# measuring stick

The vernier calliper (in parts of Germany also calliper or caliber , in Austria calliper and calliper ) is a length measuring device .

A slide, which also carries measuring arms, can be moved on a rod with usually two measuring arms. For external or internal measurements, one of the pairs of measuring legs is placed on a body from the outside or on the walls of a cavity from the inside. On the slide there is usually a measuring rod that z. B. is used to measure the depth of non-through holes.

## history

Bronze device from the Eastern Han Dynasty . (China already had contacts with the Mediterranean countries)

The oldest find of a vernier caliper comes from the Greek Giglio wreck off the Italian coast, which was used for external measurements (today's name and use as a caliper ). The ship sank in the 6th century BC. The find was made of wood. Vernier calipers remained in use later by the Greeks and Romans .

## General

Caliper with three different display methods
from top to bottom:
digital caliper, round scale and vernier display

There are different types of vernier calipers: Simple vernier calipers have at least one vernier to increase reading accuracy . Variants in which the measured value is displayed on a round scale are more accurate. Digital calipers with a digital numeric display have been available since the 1990s. These can be read off better and faster and make it possible to transfer the measured values ​​to external evaluation devices.

Vernier calipers are preferably made of hardened stainless steel in order to reduce corrosion and abrasion and to ensure dimensional accuracy over the long term. For lower requirements there are calipers made of (rusting) steel, brass or - mostly glass fiber reinforced - plastic .

Vernier calipers for use in mechanical engineering and metal construction usually have an engraved or glued-on table on the back, on which the integer nominal thread diameters are assigned the diameters of the associated core drills for preparation for cutting internal threads.

When measuring outside and inside diameters with the vernier caliper, in contrast to the micrometer, the Abbe comparator principle is not adhered to. The error caused by this ( tilting error of the first order) leads to a measurement inaccuracy that is fundamentally unavoidable in the structure of the caliper. Further error possibilities are guidance errors, system errors on the object to be measured and wear (damage, contamination). When measuring with the depth measuring rod, the Abbe principle is not violated, since the scale and measuring rod are in alignment.

Measuring with the vernier caliper is a direct measurement method , since the input and output variables are identical (in this case the length ).

## Advantages and disadvantages

Advantages of the caliper compared to other length measuring devices are:

• relatively high measurement accuracy,
• easy and quick to use,
• robust and inexpensive pocket measuring device.

Disadvantages are:

• The measurement uncertainty is greater than the resolution of the display.
• The repeatability is worse than with a measuring device with constant measuring force ( micrometer or dial gauge ).

## Designations

Use of a caliper, vernier reading

The technically correct designation for the measuring tool described in Germany is regulated in DIN 862 and is vernier caliper .

If the caliper has a locking screw, it can also be used as an adjustable gauge . This is where the names of the calliper or slide gauge , by which the device is often only known, come from. In the past, the term "caliber" (English term: calliper / caliper) was used colloquially, especially in southern Germany.

Vernier caliper with vernier reading

## Typical structure of a caliper

• Rod with fixed measuring legs (blade-shaped, for external measurements (1) also flat parts)
• movable slide with counter-measuring legs (extra rod (3) for depth measurement)
• Measuring scales on the stick ( metric (4), inch (5))
• Nonius scales on the slide ( metric (6), inch (7))
• Pressure lever (8) for releasing the spring force preventing the slide movement, or clamping screw as shown in the example above

## Use and measurement accuracy of a caliper

The measuring error of a caliper is between 0.01 mm and 0.2 mm. It depends on the measuring range, the length of the measuring arms, the wear of the slide guide and the use. A micrometer screw offers a higher resolution . The limit here is around 1 µm.

The principal error caused by tilting the caliper ( first order tilting error ) by the angle and with the distance between the measuring line and the scanning line is ${\ displaystyle \ varphi}$${\ displaystyle d}$

${\ displaystyle \ Delta x = d \ cdot \ tan \ varphi}$.

The reading point is shifted by. ${\ displaystyle \ Delta x}$

In order to keep this measurement error (better: the measurement tolerance) as low as possible, it is recommended not to apply a pronounced measurement force and to push the measuring legs together until a slight resistance is reached.

## Types of calipers

### Digital caliper

Digital caliper with capacitive linear encoder under the plastic cover
Capacitive electrodes under the display on the slide

Digital calipers replace the analog vernier with a digital display unit which, in addition to a liquid crystal display (LCD) for the numeric display, various operating elements in the form of pushbuttons, also includes a microprocessor for the calculations and usually a button cell for the power supply. Thanks to the possibilities offered by the numeric display, you can switch between different units such as metric output or the display in inches at the push of a button. The accuracy of digital calipers varies depending on the model and is in the range of 0.01 mm to 0.02 mm with a total length of 150 mm. The resolution of the display is 0.01 mm and on some models 0.001 mm. The absolute accuracy decreases with longer callipers.

In contrast to precision mechanical calipers with toothed rack and round scale, the measured value is recorded using a linear encoder . This sensor designed for distance measurement can work according to various measuring principles: In digital calipers, because of the low power requirement, primarily capacitive and, to a lesser extent, inductive linear encoders are common. Part of the sensor is attached to the movable slide below the display. The second part of the sensor is fixed in the rod, the so-called stator , below the scale. Essential work in the practical development of capacitive linear encoders in digital calipers is based on the work of Larry K. Baxter et al. back from the mid 1980s.

In the figures on the right, a digital caliper with capacitive linear encoder is shown as an example, in which the plastic cover has been partially removed and the underlying structures of the capacitive encoder are visible. This periodically arranged structure in the rod represents electrical conductor tracks, which in principle form plates of capacitors . Metal strips attached opposite, which are located below the LCD display in the slide and are shown in the second figure, are fed with several different types of pulse-width modulated square-wave signals, which are generated by the electronics of the caliper. Depending on the position of the slide, the different capacitive couplings of the geometric arrangement result in different types of signal curves on the receiving electrode, which is also attached below the LCD display. Using digital signal processing , the exact position of the slide can be determined and displayed.

There are various methods and types of implementation for the processes and linear encoders used. In addition to the physical principle, the linear encoders used in digital calipers can be roughly divided into relative and absolute encoders, capacitive and inductive. With relative linear encoders, the zero point must be set using a button before a measurement. The opening width is then determined by a counter in the electronics relative to this zero point. Due to the low energy consumption of the electronics, due to the CMOS circuit technology used , such a zero point adjustment may only be necessary after inserting a new battery into the measuring instrument. In the case of absolute linear encoders, the position information is determined by the way the structures are arranged in the encoder; zero point adjustment is not necessary. Depending on the model, some digital calipers also offer a hold function of the display at the touch of a button, in which a current measured value can be frozen in the display for later reading.

Furthermore, digital calipers usually have a serial data interface, which can be designed as an RS-232 interface, which allows the measurement data to be automatically picked up for external storage or display of the measured values ​​on larger, external displays.

### Other variants

• Kluppe ( vernier caliper for measuring the diameter of tree trunks in forestry)
• Button measure
• Pocket caliper with linear scale and vernier
• Prism caliper with prismatic guides and increased reading accuracy (0.02 mm instead of 0.05 mm) thanks to scales in one plane, thus avoiding parallax errors
• Workshop caliper with linear scale and vernier
• Pocket caliper with round scale (watch caliper)
• Workshop caliper with round scale (watch caliper)
• Precision caliper for gear teeth
• Special calipers such as grooved calipers, three-point calipers, etc.
• Depth caliper (standard caliper with integrated measuring bridge without knife edges)
• Height caliper (measuring the height of an object surface above the measuring table level)

## Web links

Commons : Vernier Caliper  - collection of pictures, videos and audio files

## Individual evidence

1. In Austria only the terms "slide gauge" and "slide gauge" are defined by the official Austrian dictionary (43rd edition, complete edition with the official set of rules, Österreichischer Bundesverlag, Vienna, 2016, ISBN 978-3-209-08514-6 , p . 624). It should be noted, however, that according to DIN 862 a caliper is technically correct not a teaching .
2. Example of the Federal Chamber of Architects and Consulting Engineers from 2009, accessed on November 8, 2015
3. Mensun Bound: The Giglio wreck: a wreck of the Archaic period (c. 600 BC) off the Tuscan island of Giglio , Hellenic Institute of Marine Archeology, Athens 1991, pp. 27, 31 (Fig. 65; Engl.)
4. a b Roger B. Ulrich: Roman woodworking , Yale University Press, New Haven, Conn., 2007, ISBN 0-300-10341-7 , p. 52f.
5. hand tool . Encyclopædia Britannica from Encyclopædia Britannica 2006 Ultimate Reference Suite DVD, accessed July 29, 2008
6. www.beuth.de
7. Technical data sheet - Digital Calipers. Mitutoyo Inc. , accessed April 20, 2017 .
8. Patent US4586260 : Capacitive displacement measuring instrument. Applied on May 29, 1984 , published May 29, 1986 , Applicants: The L. S. Starrett Company, Inventors: Larry K. Baxter, Robert J. Buehler.
9. ^ Larry K. Baxter: Capacitive Sensors: Design and Applications . 1st edition. Wiley-IEEE Press, 1996, ISBN 978-0-7803-5351-0 , Chapter 18. Vernier caliper.