LIMITATIONS TO CONVERSION OF BOARD-FOOT LOG RULES 83 



or 5.55 per cent. As the increase in per cent of correction in the total scale becomes 

 less with increasing thickness of boards sawed, this method is more accurate than 

 that of computing the average dimensions of the products sawed. In the above 

 case the latter would have been lj inches, calling for a correction of 7.1 per cent 

 instead of 5.55 per cent. 



Correction for thin lumber based on superficial contents. In a similar way, 

 log rules for 1-inch lumber may be corrected to give the product in superficial board 

 feet for lumber sawed to thicknesses less than 1 inch. Since the board, of whatever- 



thickness, measures 1 superficial foot, the "per cent of utilization" will be , t being 



t-\-K 



thickness of board, K, saw kerf. For f-inch kerf and 1-inch lumber, the standard 



1 



1 t I rr 



per cent is - — - =80 per cent. Then the correction per cent is . 



1+K 



TABLE XVI 



Correction Per Cents for Contents op Logs in Superficial Board Feet 

 for Lumber Sawed Less than 1 Inch in Thickness 



77. Limitations to Conversion of Board-foot Log Rules. It is thus 

 seen that a correction of the total scale of logs regardless of diameter or 

 length can be made whenever thid correction takes the form of a straight 

 per cent of the volume of the scale. In addition to the effect of saw kerf 

 and thickness of boards, this principle applies to cubic rules erroneously- 

 used for board feet (§ 38). But no true board-foot log rule can be con- 

 verted by a constant or flat per cent into the values of any other log 

 rule, unless the deduction for waste from slabs and edgings is identical 

 for both rules, and the difference is wholly due to the use of different per 

 cents of waste for saw kerf. Otherwise, the conversion factor will vary 

 with diameter of log. Since tables of tree volumes and the scale of a 

 number of logs include logs of different sizes, such volume tables or 

 scale totals must be remeasured in the log in order to determine the 

 values for any other than the log rule originally used. 



