3.2i/°i: 
 
 TECHNICAL     NOTE     number 
 
 156 
 
 UNITED    STATES     DEPARTMENT    OF     AGRICULTURE 
 
 FOREST     SERVICE 
 
 MADISON     5.     WISCONSIN 
 
 FOREST     PRODUCTS     LABORATORY    i/N'V   ( 
 
 REVISED^ " ^^*xLvJL252 
 
 LE  OF  RELATIVE  HUMIDITY  AND  E&U^LIB-BITJMjrfOlSTURE 
 CONTENT  FOR  DRY-  AND  WET-BULB ■  H YGROM ETE8C 
 
 Dry-  and  wet-bulb  hygrometers  are  extensively  used  around  sawmills 
 and  woodworking  factories  to  measure  seasoning  and  storage  condi- 
 tions. The  following  table  is  for  use  in  determining  relative -humidity 
 and  equilibrium-moisture-content  (EMC)  values  from  dry-  and  wet-bulb 
 thermometer  readings. 
 
 The  hygrometer  should  be  placed  at  the  location  where  information  on 
 relative  humidity  or  EMC  is  desired.  It  should  not  be  subjected  to  direct 
 radiation  from  heating  coils  or  to  blasts  of  hot  air  from  space  heaters. 
 However,  some  air  should  circulate  around  the  thermometer  bulbs.  The 
 hygrometer  should  remain  in  place  for  several  minutes  before  readings 
 are  taken  so  as  to  make  certain  that  true  temperatures  are  obtained. 
 The  wick  on  the  wet-bulb  thermometer  should  be  clean  and  the  water 
 pure. 
 
 Read  the  two  temperatures  and  subtract  the  wet-bulb  reading  from  that 
 of  the  dry  bulb.  The  difference  is  the  wet-bulb  depression.  On  the 
 table,  locate  the  vertical  column  of  figures  headed  by  the  appropriate 
 wet-bulb  depression  and  the  horizontal  row  of  figures  beginning  at  the 
 extreme  left  with  the  observed  dry-bulb  reading.  The  figures  marking 
 the  intersection  of  these  rows  give  the  relative  humidity  and  EMC. 
 
 EXAMPLE:  Wet-bulb  temperature  132°  F.  ,  dry-bulb  temperature  140° 
 F.  ;  the  wet-bulb  depression  is  8°  F.  The  figures  marking  the  inter- 
 section of  the  vertical  column  for  8°  F.  wet-bulb  depression  with  the 
 horizontal  row  beginning  with  140°  F.  at  the  extreme  left  are  79  and 
 13.2.  The  roman-type  79  is  the  relative  humidity  in  percent  and  the 
 italic-type   13.  2  is  the  EMC  in  percent. 
 
 iu    90608    F 
 
 Aqr  lc 
 
Relative  Humidity 
 
 1  and  Equilibrium  Moisture  Content  *  Table  for  Use  With  Dry 
 
 ■Bulb 
 
 Temperatures  and  Wet-Bulb  Depr 
 
 Temperature  dry 
 
 Wet  bulb-depression  (•  F.) 
 
 buib(  r., 
 
 1 
 
 2 
 
 2 
 
 < 
 
 > 
 
 6 
 
 7 
 
 s 
 
 • 
 
 10 
 
 " 
 
 12 
 
 ,3 
 
 .. 
 
 16 
 
 ,r, 
 
 ■7 
 
 18 
 
 10 
 
 20 
 
 2, 
 
 22 
 
 23 
 
 21 
 
 26 
 
 JL 
 
 L 
 
 2, 
 
 ,, 
 
 so 
 
 32 
 
 34 
 
 .,„ 
 
 38 
 
 4,1 
 
 ■•■ 
 
 50 
 
 20 
 
 r 
 ,;; 
 
 81 
 9< 
 
 '.4 
 
 82 
 
 w 
 
 It  1 
 
 'J 
 0 
 0 
 
 18.0 
 
 2 
 
 60 
 
 u, 
 
 81 
 
 S 
 
 8! 
 
 30 
 82 
 
 "sr 
 
 IS  1 
 80 
 
 13  . 
 85 
 
 86 
 
 8! 
 10.7 
 
 ei 
 
 n  i 
 
 SI 
 
 81 
 8 
 
 29 
 37 
 
 10.  i 
 71 
 
 31 
 62 
 
 a 
 ro. 
 
 26 
 
 10.1 
 66 
 
 39 
 58 
 
 62 
 
 ro.s 
 
 10.. 
 
 6 
 
 26 
 
 ; 
 
 38 
 
 6: 
 
 61 
 
 36 
 SO 
 
 6 
 
 32 
 
 61 
 
 13 
 39 
 
 68 
 6 
 
 3: 
 
 5 
 
 20 
 
 & 
 6' 
 
 30     28 
 38     86 
 
 62     60 
 
 5',       f. 
 
 67     H 
 
 1  i:  9 
 
 to     E 
 
 2i.     a 
 
 36     31 
 
 .in     :». 
 
 1 s  g  i 
 
 in 
 
 M     63 
 
 57       51 
 
 30 
 
 .'« 
 38 
 
 48 
 61 
 
 ;; 
 
 1      97 
 
 ;;; 
 
 37 
 
 4- 
 60 
 
 13 
 
 26 
 
 29 
 
 5   • 
 
 35 
 
 S.I 
 
 7 
 
 4. 
 
 TO 
 26 
 
 35 
 
 32 
 
 K 
 
 7 
 
 J! 
 
 81 
 
 2 
 
 31 
 SI 
 39 
 
 0.7 
 
 H 
 
 si 
 
 31 
 
 '28 
 
 2 
 
 5 
 
 a 
 
 10 
 
 160 
 
 H 
 
 170. 
 
 190..- 
 
 200 
 
 210 
 
 2-1 
 
 »  Equilibrium  mo 
 
 ltal 
 
UNIVERSITY  OF  FLORIDA 
 
 3  1262  09216  7088