compensated for in the electronics, especially when used in conjunction with servo-controlled balances.

2.4.9 Beam Balances

Beam balances fall into the unequal arm balance category. Generally, they are not capable of weighing very light objects (<0.1 g). They obtain greater precision by having a separate beam for each decade of weight capability (by splitting up the measurements on different beams, you obtain greater distance for each subdivision of weight. Otherwise, the gradations are closer together, making it more difficult to distinguish between them).

As seen in Fig. 2.27, the higher weight readings (the 100and 10-unit beams) of a beam balance have notches at regular intervals, which forces the sliding weight to consistently land in the same spot each time the weight is moved to that location. This design helps ensure precision (repeatability) for those weight ranges as well. The 1.0-unit beam has no notches, which allows the user to slide the counter weight along the beam until the balance arm is in balance.

Before using a balance, be sure that it is level with the surface of the table. Some beam balances have a built-in level for this purpose. Screw the legs of the balance until the balance is level. Balances that do not have built-in levels are not expected to have the accuracy of those requiring precision-leveling. Regardless, you should always level the balance as best you can by eye.

Before an object is weighed, the balance should first be nulled, or zeroed. This zeroing procedure is done with nothing on the balance pan and by placing all of the weights (on each beam) to their zero points. If the balance does not indicate "centered," rotate the balancing screw (on the pan side of the balance) as necessary to zero, or "balance" the balance. Balances that require you to complete (finish) the balance process with the beam in the same balanced (null) position at which you started are called null-type balances.

To weigh an object, place the unknown object on the center of the balance pan. Move the heaviest weight over, a few notches at a time, until the beam side is heavier than the pan side. Move that weight back one notch, then take the next heaviest weight and slide it over, a few notches at a time, until the beam side is again heavier than the pan side. Move that weight back one notch. Repeat this process for all of the weights on notched beams.

The lightest-weight beam has no notches. Slide the counter-weight until the beam and pan are balanced. Reading the weights on the balance involves adding up the weights on the various beams. Figure 2.27 shows the counter-weights on the three beams at "70," "8,"and "0.76." Thus, the reading of this triple-beam balance is "78.76."

After you complete a weighing and record the value in your log book, remove the object and return the counter-weights to their zero value.

decade are required. An example of the weights necessary to go from 0 to 10 would be 1, 1, 3, and 5 (the 0 to 10 range as shown in Table 2.24). The selection of weights is handled internally by the crankshaft. The user simply rotates the dial until the proper deflection is noted on the balance's viewing screen.

The counterbalance weights within a single-pan balance are described as having an apparent mass. The term apparent mass is used because what you are reading is different from true mass due to the effects of air buoyancy. If corrections are made for air buoyancy (see Sec. 2.4.3), you have true mass. In more recently manufactured single-pan balances,* the apparent mass has been changed from 8.3909 g/cm3 (brass) to the current 8.0 g/cm3 (stainless steel). This change was made to better approximate the density of grade S weights (see Sec. 2.4.13 and Table 2.26). The density of the apparent mass used within a balance should be identified somewhere on the balance and/or in its literature. If you are using a balance with the older apparent mass and switch to a balance with the newer apparent mass, you would need to add a 0.0007% correction to the total weight. This correction is equivalent to 7 mg in 1 kilogram and is so significantly lower than the tolerance of most balances that the correction is not really necessary.

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:

1.Because the weights are inaccessible to the user, there is no chance for the weights to be accidentally touched. Analytic balances are so sensitive that fingerprints, or corrosion from fingerprints, can alter the weight of analytic weights.

2.The single-pan balance can make use of optics and/or servomotors (see next section) to obtain the smallest weight measurements. By eliminating mechanical deflection of weights for the final part of the mea-

*This was changed in the mid-1970s.

surement process (such as a gold chain* or rider1), repeatability is substantially improved.

3. 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.

There are many different brands and models of single-pan balances. Each has its own mechanism for weighing an object. As in all other lab equipment, the original manual, or a copy of the same for information on how to use the balance, should be readily available. Regardless, the general procedures are typically similar: The larger weights are invariably manipulated by dials for each decade of weight. These dials are rotated until the appropriate weight is obtained or until the final weighing is complete.

Weighing an object on a single-pan balance typically involves first making sure that the balance is nulled. This procedure is done by setting all dials to "0," rotating the lever arm to the free arm position, and manually setting the servomotor or optical window to "0.00" to verify its "null" starting point. Once the balance is set, return the lever arm to the arrest position before placing the object to be weighed on the center of the pan, inside the door of the balance. For very sensitive weighing, do not handle the object with your bare hands. Select tongs that are sufficiently long so that your hands do not get into the weighing chamber area. Close the door and wait a few minutes to allow the internal sections of the balance to regain equilibrium for both drafts and temperature.

The weighing procedure is made by rotating the lever arm to the semi-release position. Slowly rotate the weight knobs from lighter to heavier. When the scale deflects, back up one weight calibration (for instance, if the scale deflects when the gram dial is rotated to "7," rotate the dial back to "6"). Continue the process for each weight dial until the final weight measurement is attained.

It is not possible to specifically explain how to make the smallest weight division weighing for single-pan balances in a book like this one because there are so many different ways it can be done depending on the brand or model of balance. Regardless, the general procedure for a final measurement is made by rotating the arresting arm from the partial freedom position to the full freedom position. Then dial, turn, or observe the smallest weight division for the completed weight. Once the weight has been determined, return the arresting arm back to the full arrest

A gold chain was used on one design of the single-pan balance. The gold chain was unwound onto the balance pan until the sample and internal weights balanced. The length of chain on the pan was directly equal to its proportional weight and therefore the length of chain was used to make the final measurement digit.

tThe rider is a sliding counterbalance that is used to make the final weight calibration on two-pan balances.