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the dry bulb temperature and the wet bulb temperature can be used to determine the relative humidity. The wet bulb depression is merely the difference between the dry bulb and the wet bulb temperatures. To show what this looks like, I’ve included part of the table from the Dry Kiln Operator’s Manual with the entry points for the previous example marked as before (Table 3). Much more of this table is included as Appendix B. Table 3. Relative humidity and equilibrium moisture content for some specified dry bulb temperatures and wet- bulb depressions. For each dry bulb temperature, the bold-font data in the first row are relative humidity values and the italicized numbers in the second row are the EMC values. (Source: Dry Kiln Operator’s Manual, Table 1-6) DB °F 60 65 70 75 WB Depression (°F) 1 234 94 898378 - 19.9 17.4 15.6 95 908480 - 20.3 17.8 16.1 95 908681 - 20.6 18.2 16.5 95 918682 - 20.9 18.5 16.8 (Relative humidity (%) in bold font, EMC (%) in italics) 5 6 7 8 9 101112131415161718 73 68 63 58 53 484339343026211713 13.9 12.7 11.6 10.7 9.9 9.1 8.3 7.6 6.9 6.3 5.6 4.9 4.1 3.2 75 70 66 61 56 524844393632272420 9.7 8.9 14.4 13.3 12.1 11.2 10.4 RH 8.3 7.7 7.1 6.5 5.8 5.2 4.5 77 72 68 64 59 55514844403633EM29C25 14.9 13.7 12.5 11.6 10.9 10.1 9.4 8.8 8.3 7.7 7.2 6.6 6 5.5 78 74 70 66 62 585451474441373431 15.2 14 12.9 12 11.2 10.5 9.8 9.3 8.7 8.2 7.7 7.2 6.7 6.2 192021 951 2.3 1.3 0.2 16138 3.8 3 2.3 221915 4.9 4.3 3.7 282421 5.6 5.1 4.7 22 - - 6 1.4 12 2.9 18 4.1 23 - - 2 0.4 9 2.3 15 3.5 Start at the line for 70°F and look right until you find 40% in the RH data in the same row. Notice that the same EMC as noted earlier (7.7%) is listed in the line directly below 40% RH. If you look at the column label above this pair of values you will see that the difference between the dry bulb and wet bulb temperatures (i.e., the wet bulb depression) would be 14°F. For this example, when the DB is 70°F and the WB depression is 14°F the wet bulb reading will be (70-14)=56°F. There are a number of inexpensive digital relative humidity meters on the market that you can use to monitor the conditions in a shed or retail area. Digital meters are more accurate than the analog dial type. Wireless digital sensors are also available. If you want to verify a meter’s calibration for relative humidity, put it in a plastic baggie with a small container of table salt and water, thoroughly stirred, with enough of a surplus of salt that not all of it dissolves. Close the baggie and wait; after 24 hours the meter should read 75% RH. 43PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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