Hysteresis in the balance is caused by excessive stretching of the springs, and it is primarily due to overloading or to the accidental dropping of an object onto the pan. Microbalances are very sensitive to overload and shock. When using a microbalance, set the lever to the rest position when adding or removing material; turn the lever to the weigh position to register the weight. In some cases, drift due to hysteresis can be eliminated by allowing the balance to stand without weighing long enough for it to recover. If stretching of the springs is excessive, an expensive balance overhaul may be needed. In the case of electronic force restoration balances, springs are replaced by flextures, and the term creep is more appropriate than hysteresis.
QUALITY ASSURANCE PROCEDURE FOR MEASUREMENT OF BALANCE DRIFT
Over an extended period of time, balance drift and other day-to-day variations are monitored by weighing a fixed check-weight on a regular basis; this check should be performed after the balance has been calibrated at the ambient laboratory temperature. The check should be made before the first weighing of the day or after any event that might disturb the balance's calibration (power failure, moving the balance to a new location, etc.). The check-weight may be any object whose mass remains constant and does not exceed the load limit of the balance. A balance weight makes a reliable check-weight. Each balance should be provided with a check-weight, which should be stored in a protective container near the balance.
Perform the following procedures to reduce balance errors and the possibility of an incorrect reading because of drift:
Make certain that the electrical power to the balance is on and that the level bubble is in the center of the indicator.
Calibrate the analytical balance or the microbalance. [NOTESome balances have a calibration lever, which must be returned fully to its original weighing position. Do not depend upon any prior calibration.]
The first person to use the balance each day should weigh the check-weight and record the weight in the log book for comparison with previous readings. If a deviation greater than those indicated below for Analytical Balances and Microbalances is observed, the balance should be reported for service. [NOTECheck-weights tend to gain weight upon standing because of mishandling and exposure to contaminants in the atmosphere. These weights can be cleaned by wiping with a lint-free cloth moistened with a small amount of an appropriate solvent such as diethyl ether.]
Select a check-weight of an appropriate mass to examine an analytical balance. If possible, set the balance to read to 5 decimal places. Follow the manufacturer's operating instructions. Pick up the check-weight with a forceps, place it carefully on the balance pan, and weigh it. [NOTE
Do not drop the weight on the balance pan, because damage to the balance could result.]
Place the weight in the center of the pan to eliminate corner-weighing differences. The accuracy of the weight is not important: the only factor of interest is whether any drift has occurred. If no drift has taken place, the value should remain constant. Periodic weighing of a fixed weight will determine whether the boards (or knife edges in mechanical balances) in the instrument are defective. The check for drift at the most sensitive position will show whether a problem exists; the variation in the observed weight does not exceed ±0.2 mg. For example, with a 20-g weight, if the mean value of the readings were 19.9984, the tolerance would be from 19.9982 to 19.9986 g. Thus, several readings must be taken before one can establish a tolerance. [NOTE
The check-weight need not be of high accuracy, but it is essential that its mass remain constant. In addition, the tolerance does not correspond to the value of 0.1%, specified under Weights and Balances 41
, for weighing material accurately. Rather, the tolerance is purposefully tight to reveal possible drift or calibration errors; this tolerance is readily achievable with modern electronic balances.]
Proceed as directed for Analytical Balances, but use a check-weight appropriate for the particular balance. For example, a 100-mg check-weight might be selected for a balance that has a load limit of 150 mg; or a 10-mg check-weight might be used for an ultramicrobalance with a load limit of 15 mg. (The operator must know the maximum capacity of the balance to select the correct check-weight.) The balance indicates the weight in milligrams. Record the weight as soon as the reading is stable for a few seconds. The variation in weighings ought to be within a range commensurate with the specifications given by the balance manufacturer, but not greater than 0.1% of the amount of material typically weighed on the particular balance. For example, if 10-mg samples are routinely weighed, the variation in the weighings of the check-weight cannot exceed 0.01 mg.