36 



Hardwood Record — Veneer & Panel Section 



September 25, 1919 



How to Determine Relative Humidity 



Editor's Note 

 The accompanying article by L. V. Teesdale of the Forest Proiiucts Laboratory. Madison, Wis., explains methods 

 of determining the relative humidity in the dry kiln or in the free atmosphere by appropriate instruments. 



Relative humidity in the dry kiln or in the atmosphere may be 

 determined with a wet and dry bulb hygrometer. A hygrometer 

 is made by attaching two similar thermometers to a convenient 

 stand, the bulb of one being kept moist by means of a wick which 

 dips in a vessel of water. These instruments depend for their ac- 

 tion on the fact that the drier the air the more rapid will be the 

 evaporation from a wet body exposed to the air. Since evapora- 

 tion requires heat, it follows that the extent to which a wet body is 

 cooled by evaporation will depend upon the ability of the surround- 

 ing air to absorb moisture. If the air is saturated, no evaporation 

 takes place and the wet and dry bulb thermometer read alike. If 

 the air is not saturated, evaporation will take place and the cooling 

 effect of evaporation will cause the wet bulb to indicate a lower 

 temperature than the dry bulb. The greater the evaporation the 

 greater the difference between the two bulbs, hence the drier the 

 air fic lower the relative humidity. The dry bulb gives the 

 actua^ temperature regardless of the humidity. 



To determine humidity by means of a hygrometer, place the in- 

 strument in a fair circulation of air, or if circulation is feeble, fan 

 the instrument to induce circulation and evaporation. Read the 



temperature of the wet bulb and subtract it from the dry bulb 

 reading. On the relative humidity table find the temperature of 

 dry bulb in the left hand column, and follow across the table to the 

 column under ' ' difference between wet and dry bulb tempera- 

 tures, " as given at top of table. The figure given in the table at 

 this intersection is the relative humidity. 



Example. Dry bulb temperature, 140° F. 



Wet bulb temperature, 132° F. 



Difference, 8° F. 



Find the dry bulb temperature in the left hand column, and fol- 

 low across the table to the column under difference in dry and wet 

 bulb temperature. At 8° difference we find the figure 79, which is 

 the relative humidity. 



Cleanliness Necessary 

 Clean, soft water (preferably distilled water) should be used in 

 hygrometer wells. To fill wells, a bottle similiar to a throat ato- 

 mizer should be so arranged that a small stream of water may be 

 blown into the small ojjening of the well. The wicks should be silk, 

 and can be bought in quantity from the hygrometer manufacturer. 

 Wicks become filled with deposit from substances absorbed from 

 the air, which retard the evaporation, thus making readings in- 



EELATIVE EUMDITY TABLE { Continued on page 40) 



Difference Between Tet ar,a Ery Bull. Thenr.OEeterf - Ir Eegrees Fahrenieit. 



9 10 11 



12 IS 14 15 16 17 16 19 20 21 22 23 24 26 26 27 28 29 30 51 32 33 34 35 36 37 38 39 



40 



53 46 44 



69 66 51 

 64 61 67 

 67 65 61 



70 68 64 



71 69 65 



71 69 65 



72 69 66 

 72 70 66 



72 70 67 



73 70 67 



75 71 66 



74 71 66 

 74 71 68 



74 72 69 



76 72 69 



75 72 70 



75 73 70 



76 73 71 

 76 75 71 

 76 74 71 



76 74 71 



77 74 72 

 77 74 72 

 77 75 72 



77 76 73 



78 75 73 

 78 75 73 

 70 76 73 



78 76 74 



79 76 74 

 79 77 74 

 79 77 74 

 79 77 75 



79 77 75 



80 77 75 

 60 78 75 

 80 78 76 

 80 78 76 



80 7P 76 

 f)l 76 76 



81 78 76 

 81 79 77 

 01 79 77 

 81 79 77 



81 79 77 



82 79 77 

 82 60 78 

 82 00 70 

 82 80 78 



nz 80 79 



39 54 



47 4S 

 64 60 

 66 65 

 62 69 



62 59 



63 60 



63 60 



64 61 



64 62 

 66 62 



65 6S 



66 63 

 66 64 



66 64 



67 65 



67 65 



68 65 

 68 66 



68 66 



69 67 

 69 67 



69 67 



70 68 

 70 68 



70 60 



71 69 

 71 69 



71 69 



72 70 

 72 70 

 72 70 



72 71 

 78 71 



73 71 

 73 71 



73 72 



74 72 

 74 72 

 74 72 



74 73 



75 73 

 75 73 

 75 73 

 75 73 



75 74 



76 1i 

 76 74 

 76 74 



76 75 



77 76 



30 



40 '^1 

 47 44 

 62 49 

 67 54 



57 54 



58 55 

 60 56 



59 66 



60 67 



60 67 



61 58 



61 59 



62 69 



62 60 



63 60 



63 61 



64 61 

 64 61 



64 62 



65 62 



66 63 

 66 63 

 66 63 



66 64 

 £6 64 



67 65 

 67 55 



67 65 



68 66 



68 66 

 66 66 



69 67 

 69 67 

 69 67 



69 68 



70 6fi 

 70 68 

 70 68 



70 69 



71 69 

 71 69 



71 70 



72 70 

 72 70 

 72 70 

 72 70 



72 71 



73 71 

 73 71 

 7'- 72 



35 29 

 41 38 

 47 44 



61 48 



62 49 



52 50 



53 51 



54 61 

 66 62 



55 63 



56 63 



56 54 



57 64 

 57 65 



68 56 

 56 56 



69 67 

 69 67 

 60 56 



60 58 



61 69 



61 59 



62 60 

 62 60 



62 60 



63 61 

 63 61 



63 61 



64 62 



64 62 



65 63 



66 63 

 66 63 



65 64 



66 64 

 66 64 



66 65 



67 65 

 67 65 

 67 66 



67 66 



68 66 

 68 66 

 68 67 



68 67 



69 67 

 69 67 

 69 60 



69 68 



70 69 



35 32 

 41 39 



46 44 



47 45 



48 46 



48 46 



49 47 



50 48 

 61 49 



51 49 



52 60 

 55 51 



53 51 



54 62 



54 52 



55 53 



55 53 



56 54 



56 54 



57 55 



57 55 



58 56 

 58 56 



58 57 



59 57 



59 57 



60 58 

 60 58 



60 59 



61 59 

 61 59 



61 60 



62 60 

 62 60 



62 61 



63 61 



63 61 

 OS 62 



64 62 

 64 62 

 64 63 



64 63 



65 63 

 65 63 



65 64 



66 64 

 66 64 



66 65 



67 66 



29 

 36 34 



42 39 



43 40 

 4S 41 

 4^- 42 



45 43 



46 44 



47 44, 



47 45 



48 46 



49 45 

 49 47 



60 46 



51 46 



61 49 



62 49 



52 50 



53 50 

 53 51 



53 51 



54 52 



54 52 



55 53 



55 53 



56 54 



55 54 



57 55 

 57 55 



57 56 



58 56 



58 66 



56 57 

 69 67 



59 5rt 



59 50 



60 58 

 60 59 



60 59 



61 59 

 61 60 



61 60 



62 60 

 62 60 



62 51 

 52 61 



63 62 

 63 62 

 61 6? 



31 29 



37 35 



38 36 



39 37 



40 30 



41 39 



41 39 



42 40 



43 41 



44 42 

 44 42 

 46 43 

 46 44 



46 44 



47 45 



47 46 



48 46 



49 47 



49 47 



50 46 



50 48 



61 49 



51 49 



62 50 



62 50 



63 51 

 53 51 



53 62 



54 52 



54 53 



55 53 



56 53 

 65 54 



55 54 



56 64 

 55 55 



57 55 

 57 55 



57 56 



58 56 



68 56 

 50 57 



59 57 

 59 57 



69 6f 



59 56 



60 58 



61 6C 



26 25 



33 31 



34 32 



35 33 



36 34 



37 35 



37 36 



38 37 



39 37 



40 38 



41 39 



41 40 



42 40 



43 41 



43 42 



44 42 



44 43 



45 43 



46 44 



45 45 



47 45 



47 46 



48 46 



46 47 



49 47 

 49 46 



49 48 



50 46 

 5C 49 

 61 49 



61 60 



62 60 



62 61 



52 51 



63 51 



53 52 



63 52 



64 53 



54 53 



55 53 

 55 54 



55 54 



56 54 

 56 55 

 56 55 



68 57 



29 27 



30 28 



31 29 



32 30 

 83 31 



34 32 

 36 33 



35 34 



36 34 



37 35 



38 36 

 36 37 



39 37 



40 36 



40 59 



41 40 



42 40 



42 41 



43 41 



43 42 



44 42 



44 43 



45 44 



45 44 



46 45 



46 45 



47 45 



47 46 

 46 46 



48 47 



49 47 

 49 47 



49 48 



50 48 



50 49 



51 49 

 51 50 



51 60 



52 50 

 68 61 



52 51 



53 51 

 53 52 



53 52 



54 52 

 64 63 

 56 64 



27 



28 



29 26 



30 20 27 



31 29 28 



32 30 26 



33 31 29 



34 32 30 

 34 53 31 

 36 83 32 



36 34 33 



37 35 33 

 37 36 34 

 36 36 35 

 39 57 35 



39 SO 35 



40 30 37 



40 39 37 



41 39 38 



42 40 39 



42 41 39 

 45 41 40 



43 42 40 



43 42 41 



44 42 41 



44 4E 42 



45 43 42 

 45 44 43 



45 44 43 



46 45 44 



47 45 44 

 47 45 44 



47 46 45 

 40 47 45 



48 47 46 



49 47 46 

 49 48 46 



49 48 47 



50 48 47 

 50 49 48 



50 49 48 



51 49 48 

 51 50 49 

 51 50 49 

 53 52 51 



27 



28 



29 27 



30 28 



30 29 



31 29 



32 30 

 S3 31 

 85 32 



34 32 



35 33 



35 34 



36 35 

 S6 35 



37 36 



38 36 



58 37 



39 36 



59 38 



40 39 



40 39 



41 40 



41 40 



42 41 



42 41 



43 41 

 43 42 



43 42 



44 43 



45 43 

 45 43 

 45 4-4 



45 44 



46 45 



46 45 



47 45 

 47 45 



47 46 



48 46 



49 48 



27 

 27 

 26 27 



29 26 



30 28 



30 29 



31 30 

 31 31 

 52 31 



33 32 



34 33 



34 33 



35 £4 



36 35 



56 55 



57 55 



37 35 

 36 57 

 56 37 

 39 56 



39 36 



40 39 



40 39 



41 39 

 41 40 



41 40 



42 41 



42 41 

 45 42 



43 42 



43 42 



44 43 

 44 43 



44 45 



45 44 

 45 44 

 47 46 



27 



£8 



29 27 

 £9 28 



30 26 



31 29 



31 30 



32 30 



32 31 



33 32 



34 32 

 £4 33 



35 S3 

 55 5-' 



36 S4 



37 36 

 58 36 

 56 57 

 09 57 



39 58 

 E9 58 



40 £9 



40 £9 



41 59 



41 40 



42 40 

 42 41 



42 41 



43 41 

 45 42 

 45 4S 



27 



28 27 



28 27 



29 28 



30 £9 



30 29 



31 30 



31 30 



32 31 



33 32 

 3S 3£ 



34 33 



54 S3 



35 54 



35 34 



36 35 



55 £5 



37 35 

 £7 56 



37 36 

 £8 £7 



38 57 



39 38 



39 38 

 £9 50 



40 £9 



40 39 



41 £9 



42 41 



27 



28 27 



28 28 



29 28 



30 29 



30 29 



31 30 



31 SO 



32 31 



32 31' 



33 32 



33 3£ 

 54 55 



34 33 

 £5 34 



35 34 

 56 £5 



35 35 



36 35 



37 36 



37 36 

 56 87 



38 37 

 38 37 

 40 39 



