3258 Chapter 27 



Errors may be introdued by scaling even inches. If measurements are always 

 rounded downward — i.e. , logs 12.0 to 12.9 inches tallied as exactly 12 inches — 

 the average downward bias for the Doyle rule is (Row and Guttenberg 1966): 



Scaling diameter Downward bias 



Inches Percent 



6 35.9 



9 17.3 



12 11.5 



15 8.6 



18 6.8 



Scribner Decimal C log scale. — Scribner did not base his log scale on a 

 formula; instead he drew circles of different diameters and plotted the ends or 

 cross sections of boards which might be sawn within each circle, computed the 

 board cross sectional area in square inches, divided this value by 12 to get board 

 feet per foot of log length, and finally, multiplied by the log length. As a result of 

 this method of computation, values for successive inch classes increase in an 

 irregular manner. In the Scribner Decimal C rule, the last figure in the scale of a 

 log is rounded to the nearest 10 (e.g. , a log scale of 1 14 bd ft is rounded to 1 10). 

 Log contents according to the Scribner decimal C log scale are shown in table 

 27-15. 



Grosenbaugh (1952, p. 12) expressed the Scribner scale in a regression 

 equation as follows: 



V = 0.0494D2L - 0.124DL - 0.269L (27-5) 



For precise computations, volume of 16-foot logs based on nearest tenth-inch 

 scaling diameter are useful (table 27-16). These values were computed by the 

 following equation: 



V = 0.79D2 - 2D - 4 (27-6) 



In these equations, 



V = volume, board feet 



D = scaling diameter, inches 



L = scaling length, feet 



Gross log scale may be reduced because of defects. Appropriate deductions 

 (board feet) can be read from table 27- 1 7 if the length and cross sectional area of 

 the defects are known. 



International V4-inch log scale. — This formula-based scale accounts for 

 taper in logs by evaluating them in 4-foot lengths. Content of a log is computed 

 by summing the contents of the 4-foot lengths comprising it and assuming that 

 taper increases diameter 1/2-inch in each 4 feet of log length. Saw kerf is assumed 

 to be '/4-inch. The formula for each 4-foot length of log is as follows: 



V = 0.905 (0.22D2 - 0.7 ID) (27-7) 



Log contents computed from this formula are usually rounded to the nearest 5 

 board feet as shown in table 27-18. 



