical
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location, climate, and season. The number of openingsin the enclosure, the airtightness of the structure, thedistance of equipment from the space, and the amount ofair to be extracted or exhausted by means other than DHequipment also influence the DH capacity. Relativelyairtight enclosures generally require less DH volumebecause little or no additional air or moisture isintroduced into the space. Relatively large spaces usuallyrequire fewer air exchanges. Equipment contractorsusually have guides that give volume data for theirequipment.
The flow capacity of a dehumidifier for a given number ofair changes per hour is calculated using the formulashown in Equation (1):
XRACVi 60
))((=
Where:§ Vi is the internal volume of the space minus the
volume of any obstructions in ft3
§ RAC is the required air changes per hour§ X is the air-flow capacity in ft3/min that corresponds
with the specified air change rate
Or, for DH equipment with a capacity expressed in m3/h,the flow capacity is calculated using the formula shown inEquation (2):
XRACVi ))(( =
Where:§ Vi is the internal volume of the space minus the
volume of any obstructions in m3
§ RAC is the required air changes per hour§ X is the air-flow capacity in m3/h that corresponds
with the specified air change rate
Example One: Find the capacity of dehumidificationequipment that can dehumidify a110-ft-diameter enclosed space witha 6.0-ft-high floating roof.
1. Find the internal volume (Vi):
h dVi 4 2 π
=
=iV (110 ft)(110 ft)(0.7854)(6.0 ft)
=iV 57,000 ft3
(1)
(2)
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NACE 6A192/SSPC-TR 3
Where:d = diameterh = height
2. Set the required air changes (RAC) per hour: 2
3. Multiply the internal volume by the required airchanges to determine the amount of air needed perhour: (57,000 ft3)(2) = 114,000 ft3/h
Example Two: Find the capacity of dehumidificationequipment that can dehumidify a 60-m-diameter, 12-m-high enclosedspace.
1. Find the internal volume (Vi):
h dVi 4 2 π
=
=iV (60 m)(60 m)(0.7854)(12 m)
=iV 34,000 m3
2. Set the required air changes (RAC) per hour: 1
3. Multiply the internal volume by the required airchanges to determine the amount of air needed perhour: (34,000 m3/h)(1) = 34,000 m3/h
Impact of Contaminants and RH
Reducing RH at the surface inhibits corrosion and rustbloom. The appropriate RH to be maintained duringblasting depends on the amount of surface contaminantspresent. The amount and type of surface contaminationcan greatly influence the rate of rust bloom. As a generalrule, assuming surface RH is constant, the rate ofdeterioration (e.g., corrosion and rust bloom) increaseswith increased contamination.
The critical relative humidity of a material is defined asthe humidity level above which the corrosion rateaccelerates rapidly. For example, clean iron in pure,clean air does not corrode until the RH is approximately90%. However, when a small amount (0.01%) of sulfurdioxide is present in the air, the critical relative humidity islowered to 65%, and the steel begins to corrode at thismuch lower RH level. More severe corrosion, which canoccur if the surface is exposed to a 3% sodium chloridesolution, lowers the critical relative humidity to 55%.Appendix A provides more information about the relativehumidities at which different contaminants can bring
moisture to the surface. This absorbed moisturedissolves in the contaminant, forming an electrolyte thataccelerates rust bloom. Properly cleaned steel corrodesat a much slower rate in high-humidity environmentsprovided surface contaminants are not present.
It is generally recognized that a larger dew pointdifferential is needed for equivalent rust bloom protectionat conditions of greater surface and atmosphericcontamination. Steel surfaces can be tested for thepresence of soluble salts and other nonvisible conta-minants so that these factors can be taken intoconsideration. Testing methods are utilized either in theinitial stage of projects, when specified, or at any timeduring the project if detrimental levels of soluble surfacecontamination are suspected.
The traditional cycle of blasting, cleaning, and primingcan mask contaminants by not allowing enough time forambient humidity to react and begin to show rust bloom,and thus show evidence of the contaminants.
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Example: A surface is blasted on a day when thedew point is marginally acceptable. Thesteel “turns” (i.e., rust bloom appears).Overnight, a weather front brings drierweather. The next day, the same area isreblasted and does not show evidence ofrust bloom.
The reason that the area does not show rust bloom onthe second day of blasting is that the dew pointdifferential has decreased, and is not great enough forflash rusting to occur. However, this does not indicatethat the surface is not contaminated. Contaminants canbe masked by the lowered dew point differential.
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Dehumidification, if not used correctly, can maskcontaminants. Exposure to dehumidification for a shortperiod of time might not allow the corrosion process toappear as rust bloom. If dehumidification is properlyemployed and the blast is held for an extended period oftime, during which rust bloom occurs, the surface mightbe tested for contaminants. A brief list of surface RHvalues that initiate accelerated rust bloom for differentcontaminants is found in Appendix A. Table 1 illustratessurface RH according to surface temperature and dewpoint differential.
TABLE 1: Dew Point Differential and Surface RH
Difference Between DewPoint and SurfaceTemperature
Approximate RH at Surface82 to 86% 74 to 80% 54 to 62% 43 to 51% 34 to 42% 27 to 35% 19 to 26%
Through field experience in controlling conditions inenclosed spaces, it is known that lowering the dew pointtemperature well below the surface temperature iseffective in slowing the corrosion rate. The typical mini-mum dew point differential specified for holding a blastover an extended period of time is 17 to 25°F (9 to 14°C)with a relative humidity not to exceed 40 to 55%. It hasbeen found that rust bloom accelerates if the surface RHexceeds 40 to 55% RH (see Table 1).
Because it is often not practical to obtain a surface RHreading in the field by instrumentation alone, the followingdew point differential method is often used:
To compute the surface RH, the dew point of thesurrounding air is subtracted from the surface temper-ature. This can then be plotted on a psychrometric chart(Figure 3) or a dew point chart (Table 1). Theintersection of the surface temperature value and the dewpoint value on the chart indicates the RH of the air at thesurface. Because this thin layer of air at the surfaceassumes the temperature of the surface, this value canbe assumed to be equivalent to the RH at the surface.
Figure 3 can be used to determine the surface relativehumidity. The dry bulb temperature represents the actualsurface temperature. The vertical line can be followed upto where it intersects the horizontal line representing theactual dew point. The surface relative humidity at thatdew point can then be read. For example, a wet bulbtemperature of 75°F with a dry bulb temperature of 100°Findicates an absolute RH of 30 to 40%.
The dew point differential method allows the specifier tomonitor the inside surface RH conditions in an enclosedspace by measuring the differential between the surfacetemperature and the air dew point. Table 1 shows therelative surface humidities that correspond to typical dewpoint-to-surface temperature differentials. For example, ifa specifier determines that it is beneficial for a particularsurface to not contact air with an RH greater than 40 to55% RH, he/she can specify that a 17 to 25°F (10 to14°C) dew point differential be maintained whenever thesteel is not coated. This computation is approximatelythe same at all dry-bulb temperatures.
Uses of Dehumidification and Temperature-Control Equipment
Improved productivity and scheduling can be achieved byeliminating the daily blast/clean/prime cycle. Properlydesigned, installed, and operated DH equipment canreduce the risk of coating failure with the same, orimproved, productivity.
When using dehumidification and temperature control,installation of multiple-coat linings can be divided into twodistinct phases:
• Phase One: From surface preparation throughcomplete application of the first coat of coating/liningmaterial
• Phase Two: The application of additionalcoating/lining material in a multiple-coat system,including drying and curing
Each of these phases involves separate considerationsthat influence the selection and operation of climate-control equipment.
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FIGURE 3: Psychrometric Chart
Phase One: From surface preparation through com-plete application of the first coat ofcoating/lining material
During abrasive blasting, large amounts of dust canaccumulate in an enclosure. At the same time, the newlycleaned, unprotected steel is especially vulnerable tocorrosion or rust bloom. In the past, it was common toblast the surface, clean the area, and prime the blastedsurface in a single shift. This was done to prevent therust bloom of freshly blasted steel between shifts. Adisadvantage of the blast/clean/prime cycle is that it cancause overblast damage to freshly primed surfaces.Dehumidifiers have been used to protect the surface fromthis type of rust bloom for an extended period of time.This can allow for more time and attention to inspect andclean the surface prior to coating.
When blasting operations cease and the enclosure is leftuntil the next shift, steel is exposed to weather changes ata time when personnel are not on site to protect thesurface. Dehumidification equipment is used to maintaina dew point differential great enough to avoid rust bloomduring unexpected weather changes. In circumstancesother than those involving insulated tanks, it is generallyconsidered more cost-effective to dehumidify the air thanto heat the surface to maintain a given dew pointdifferential. Dew point differentials greater than thosespecified on coating data sheets are often specified inorder to provide a greater safety margin for weatherchanges when blast-cleaned steel is at risk.
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Freshly blasted surfaces are typically allocated thegreatest practical dew point differential (lowest possiblesurface RH). Also, large volumes of fresh air are oftenintroduced into the enclosed space to replace the airexhausted to control dust. If dust collectors are beingused, a percentage of the air can be recirculated.Because of the large airflow and dew point differentialrequirements, the dehumidification equipment usedduring this stage of the project is often of a larger volumethan that used later. This is maintained until the first coatcan be completely applied.
During blasting and holding periods, the temperature ofthe air inside the enclosure is not normally a concern,except as it affects worker comfort, productivity, andsafety. During warm seasons, the contractor is careful toensure that the air temperature inside the enclosure doesnot exceed the limits established by regulatory agenciesfor personnel safety. Changing the air temperature doesnot reduce the dew point or absolute humidity of the air.Therefore, heating the air does not prevent condensationor high RH at the surface unless the skin temperature isalso appreciably raised. This is illustrated by plotting onthe psychrometric chart (Figure 3).
Phase Two: The application of additional coating/lining material in a multiple-coat system,including drying and curing
Many specifiers elect to retain dehumidification, cooling,and heating equipment on site during Phase Two. Eventhough the steel is now fully protected from flash rusting,
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the curing cycles of most coatings are affected in someway by the humidity of the surrounding air. Although inthe case of a few coatings (such as moisture-cureurethanes and solvent-based inorganic zinc) overdryingcan retard the cure, DH is usually beneficial. The coatingsystem supplier can advise the specifier on the optimalhumidity for coating cure.
Just as air is exhausted during blasting operations forparticulate control, air is also exhausted during coatingoperations to prevent the build-up of hazardous vapors.Consequently, the incoming fresh air is dehumidified byadequately sized DH equipment to remove the moisturefrom that air or to maintain the proper balance of DH andexhaust air.
Most coatings have specified application temperatureranges. These specifications usually refer to the temper-ature of the substrate rather than the temperature of theair. However, because the air temperature can affect thesubstrate, some specifications include a maximum and/orminimum air temperature. Surface temperature is mostoften a problem during colder weather. Hot air movestoward cold air. The greater the temperature differential,the faster the hot air moves toward the cold. In addition,steel loses heat depending on how fast the air is movingacross it. Therefore, cold and windy weather can presentdifficulties.
In most cases, the primary concern during Phase Twodehumidification operations is maintaining the surfacetemperature high enough, although a maximum temper-ature is sometimes also specified. Surfaces are usuallyheated by supplying hot air to the enclosed space.
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Because steel alone has nearly no resistance to heatloss, the air is supplied much hotter than the specifiedsurface temperature to offset the heat loss through thewall of the enclosure. The inside “wall temperature” of alarge steel tank is within 1°F (0.6°C) of the outside steeltemperature. In very cold or very hot weather, exteriorinsulation or a temporary enclosure can be used toreduce the size and cost of the temperature-controlequipment.
The surface temperature provides part of the energyneeded to cure the coating. Too much heat from thesurface can interfere with proper cure, and too little heatcan cure the surface of the coating before the solventsevaporate, trapping those solvents and slowing overallcuring time. Usually, surface temperatures of 50 to100°F (10 to 38°C) are acceptable for coating cure, andfaster curing rates occur at elevated temperatures. Thespecific coating used governs the surface temperaturerange for proper curing.
With the exception of coatings that require a “forced cure”(high-temperature cure), the minimum air temperaturesused are generally modest (usually 50 to 70°F [10 to21°C]). However, extremes of air temperature, likeextremes of surface temperature, can be detrimentalduring cure. If the inside air is too cool, curing can takeso long that the project is delayed or the coated surface isput back into service without complete coating cure. Airtemperatures of 65 to 100°F (18 to 38°C) are generallyconsidered adequate for coating cure.
Inspection Instrumentation
Inspectors use a variety of instruments to measureenvironmental conditions and the performance of temper-ature- and humidity-control equipment. Each type ofequipment has its proper use, and each project canbenefit from different levels of accuracy from theseinstruments.
Humidity Readings
Most instruments used to measure humidity in the fieldare not designed for high levels of accuracy and repeat-ability. Accuracy greater than ±2% relative humidity (RH)is not generally available outside the laboratory. Further,instruments can drift from their specification accuracybecause of sensor-surface contamination. These pos-sible problems are often overlooked by those who specifyhumidity levels for a project, and dew point differentialsare often not specified in light of the uncertainty of thesemeasurements. For example, if a low-cost sensor has atolerance of ±5% and the apparent reading it displays is95°F (35°C), 50% RH, the true value can be as high as55% or as low as 45%. That range allows a difference in
moisture content of 114 grains/lb (16 g/kg) versus 86grains/lb (12 g/kg) and a differential between dew pointsof 71°F (22°C) and of 63°F (17°C).
In critical situations, not only the instrument to be used,but also the frequency of its calibration can be specified.In less critical situations, the project manager can allowany instrument to be used, as long as the type andaccuracy of the instrument is documented along with itsreadings.
A common problem with relative humidity readings is afailure to obtain a simultaneous temperature reading.Relative humidity by itself is not a useful value for moni-toring coating projects. Unless both temperature (dry-bulb temperature) and relative humidity (wet-bulb temper-ature) are recorded, the dew point cannot be calculated.
Psychrometers or Wet- and Dry-Bulb Thermometers
The key to gaining accurate wet- and dry-bulb temper-atures is to achieve rapid evaporation of water from thewet-bulb thermometer. If evaporation is impeded, the
wet-bulb instrument records a humidity reading that ishigher than the true humidity. Only an absolutely cleanwick provides accurate wet-bulb thermometer readings.Even the oil from a human fingerprint can prevent properevaporation from the wick. Clean, distilled water is usedto wet the wick in order to prevent contamination fromdirt, oil, or other sources. Also, the reading can be falselyhigh if the air does not flow smoothly around the wet wickat a velocity of at least 500 ft (150 m) per minute.
For these reasons, readings from hand-held, wet- anddry-bulb devices like the sling psychrometer have areputation of being accurate only within +5%. Aspirated(fan-powered) psychrometers are less subject to errorsdue to inadequate airflow, but they remain subject toerrors due to inadequate wetting and wick contamination.Therefore, several wet-bulb readings are usually taken.When such instruments are used, any measurement erroris generally above, rather than below, the true wet-bulbreading. The lowest of these is usually closest to the truevalue.
Electronic Hygrometers
The accuracy, repeatability, and response time ofelectronic instruments varies widely. Electronic measure-ments are not inherently more accurate than psychro-metric measurements. Even readings taken with a verycostly instrument can be less accurate than good wet-and dry-bulb temperature readings. A more costly devicecan initially be very accurate, but can drift over time in therugged field environment of blasting and coatingoperations. A good technician is aware of the accuracylimitations of the device used, and records the RHaccuracy of the instrument along with the readings taken.
The sensor element of an electronic hygrometer canchange behavior if it becomes contaminated with dust oradsorbed vapors. Therefore, the sensor is typically pro-tected with some form of air filter. However, such filtersslow the response time of the sensor. On-site measure-ments are typically taken at a variety of temperature andhumidity levels. Most technicians are aware thatalthough an electronic instrument can display a valueinstantly, the sensor might not have yet reached equil-ibrium with the air being measured. For example, if ameasurement is taken in the cold, saturated air leaving acooling coil, the sensor can take five minutes to an hourto reach equilibrium for accurate measurements in a verydry environment. In general, moving from a dry environ-ment to a more humid one provides faster response timesthan moving from a humid environment to a drier one.
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Air Temperature Readings
Dry-bulb air temperature readings are much less subjectto error than either humidity or surface temperaturemeasurements. Accuracy of ±1°F (±0.6°C) is commoneven for a low-cost thermometer. However, one commonsource of error is the influence of radiational heatsources. A reading taken in the shade can be severaldegrees lower than a reading taken 1.0 in. (2.5 cm) awayin full sunlight. The sensor bulb of the thermometer orelectronic temperature sensor is protected from sunlightand other sources of radiational heat (such as theglowing elements of electric heaters) for accuratemeasurements.
Surface Temperature Readings
Obtaining accurate surface temperature measurementspresents some challenges in the coating environment.The surfaces in question are likely to be rough, and theycan be wet with uncured coatings. Each of the commonsurface thermometers commonly used in the field hasadvantages and limitations.
Bimetallic Magnetic Surface-Contact Thermometers
These devices are used widely in the industry, and areinexpensive. However, they are only used on a ferrousmetal substrate with a smooth surface for reasonableaccuracy. Also, they cannot be used to monitor thetemperature of surfaces as they are being coated,because the thermometer must remain in contact with thesubstrate while the reading is being taken.
Electronic Surface-Contact Thermometers
Another low-cost alternative is the electronic surface-contact thermometer. These are generally accurate to+1°F (+0.6°C), and have a faster response time thanbimetallic thermometers. The sensor maintains fullcontact with the surface during a reading. Electronicdevices intended for measurements in air do not provideuseful results if used to read surface temperatures.
Infrared Surface Temperature Measurement Devices
Infrared devices have the advantage of not having toremain in contact with the surface while a temperaturemeasurement is being taken. Also, these more costlydevices can take highly accurate temperature readings atlong distances away from the surface. In general,however, the sensor head is placed as close as possibleto the surface.
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References
1. NFPA 33 (latest revision), “Standard for SprayApplication Using Flammable or CombustibleMaterials” (Quincy, MA: NFPA).
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2. ASHRAE Handbook of Fundamentals SI Edition,Chapter 19, Sorbents and Desiccants (Atlanta, GA:ASHRAE, 1988).
Chemical reactions are very temperature-sensitive andare limited to creating a thin oxide coating, usually
invisible, on a freshly prepared metal surface at roomtemperatures. This oxide layer is formed almostimmediately and the metal shows little sign of furtherdegradation in a dry atmosphere. This is called a Stage 1(dry) surface. At low RH, a molecular layer of watermolecules adheres to the surface.
Wet Corrosion
As the relative humidity increases, the moisture filmincreases exponentially in thickness (Figure A2). Thethicker moisture film behaves progressively more like anelectrolyte and creates corrosion cells. These cells can
NACE 6A192/SSPC-TR 3
be created by a wide variety of factors such as differentialaeration, presence of different atoms in the crystal lattice,pores in the protective oxide layer, or even the presenceof grain boundaries. The corrosion rate increases as the
11
moisture film thickens. This is called a Stage 2 (damp)surface.
FIGURE A2: Relationship Between Adsorbed Layers of Water Moleculeson a Clean, Finely Polished Iron Surface
Chemical Factors
If trace chemical contaminants are present in the air,these can form a fine film of surface salt that can behygroscopic. The presence of any soluble salt enhancescondensation because the vapor pressure above a saltsolution is lower than that over pure water.
The hygroscopic characteristics of salts can be quantifiedin terms of the equilibrium relative humidity in a closed airspace above the saturated solution. This relativehumidity represents the condition in which the saltsdissolve in moisture absorbed from the air. Lowerrelative humidity values are associated with the morehygroscopic salts. The relative humidity range overwhich condensation occurs on different salts is illustratedin Figure A3.
0
20
40
60
80
100
% RelativeHumidity
Zinc Chloride
Calcium Chloride
Zinc Nitrate
SodiumNitrate
AmmoniumSulfate
ZincSulfate
PotassiumNitrate
PotassiumSulfate
Figure A3: Relative Humidities at Which Salts Begin to Dissolve
