2.4.11 The Top-Loading Balance

do not need to bother trying to weigh with a servomotor on such a surface.

2.Electromagnetic interference. This interference can come from any elec- tromagnetic-emitting field or source such as a CRT (from computer screens), RF generator, and radio transmitter. Using a hand-held radio transmitter, one can test the effects of electromagnetic interference on a balance. Erratic behavior of the balance's display may also be caused by interference on a floor above or below the balance location.

3.Dust contamination. Although it is easy to associate problems with dust on mechanical balances, it is less apparent why dust would affect an electronic apparatus. The answer is that because there is movement within the servomotor itself, dust collections between the magnet and electric coils is likely to cause erratic measurements. Additionally, if dust-sized ferrous particles find their way to the electromagnet, the servomotor could be shorted and rendered useless.

Analytical top-loading balances (those that can measure one-thousandth of a gram or smaller) have covers or doors to isolate the balances from drafts. These covers also provide limited protection from accidental spills. Typically, generaluse, top-loading balances do not have covers and are therefore subject to damage from accidents. Plexiglass covers may be obtained for many models of top-load- ing balances to protect them against such problems. Even a cardboard box placed over a balance will help reduce dust and limit accidental spills on the balance.

One problem inherent in top-loading balances is that the weight of objects to be weighed can vary when placed at different locations on the weighing pan.* Although this inconsistency should not apply, the problem is often complex and has to do with the geometry of how a top-loading balance is made and where the weight is distributed. Fortunately, testing for this problem is easy: Make several weighings of the same object at various locations on the balance pan. You may want to pre-mark the pan with some numbered geometric pattern (such as a star) to readily identify the location of any weight changes.

2.4.12 Balance Verification

If you have determined that your balance is making inaccurate measurements and you have eliminated human error, you not only cannot trust any future weighings, you must question all past weighings to the point of the last balance verification. By maintaining written records of balance accuracy tests on a routine basis, the reliability of past measurements can be verified. Otherwise, every weighing made between the last verification and the first appearance of faulty readings is in

'Hanging pan balances, by their design, cannot have this problem.

138 Measurement

doubt.* If you find errors during equipment testing, you need to track their source and correct the problem. Otherwise, all future data will also be in doubt.

The tolerance of a given balance is based on the level of accuracy that the balance is designed to provide. The greater the tolerance, the less the precision. The less the tolerance, the greater the precision. The tolerance of a balance is a percentage of its last significant figure (in fact, tolerance is often defined by the last significant figure). If you have a balance which is accurate to ± 0.1 gram, you should not report a reading of 0.02 grams.

When we discuss a balance's quality, we generally are referring to its reliability and accuracy. A balance, no matter how sensitive, is not a quality balance unless it is reliable and accurate in its measurements. Because the accuracy of a balance can decrease from wear, dirt, or contamination, routine periodic verification is required. The manufacturer can provide suggested verification schedules that may have to be increased or decreased depending on the conditions in your lab.

All balances should be checked for:

1.Precision. Does the balance read the same weight over a series of measurements for the same object?

2.Accuracy and linearity. Does the balance read the same weight as that given for a calibration's nominal weight, and does the balance provide the same accurate weighings over the full weight range of the balance?

3.Readability. Is there accurate, repeatable deflection at the smallest unit of measurement that the balance is supposed to read, including any vernier or micrometer calibration (if present)?

4.Settling time. Does the balance take the same amount of time to settle at the final weighing as it did to null?

5.Response to temperature. Does the balance provide the same reading at 20°C as it does at 25°C?

6.Responses to other environmental disturbances. What are the effects on the balance of drafts, vibrations, electromagnetic fields, magnetic fields, and other conditions?

Tests 4, 5, and 6 help identify under what conditions you should not bother making weighings. They should be reevaluated each time balance verification is made, because general wear and tear may exacerbate any environmental influence.

All electronic balances should also be checked for:

7. Warm-up variations. Some balances may indicate different weight values for the same object depending on how long the balance has remained in operation. See if any of the six previous tests is affected

If the amount of inaccuracy is less than your experimental limits, there is no reason to throw out any measurement.

" From ASTM Designation E 617 "Standard Specification for Laboratory Weights And Precision Mass Standards," reprinted with permission.

if the balance has just been turned on, left on for one-half hour, or left on for several hours.

Finally, top-loading balances should also be checked for:

8. Off-center errors. Does the balance make consistent weight measurements when an object is placed at different locations on the balance pan?

2.4.13 Calibration Weights

Calibration weights should only be used to calibrate or verify the accuracy of a balance. Calibration weights should never be used to make weight determinations. They should never be handled directly with hands, and they should be stored in safe locations away from environmental dangers.

A balance should be verified using the same calibrated weight each time. Because calibrated weights have expected variations in tolerance, using different weights may yield varying test results and could lead you to believe your balance requires constant (minor) recalibration when, in fact, no such calibration is required.

The ASTM has categorized laboratory weights into the following divisions:

1. Types (I and II). Type refers to how the weights were constructed. Type I is of better quality than Type II. See Table 2.25.

2.Grades (S, O, P, and Q). Grade refers to how the surfaces of the weights are finished. S is better in quality than O and, likewise in turn, P and Q. See Table 2.26.

3.Classes (1,1.1, 2, 3, 4, 5, 6). Class refers to the amount of weight tolerance. The lower the Class number, the smaller the tolerance. Class 1.1 is a specialized class for calibrating low-capacity, high-sensitivity balances. See Table 2.27 & Table 2.28.

Table 2.26 Laboratory Weight Grades"

" From ASTM Designation E 617 "Standard Specification for Laboratory Weights and Precision Mass Standards," reprinted with permission.

Table 2.27 Tolerance by Class of Weights

a From theASTM document E 617, Table X3.1, Class 1Metric, reprinted with permission. * From theASTM document E 617, Table X3.3, Class 2 Metric, reprinted with permission. c From theASTM document E 617, Table X4.1, Class 3 Metric, reprinted with permission.

Table:2.28 Tolerance by Class of Weights