Ensuring accuracy and calibration of calipers

In this blog, we will discuss the accuracy and calibration of digital, dial, and vernier calipers, as well as the importance of keeping instruments properly calibrated.

Metrology is often referred to as the ‘science of measurement’ and calibration, along with verification, are its’ two pillars.

Calibration is the process of adjusting measurement equipment to align with known values or to meet specific standards. Measuring equipment can lose their precision over time, so will need to be ‘re-calibrated’. Re-calibrating restores accuracy by identifying and correcting deviations.

Within industry sectors that depend on highly accurate measurements - such as aerospace engineering, medical device manufacturing & motorsport - accuracy is a pre-requisite. You’ll be required to ensure consistency in measurements. Not doing so can lead to significant issues such as product recalls or non-compliance with regulatory standards.

Verification is the process of checking that the measuring meets the specified accuracy standards and or tolerances. 

During verification, you’ll ensure that the equipment stays within its’ designed operating parameters (so it delivers integrity of data produced) by comparing its’ performance against a recognised standard. 

This will ensure the instrument delivers consistent and reliable results. 

• Vernier calipers are traditional but sophisticated precision measuring tools used to accurately measure linear dimensions like length, width, thickness, and diameter.

• A Vernier caliper consists of a main scale and a sliding Vernier scale. This enables precise readings by using a small, movable auxiliary scale with finer graduations to increase accuracy.

• The main scale provides the basic measurement in millimetres or inches.

• A sliding (Vernier) scale then allows the user to divide the main scale's subdivisions into smaller increments, allowing for more accurate measurements.

• By aligning a mark on the Vernier scale with a mark on the main scale, you can determine the fractional part of a unit of measurement.

• The Vernier caliper has two sets of jaws, one for measuring external dimensions and one for measuring internal dimensions.

• A depth caliper probe enables the measurement of the depth of holes or recesses.

• Digital calipers are versatile and can measure both internal and external dimensions, as well as depths and distances.

• They are predominantly used to determine the length, width, thickness or diameter of an object.

• Measurements in either metric or imperial units are shown on a digital display. Accurate measurements are provided, frequently to within 0.01mm (0.0005 inches).

• When compared to traditional Vernier calipers, Digital calipers offer easier readability. This removes the need for visual estimations or calculations.

• They often include features like a ‘reading hold’ for awkward measurements or data transfer capabilities.

• If you need to take differential measurements, a ‘Zeroing’ function is usually used, this allows you to reset the measurement to zero at any point along the slide.

• Dial calipers are another precision measurement tool, but one that uses (you guessed it), a dial to indicate measurements. The dial display makes it easy to read measurements, even for those unfamiliar with Vernier scales.

• The dial provides accurate readings for both internal and external dimensions, as well as depth measurements, including step distances.

• Readings are typically to the nearest hundredth of a millimeter or thousandth of an inch.

• When using a Dial caliper, a rack and pinion mechanism connects the movement of the jaws to a pointer on a dial. As the jaws move, the pointer rotates on the dial to provide the measurement.

• The Dial caliper’s main scale provides a general measurement, and the dial provides more precise measurements.

According to the international standard ISO 13385-12019, two accuracy specifications cover the most important metrological characteristics of calipers.

1.     the partial surface contact error, E (which applies to any measurements made using the outside measuring faces)

2.    the scale shift error, S (which applies to measurements using any other measuring faces available on the caliper, such as inside, step, and depth measurement)

The difference between the above is that all specifications apply when the zero is set using the outside measuring faces and larger errors may occur when using the other measuring faces.

ISO 13385-1:2019 is a UK-accredited standard used for the calibration of calipers.

It focuses specifically on the design and metrological characteristics of these instruments.

The most important aspect of caliper calibration is to verify conformance with the accuracy specifications in ISO 13385-1:2019.

• ISO 13385-1:2019 covers various types of calipers, including Digital, Dial, and Vernier calipers.

• The standard defines key terms and characteristics relevant to caliper accuracy, such as partial surface contact error (E) and scale shift error (S).

• It also specifies default values for the maximum permissible errors (MPE) of calipers.

• These values are presented in millimeters or inches and depend on the caliper's size and measurement type (e.g., internal, external, depth).

• The default size for testing inside measurements for example, is between 20 mm and 50 mm.

• The standard also specifies default sizes for testing, for example, between 20 mm and 50 mm for inside measurements, and less than 50 mm for depth and step measurements.

• Finally, ISO 13385-1:2019 includes requirements for test methods to verify conformance to the MPE. 

At MSC we stock a comprehensive range of over 180 calipers, which you can browse here. Rob Hooper, Senior Category Manager at MSC tells us more about the range:

“Mitutoyo is our go to for calipers, having huge, trusted brand presence within the market. We have a great relationship with them and have some of their measuring equipment within our Tech Centre too, which we use for training purposes and customer presentations, whether via video link or on site. For a six-inch caliper you’d want to look for the Mitutoyo 500 series (500196-30).

We also work with brands such as Bowers, who have a great range of product from entry level equipment (such as the Moore & Wright brand), up to the Sylvac brand, which is very specialist.

Finally, we have our own brand, SPI (Swiss Precision Instruments). This is an entry level product that does ‘exactly what it says on the tin’ and which we sell a lot of to customers that are more budget conscious. These also prove to be a good choice for companies that find they are breaking a lot of calipers or for use by apprentices.”

We hope that this blog has been useful in helping you to ensure accuracy through properly calibrated calipers.

Find the right caliper for your measurement requirements here at: https://www.mscdirect.co.uk/measuring-and-inspection/calipers

Or head over to the Mitutoyo caliper range here, where you will find Mitutoyo digital calipers, Mitutoyo vernier calipers and much more!