< Calibration of instruments

According to the International Vocabulary of main and general terms of Metrology (VIM), the calibration is a ensemble of operations aimed to settle, in certain pre-set conditions (see Calibration Procedures), the correspondence between the values indicated by an instrument and those values known refered to a master, ensuring thus the traceability of the lectures given by the instrument to the basic units of the International System (SI), adjusting the instrument if necessary and expressing this correspondence by means of a table or a graphic.

Abad Controls, s.l., certified company according the ISO 9001:2000 Norms, issues calibration tests for pressure instruments with accuracies 0.6, 1.0, 1.6 and 2.5 with demonstrable traceability (upon request up to the primary national or international master). Anyhow, notice that our test certificates are not official tests; in that case we refer to any of the national laboratoires authorized for that purpose.

 Reasons to calibrate

The aging of the components, changes of temperature and the mechanic stress suffered by the instruments slowly deteriorates their functions. When this takes place, the measures began to lose reliability and therefore the performance as well as the quality of the product decreases. This depletion can be detected and limited through the calibration process.

A proper calibration of the instruments provides the certainty that our instruments gather the required specifications. Therefore the point of these confirmations is:

- Maintain and verify the correct performance of the equipment.

- Give response to the required quality standards, (e.g.: requirements related to the ISO9000 Norms).

- Assure the reliability and traceability of the measures.

The calibration of an instrument allows to know its uncertainty, an essencial value within a quality system to group the instruments in metrologic categories for its further use.

Traceability

Is the property of an instrument that allows to refer it to proper measure masters, usually national or international masters, by means of an unbroken chain of comparisons. According to the ISO 9000 Norms, all the masters used in instrument calibration should be traceable higher rank masters (also called primary masters) belonging to an authorized laboratory, it is, tested in turn by an official organization with a master of better accuracy. Those primary masters should attend their own test certificate, where will clearly appear the source, the uncertainty, the calibration date and the environmental conditions when the results were obtained.

A unique calibration mark or TAG is documented for every instrument, that will also be reflected on the corresponding certificate. On the certificate it will be documented as well the type, serial number and uncertainty of the master used, calibration date, stated interval recalibration, calibration method, obtained calibration results, maximum error permitted, master traceability, environmental conditions and names of responsible) of calibration as well as the calibration department chief signature.

The masters used are at least four times better for class 0.25, and until forty times better for class 2,5.

Duration ot the test certificates

It is not stated a standard interval of metrological calibration. It depends on the case of application and frequency of use, accuracy required, etc. AbCo recommends an annual recalibration of the pressure gauges.

< Types of certificate

1. AbCo Calibration Report

Made according the internal Test Procedure Nº 837-1.
This certificate has two pages. On the first one it is detailed the characteristics of the instrument to be calibrated, as well as those of the master used and its traceability. In the second one are transcribed the readings rising (increasing value) and falling (decreasing value), reflecting the error % span on a calibration graphic.

2. AbCo Calibration and Uncertainty Report

In this case we apply the Calibration Procedure P-002 edited by the Industry and Energy Ministery of Spain.
That certificate is composed of three pages. On the first one are related the features of the instrument to be calibrated as well as the master and its traceability. On the second one are indicated the obtained results (3 cycles) upwards (increasing) and downwards (decreasing), and drafting from these results the medium value, correcting value, the histeresys and the errors % of span, reflected on a graphic. On the third page are indicated the typical deviations and uncertainties obtained, assigning to the instrument the largest one.

< Calibration procedures

CALIBRATION PROCEDURE Nº 837-1

1. The instrument to be calibrated will be introduced inside the room where the calibration will take place, in order to make environmental conditions as similar as possible as those of the reference master. The test room is equipped to achieve the environmental conditions (temperature, humidity, etc) required.

2. Both instruments (master and the one to be calibrated) are connected to the pressure or vacuum generator depending the range:

- Pneumatic pump for verification through compurgation. Fluid used dry and filtered air. For ranges from o up to 6 bar.

- Hydraulic pump for verification through comparison. Fluid used distillated water. For ranges from 6 to 800 bar.

- Vacuum pump, for ranges from 0 up to -1 bar.

3. Measure units (bar, kg/cm², psi…) are introduced on the master pressure transmitter used to make the verification.