Inaccuracy weighing

It is likely that volumetric measures were used for quantity deterrnination when commodities were first bartered however, it has been established with certainty that weighing scales or balances have been in use for at least 7,000 years (1). Measuring by weight instead of by volume eliminates some very considerable inaccuracies from, for example, changes in specific gravity of liquids with temperature, or changes in density of solids owing to voids. 

Furthermore, if one takes into consideration that the combined inaccuracy of any point is determined by the law of propagation of error from the individual inaccuracies of each step leading to the final measurement, it becomes obvious that it does not make sense to use excessive accuracy at one step if another step in the chain of events has been much less accurate. For example, it is not necessary to use a high precision scale for weighing during the analysis of a sample that was taken from only one location without homogenization by splitting. 

The unidirectional flow velocity should be such that it does not disrupt the sensitivity of balances in weighing areas. Where necessary the velocity may be reduced to prevent inaccuracies during weighing, provided that sufficient airflow is maintained to provide containment. 

To increase confidence in the results a number of replicates wiU be needed. You will need to consider how many repeats will be suitable based on the technique and the level of accuracy required. The replicate samples should aU be prepared from the start and should not be, for example, just a variety of dilutions from a single stock solution. Any inaccuracy in the initial weighing or making up of the stock solution will lead to subsequent dilutions also being inaccurate (although the precision may be good). Remember accuracy and precision do not mean the same thing ... 

Electronic scales use a weighing device called a load cell underneath the platform. The load cell, an aluminium alloy beam, eliminates the need for springs, cogs, or other moving parts which can wear, break, or cause inaccuracy in mechanical scales. 

Maximal measurement uncertainty at volume measurements could be calculated as well. However volume measurements are generally only performed as second choice if weighing would pose too many drawbacks for the reliability of the preparation process (see Sect. 29.1.2). Therefore, the acceptable measurement uncertainty for volume measuring depends strongly on the nature of the process in question. Usually only inaccuracy is taken into consideration. It is limited by requiring a minimum filling of the device to be used, as is explained in Sect. 29.1.7. More exact calculations would be irrelevant. 

The measurement uncertainty of weighing will usually lead to a deviation due to inaccuracy and imprecision of not more than 1 % [10]. Volume measurements however, may show a deviation of 1.5 % under the most favourable conditions. Under less favourable conditions this can easily rise to 3 % or more (see Sect. 29.1.7). 

For some steps of the preparation processes, however, weighing is not or hardly possible or volume measurement has advantages for reasons that outweigh inaccuracy. Examples include the adjustment to volume of large amoimts of solutions, working in a laminar flow cabinet, and preparing parenterals prior to administration on the ward. 

A typical e-value for an anal3 tical balance (acciuacy class I) is 0.001 g, indicating an operational inaccuracy limit of 0.001 g in the lower part of the weighing range. For less sensitive precision balance (accuracy class II) a typical e-value is 0.1 g. 

When using graduated pipettes for more viscous liquids, an additional inaccuracy must be taken into account. Viscous fluids are therefore preferably weighed (see Sect. 29.1.2). 

Precision balance (Beam balance) With a resolution of 3 mg, for instance, these balances dispose of a capacity of 30 kg. They are also applied for the production of calibration gas mixtures according to the method of re-weighing with an inaccuracy of up to 0.01% rel. (Fig. 9.1). 

At a capacity of 150 kg, for example, electronic balances show a weighing inaccuracy of up to 0.1 g. Meanwhile, there are also solutions for the re-weighing method with fully-automatic connection and disconnection of the filling pipe under the application of electronic balances [9.13]. 

The importance of the analytical control depends on the preparation tolerance of the mixture method applied. In case of preparation methods with higher inaccuracies it is common practice to determine the mole fraction by means of analysis. Even if the mixture is gravimetrically produced, analytical control cannot be renounced. On the one hand, it can never be excluded, for instance, that systematic weighing errors or individual errors of the operator occur. On the other hand, above all in case of corrosive components, adsorption effects and reactions with the inner surface of the gas cylinder cannot be excluded. Such effects can only be detected analytically. 

Measurement of mudcake properties. The mudcake model used required independent lab measurements for permeability, porosity, and solid fraction. This implied the need for tedious, time-consuming tasks involving weighing, drying, sorting, and so on, procedures not unlike those reported later by other authors (e.g., Holditch and Dewan, 1991 and Dewan and Chenevert, 1993). The inaccuracies present in such tests pose hurdles to practical field implementation, since any formation predictions obtained would only be as... 

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