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30 MARCH 2016 COATINGSPROMAG.COM during dry abrasive blasting and coating operations is much more appropriate. Te 17–25° F diferential between the dew point temperature and the surface temperature will always ensure that the surface RH stays in the 40–55 percent safe range. See Table 1 as summarized in NACE 6A192/SSPC-TR-3. A psychrometric chart can also be used to compare the results between the two ranges. See Chart 1, which takes four representative outside conditions: 50° F (10° C), 60° F (15.6° C), 70° F (21.1° C), and 80° F (26.7° C). It is generally assumed that the steel surface tempera- ture is going to be at or near the outside ambient temperature. From this graphic, it is evident that a 5–7° F dew point spread will not result in an 80 percent maximum RH. It is also evident that a minimum dew point spread of 17–25° F will result in a maximum surface RH of 55 percent. Te intent is to signifcantly slow the corrosion rate to prevent fash rusting issues. It is well known that clean carbon steel corrodes much slower when the relative humidity is below 50 percent. Tis specifcation ensures the maximum protection for quick changes at the jobsite in terms of fast-moving weather changes or condi- tions, such as rain or snow. A specifer can expect several results from follow- ing this dew point spread: 1. It w ill hold the conditions for an indefinite per iod of time. T he range prov ides the needed abilit y to blast out the entire tank w ithout a fear of losing it. In other words, it w ill help " hold the blast." Utilizing the17-degree range w ill ensure that you w ill a lways be well below even the tightest R H requirements from most coatings manufacturers. 2. It will provide greater predictability of the work schedule. A larger dew point spread will allow the contrac- tor the needed time to complete ongoing work without fear of losing any freshly blasted steel. This helps keep the contractor on schedule and moving forward. 3. It will give protection from fast-mov- ing weather fronts. Weather can change very quickly. Smaller dew point ranges might leave a project in jeopardy by not allowing the needed time to coat up. A larger dew point range will often provide the contrac- tor with a safety net that will provide the time to make the needed adjust- ments to help protect the blast and coat the substrate. Specifying the Differential Certainly there are projects that can meet the five-degree str ingent target Chart 1. Examples of Potential Surface RH for Four Representative Outside Temperatures Specifying Success