Measuring Outdoor Climatic Conditions

Referenced products:

DeFelsko manufactures a hand-held, non-destructive instrument that is ideal for measuring climatic conditions before, during and after the application of coatings or linings in external environments.

Product photo of the PosiTector DPM Dew Point Meter

Measurement Challenges

The primary challenge is to establish a simple means of verifying surface and climatic conditions that are acceptable for the application of protective coatings and linings.  

The ease of taking periodic measurements is also important as conditions are always changing and different microclimates can exist in close proximity to one another.  

The ability to log results may also be important as evidence of the observation of these conditions before, during and after the job.

Climatic Observation Solution

The multi-purpose PosiTector DPM Dew Point Meter is ideal for continuous measuring of air temperature, surface temperature, and percent relative humidity.  By placing the instrument in the vicinity of the structure to be coated or lined, the Dew Point Meter has the built-in software capability to calculate the dew point temperature and the difference between the surface and dew point temperatures. Figure 1 illustrates a sample LCD display provided by the Dew Point Meter.  It shows ambient air temperature (Ta), surface temperature (Ts), relative humidity (RH), dew point temperature (Td), and surface temperature minus dew point temperature, which is a critical indicator of the probability of condensation forming.  If the PosiTector DPM senses a smaller difference (delta value), three things happen: the contrast of the display reverses, the red LED emits, and the meter’s alarm beeps.  See Alarm Mode demonstration below.

Product photo of the PosiTector DPM with Alarm mode screen
Figure 1. PosiTector DPM Alarm Mode

At any time, all five of the climatic conditions being observed and calculated, plus the date and time, can also be stored in memory with the simple press of a button. The Dew Point Meter also has a unique Auto Logging feature which automatically records datasets containing all 5 climatic conditions at a user selected time interval. This is useful for maintaining a complete record of environmental conditions and trends leading up to, during, and after application of the coating.                     

The design of the Dew Point Meter protects it from microclimate changes caused by factors such as the operators’ hand or the effects of direct sunlight, while still being sensitive enough to identify small changes in temperature in shaded areas, crevices and exposed areas.

Alternative Solution

Traditionally, relative humidity is calculated using the wet and dry bulb temperature measurements taken from whirling or electronic hygrometers (also referred to as sling psychrometers).  In addition for the need to continuously whirl (rotate) these instruments at a steady pace for 90 seconds, the temperature readings must be immediately taken since they will begin to change as soon as the whirling stops.  Calculations for relative humidity and dew point temperature must then be estimated using psychometric tables, graphs and/or slide rules.  Finally the results need to be compared to yet another separate piece of equipment, the surface temperature thermometer.  For periodic observation this process must be repeated throughout the coating and lining application process.  If continuous measuring of air temperature and relative humidity is desired then additional recording instruments are required.

The multi-functional, single-hand operated PosiTector DOM Dew Point Meter is an instrument that is simple to use as it makes all of its calculations electronically using feedback from its sensitive and accurate sensors, high quality electronics and software. The result is a single instrument with the ability to take immediate and continuous readings that are far less subject to operator interpretation and measurement errors, as well as calculation and rounding errors associated with the use of look up tables. The advantage of the Dew Point Meter is well demonstrated by the pictured application, which involves the coating of a large water tower. Consider the requirement to measure and record climatic conditions while painting, then compare the two methods and required equipment when working on a scaffold 50 meters (160 feet) off the ground.

Free Consultation

For current pricing or to order this instrument, please see our Sales page. If you require additional technical information or have questions relating to your particular application, we encourage you to take advantage of our years of experience in recommending the best gage for your application.

Background on Climatic Measuring

What is the Application?

Surface preparation and coating application should be performed under optimum environmental conditions to help prevent potential coating failure. A major factor affecting the long term performance of coatings on steel structures is the climatic conditions during pre-treatment and coating application. The PosiTector DPM Dew Point Meter is an electronic device that enables painting contractors, inspectors and owners to measure and record all applicable environmental conditions. 

Why Measure?

The primary reason for measuring climatic conditions is to avoid rework and the premature failure of protective coatings.  Recommendations and requirements are covered under various internationally recognized standards.  The following is a brief summary of recommendations and requirements for climatic conditions as referenced from international standards…

Temperatures—It is important to measure both surface and air temperatures as they are often different. Application at incorrect temperatures can cause defects including: blistering, pinholing, cratering, dry spray & mud cracking.  Factors such as direct sun, shade, shape and mass of the object to be coated can create unexpected thermal behavior at the surface. At night, metal structures radiate heat and are cooled below air temperature, while during the day, they absorb heat and are usually warmer than the air temperature. By measuring surface temperature it is possible to avoid potential application problems if air or surface temperatures become too hot or too cold for satisfactory film formation. Surface and air temperatures directly affect the rate of cure of the applied coating or lining system and are major factors in the formation of surface moisture or dew. ASTM D3276—"Standard Guide for Painting Inspectors (Metal Substrates)" states that the minimum air/surface temperature is usually 40ºF (5ºC). But it may be as low as 0ºF or –18ºC for “cold-curing” one or two component systems. Painting should not be undertaken when the temperature is dropping and within 5ºF (3ºC) of the lower limit.

Relative HumidityRelative Humidity is the amount of moisture in the air expressed as a percent of overall volume. High relative humidity can retard the rate of solvent evaporation. For this reason, the maximum relative humidity at which coatings or linings can be applied and cured is generally set at 85%.

Dew Point TemperatureDew Point is the temperature at which moisture will begin to form on a surface.  The dew point temperature is a function of air temperature and the relative humidity. (see ISO 8502-4 Annex A). It is the temperature to which a volume of air must be cooled in order to reach saturation.  A number of major standards organizations call for careful dew point control.  Moisture on a freshly blasted steel substrate will cause the steel to rust. If a thin, invisible film of moisture is trapped between the applied coating and the substrate, the system will likely fail prematurely. To ensure that dew point problems do not exist, ISO 8502-4 and SSPC guidelines state that the surface temperature must be a minimum of 5ºF or 3ºC above the dew point during the 3 critical phases of coating…preparation, application, and cure. 

Where is the market? 

Contractors involved in the surface preparation, protective coating or lining of large structures are ideal candidates for the Dew Point Meter. Coating and lining contractors are an incredibly large market in an industry facing increasing costs for new, replacement and maintenance costs on existing structures. These cost pressures makes equipment such as the Dew Point Meter, capable of helping to ensure successful coating and lining application, extremely valuable in reducing costs associated with rework and premature structural damage due to inadequately applied protective coatings.

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