Neglect or abuse can damage a balance resulting in no deflection for the original smallest units. The result is a change in the readability for the damaged balance to a new, and larger, value.

Linearity is the ability of a balance to accurately read the entire range of weights that it was designed to weigh. A balance which accurately weighs 10 mg, but poorly weighs 100 mg, but again accurately weighs 200 mg on up to its full-scale calibration, is said to have poor linearity.

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.

Accuracy is the ability of a balance to precisely and repeatedly read a weight. The ability for a balance to read a particular weight is based not only on the above attributes and characteristics, but also on three other factors:

1.The quality of the machine

2.The quality of the weighing process conditions [typically dependent upon the balance's location (see Sec. 2.4.5)]

3.The skill of the person operating the balance

Each of these factors is dependent on the other two, and the failure of any one of them can affect the accuracy of a weighing. For example, do not expect great accuracy from a balance that is located above a radiator. Likewise, do not expect accuracy from a balance which has just demonstrated poor precision. Finding the source of errors in weighing is a step-by-step process. You must rule out each problem before moving on to the next level.

2.4.5 Balance Location

By their nature, balances are fragile pieces of equipment. The more sensitive a balance is, the more susceptible it is to environmental influences. The following should be considered before placing permanently in any location:

1. Balances should be away from sources of vibration. Rooms used for balance work should be away from elevators and ventilation motors. They should be located near support walls (as opposed to walls that just separate rooms), which can help dampen vibration. To observe the amount of vibration your balance is receiving, lay small dishes of water at several locations on the supporting table you plan to use and float microscope cover slips on the surface of the water in each dish. Shine a light on each cover slip in turn, such that the light is reflected onto an adjacent wall. Make observations over several times during the day. Have someone walk around the room, close or open the door, and perform other activities that may cause vibrations. The more sta-

room. For drafts, baffle vents, place screens around the balance, or use other similar stop-gap measures.

2.4.6 Balance Reading

Consider the following scenario: Many people each make one reading on the same balance, using the same weight for each reading. All readings are within one-hundredth of a gram except that of one user, whose reading was one-tenth of a gram off. It is safe to assume that the user does not know how to use, or read, that particular balance or how to make an accurate weighing on any balance. Either way, this user needs some help in balance use.

To best examine a person's ability to use a balance, have him or her make multiple weighings (e.g., 10 times) using the same balance and weight. If he or she consistently makes the same type of error, the readings will have very little variation. If he or she makes a variety of random errors, the readings will vary with no consistent pattern or be caused by not preventing the effects of electrostatic forces (see Rule 11 below).

It is impossible to provide instruction for all types of balances in a book such as this one. However, it is possible to provide simple, generic, general rules for the operation of a balance. These general guidelines for balance operation are the same regardless of the specific type of balance:

1.Never force a lever, pan, door, or other part of a balance. If balance parts do not move smoothly and easily, there is probably a reason, and brute force will not be the cure.

2.Never drop a weight or a sample, or let one fall on any part of the balance (for that matter, do not drop a weight or sample at any time).

3.Never handle a calibration weight with your fingers. Even if the tolerances you are working with are well above the limits for weighing a fingerprint, the oils from your skin could corrode the weights. Besides, if you get in the habit of handling low-quality weights (see Sec. 2.4.12) with your fingers, you are likely to continue the habit with high-quality weights.

4.Never handle an object to be weighed with your fingers when making analytical weighings, because your fingerprints will alter the weight. Use tongs or tweezers.

5.Never let a spill sit. Keep some paper towels within reach of a balance to help mop up any spill immediately. Such a spill may pose a health danger or could damage the balance.

6.Never place or remove a weight on a balance without first verifying that the pans or trays are in the arrested position—that is, the pan(s) are securely supported and cannot move. Otherwise the action could cause the blades to slip from their pivot points and/or damage the blades.