Willrich Precision Instruments: Calibration and Precision Measurement Solutions
Since 1971, Willrich Precision Instruments has been a trusted provider of precision measurement solutions and calibration services for decades. Known for its expertise in gauges, CMM inspection, and ISO/IEC 17025 accredited calibration, we have become a reliable partner for industries that require accurate and traceable measurements.
As manufacturing tolerances become increasingly strict, precise measurement is central for maintaining and improving product quality. Reducing variability depends on minimizing both data bias and measurement variance.
Our calibration services and precision instruments support proper measurement techniques. Inaccurate measurements lead to poor quality outcomes. This is why our expertise and certified calibration services play a key role in improving quality control processes.
The Importance of Precision Measurement Tools in Today’s Manufacturing
Modern manufacturing demands accuracy at levels unheard of just a few decades ago. Components are now produced with tolerances measured in microns; even the slightest variation can lead to rejected parts or costly rework.
Consistent measurement also supports quality improvement efforts by reducing bias and variability in collected data. Reliable data enables better decision-making, improved production efficiency, and increased customer satisfaction.
On the other hand, untrustworthy measurements compromise product quality and lead to increased waste. This is why manufacturers rely on proven brands such as Mitutoyo (a leader in precision measuring tools) to achieve accurate and repeatable results.
Calipers and the Progression of Units of Length
Units of length were based on the human body for much of human history. The foot, the fathom (the span of outstretched arms), and the cubit (measured from the bent elbow to the tip of the middle finger) were widely used. While imprecise, these measurements were accessible to everyone. It served well enough until the industrial era demanded greater accuracy and consistency.
The French Revolution had brought about significant political and scientific upheaval by the late 18th century. Scientists and philosophers in France argued that a true standard of length should come from a natural constant. They believed this would keep it unchanged forever.
After a seven-year expedition, two French astronomers determined the metre by triangulating the distance from Dunkerque to Barcelona. Ironically, this new “universal” unit was only about twice the length of the ancient Egyptian cubit. The actual size of the unit mattered less than achieving global agreement on its use.
The famous declaration À tous les temps, à tous les peuples (“for all time, for all peoples”) acted as an early mission statement to promote the metre as the universal standard. Still, Imperial measurements retained practical benefits. Fractions like 1/4 and 1/8 are convenient, and the inch, along with its 12-inch foot, remains a sensible and familiar unit of measurement.
The Quest for a Universal Measurement Standard
In the eighteenth century, after Sir Isaac Newton’s death (1642–1727), the center of scientific progress shifted from England to France. Well before the French Revolution, French scholars and scientists were already debating the need for a new, universal unit of length. They wanted it based on a physical constant rather than arbitrary human references.
One early idea was to define the standard using the length of a pendulum. However, a pendulum’s period varies slightly with changes in gravity, especially at different latitudes. This proposal was rejected despite suggestions to standardize it at 45°N latitude.
Instead, the French Academy of Sciences approved a far more ambitious plan: to define the unit as one ten-millionth of the distance from the North Pole to the equator along the Paris meridian. They named this new unit the mètre, derived from the Greek metron and Latin metrum, meaning “measure.” They decreed that it would follow a decimal system.
Measuring an entire quadrant of the Earth was nearly impossible. Hence, astronomers focused on the longest practical land segment of the Paris meridian, stretching from Dunkerque to Barcelona. Both cities are at sea level, making calculations easier.
Casini de Thury and La Caille had made previous measurements along a similar route (from Dunkerque to Collioures) in 1739–40 for the first accurate maps of France. However, the Academy sought even greater precision using the latest instruments.
Once surveyed, this section was used to extrapolate the full quadrant length. They also took into account corrections for the Earth’s slightly flattened, oblate shape. The result was 5,132,430 toises (a toise being approximately six feet), which was then divided into ten million equal parts to define the meter. Despite the scientific achievement, public acceptance proved difficult. Napoleon later reinstated the old units, nearly ending the metre’s adoption before it had even begun.
The Origins of a Measurement Standard: The Egyptian Cubit
Many of the earliest units of length were based on the human body, as it provided a convenient and universally available reference. The fathom (used in numerous ancient cultures) likely came from the span of rope a person could hold between outstretched hands.
In ancient China, the shaku (later adopted in Japan) was measured from the edge of the thumb to the tip of the middle finger. Such standards were practical because most people had similar body proportions and could use them without special tools.
The Egyptian cubit followed the same principle. It measured the distance from a bent elbow to the tip of the middle finger of a ruling Pharaoh. His arm was considered the authoritative standard. Even without translation, hieroglyphic inscriptions depict symbols representing this unit.
The cubit was roughly 500 mm long. It was divided into 28 smaller parts, each approximately 18 mm in size. These parts were further subdivided into halves, thirds, quarters, and sixteenths. These were the smallest recorded graduations.
Curiously, when the metre was established centuries later based on a natural constant, its final length was almost exactly double that of the ancient cubit. Had the French astronomers who calculated the Earth’s Great Arc chosen to divide it into twenty million parts instead of ten million, the cubit might have become the global standard of measurement today.
What Are Calipers and How Are They Used?
Calipers are versatile measuring instruments used for a wide range of applications, be it Vernier, Dial, or Digital. They can measure internal and external dimensions, depths, and even step distances. Professionals across many fields rely on them, including machinists, mechanics, scientists, dentists, chemists, archaeologists, and anthropologists.
Fundamentally, anyone who needs precise measurement tools uses them. Because they are used in such varied environments, calipers are often subjected to rough handling. To withstand this, most caliper jaws are heat-treated and hardened to a minimum hardness of 62 HRC.
Modern calipers are typically made from stainless steel, which is durable enough for standard use. However, measuring extremely hard or abrasive materials can still cause wear over time. To increase durability, many calipers use tungsten-carbide jaw inserts. These inserts provide exceptional hardness and significantly extend their working life.
Dial calipers offer the same functions as vernier calipers. Still, they are easier to read, thanks to mechanical magnification that can reach a ratio of 100:1. However, this added complexity makes them more expensive and more vulnerable to contamination from dust and swarf. Their resolution is typically 0.001 inch, with an uncertainty of ±0.001 inch for a 0–6 inch range.
Some calipers also feature carbide inserts on the inside jaws. If the outside jaws become bent, you can restore them by lapping with a disc-shaped stone until they return to proper parallel alignment. In cases where the inside jaws deform, heating and carefully bending them back into shape can restore functionality.
Calipers and the Vernier Scale: The Most Versatile Measuring Tool
Calipers are considered one of the most practical and versatile precision measurement tools. Its multiple measurement functions, user-friendly design, durability, broad measuring range, and affordable cost contribute to its reputation. They are a key general-purpose instrument in many toolkits.
However, calipers are not fully compliant with Abbe’s Principle, which states that for maximum accuracy, the measurement axis must align directly with the scale axis. When the two axes are not coaxial, measurement errors can occur. Fortunately, most of these potential inaccuracies can be minimized by following a few careful handling techniques.
How to Read a Vernier Caliper
A vernier caliper uses a secondary sliding scale to measure with much greater precision than the smallest divisions on the main scale alone. This secondary scale (called the vernier scale) subdivides the main scale’s increments by aligning its markings with those on the primary scale. The modern version of this system was introduced by French mathematician Pierre Vernier (1580–1637).
Reading a vernier caliper (especially a metric model) is relatively simple. Although it does require sharp eyesight or a magnifying glass since, unlike a dial caliper, it offers no mechanical magnification. Users also need to avoid parallax error because the two scales sit at slightly different levels.
On a typical metric vernier caliper, each vernier division is 0.02 mm shorter than a 1 mm main scale division. As the jaws open, every 0.02 mm of movement brings the next vernier marking into perfect alignment with a main scale marking. This allows the user to read measurements in 0.02 mm increments.
On an inch-based vernier, the main scale is divided into 0.025-inch increments. On the other hand, each vernier division is 0.001 inch shorter than two main scale divisions. This wider spacing makes the inch version easier to read. The operating principle remains the same, providing a resolution of 0.001 inch.
How to Read Dial Calipers
Dial calipers offer the same versatility as vernier calipers. Still, they are much easier to read thanks to their mechanically magnified scale, which can amplify readings by up to 100:1. This magnification makes them ideal for users who prefer quick and precise measurements at a glance.
However, the intricate moving parts that enable this feature make dial calipers more expensive than vernier models. They are also more susceptible to damage or malfunction from dust, swarf, or other contaminants.
How to Read Digital Calipers
Traditional digital calipers operate using a simple binary counting system. Beneath the slider, alternating light and dark bands are arranged along the track, and the caliper counts these bands as the slider moves. Because it only tracks how many bands have been passed, it cannot determine the slider’s exact position on its own.
Hence, most digital calipers require users to close the jaws fully and zero the display after powering on. It resets the system before taking any measurements.
This was a clear disadvantage compared to vernier calipers, which can be read at any point within their range without requiring a reset. Some still argue that vernier calipers are the best choice because they are simple, inexpensive, and as accurate as digital versions. However, vernier calipers are more difficult to read, especially when using inch measurements. They also cannot easily switch between metric and imperial units and lack the ability to toggle between absolute and incremental measurements.
Proper Measuring Technique for Calipers
Before taking measurements, open and close the jaws several times to confirm accuracy. Each time you close them, the display should return to zero, which should take only a few seconds to verify. Although ABSOLUTE-type calipers do not require zero-setting, it is still good practice to close the jaws and verify the zero setting for proper functioning.
Once confirmed, begin measuring. Always take multiple readings, as the first measurement is often inaccurate. Continue until you get consistent results. For example, if repeated measurements stabilize at 73.88, disregard earlier inconsistent readings, as they are likely errors.
A proper measurement should take no more than three or four seconds. Gently move or “wiggle” the caliper to align it correctly against the work surface. Apply only light pressure: touch the workpiece, release, and repeat to find the actual reading.
In theory, the correct value is the smallest consistent reading obtained during repeated trials, as misaligned jaws usually cause higher readings. For instance, if repeated measurements settle at 84.73, that should be accepted as correct. A slightly higher reading (such as 84.75) is likely incorrect.
If measurements do not repeat, an inconsistent measuring force is usually to blame. Proper technique is a must for accurate results. With practice, you can reduce operator bias (which can be as high as 50 μm (0.002 inch)) to nearly zero.
Accuracy Levels and Calibration for Calipers
The accuracy of a caliper (often referred to as its instrumental error) varies with its measuring range. The typical uncertainty is ±0.02 mm (±0.0008 inch) for calipers measuring 0–200 mm (0–8 inches) when zeroed with the jaws fully closed.
The uncertainty increases by approximately ±0.01 mm (±0.0004 inch) for every additional 200 mm of range, progressing steadily up to a 1000 mm range.
This gradual loss of accuracy can be minimized by setting a dial or digital caliper against gauge blocks that are close to the target measurement. For example, when measuring around 150 mm, presetting the caliper to a 150 mm gauge block effectively cancels out any accumulated error up to that point.
In this case, the caliper would read exactly 150.00 mm. An uncertainty (k = 2) of ±0.01 mm can be quoted for measurements near this value. Vernier calipers require the use of a recorded calibration correction value to achieve similar accuracy.
Micrometer standard bars can substitute for long gauge blocks for larger measurements. For instance, a 300 mm standard bar is guaranteed to be within ±0.007 mm (±0.00025 inch for a 12-inch bar) of its nominal length when purchased. These bars also include a calibration correction value, marked on one of their insulators. This value can be added to the nominal measurement when maximum accuracy is needed.
Willrich Precision Instruments supports industries that depend on accurate, traceable measurements. Our precision measurement tools and services cover gauges, optical comparators, hardness testers, vision systems, and more.
Let’s improve your measurement accuracy today. Reach out, and we’ll help you achieve results you can trust.
