The Top-Loading Balance

Because the neutral (or null) position can easily be established with a weight on the pan, recalibration and taring to the null position (before actual weighing) are accomplished by pressing a button. 

Lest you think that the laboratory balance has been made perfect by the servomotor, realize that the servomotor is electronic and therefore is susceptible to various types of interference. Sources of interference include  

Electromagnetic interference. This interference can come from any elec- 

Dust contamination. Although it is easy to associate problems with dust 

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Place the top-loading balance on a clean even surface in a location sheltered from wind, dust, and rain. Ensure that the balance pan is level center the level bubble inside the window, if necessary. Clean the pan with the brush. 

Weigh the pycnometer with the sample on the top-loading balance. This is the Sample Weight in the calculations to follow. 

Thermobalances are available in two different designs, i. e. a horizontal construction (Fig. 5-1) and vertical construction. For the vertical construction there are two different versions the top-loading balance, which has the oven above the balance (Fig. 5-2), and the thermobalance with the sample tray hanging on a thin hangdown wire into the oven below the balance (Fig. 2-4). The suppliers claim some advantages for the horizontal arrangement such as complete lack of buoyancy, the avoidance of condensation in the balance cabinet, and the reduction of sensitivity to vibration. However the last two points cannot be confirmed by the author s experience. Despite an efficacious gas flow, a diffusion of condensates was observed opposite to the flow direction of the gas. The sensitivity to vibrations seems to be comparable with the vertical design. Of the vertically constructed balances, the top loader is more convenient to manipulate. 

The other option is weighing. Since low-tech glassware or disposable pipettes could continue to be used, obtaining a tare and a total weight on a top-loading balance would add only a few seconds for each sample, but inherently be much more accurate and precise than any operation involving just pipettes and graduated flasks. s... 

Bzik, T. J., Henderson, P. B., and Hobbs, J. R, Increasing the Precision and Accuracy of Top-Loading Balances Application of Experimental Design, Anal. Chem. 70, 1998, 58-63. 

Balances with few such significant figures to the right of the decimal point (zero to three) are often referred to as ordinary balances or top-loading balances (precision is 100 to 1 mg). A top-loading balance is an electronic ordinary balance with a pan on the top, as shown in Figure 3.3. The electronic... 

When a weight measurement enters directly into the calculation of a quantitative analysis result that is to be reported to four significant figures, is an ordinary top-loading balance satisfactory for this measurement Explain. 

Obtain samples of various foods and weigh approximately 1 g of each on a top-loading balance. Record and label each weight (as to the type of food) in your notebook Also in your notebook, give each an identifying number. Place each sample in a 100-mL round-bottom flask labeled with the identifying number. 

A top-loading balance is an electronic balance with the sample pan on the top. It is not enclosed, which means that it is capable of measuring only to the nearest hundredth of a gram. See Figure 3.3. 

In general, solid solutes should be weighed on weighing paper or plastic weighing boats, using an analytical or top-loading balance. Liquids are more conveniently dispensed by volumetric techniques however, this assumes that the density is known. If a small amount of a liquid is to be weighed, it should be added to a tared flask by means of a disposable Pasteur pipet with a latex bulb. The hazardous properties of all materials should be known before use and the proper safety precautions obeyed. 

Top loading balances show the highest accuracy (Fig. 2.10). Objects can be weighed very rapidly with these balances because the total weight, to the nearest 0.001 g, can be read directly off either an optical scale (Fig. 2.11) or a digital readout. Balances of this type are very expensive and one should be used only after the instructor has demonstrated their use. 

A calibrated weight obtained from the National Bureau of Standards had a value of 10.000 g. When it was used on a student s top loading balance, the balance showed the following readings 9.503, 9.499, 9.500. Comment on the balance s accuracy and precision. 

On a top loading balance, a beaker weighed 102.356 g. Express the quantity in kilograms and milligrams. Show your work. 

Using a single pan, triple beam balance (Centogram) or a top loading balance (if available), determine the mass of the block (3). Record the mass to the nearest 0.001 g. Calculate the density of the block (4). Repeat the measurements for a second trial. 

Off-center errors are problems specifically associated with top-loading balances. Placing a balance pan above the fulcrum places different torques and friction on balance pieces that do not exist when a balance pan hangs. The problem is exhibited if an object has different weight readings when moved to various locations across the surface of a top-loading balance pan. 

Fig. 2.25 Applications of the Roberval principle used with top loading balances. From The National Bureau of Standards Handbook 94, Figure 6, The Examination of Weighing Equipment by M.W. Jensen and R.W. Smith, U.S. Printing Office, 1965, p. 155. Reprinted courtesy of the national Institute of Standards and Technology, Technology Adminstration, U.S. Department of Commerce.

The single-pan electronic balance [Fig. 1 l(m)] capable of weighing to either 0.01 or 0.001 g and having a capacity of 100-250 g is very useful. Weighing is a pleasure with these balances. Although the top-loading digital balances are the easiest to use, a triple beam balance will work just as well. 

Figure 2-3a shows an electronic balance with the pan located below the cell. Higher precision is achieved with this arrangement than with the top-loading design shown in Figure 2-3b. Even so, top-loading electronie balances have a precision that equals or exceeds that of the best mechanical balances and additionally provides unencumbered access to the pan.

Although an analytical balance can be used for calibration, weighings to the nearest milligram are perfectly satisfactory for all but the very smallest volumes. Thus a top-loading balance is more convenient to use than an analytical balance. Weighing bottles or small, well-stoppered conical flasks can serve as receivers for the calibration liquid. 

What if I have a top-loading balance without a cage—do I have to have only closed containers on it while weighing Yes. Cuts down on the mess. An open container on a balance pan is usually an irresistible invitation to load product into it while on the pan, spilling the product onto the pan. 

Fluidized bed feeders generally give more consistent homogeneous p.c. flow for low feed rates than other types of feeders (13-15) The fluid bed feeder used in this investigation is shown schematically in Figure 4. The feeder rested on a top loading balance in order to continuously monitor the p.c. feed rate. It delivered between 0.15 and 0.3 grams of coal with approximately 0.5 liters of air per minute. The p.c. particle size was kept between 60 and 325 mesh (250 to 45 pm) to insure consistent performance. 

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