The published extrusion data shows natural convection performance for a three inch section with a centrally located point source heat load. Because the heat load is assumed to be at a point rather than uniformly distributed, thermal resistance does not change linearly with length. (The ends of a very long extrusion would be cooler than the center and therefore the transfer of heat to the surrounding air is little, if any.) It is therefore necessary to apply a correction factor to published data for extrusion lengths shorter or longer than three inches. The corrected thermal resistance for different lengths of extrusion is obtained by multiplying published °C/W/3-in data by the appropriate factor from the following table:

Heat Sink Length	Correction Factor
1.0 inch (25.4mm)	1.80
2.0 inch (50.8mm)	1.25
3.0 inch (76.2mm)	1.00
4.0 inch (101.6mm)	0.87
5.0 inch (127.0mm)	0.78
6.0 inch (152.4mm)	0.73
7.0 inch (177.8mm)	0.67
8.0 inch (203.2mm)	0.64
9.0 inch (228.6mm)	0.60
10.0 inch (254.0mm)	0.58
11.0 inch (279.4mm)	0.56
12.0 inch (304.8mm)	0.54
13.0 inch (330.2mm)	0.52
14.0 inch (355.6mm)	0.51
15.0 inch (381.0mm)	0.50

This Extrusion Length Correction Factor Table can also be used to determine the length of extrusion required to obtain a desired thermal resistance. Divide the desired thermal resistance by the published thermal resistance for a three inch section to obtain the correction factor, which can be used to determine the correct length.

EXAMPLE: Extrusion 62465 has a thermal resistance of 8.0 °C/W/3-in. A five inch piece will have a thermal resistance of 6.24°C/W, using the appropriate length correction factor of 0.78 [i.e. 8.0°C/W x 0.78 = 6.24°C/W].

Since the thermal resistance of 6.24°C/W is at a temperature rise of 75°C, the resistance of the heat sink at a temperature rise of 50°C will be increased by the temperature correction factor of 1.106 from the Temperature Rise Correction Factor Table. Therefore the new natural convection thermal resistance at 50°C is 6.90°C/W [6.24°C/W x 1.106 = 6.90°C/W].