Measure from any unobstructed sighting position and at any height on the tree-stump, dbh or merchantable height. Two pentaprisms, mounted in a tubular metal frame, permit simultaneous viewing along parallel lines of sight. The prism on the left side is fixed while the right prism is movable along the measuring scale. The image of the right side of the stem as viewed through both prisms can be aligned with the left side. At the alignment position, the outside bark diameter of the stem may be read directly from the measuring scale. Measures diameters 3” to 24” or 7 cm to 62 cm. Accurate to ±0.1 inch; ±2 mm at 50 feet.
Using the Wheeler Pentaprism Caliper
Holding the caliper 3” to 4” from your eye, look through the viewing slot. Through the upper part of the slot you will see the left edge of the tree. In the lower part of the slot you will see the right edge of the tree reflected in the stationary prism. Slide the movable prism until the right edge reflection is brought into direct vertical alignment with the left tree edge, midway between the two vertical guidelines. Then read the tree diameter as indicated on the scale by the pointer.
To calibrate the Pentaprism Caliper, set up a cylindrical target of known diameter. Measure it with the caliper from a distance of 10 feet and again from a distance of 50 feet. Adjust the point position to make the scale read the same as the known diameter target.