Pan balance

The type of balance referred to abo e has now been superseded by far more accurate and more rapid balances w hich do not retynre the method of weighing described (p. 466). This book may, however, be used in laboratories which do not possess these balances, or use single-pan balances. 

With these top pan balances it is not necessary to shield the balance pan from gentle draughts, and weighings can be accomplished very rapidly and with the usual facility of the results being recorded with a printer. 

For small test quantities or for some premium quality production, direct reading one-pan balances are used. They are faster than analytical balances and provide an accuracy within one percent. Automatic weighing machines are also available... 

A single-pan balance (Fig. 23) is intended for rapid weighing without the use of weights. The balance is designed for weighing loads with a mass from 0 to 200 g. 

When a two-pan balance is "balanced," the total torque on the balance is zero, and you can set the clockwise torque equal to the counterclockwise torque because the lever principle,... 

For a two-pan balance, the lever arms are equal (Lt = L2), the pan weights are equal, and the effect of gravity cancels, so that "at balance" the situation simply reduces to the fact that the effective masses of the object (o) and the weights (w) are equal. That is,... 

This same expression can be derived for single-pan balances that use the method of substitution of weights. The fact that the lever arms are unequal and that there is a constant load on the balance does not alter the final expression. 

Briefly, there are two kinds of mass gravitational mass and inertial mass. Gravitational mass is what is measured on any kind of weighing machine (classically a pan balance, in which the mass to be measured is weighed against a collection of standard masses) inertial mass is what is measured in a collision experiment between the mass to be measured and a standard mass. In each case, the measured quantity is measured relative to some chosen standard, and therefore has no absolute significance. 

Table 2.12 Corrections for Water Weight Determinations for Borosilicate Glass Using a One Pan Balance (Nominal capacity 100 mL)...

Corrections required when weighing volumetric flasks are somewhat different than straight volumetric readings. Both single- and double-pan balances have four common parameters which can affect the accurate weighing of liquids, but the single-pan balance has one separate parameter of its own. The common parameters are water density, glass expansion, and the buoyancy effect. 

As Table 2.13 shows, for most laboratory work, the parameter that is likely to have the greatest effect on measurements when using a two-pan balance is water temperature. Any weight measurements made when the liquid temperature is not 20°C can be corrected by using Table 2.12. This table is used when the volumetric flask is Type I, Class A (borosilicate glass). It may also be used for Type I, Class B... 

Never leave a single-pan balance without first setting the pan(s) to the... 

Analytical balances are those of Class I and Class II (see Table 2.21) and require a division of the scale s capacity that is typically smaller than 10 5 of the total capacity with a readability of 0.1 ig to 0.1 mg (10 7 to 10-4 g). Analytical balances have the greatest precision for the most demanding work. For many years, only a properlymade two-pan balance could achieve the precision required to obtain the required level of accuracy. Despite their accuracy, weighings made on two-pan balances were slow and inefficient. Eventually, technology improvements in single-pan balances brought them to meet and eventually exceed the capabilities of two-pan balances, yet maintain their speed and efficiency. 

Tt is unlikely that you will find the classic two-pan balance in use in laboratories. In a way this omission is a pity because they are typically beautifullymade instruments. For those of you who may still use them, I cannot fault you for your romantic actions. For those of you who are considering getting rid of one to make room, please do not. You do not need to use it, just keep it and let its beautiful construction remind you of the art that is possible in technology. Because of the rarity of two-pan balances in contemporary labs, there will be no discussion of their use or operation in this book. 

Regardless of whether a single-pan balance is completely mechanical or a hybrid, there are several advantages of the single-pan balance over the two-pan balance ... 

Single-pan balances typically have damping devices that help to minimize the free swinging of the lever arm. This device speeds the weighing process immensely over the time spent counting swings for two-pan balances. 

---