30 



25 



" 20 



o 15 

 o 



o 



10 



DESORPTION AT75°F + 5' 



ADSORPTION AT 75°F + 5° 



— 



1 



1 



10 20 30 40 50 60 



RELATIVE HUMIDITY (PERCENT) 



70 



80 



90 



100 



Figure 5. — Equilibrium moisture content data for adsorption and desorption with least 

 squares fit curves from equations of the form, EMC = aH^ + c • EXP [(H - 100) /d] for 

 ponderosa pine needles. 



for each relative humidity and the difference used for a least squares fit to an ex- 

 ponential function, c • EXP [ (H - 100)/d], from 50 percent relative humidity up to the 

 maximum relative humidity. The overlap from 50 to 60 percent relative humidity pro- 

 vides a smooth transition and fit to the data. The coefficients, a, b, c, d, along 

 with ri and r2, correlation coefficients of moisture content to relative humidity for 

 each portion of the equation, were determined to be: 



Adsorption 

 Desorption 



0.891 

 1.651 



0.612 

 .493 



0.9998 

 .9976 



17.54 

 19.35 



8.91 

 10.88 



0.9818 

 .9893 



The curve in figure 5 shows the goodness of fit and shows the separation of the adsorp- 

 tion and desorption curves to be about 1.5 percent moisture content. Above 65 percent 

 relative humidity, the separation continues to increase, reaching about 3.2 percent 

 moisture content at 90 percent relative humidity. 



Although the effect of temperature on EMC was not part of the tests reported here, 

 the EMC curves were compared with earlier adsorption data (Anderson 1964; Rothermel 

 and Anderson 1966; Anderson 1969). In the earlier tests, ponderosa pine needles were 

 first ovendried. Then the needles were conditioned over saturated-salt solutions for 

 different humidities at 90° F (32° C) until the needle moisture content had stabilized. 

 These data were also fit to equation 2 and compared to the 75° F (24° C) data (fig. 6). 

 A difference due to temperature appears from 20 to 80 percent relative humidity, but 

 beyond these conditions the EMC values are essentially the same. 



9 



