Single-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The type of balance referred to abo e has now been superseded by far more accurate and more rapid balances w hich do not retynre the method of weighing described (p. 466).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. 

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. 

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. 

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. 

Calibration correction tables for single-pan balances are provided in Table 2.14, which is used in the same manner as the previous example. Like Table 2.12, this table is used when a volumetric flask is Type I, Class A (borosilicate glass). It may... 

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. 

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. 

Electronic single-pan balances with digital readouts are now favoured over mechanical types and are common in most laboratories. There are essentially two types of balance ... 

Fig. 4.3 Single-pan balance (a) general purpose, two decimal places (b) analytical, four decimal places.

The single-pan balance is a commonly used balance in the clinical laboratory. It is most often electronically operated and self-balancing. Such a balance may be coupled directly to a computer or recording device. In the electronic singlepan balance, a load on the pan causes the beam to tilt downward. A null detector senses the position of the beam and indicates when the beam has deviated from the equilibrium point. 

The density of masses used in single-pan balances (or to calibrate electronic balances) ranges from 7.8 to 8.4 g/cm, depending on the manufacturer. Use of 8 g/cm is adequate for most purposes. If greater accuracy is required, the specifications for the balance to be used should be consulted for the necessary density data. 

A triple-beam balance with a sensitivity less than that of a typical top-loading auxiliary balance is also useful. This is a single-pan balance with three decades of masses that slide along individual calibrated scales. The precision of a triple-beam balance may be one or two orders of magnitude less than that of a top-loading instrument but is adequate for many weighing operations. This type of balance offers the advantages of simplicity, durability, and low cost. 

Single-pan balance An unequal-arm balance with the pan and weights on one side of the fulcrum and an air damper on the other the weighing operation involves removal of standard weights in an amount equal to the mass of the object on the pan. 

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