Mass calibration

In the mass calibration laboratory, based on the scope of work, the certification of qualification and national and international standards of weight calibration, calibration of scales, calibration of scales, calibration of baby’s height and weight are calibrated by experienced and trained experts.

Stability and change of specifications with time (drift) of standard reference weights

Possible changes in the mass of the reference standard weights from the time of their previous calibration should be considered. These changes can be estimated from the results of their successive calibrations. If such a date is not available, it is common to assume that their mass change is equal to the uncertainty of measuring two consecutive calibrations. The stability of weights is affected by the materials and their quality of construction, surface smoothness, instability of the materials regulating their mass, physical damage and burnout, and environmental pollution. If the environment or method of using standard weights changes, its stability needs to be evaluated. The calibration distance for reference standard weights depends on their stability.

Weighing device (scales) or weighing process

The performance of the scales used for calibration should be evaluated and its contribution to the uncertainty of the whole weighing process should be determined. In this performance appraisal, all items related to the weighing instrument that have a significant effect on the weighing process should be considered. The weighing process evaluation may include all or several of the following:

  1. A) Measurability of measurement
  2. B) The degree of linearity in the range used
  3. C) Readability, resolution in digital displays and weight classifications of analog markers
  4. D) Place the weights away from the center of the scales in the calibration process, especially if a group of weights are placed in the scales at the same time;
  5. E) Magnetic effects, for example due to magnetic weights or due to force leveling motors that affect cast iron weights;
  6. C) Temperature effects such as temperature difference between weights and scales
  7. G) Arm length error in the scale with two equal arms

Effects of air buoyancy

The accuracy of the amount of corrections required due to the effect of air buoyancy on mass calibration depends on knowing the density of the weights used and how the air density is determined. The density of the weights can be determined by some laboratories, but in most criminal cases, they presuppose pre-determined values. An equation is usually used to determine air density after measuring air temperature, pressure, and humidity. This equation is stated in a document published by the British NAMAS Institute with the number NIS 0416. If the degree of accuracy is considered, it is necessary to measure the amount of carbon dioxide in the air. Pre-determined values ​​for air density in the range of 1.079 to 1.291 kg per cubic centimeter are assumed to be in the range of 30% to 70% humidity, air temperature 10 to 30 degrees Celsius and atmospheric pressure from 950 mm to 1050 Millibar is accessible. For mass comparisons (mass calibration by comparing the weight of the calibrated weight with the reference standard weight mass), the buoyancy uncertainty is typically as follows for different weights:

For ordinary weights made of stainless steel, nickel, brass, German silver: ±1ppm

For cast iron weights: ±3ppm

For aluminum weights: ±30ppm

If in the mass calibration laboratory mass comparisons are made in a certain range of temperature, pressure and humidity, the buoyancy uncertainty can also be smaller, and if the environmental conditions are such that the density If the air is 1.2 kg per cubic centimeter, the effect of buoyancy and its correction can be ignored. If the effect of buoyancy on the measurement results is corrected, the uncertainty can be virtually ignored, but the uncertainty of the corrected value must be evaluated.

Some scales show the mass directly from the force applied to them. For example, scales that electromagnetically, using a coil, compensate for the force applied to the scale and the change in the output voltage of their coil is displayed as a mass. These scales will usually be subject to larger buoyancy uncertainties. For scales that use weights made of stainless steel, nickel, brass, German silver, the uncertainty of the buoyancy effect can be as high as ±16ppm, for cast iron weights as high as ±18ppm  and for aluminum weights as high as ±45ppm  will be.

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