|  gauge usually has three sections of measurement.  Items measuring gauges. A = for measuring external dimensions. B = for measuring internal dimensions. C = to measure depth. The strip (or major scale) is graduated in millimeters or 0.5 mm if the metric system is low or knockout fortieth of an inch or if it is under the English system. The Vernier (vernier or scale) in the cursor, allowing readings below the following decimals. Metric 1 / 20 mm or 1 / 50 mm
English System 1 / 128 inch. or 1 / 1000 inch. The following lengths of gauges are widely used:
Metric 150 mm, 200 mm, 300 mm 6 inches
English system., 8 inches., 12 inches.  This gauge is equipped with a button instead of the traditional screw brake.
If the button is pressed, the cursor can be slid along the strip, when the button is released, the cursor will stop automatically.  This type is equipped with a screw which is used to move the cursor slowly when used as a gauge fixed, this allows easy adjustment of the cursor.  This guy called dial indicator is equipped with a dial indicator in place of a vernier to allow easy reading of the scale. precautions when measuring.
| Point 1: Make sure the gauge is not damaged. | If the gauge is often handled roughly, be rendered useless before completing its normal service life, to keep always useful not stop taking extreme care following 1) Before making measurements, clean dust and dirt surfaces measurement cursor and rail, particularly remove the dust of the sliding surfaces, and dust can often obstruct the sliding of the cursor. 2) Make sure that surfaces measurement of the jaws and the peaks are free of kinks or chipping. 3) Verify that the sliding surfaces of the strip are free from injury. For correct measurements, check the tool arranged as follows: 1) Be sure that when the cursor is completely closed, the zero on the scale and vernier strip are aligned with each other, also check the measuring surfaces of the jaws and the peaks as follows: - When light passes between the contact surfaces of the jaws, contact is correct. - Contact of the peaks is better when a uniform band of light passes through the measuring surfaces.  2) Place the gauge down on a flat surface with the depth gauge down and push the depth gauge, if the zero graduation on the strip and the vernier scale are misaligned, the meter depth is abnormal.  3) Verify that the cursor moves freely but not loosely along the strip. | Item 2: Set the proper gauge on the object being measured | Place the item on the bench and measure it, hold the gauge in both hands, place your thumb on the button and push the vernier jaws against the object to be measured, use only a gentle force. 
correct method of handling the gauges external measurement.
Place the item as deep as possible between the jaws.  If the measurement is made at the end of the jaws, the cursor could bend resulting in an inaccurate measurement.  Hold the object square to the jaws as shown in (A) and (B), otherwise you will not get a correct measurement.  internal measurement. This measure is to make mistakes unless carried out carefully u enter the peak entirely within the object to be measured, ensuring adequate contact with the measuring surfaces and take reading.  By measuring the inside diameter of an object, take the maximum value (A-3) to measure the width of a groove to take the minimum value (B-3).  is good practice to measure in both directions AA and BB in A-3 to ensure proper measurement.
Measurement of small holes. The measurement of small inner diameter is limited, we are exposed to confuse the apparent value "d" real value "D"  The best value "B" in the figure or the lowest "D" is the error. depth measurement. the measurement of depth, do not let the tip of the instrument tip, be sure to keep it level. 
corner of the object is more or less round, therefore, turn the bar resaque deep into the corner.  Examples of methods of measurement,
right and wrong. | Item 3: Keep the gauge properly after use. | When using the gauge, the surface of the scale is often played with his hand, so after use, wipe the tool clean with a cloth and apply oil to the sliding surfaces measurement before placing the instrument in its case. Be careful not to put any weight on the gauge, you could strain the power strip.  not strike ends yu peaks of the jaws or use them as a hammer. 
not hit the ends of the jaws
not use the gauge to measure a moving object. 
not measure an object while in motion. How to read the gauge (metric): Example 1. (Metric)  Step 1.
The zero point of the vernier scale is located between 43 mm. and 44 mm. on the scale of the strip. In this case read the first 43 mm 43 mm. Step 2.
on the vernier scale, locate the graduation line with the graduation of the scale of the strip. This ranking is "6" .6 mm Step longitudinal plane 43 + 0.6 = 43.6 mm Example 2. (Metric)  Example 3. (Metric)  Example4. (Metric)  How to read the gauge (English system)
Example 1. (English)  Step I. The zero point of the vernier scale is located between 2 4 / 16 "., And 2 5 / 16 inch. On the scale of the strip. In this case, read 2 4 / 16 "., First 2 4 / 16".
Step II. On the scale of a vernier, find the graduation which is in line with a graduation on the scale of the strip. This setting is "6", the 6 on the vernier indicates
6 / 128 .---------> 128 in. / in. Final Step. Step I + Step II  The correct reading is 2 19/64 inches.  Step I + Step II 4 3 / 16 + 4 / 128 = 4 24/128 + 4 / 128 = 4 28/128
= 4 7 / 32 The correct reading is 4 7 / 32 ".  Step I read 2400 in., First Step II Graduation 18 on the vernier scale is in line with a graduation scale of the strip, this reading is 18 inches pulg./1000 or 0018. Step I + Step II
= 2400 + 0018 = 2418 in.. The correct reading is 2,418 inches.  step I + Step II = 4,450 + 0,016 = 4,466 inches. The correct reading is 4,466 inches.
Information from: http://www.rolcar.com.mx/Mecanica% 20of% 20the% 20sabados/instrumentos_de_medicion.htm
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