DeFelsko PosiTector DPM v.3.1 Full Manual - Manual (Page 9)

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Why is Measurement Important? Why is Measurement Important?
For industrial painters, 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.
Temperature – 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. 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 5ºC (40ºF).
Relative Humidity - 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%. This can be set as an alarm in the
Advanced model.
Dew Point Temperature - 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 3ºC or 5ºF above the dew point during the 3 critical phases of
coating preparation, application, and cure.