- Micrometer screw gauge
- Scrap paper
Caliper, the thickness of a single sheet of paper, can be accurately measured down to a thousandth of an inch. Thickness measurements with this level of accuracy form an integral part of the paper and printing industry where inventory checks, issuing of quotations, and standard quality control all require accurate paper thickness measurements. The most common gauge used for taking a paper thickness measurement is the mechanical micrometer screw gauge. Correct use of this small gauge will help avoid costly mistakes that inevitably arise from using paper of inappropriate thickness.
Clean the screw micrometer jaws by gently closing the jaws on a piece of scrap paper. Open the jaws and remove the paper. Repeat this cleaning procedure before every measurement to remove any dust, paper fibers or grease particles that may contribute to an inaccurate measurement.
Insert the paper that is to be measured between the jaws of the screw micrometer. Close the jaws. A spring-loaded ratchet produces a clicking sound when the jaws are sufficiently closed to take a measurement. Never tighten the ratchet beyond three clicks.
Take the first part of a two-part measurement. The first part of the measurement is read from the horizontal scale on the sleeve of the micrometer, where the thimble with the rotating scale crosses the horizontal scale. The digits on the horizontal scale represent 100 thousandths of an inch, the long line markings indicate 50 thousandths of an inch, and the short lines on the horizontal scale mark 25 thousandths of an inch. For example, if the thimble intersects the horizontal scale between the 5 and the 6 on the horizontal scale -- after the first short line that follows the 5 but before the next long line on the horizontal scale -- then the first part of the measurement will be between 0.5250 and 0.5500 inches. Report the first part of the measurement as the lower number of this range, using 4 decimal places after the point: 0.5250 inches.
Take the second part of the two-part measurement from the rotating vernier scale of the micrometer thimble. The digits on the thimble scale are tens of thousandths of an inch, and the short lines are midpoints between the digits. Find where the axis of the horizontal scale intersects the thimble scale, as this is where the measurement is taken. For example, if the horizontal axis intersects the thimble scale between 13 and 14, but before the midpoint line on the thimble scale, then the second part of the measurement will be between 0.0130 and 0.0135 inches. Once this range is established, the last digit or fourth place after the decimal point in the second measurement is estimated depending on the exact position of the thimble. Report the second measurement including your estimation: 0.0134 inches.
Add measurements one and two to produce a final thickness measurement. For example: 0.5250 inches + 0.0134 inches = 0.5384 inches. Report the sum of measurement parts one and two as your final measurement.
Repeat this procedure three to five times at various points on the piece of paper. Average the final measurements to find the most representative thickness for the paper.
A screw micrometer should always be correctly zeroed using the wrench that is supplied with the micrometer prior to taking any measurements. If such a wrench is not available to adjust the zero position of the gauge, first take a reading with the micrometer jaws closed and no paper between the jaws, and subtract this zero reading from the final paper thickness measurement.