METHODS 



Equipment 



The EMC's of ponderosa pine needles were obtained over a range of relative humid- 

 ities (16 to 88 percent) at ambient air temperature of 75° F ±5° (24° C ±2°) by means 

 of two preconditioning cabinets and a final conditioning cabinet. One preconditioning 

 cabinet provided a high humidity (95 percent) environment at ambient air temperature 

 with open trays of water. The other preconditioning cabinet provided a hot-dry environ- 

 ment of 2 to 6 percent relative humidity by simply heating the ambient air to 120° F 

 (49° C) . The final conditioning cabinet was maintained at a fixed relative humidity 

 by use of saturated-salt solutions at the 75° F ±5° ambient temperature. Temperatures 

 within the cabinets were measured with mercury thermometers accurate to ±1° F and 

 relative humidity was determined by standard psychrometer procedures and by dewpoint 

 measuring equipment accurate to ±1° F. 



The salt solutions used and the measured relative humidities for each were: 



Salt : LiCl MgCl2 Mg(N03)2 NaCl KNO3 



Percent relative humidity: 13 33 53 71 91 



The moisture content of the needles was determined by withdrawing samples for 

 xylene reflux distillation (Buck and Hughes 1939). Initial frequent sampling showed 

 the moisture content to be within 1 percent moisture content of EMC after 1 week in most 

 cases. Two samples were taken at each measurement period. 



The response time tests were conducted at 80° F (27° C) , with step changes in humid- 

 ity from 90 to 20 percent and return. A conditioning cabinet was used to prepare the 

 needle moisture content prior to the response test. Then, a programable environmental 

 chamber provided a shift in relative humidity so a moisture response could be induced. 

 The moisture response was measured as a weight change with unbonded, temperature- 

 compensated strain gage transducers, load cells, and microscales. Sensitivity of the 

 weighing systems was adjustable so weight changes to 0.01 g could be resolved. Values 

 of air temperature, relative humidity, dewpoint, solar heat, and weight loss were re- 

 corded on charts for each moisture response test. The conditions in the environmental 

 chamber were controlled to ±2° F air temperature; ±2 percent at 20 percent relative 

 humidity, and ±4 percent at 90 percent relative humidity; and ±0.01 of a solar constant. 



Airspeed within the test volume of the environmental chamber was found to average 

 less than 0.50 mi/h and be rather turbulent. Variability above the center of 

 a litter bed was found to be from near to 0.60 mi/h. This may have affected 

 the response time of the fuel beds by providing more turbulent airflow than occurs in 

 natural sites. Air velocity within a needle litter bed with a bulk density of 0.94 

 Ib/ft^ (0.015 g/cc) averaged less than 0.160 ft/s (4.88 cm/s) , or 0.11 mi/h. 



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