Calipers

The caliper is undoubtedly the most basic measuring instrument. Ease of use, wide range, low cost, and the large number of applications make it the primary general-purpose measuring instrument. A standard caliper has 2 pairs of jaws, one for external measurements and one for internal measurements. In addition, it also has a depth measuring rod for depth-gauge-type measurements, which is available in flat as well as round types.

Calipers are of 3 types:

• Digital calipers.
• Dial calipers, where the measurement is made with the help of a dial.
• Vernier calipers, or simply analog calipers.

Diagram of a vernier caliper showing a measurement of 2.475cm ± 0,005 cm on the metric scale. The parts of the caliper are:

1. Outside jaws: used to measure external dimensions (e.g. the width of an object or diameter)
2. Inside jaws: used to measure internal dimensions (e.g. the diameter of a hole)
3. Depth measuring rod: used to measure depth (e.g. of a hole)
4. Main scale: calibrated scale in mm
5. Secondary scale: calibrated scale in inches.
6. Vernier scale: used for measurements with accuracy of 0.05mm or 0.02mm
7. Inch vernier scale: used for measurements with accuracy in inches
8. Lock screw: used as a brake to stabilize a measurement

How to measure with a caliper

Measurement with a dial caliper

Calipers for special applications

Digital Waterproof Calipers IP67

For more information regarding waterproofing of digital instruments

see here

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Heavy-Duty Calipers (large)

For measuring large parts, steel rules are usually used for rough measurements. However, if higher accuracy is required, heavy-duty calipers become necessary. Long-length calipers are very convenient for the user, but they require some care during use. First of all, it is important to realize that there is no relationship between readability and accuracy.

Readability is fixed, while accuracy can vary dramatically depending on how the caliper is used. Measurement technique is important, as even a small deflection of the instrument beam will cause a large error. Therefore, measurement accuracy depends to a large extent on the method used to support the caliper.

Additional care is required so that you do not apply excessive force to the jaws. Since the large jaws are farther from the main beam than on normal small calipers, the error here will be at its maximum.

Possible causes of error

Errors during measurement with a caliper can arise for various reasons. The most important reasons are the parallax effect, the fact that the caliper as an instrument does not comply with the Abbe principle, and differential thermal expansion due to the temperature difference between the instrument and the workpiece being measured.

Although there are other error factors, such as the accuracy of the graduation, the flatness of the fixed beam, and the squareness of the jaws, these factors are included in the error limits by the manufacturer.

Internal Measurements

Place the jaws as deep as possible before measuring.

Read the maximum value during internal measurements.

Read the minimum value during external measurements.

Depth Measurements

Read the minimum value during depth measurements.

Parallax Error

Always look at the vernier in a straight line. If we look at the vernier graduation from an oblique direction (A), the apparent measurement position is shifted by ΔX as shown in the figure below. This shift is due to the parallax phenomenon caused by the height difference (H) between the planes of the two graduations. To avoid this error, JIS specifies that the height difference must not be more than 0,3mm.

Moving Jaw Error

If the moving jaw goes out of parallel with the fixed jaw, either due to excessive force or due to lack of straightness of the beam, an error will occur as shown in the figure.

Measurement and temperature

The main scale of a caliper is engraved on stainless steel, and although its coefficient of thermal expansion is equal to that of the most common material to be measured, i.e. steel (10.2 ± 1) × 10-6 / Κ, it should be noted that room temperature and the temperature of the workpiece being measured can affect measurement accuracy.

Handling

Handle the instrument carefully and avoid any impacts or drops on the floor.

Cleaning and Zeroing

• Wipe dust and dirt from the sliding surfaces with a dry soft cloth before use.
• Clean the measuring surfaces by holding a sheet of clean paper between the jaws and then slowly pulling it out.
• Close the jaws and ensure that the vernier scale (or display) reads zero before measuring.

Storage

• Avoid direct sunlight, high temperatures, low temperatures, and high humidity during storage.
• If a digital caliper is not used for more than three months, remove the battery before storage.
• Finally, do not leave the jaws of a caliper completely closed during storage, as they accumulate moisture.