Calibration of soil lab equipment
Performance of laboratory tests for soil characterization is a fundamental part of any geotechnical project. Therefore, it is essential to calibrate the soil laboratory equipment used in such tests, so that the results obtained are reliable.
Content
What is calibration?
In the field of measurement technology, calibration is the documented comparison between a measuring device to be calibrated and a traceable reference device (Bureau International des Poids et Mesures, 2008). According to this definition, in order to calibrate a given instrument, a higher precision instrument, usually called a calibration standard, must be available, which provides a verifiable value for a certain measurement, which will be used to compare it with the indication of the instrument under calibration. la comparación documentada entre un dispositivo de medición que se va a calibrar, y un dispositivo de referencia trazable (Bureau International des Poids et Mesures, 2008). De acuerdo a esta definición, para calibrar un instrumento determinado se debe disponer de uno de mayor precisión, usualmente denominado patrón de calibración, el cual proporciona un valor verificable para cierta medida, que se utilizará para compararlo con la indicación del instrumento que está siendo sometido a la calibración.
Thus, during the calibration process, the output value generated by the instrument to be calibrated is compared with the calibration standard at different points in the calibration range. If the calibration error - i.e. the error generated during calibration - is below the determined acceptance limit, the calibration will be accepted. Otherwise, an adjustment of the instrument with a subsequent contrast is required. This process is repeated as many times as necessary, until the error obtained is below the established limit.
Accordingly, the calibration process revolves around two elements:
- A. The calibration error.
- B. The traceability of the calibration standard.
Figure 1 shows schematically the relationship between the calibration process and the calibration error. Basically, the aim is to bring the error below a certain accepted value, by adjusting the equipment if necessary.
Figure 1 Device calibration process
The other important aspect is the traceability of the calibration standard. But, what is traceability and why is it important? As mentioned above, any device to be calibrated must be compared to a traceable reference device. Basically, this traceability means that the reference standard must also have been calibrated to an even higher level standard. Thus, traceability must be a continuous chain of calibrations, such that the highest level calibration has been performed at a national calibration center or equivalent, linked to the standards of the basic units of the International System of Units. Figure 2 shows the traceability process.
In equipment where it is not possible to make adjustments, such as thermometers or some field equipment, if they do not meet the marked tolerances, the devices must be replaced by others that have been previously calibrated.
Need for calibration of soil laboratory equipment
According to calibration experts, there is an old rule of thumb that goes something like this: "all measuring devices measure poorly, and calibration tells us how bad these devices are".
Of course, that is not the only reason to calibrate. Calibration of soil laboratory equipment may be necessary for some of the following reasons:
- Measurement accuracy decreases with time.
- Compliance with current regulations stipulates a calibration process with some regularity.
- The quality system requires calibration
- Reliability of the results obtained in certain tests.
- Mobilization of equipment.
- Environmental reasons.
In addition, it can be mentioned that the purpose of calibration is to maintain and verify the proper functioning of equipment, particularly the most sensitive (i.e., with lower error tolerance), which require more frequent calibration.
Calibration frequency
How often should soil laboratory equipment be calibrated? The international standard COPANT 1689:2000/OML D-10 mentions that the decision on the calibration interval is based on what it calls engineering intuition. That is, someone with experience in the measurements or instruments to be calibrated, or with information on the intervals usually assumed in other laboratories, makes an estimate for each instrument or group of instruments, as to the probable time interval in which they will remain within the tolerance range after calibration. To do so, it considers the factors shown in Figure 3. intuición ingenieril. Es decir, que alguien con experiencia en las mediciones o en los instrumentos a calibrar, o con información sobre los intervalos usualmente asumidos en otros laboratorios, hace un estimado para cada instrumento o grupo de instrumentos, en cuanto al intervalo de tiempo probable en el que los mismos permanecerán dentro del intervalo de tolerancia después de la calibración. Para ello, considera los factores mostrados en la Figura 3.
Another important factor to consider is the costs associated with equipment calibration. To take this factor into account and to optimize calibration frequency intervals, the aforementioned COPANT 1689:2000/OML D-10 standard proposes several methodologies, which will be applied to groups of equipment (e.g. equipment for performing deformation and shear strength tests - TX, direct shear, consolidation, etc.) or to individual equipment (e.g. a nuclear densimeter):
- 1. Automatic adjustment or "staircase" method, in which each time a given piece of equipment is routinely calibrated, the subsequent interval is extended if it is within tolerances, or reduced if it was found to be out of tolerances. To apply this method, it is essential to keep and use the records corresponding to each piece of equipment (for example, each piece of equipment can have a folder in which all the information associated with calibrations, maintenance, transfers, etc. is integrated).
Figure 3 Elements to consider when establishing the frequency of calibration of soil laboratory equipment.
2. Control chart method, in which significant calibration points are chosen and the results are plotted on a graph as a function of time, from which the dispersion and drift are calculated for the different calibration intervals. From these figures, the appropriate time interval is determined to establish the calibration frequency
3. Time-in-use method, whereby the calibration interval is set in relation to the time the equipment is operated. The disadvantage associated with this method is that the time of use of the equipment must be strictly controlled
Another option is to adhere to certain standards to establish the calibration frequency of laboratory equipment. In this regard, BS-EN-ISO 17892-1:2014(E) proposes certain criteria for ovens, thermometers and balances to ensure that they provide reliable values.
Calibration certificate
As mentioned in section 1 of this post, the definition of calibration includes the word "documented". This means that the calibration comparison must be recorded, for which a document is generated, usually called a calibration certificate. certificado de calibración.
A calibration certificate includes the result of the comparison and all other relevant calibration information, such as the equipment used, environmental conditions, signatories, calibration date, certificate number, calibration uncertainty, among other data.
It is very important to keep calibration certificates up to date, since they are evidence of the equipment status, and serve as a basis for applying some of the methodologies mentioned in section 3 of this post, to establish the appropriate calibration interval for each equipment.
Accreditation of a laboratory
In recent years, it has become increasingly common that a requirement for a soil laboratory is that it must be accredited, in order to carry out tests in the framework of major projects. But, what does it mean that a laboratory is "accredited"? acreditado. Pero, ¿qué significa que un laboratorio “está acreditado”?
A laboratory accreditation is recognition of the laboratory's competence by a third part. Such accreditation is carried out in accordance with uniform principles worldwide and is most commonly based on the international standard ISO/IEC 17025.
Most international accreditation bodies belong to ILAC (International Laboratory Accreditation Cooperation) and subscribe to the MRA agreement (Mutual Recognition Arrangement - https://ilac.org/).International Laboratory Accreditation Cooperation) y están suscritos al acuerdo MRA (Acuerdo de Reconocimiento Mutuo – https://ilac.org/).
References
- BEAMEX (2021) ¿Qué es la calibración? Disponible en https://www.beamex.com/es/
- recursos/que-es-la-calibracion/.
- Bureau International des Poids et Mesures (2008) “Vocabulario Internacional de Metrología – Conceptos Básicos y Generales y Términos Asociados (VIM)”. Saint Cloud, Francia.
- Comisión Panamericana de Normas Técnicas (2000) “Norma COPANT 1689:2000 – Guía para la Determinación de los Intervalos de Recalibración de los Equipos utilizados en Laboratorios de Ensayos”. Sao Paulo, Brasil.
- The British Standards Institution (2014) “BS EN ISO 17892-1:2014 Geotechnical Investigation and Testing – Laboratory Testing of Soil”. London, UK.
