# What Are Fiducials

## Video: What Are Fiducials Video: TIP #038: Place fiducials in the PCB corners and on the side where SMD components are fitted 2023, March

The word "benchmark" in translation from French means "mark". This term is used in several sciences. So, for example, in geodesy, a benchmark is a point on the earth's surface with a known absolute height. Gunners call a benchmark a point that is used for sighting. In physics, the term "reference point" is accepted.

## Starting point

The term "fiducial point" means the point on which the measuring scale is based. The easiest way to see it is on an ordinary street thermometer. Looking at its scale, you will see that in the middle there is a mark with the designation "0". Below it are minus marks, above - plus ones. The zero mark is the reference point for the Celsius scale. This is the freezing point of water at sea level. Once the Celsius scale had two reference points. The second was at around 100 ° C, that is, the boiling point of water at sea level was taken as the basis for measurements.

## What other reference points are there?

There are several temperature scales. They are based on different reference points. So, absolute zero is taken as the beginning of the Kelvin temperature scale, that is, the temperature at which it is impossible to extract thermal energy from the substance. If you read on the Celsius scale, then the absolute zero will be at the level of -273.15 ° C. Some countries use the Fahrenheit scale. In England, and especially in the United States, the Fahrenheit scale is used. Zero degrees Celsius is 32 degrees Fahrenheit and 100 degrees Celsius is 212 degrees Fahrenheit. To calculate the degree Fahrenheit, you need to subtract the melting temperature of ice from the boiling point of water at atmospheric pressure and divide the resulting difference by 180. The reference point of the Fahrenheit scale is 32 ° C. Nowadays, the Reaumur scale is practically not used. Like the Celsius scale, the Reaumur system used two reference points - melting ice and boiling water. The zero mark of this scale corresponded to zero on the Celsius scale, but the 80 ° C mark was used for the boiling point, that is, the Reaumur degree was 1.25 degrees Celsius. On the Rankine scale, the fiducial point corresponds to the Kelvin scale, but the graduation is the same as on the Fahrenheit scale.

## International temperature scale

Fiducials are based on the International Temperature Scale. The first was developed in 1927 based on the Fahrenheit scale. In the Russian scientific literature, the designation MTSh-27 was adopted. Over the past century, this scale has changed several times - in 1948, 1968 and 1990. The ITSh-90 scale has now been adopted. Like its predecessors, it is based on phase transitions of pure substances, that is, reference points. Devices are calibrated according to this principle. In everyday life, the Celsius and Fahrenheit scales are usually used, and the scale of absolute temperatures, that is, Rankin or Kelvin, is more suitable for scientific purposes. This is explained by the fact that any temperature mark on them will be positive.