Metrological control on site

Introduction

Calibration is the process of comparing the values obtained by a measuring instrument with the corresponding measurement of a reference standard (or standard). According to the International Bureau of Weights and Measures, calibration is "an operation that, under specific conditions, establishes in a first step a relationship between the values and measurement uncertainties provided by standards and corresponding indications with the associated measurement uncertainties and, in a second step, uses this information to establish a relationship to obtain a measurement result from an indication."[1].

From this definition it can be deduced that to calibrate an instrument or a standard it is necessary to have one of greater precision (standard) that provides the conventionally verifiable value, which will be used to compare it with the indication of the instrument that is being subjected to calibration. This is done through an uninterrupted and fully documented chain of comparisons until reaching the primary standard, which constitutes what is known as traceability.[2] The objective of calibration is to maintain and verify the proper functioning of the equipment, respond to the requirements established in the quality standards and guarantee the reliability and traceability of the measurements. Measuring instruments require calibration more frequently the more exact their samples are, that is, the smaller the error tolerances. In general, calibration intervals depend on factors such as the requirements given by a client or a regulation and the stability over time of the instrument to be calibrated.[3].

The calibration of any measuring instrument must be carried out under controlled humidity and temperature conditions by an accredited calibration laboratory; there are various accrediting bodies depending on each country.

In Mexico there is the Mexican Accreditation Entity, EMA, an entity that accredits the quality and technical competence of a calibration laboratory to offer metrology and instrument calibration services.

During the calibration process, the output value of the instrument to be calibrated is contrasted against a standard at different points in the calibration range. If the calibration error—an error that is revealed during calibration—is less than the rejection limit determined, either by standards, the company or the manufacturer's recommendation, the calibration will be accepted.[4] Otherwise, an adjustment of the instrument with a subsequent contrast is required. These last two steps are repeated as many times as necessary until the error obtained is less than the established limit. In equipment in which adjustment is not available, such as thermocouples, etc.; If the marked tolerances are not met, the devices will have to be replaced with others that have been previously calibrated.

Metrological control on site

Introduction

Calibration process

The calibration process begins with the design of the arithmetic measurement instrument that must be calibrated. The design has to be able to “withstand calibration” throughout its calibration interval. That is, the design must be capable of taking measurements that are within the “engineering tolerance” when used under ambient conditions for a reasonable period of time. The exact mechanism for assigning tolerance values varies depending on the country or type of industry. In general, measurement equipment manufacturers assign measurement tolerances, suggest a calibration interval, and specify the range of normal use and storage. Having a design of these characteristics increases the probability that current measurement instruments will behave as expected. The next step is to define the calibration process. The selection of the standard or standards is the most visible part of the calibration process. Ideally, the standard must have less than a quarter of the measurement uncertainty that is given by the device to be calibrated.[6] The process consists of choosing a standard that meets the previously mentioned standard on measurement uncertainty and using it to compare its measurement with that of the calibrated device. After choosing a standard with a tighter degree of uncertainty and repeating the previous operation. This process is repeated until the standard with the greatest possible certainty is reached that is available in the calibration or metrology laboratory. This process establishes calibration traceability.

It must be said that this calibration process using standards is, practically always, preceded by a visual inspection of the instrument, where it is verified that it does not present any physical damage that can be seen with the naked eye. The results of this inspection are commonly referred to as the “as-found” inspection data. Normally the entire calibration process is entrusted to a single specialized technician who will be responsible for documenting that the calibration has been completed successfully.

The process that has been explained above is a difficult and expensive challenge. The cost of technical support for ordinary equipment is generally approximately 10% of the original purchase price on an annual basis. Other more exotic and/or complex machinery can be even more expensive to maintain.

The extent of the calibration program exposes the core beliefs of the organization involved. The integrity of the organization can easily be called into question depending on the calibration program that has been established. In general, each machine in an organization has a specific calibration process planned for it. For example, if a company has several of the same machines, the older machines will be used for the least demanding jobs and, therefore, will require limited calibration. Machines that are used frequently and on which the production process depends, on the other hand, will have to be calibrated more regularly and with fairly tight tolerances. On the other hand, each machine will have to be calibrated only in relation to the operation/task it performs. This means that although the machine can actually perform many more jobs than it actually does in the production process, only the work that it actually does actively has to be calibrated. The rest of the calibration processes will be unnecessary.

This process of choosing and designing the calibration process must be carried out for all the basic instruments that are present in the organization.

Measuring instruments

• - Vernier calliper "Caliber (instrument)") or universal Vernier caliper: used to accurately measure small elements (nails, holes, small objects, etc.). The precision of this work tool reaches the tenth of a millimeter and can even appreciate hundredths in twos (when the vernier is divided into fifty equal parts).

To measure exteriors, the two long extensions are used, to measure interiors (for example, diameters of holes) the two short extensions, and to measure depths, a stem that comes out from the back and is called a depth probe. To carry out a measurement, the caliper will be adjusted to the object to be measured and fixed. The movable extension has a graduated scale (10, 20 or 50 divisions, depending on precision).

The measurement using this instrument is carried out in the following way: First, the moving part is made to slide so that the object to be measured is between the two extensions (if it is an exterior measurement). The mobile extension will indicate the whole millimeters that the measurement contains. The decimals will have to be found with the help of the vernier. For this purpose, it will be observed which division of the vernier coincides with a division (any) of those present in the fixed rule. This division will coincide with the decimal values of the measurement being carried out.

• - Turner's caliper: it is very similar to the one described above, but it has nails adapted to the measurements of pieces on a lathe. This type of gauge does not have interior legs since internal measurements can be made with the exterior ones, but it will have to be taken into account that the value of the internal diameter will have to be increased by 10 mm due to the width of the legs of the instrument (5 mm each).

• - Depth gauge: it is a measuring instrument similar to the previous ones, but it has support points that allow the measurement of depths, gaps and holes. They have different base lengths and are also interchangeable.

• - Horizontal coordinate bank: it is a measuring equipment for the calibration of measuring instruments. It is equipped with a highly precise ruler that allows errors in measuring and control tools, such as vernier calipers, micrometers, comparators, smooth and threaded rings, plugs, etc. to be checked.

• - Micrometer "Micrometer (instrument)"), micrometric bolt or Palmer: is an instrument used to measure with high precision (of the order of a micron "Micrometer (unit of length)"), equivalent to 10 meters) the dimensions of an object. To do this, it has two points that are brought together by a finely threaded nail, which has a scale engraved on its outline. The scale may include a vernier. Frequently, the micrometer also includes a way to limit the maximum torque of the nail, since the very fine thread makes it difficult to notice forces that would cause the precision of the instrument to deteriorate. The micrometer is classified as follows:.

• - * Exterior micrometer: these are measuring instruments capable of measuring the exterior of pieces in hundredths. They have ground and silver-plated hard metal contacts. They exert a medium pressure between 5 and 10 N on the piece to be measured; They have a brake to avoid damaging the piece and the meter in case it is tightened too much when the measurement is being made.

• - Dial indicator: it is an instrument that allows measurement comparisons to be made between two objects. It also has applications for aligning objects in machinery. You need a stand with a magnetic foot.

• - Displays with Digimatic input: it is an instrument that has the ability to digitally display the measurement of an analog instrument.

• - Interior checker: instrument used to take hole measurements and compare them from one piece to another. It has a dial indicator for greater precision and interchangeable parts.

• - Gramil or altitude gauge: it is an instrument that measures the angle formed by two views, encrypting the result obtained. This angle may be located in a horizontal plane and is called azimuthal angle or in a vertical plane, called zenithal angle if the origin side of the graduation is the zenith-nadir line of the station point; or height angle if this side is the horizontal line of the indicated plane that passes through the point of view or aiming point.

• - Level "Level (instrument)") of water: is a measuring instrument used to determine the horizontality or verticality of an element. It is a very useful instrument in construction in general and for industry. The principle of this instrument is a small transparent tube (glass or plastic) which is filled with liquid with a bubble inside. The bubble is smaller than the distance between the two marks. If the bubble is between the two marks, the instrument indicates an exact level, which can be horizontal or vertical.

• - Tachometer: it is an instrument capable of counting the number of revolutions of an axle per unit of time.

• - Voltmeter: instrument to measure the potential difference between two points.

• - Ammeter: instrument for the intensity of current flowing through a branch of an electrical circuit.

• - Multimeter: instrument capable of measuring different electrical measurements such as voltage, resistance, normal current intensity in a circuit, as well as some other functions that the instrument has depending on the manufacturer and model.

• - Stroboscope: It is an element capable of counting revolutions and vibrations of a machinery, without having physical contact, through the field of action that it generates.

• - Thread and thickness gauges: they are comparison rules to see the type of thread of a nail or the width of an element. The thread gauge can be a Whitworth metric thread.

• - Scale: instrument that is capable of measuring the weight of a certain element. There are different sizes and different weight appreciation ranges.

• - Go-no-go gauge (cap gauge):.

• - Instruments for optical inspection:.

• - Thermometer: instrument that allows temperature measurements.

• - Roughness meter: is an instrument that, using waves, is capable of measuring the roughness "Roughness (mechanical)") of the surface of an object, without the need for visual magnification of the surface of the object.

• - Laser, as a measuring instrument.

• - Durometer: electronic instrument that allows measuring and testing the hardness of different materials, whether metal, ceramic, plastic or stone.