Calibration of balances

Suppose a very large number of measurements could be made under conditions of measurement that allow all possible variation that could occur, including systematic effects from calibrations of balances, glassware, and so on. Also suppose that the material being measured was identical for all of these measurements, which were correctly applied with any identified systematic effects corrected for. The reality of measurement uncertainty is that these measurements would not be identical but would scatter around the value of the measurand. In the absence of any other information, this dispersion is assumed to be normally distributed, which can be described by two parameters, the mean (p) and the standard deviation (cr). It is not... 

Mass of Cd, m g (SI unit)3 5 x 10-4 Certificate of calibration of balance giving period of validity of calibration and uncertainty... 

General principles of calibration of course apply to speciation analysis (Griepink, 1993 Quevauviller et al., 1996a). All efforts made to obtain a good sample and perform the extraction under the proper conditions are spoiled if the calibration is wrong. Basic principles of QA apply here, including calibration of balance and volumetric glassware, verification of the calibrant purity and stoichiometry, verification of the stability of stock solutions, etc. 

Class 1.1 weights are used for calibration of low-capacity, high-sensitivity balances. They are available in various denominations from 1 to 500 mg. The tolerance for any denomination in this class is 5 xg. They are recommended for calibration of balances using optical or electrical methods for accurately weighing quantities below 20 mg. 

A calibration schedule details the calibration of balances, volumetric glassware, automatic pipettes, thermometers, pH and conductivity meters, wavelength and photometric scales etc. The schedule consists of periodic external checks, employing a suitably accredited calibration service, supported by more regular in-house performance checks. 

Calibration of balances calibration of ultraviolet grating for wavelength accuracy exchange of lamps system suitability testing analysis of quality control samples and evaluation of results using control charts. 

The user can set the display to zero (tare) with any weight on the pan, up to the maximum adjust the degree of damping (stability) and select the weighing units required (although in the laboratory we invariably use grams). The calibration of balances is done with standard masses made of stainless steel with a density of 8.0 g cm-3. 

Besides the general rules given above, particular and severe requirements underpin the BCR certification studies. In all cases, detailed protocols and reporting forms are prepared, requesting each participant to demonstrate the quality of the measurements performed, in particular the validity of calibration (including calibration of balances, volumetric glassware and other tools of relevance, use of calibrants of adequate purity... 

Instrumental and reagent errors. These arise from the faulty construction of balances, the use of uncalibrated or improperly calibrated weights, graduated... 

The quality control unit in a cosmetics company supervised the processing of the weekly batch of shampoo by determining, among other parameters, the viscosity and the dry residue. Control charts showed nothing spectacular. (See Fig. 4.10, top.) The cusum charts were just as uneventful, except for that displaying the dry residue (Fig. 4.10, middle and bottom) The change in trend in the middle of the chart was unmistakable. Since the analytical method was very simple and well-proven, no change in laboratory personnel had taken place in the period, and the calibration of the balances was done on a weekly basis, suspicions turned elsewhere. A first hypothesis,... 

Note This experiment assumes that a permanent log and a quality control chart are constantly maintained for each analytical balance in use in the laboratory. Each day you use a given analytical balance and log in with your name and date. The following calibration check should be performed weekly on all balances. If, according to the log, the calibration of the balance you want to use has not been checked in over a week, perform this procedure. Review Section 3.3 for basic information concerning the analytical balance. 

Check the calibration of the analytical balance you have chosen to use by weighing this standard weight on this balance. When finished, store the standard weight in the specified protected location. 

What does it mean to check the calibration of a balance ... 

Gravimetric analyses do not usually require standard solutions of any kind and do not require the calibration of any equipment beyond a balance. 

In a typical experiment, a small volume of an insoluble surface-active material (dissolved in a water-insoluble solvent such as benzene) is placed atop a clean water surface. As the solvent evaporates away, a film remains and the moving barrier can be adjusted so that the surface film exerts pressure on the mica float. A calibrated torsion balance is used to measure the force that the film exerts on the float. That force divided by the length of the float is the force per unit length or the surface pressure. For studies of lipolysis kinetics , a Langmuir trough can be constructed so that one can measure lipase action under first-order and zero-order conditions. 

It is possible to check the calibration of a pipet, flask, or buret. The process involves weighing with a calibrated analytical balance. The volume of water (temperature noted) delivered or contained by the glassware is weighed. Then the analyst converts this weight to volume (using the density of water at the temperature noted), corrects the result to 20°C (the usual temperature of the factory calibration), and compares it to the factory calibration. If the difference is not tolerable, the piece of glassware is either not used for accurate work or a correction factor is applied. It should be pointed out that the thermometers used must be properly calibrated and that the timer used to measure the delivery time for the burets and pipets must also be calibrated. 

Write an SOP for the calibration of an analytical balance. Include a revision number, an indication of what SOP it replaces, the effective date, the purpose, the scope, the outline of the procedure, and references. Also include your name under "prepared by" and another name for "approved by."... 

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