I must disagree with Jack's interpretation of Table 2 of SAE AMS 2431/4C. The area of field is clearly stated to be in units of a square inch - with the equivalent of 1 square inch being given, correctly, as 645 square millimeters. For AWS 17 the required field of view is 0.0625 of a square inch - which corresponds to 40 square millimeters. 40 square millimeters is the area of a square that has sides of 6.32 mm. Using "square packing", spheres with a diameter of 0.43 mm could be arranged 15 by 15 giving a total of 225 spheres in each of the 11 field areas to be viewed.
For consistency, the field area tested should be square - a spherical field area of the same area as a square field area would accommodate a slightly different number of spheres.
