Humboldt State University ® Department of Chemistry

Robert A. Paselk Scientific Instrument Museum

 
The following complete booklet is posted with permission of the copyright holder, Fulcrum Inc., successor to Christian Becker.
 
Web version © Copyright 2000 R. Paselk
 
I have attempted to make the layout and appearance similar to the original. Thus all pages are presented, even the blank "memoranda" pages.
 


 
 

PREFACE


This booklet is provided for the basic purpose of advising the laboratory worker, student and scientist on the care and use of the Precision Balance. It is based upon the experience of over 100 years of manufacturing Balances, fortified by many sugges-tions from customers and scientific organizations. Our thanks go out to them for their assistance in round-ing out the practical usefulness of this work.

We are presenting here only the basic fundamen-tals. No attempt is made to discuss specialized techniques of weighing, or the operation of special -purpose balances. It is obviously impossible to create a comprehensive text book within this small compass. We therefore treat only those situations which arise in general laboratory practice.

Christian Becker


 

Copyright 1952


THE TORSION BALANCE COMPANY

Clifton, N. J.


 

 

CONTENTS


PRECISION WEIGHING page 7
A Pinnacle of Scientific Exactness
 
THE PRECISION BALANCE page 8
Principal Parts and accessories
 
ASSEMBLING A BALANCE page 11
MOUNTING AND LOCATION OF THE BALANCE page 12
1. Firmness of Support.
2. Freedom from Vibration.
3. Protection from Air Currents.
4. Protection from Dust Fumes.
 
GENERAL PRECAUTIONS page 15
To be observed in using a Balance.
 
METHODS OF WEIGHING page 19
Zero Methods:
1. Simple direct weighing.
2. Simple Substitution weighing.
Deflection Methods:
1. Simple deflection method.
2. Other deflection methods.
 
GENERAL CAUTIONS IN ALL WEIGHING PROCEDURES page 25
Parallax in Readings. Temperature Equality.
Equality of Relative Humidity.
 
DAILY ROUTINE IN CARE OF ANALYTICAL BALANCES page 26
CHECKING ANALYTCAL BALANCES page 27
General observations. Checking the Sensitivity. Check-
ing the Sensitivity Reciprocal. Checking the weighing
chain. Checking the rider concentricity. Checking the
beam notches. Checking the damper.
 
WHAT TO DO IF BALANCE IS NOT OPERATING PROPERLY page 31


 

PRECISION WEIGHING


7


PRECISION WEIGHING

A Pinnacle of Scientific Exactness

Precision weighing-or properly speaking, the accurate measurement of mass - is a foundation stone of modern science. It became a science in its own right as man progressed from the age of alchemy to the systematic study of chemistry. Lavoisier made the transition when he isolated hydrogen, oxygen, and nitrogen. The traditional concept of 'fire, air, earth and water' was swept away, and a new understanding of the universe, with elements as building blocks, took its place.

These new elements were predictable, stable. As soon as it was understood that weight was related to the nature of individual elements, infinitely precise weighing devices became a necessity. The laboratory balance was born.

Later, it evolved into a dependable instrument when the problem of protecting the knife-edge pivot bearings was solved by a stirrup-like arrangement which lifted the knife edge off the bearing surface when not in use.

Then, early in the nineteenth century, Justin von Liebig created the still more precise long-beam balance. Weights themselves, according to most opinion, were standardized some time around the French Revolution, and the art of infinitely close-tolerance weighing became the heart of man's search for scientific truth.

In the development of the modern Analytical Balance the name Becker has always been outstanding. Christopher Becker and his two sons, Ernst and Christian, started the manufacture of analytical balances and weights in the United States in 1835. At the end of the Civil War, manufacturing was resumed until, in 1884, the two sons established the business under the name, Becker Bros. Christian Becker, as it is known today, was established in 1892 when Ernst died, and his brother adopted the present name.

Based on the contributions of the Becker family, the precision balance of today is the finest instrument of its kind ever developed. It measures far more delicate differences in mass. The weights are of a still higher order of precision. The balance mechanism is better protected, easier to use, and easier to guard from abuse.

The infinitesimal fractions of weight which it will measure are still the most scientifically important measurements of mass. Every man entrusted with the manipulation of a balance does well to know his balance, to respect its capacity for scientific exactness, and to handle it accordingly.


PRECISION WEIGHING

8


 

PRECISION WEIGHING


9


PRECISION WEIGHING

10

RIDERS

For precision weighing the rider balance is one of the many refinements employed. The principal elements . . . the rider, the rider lifter, and the lifter control knob outside the case . . . enable the opera-tor to make weight adjustments without opening the balance door. Thus, delicate weighing procedures are not endangered by external factors. The notched beam permits the adding of additional weight up to one gram, in 100 mg. amounts, without opening the balance door.

CHAIN-WEIGHING DEVICE

Another refinement is the Chainomatic® attachment which, com-bined with a notched beam, speeds the weighing process. All weight adjustments from 0.1 mg. to 100 mg. can be made outside the case, by increasing or decreasing the length of the chain. This can be done while the beam is swinging. It results in an important saving of time.

MAGNETIC DAMPERS

The magnetic damper brings the indicator to rest quickly by stopping the swinging or oscillation of the beam. It means still greater rapidity in weighing while preserving the delicate knife edges.

PROJECTION SYSTEMS

The projection of a reading through a magnifying optical system makes it possible to perform fast, repetitive weighings. The ideal system provides a direct reading right from the screen equipped with a vernier or similar device for fractions of divisions on the scale Balances equipped with this device must be highly accurate, as any small error is magnified through the projection system.


 

ASSEMBLING


11

ASSEMBLING A BALANCE

For the protection of its all-important knife edges and delicate parts, a Precision Balance is disassembled for shipment, and its parts are packed separately.

While only the experienced laboratory worker should undertake the delicate task of assembling a balance, it may be well for all to understand something of how it is done, and how the delicate parts are handled.

It is not our purpose in this booklet to give complete and specific instructions on unpacking and assembling an analytical balance. Natu-rally, this procedure will vary with the type and make of balance. Therefore, we suggest that you follow the manufacturer's assembly instructions, which are ordinarily supplied with a new balance. When a balance is unpacked, retain the special box for the beam and other loose parts in case the balance need be disassembled for transporting or shipping at a later date.

When entrusted with the unpacking and assembly of a new balance, there are a few general precautions, however, which the individual should observe in every case:

1. Move slowly and carefully, steadying the hands against a solid object.

2. Before you start to assemble the balance, make sure you know where parts go and how to put them in place.

3. Be extremely careful of the knife edges . . . a single blow on one edge can ruin the balance.

4. Remember, an analytical balance is a precision instrument, be gentle and cautious in all assembly procedure.


MOUNTING AND LOCATION

12

MOUNTING and LOCATION of the BALANCE

It is fundamental that any balance for precision weighing be (1) solidly, rigidly supported, (2) free from vibration, (3) shielded from air currents, excessive humidity and varying temperatures, and (4) protected from corrosive fumes or vapors.

The sensitivity of the balance and the accuracy of its readings demand a thoughtful selection of location and preparation of the support. Consider the following factors.

1. FIRMNESS OF SUPPORT.

The supporting surface must be extremely rigid. When the beam is released and equilibrium is obtained, any pressure on the support must not cause any visible movement of the pointer.

The balance should be placed upon a solidly constructed bench or table. The type of construction of the building in-volved dictates whether the table or bench should be isolated from the wall or rigidly attached to the wall. If isolated from the wall, the bench or table must rest firmly on the floor and must be anchored securely.

The level of the support should be permanent. This elimi-nates repeated relevelling of the balance case and the readjust-ment of the zero point. The major thing to guard against is a lack of rigidity in the support which might permit tilting during the weighing operation.

2. FREEDOM FROM VIBRATION

The balance support should be free from any vibration that produces a visible effect upon the operation of the balance itself. Some modern balances have built-in vibration dampers. Otherwise, satisfactory elimination of vibration at the balance may sometimes be obtained by mounting on a heavy lead slab, supported on a sponge rubber cushion of medium resistance.


 

MOUNTING AND LOCATION


13

3. PROTECTION FROM AIR CURRENTS, CHANGES OF TEMPERATURE, and HUMIDITY

Balances should be mounted in a draft free area so that when the balance door is opened, air currents will not be set up inside the case. Any perceptible draft in the vicinity of the balance case interferes with weighings.

A balance should not be located near hot or cold objects, such as cold windows, radiators, electric ovens, stills, or areas that are at a different temperature than the rest of the room. When subjected to uneven temperature conditions, convection currents may be set up within the balance case. Such currents, no matter how minute, can spoil the regularity with which successive swings of the beam decrease in amplitude. (This is called irregular falling off.) It may even cause the zero readings of subsequent readings to vary.

Sunlight, or illumination from high-powered lamps in close proximity, should not be allowed to shine on the balance. Symmetrical illumination of the balance is preferred so that one arm of the beam or one side of the case, is not heated more than the other. Even symmetrical radiation may sometimes produce uneven heating effect. Therefore, it is essential to limit the total amount of illumination, or other radiation. These cau-tions may leave much unsaid, but there is no absolute rule to assure perfect temperature and radiation conditions.

For normally adequate illumination, common sense must govern. Thirty to fifty watt lamps may be used a few feet from the case. This will not harm most analytical work.

In general, temperature must be constant during weighing, as well as for a considerable period beforehand. The constancy required varies for the weighing of objects of different size and character, and for different types and styles of balances. A change of a few degrees in 4 or 5 hours is not likely to cause trouble in ordinary analytical work. It should be noted, however, that falling temperature is much more likely to cause trouble than is a rising temperature.


MOUNTING AND LOCATION

14

Humidity in the normal range of 35% to 60% need not be controlled, unless the zero reading of the balance itself is abnormally affected, or the substance weighed is particularly hygroscopic.

IMPORTANT
In order to avoid the danger of changes in atmos-pheric conditions, many laboratories have found it advantageous to install Weighing Rooms, with air conditioning for maintenance of constant tempera-ture and humidity.

In order to avoid the danger of changes in atmos-pheric conditions, many laboratories have found it advantageous to install Weighing Rooms, with air conditioning for maintenance of constant tempera-ture and humidity.

4. PROTECTION FROM DUST cod FUMES in SURROUNDING ATMOSPHERE.

Dust, or other foreign particles, may cause undue wear on knife edges, bearings, and precision finished parts. Naturally, metallic dust is more dangerous than non-eroding material, such as lint. Corrosive, sticky, or condensing fumes, may also have an adverse effect upon the balance.

For any kind of balance, local conditions will determine whether dust covers will suffice, or whether a separate weighing room is necessary. The seriousness of dust and fume conditions is often underestimated, and laboratories should take special precautions in this respect. Good judgment will balance all the elements of cost and time against the danger of appreci-able error.


 

GENERAL PRECAUTIONS


15

GENERAL PRECAUTIONS

to be observed in using a Balance

The function of an Analytical Balance is that of weight measurements to a high degree of accuracy. As has been said, the Balance s a scientific instrument, and while capable of long years of service without loss of efficiency, it needs protective care at all times. Here are guidance suggestions which should be read thoughtfully, and thoroughly understood, before any weighing is attempted.

1. Above all, the stirrups and beam must be lifted off their pivot knife edges by the beam arrest. They must remain so at all times, except when a reading is being taken.

The balance must be arrested (pan support and beam sta-tionary) while weights or material are placed on or removed from a scale pan. As a general rule, the balance should be completely arrested while making any changes that move any parts of the instrument.

Only when the balance is ready for a reading to be made should the stirrups and beam be lowered carefully-onto their knife edges. They must be lifted, or arrested, again immediately after the reading is taken.

2. The balance case should be kept closed. Even in weighing it should be opened only to place material or weights in the pans, and immediately closed.

The accuracy of balance readings cannot be depended upon, unless the case has been closed long enough to let the air within become quiet. This should not take more than a few seconds, unless temperature change has set up convection currents.

3. Placing the hands inside the balance case, or pressing the head against the front door, can introduce appreciable errors in weighing. However, ordinary analytical weighing does not usually require such extreme precautions.


GENERAL PRECAUTIONS

16

4. In adjusting the zero reading of a balance, complete arrest-ment should be made if the rider is moved on the beam. If an "equilibrium adjuster", "autodex" or any other device specifically designed to adjust the zero point is used, the balance need not be arrested. However, when in doubt, follow the manufacturer's operating instructions.

5. The stirrups and the balance must never be released with a quick jerking motion. Remember that "releasing" the beam is, in reality, setting a razor-sharp edge down on the hard agate bearing-do it gently! Never release the beam and stirrups further than necessary, to determine whether weights on the scale pan need be changed to effect balance.

6. The pan arrestments may be released as suddenly as desired. However, if pan arrestment causes a "kick" or flicker of the index reading, it may be avoided by a very sudden release. (On some balances, it may be avoided by a slow release.) In any case, careful experimentation will determine the most suitable method for releasing the balance.

7. In arresting a balance that is swinging free, always bring it to a stationary position by the pan arrests first. This can be accomplished most efficiently by engaging the pan arrest when pointer is close to the equilibrium point. Care should be taken to avoid arresting the free swinging balance when the deflection of the indicator or pointer from zero exceeds two divisions.

8. A studied development of skill in releasing and arresting the balance will pay dividends in avoided wear or damage. Speed should be sought through skill and overall efficiency, not in rushing.

9. Materials to be weighed should almost never be placed directly on the scale pan. An exception to this rule might be a solid piece of glass or nickel. Even inert powders, that would not corrode the pans, must be weighed in watch glasses or other holders. Any containers placed on the pans should be wiped clean of chemicals or other materials that might corrode the pans. Such a procedure keeps the pans clean.


 

GENERAL PRECAUTIONS


17

10. From the start to the finish of any single analysis always use the same set of weights, placing them on the pans with the forceps.

11. Although no error is introduced by dust that remains on the pan during weighing, dust may cause other inaccuracies in the operation of the balance. Pans should be dusted with a camel hair brush. The floor of the balance case should also be kept perfectly clean. After weighing is completed, weights should be placed in the weight box and placed in the balance drawer. Do not leave weights on the floor of the balance case when the balance is used only occasionally.

12. Balances should never be overloaded. Any reputable bal-ance will carry considerably more than its rated capacity without any obvious effect. Yet, overloading is detrimental to the precision operation of a balance, and the possible damage is in direct relation to the amount of the overload.


MEMORANDA

18

 


 

METHODS OF WEIGHING


19

METHODS OF WEIGHING

There are a number of different techniques in determining weight by the use of the analytical balance. No single method is preferable under all conditions. Students and laboratory technicians should be-come familiar with all the various techniques as a general background. Then they can select the method suited to conditions present and the equipment available.

First, the student or laboratory worker, should understand cer-tain terms and procedures basic to all weighing operations:

1. The "sensitivity" of a balance is usually expressed in terms of the amount of weight which will cause the pointer to deflect or move 1/4 of a division on the index scale. Sensi-tivity is sometimes also expressed in terms of the change in the equilibrium point caused by a 1 mg. weight, the change being expressed in scale divisions per milligram.

The "sensitivity reciprocal" is defined as the amount of weight required to cause a change in the position of rest of the pointer equal to one division of the graduated index.

2. A careful distinction should be made between the two general methods of weighing. A "zero method" is one in which a rider, chain, or other balancing device is adjusted, until the equilibrium position is the same as the zero reading. In "deflection weighing," the actual departure from the zero reading is used to determine a part of the recorded weight of the object.

3. The term "zero" is usually applied to the center of the index scale, rather than to a reading produced by the balanc-ing of the beam.

4. The "zero point" of an unloaded balance is the mean of the swings in each direction. This may or may not be at the zero of the index scale. Theoretically, the equilibrium posi-tion or "rest point" should be at the zero on the index.


METHODS OF WEIGHING

20

5. Only balances that have regularly installed damping devices to stop swinging of the beam come to rest quickly. When damped balances approach their final position very slowly. avoid taking the reading until the balance has actually reached a rest point. Withdrawing the magnet somewhat to reduce the degree of dampening will allow the balance to swing more freely so as to reach a more positive rest point.

6. On free-swinging balances, the motion should not be stopped or controlled by touching the pan or other parts of the balance. Length of swing may be controlled by moving the weighing chain, or by careful use of a rider weight. Usually, skillful manipulation of the pan arrests will assure a reason-able length of swing.

7. Equilibrium position may be either estimated or computed. On the former, note the center point from which the indi-cator seems to move to each side. For short swings, this method is surprisingly accurate; but on long swings, it is not practical.

8. A computed equilibrium position is calculated from the plus and minus readings at the turning point, or extreme end of each swing. At least three successive swings should be taken, and an average of both the plus and minus figures calcu-lated. The equilibrium point is then the average of the plus and minus results. To eliminate plus and minus calculations, some prefer to number the index scale from left to right, making the center line number 10.


 

ZERO METHOD


21

ZERO METHODS of Weighing

1. SIMPLE DIRECT WEIGHING

First, determine the zero point. Then place the object to be weighed on the left pan. Weights are then placed on the right pan (with forceps) and rider, chain, or other weighing accessories are adjusted until the rest position is the same as the zero point. Total weight is equal to the sum of the weights placed on the pan, plus the amount of adjustment of rider or chain.

When weighings are made only occasionally, it is essential that the zero point be determined before each weighing. For occasional weighing, adjustment of the zero point may be accomplished by means of the "autodex" or equilibrium adjuster, if the balance is so equipped. However, when weighings are made frequently in immediate succession, the zero point need be checked only as often as experience proves advisable. In the latter case, it is worthwhile to have the zero point set at the center of the index scale, or any other convenient point. This will speed up the weighing operation considerably.

2. SIMPLE SUBSTITUTION WEIGHING

The great advantage of th s method lies in the fact that it eliminates the effect of any inequality in the arms of the balance beam. In this method, it is not necessary to determine the zero point. The procedure in substitution weighing is as follows:

The object to be weighed is placed on the right-hand pan. It is then counterbalanced by weights or other material, without any adjustment of rider, chain or other weighing accessories. The equilibrium, or rest point now determined is recorded. Now the object being weighed is removed, and all counter-poising material is left untouched. Weights, rider or chain are then adjusted until the previous equilibrium position is reached. The weight of the object is the total of weights, plus or minus all other adjustments.


MEMORANDA

22

 


 

DEFLECTION METHODS


23

DEFLECTION METHODS of Weighing

1. SINGLE DEFLECTION METHOD (single swing readings)

In this method, a permanent overload is given to the balance by adjusting the beam or equilibrium point, so that releasing the balance causes the pointer to swing three to seven divisions to the right. Make a note of this first turning point, making sure that pan arrests are so adjusted as to avoid lateral vibration of pans when balance is released. In actual weighing, the object to be weighed is placed upon the left pan. Weights are adjusted until the balance swings or releases at the same turning point that was noted when the pans were empty. The total weights used to accomplish this release is the weight of the object.

The advantage of this method lies largely in its rapidity. In many cases, it may be dangerous, depending upon the type of balance used. Extreme caution should be observed when using this "short cut" method.

2. OTHER DEFLECTION METHODS

In all weighing, a final small portion of the weight may be calculated from the deflection of the beam and the sensitivity of the balance. This saves the time of adjusting the rider, chain or other devices necessary to bring the pointer to rest at the zero point.

The relative advantage of the deflection method as against the zero method, depends upon the equipment available, the work being done, and the qualifications of the operator.

The weight equivalent of the deflection may be calculated as follows:

Weight equivalent = (R-Z)/X

where

R = final equilibrium position

Z = zero point

X = change in rest point of the balance caused by 1 mg. change in weight


DEFLECTION METHODS

24

The following example will make the method clear:

Final equilibrium position=+5.0 div.
Zero point = + 1.0 div.

weight = 3.0 divisions

Substituting in the above equation:

Weight equivalent = (5.0 - 1.0)/3.0 = 1.33 mg.

(0.0013 for ordinary analytical work)

The weight equivalent may need to be added to, or sub-tracted from the rest of the weight determination, depending upon the direction of the deflection. In using this method in analytical work, the weights are ordinarily adjusted to the near-est milligram and the tenths or hundredths of a milligram are then calculated.

It must be kept in mind that the amount of change in the rest point caused by 1 mg. change in weight may vary with the load on the pans and should therefore be determined for vari-ous loads. When using the above equation, use the value apply-ing to the load on the pans.


 

GENERAL CAUTIONS


25

GENERAL CAUTIONS
in all weighing procedures

PARALLAX IN READINGS

When an index scale is read by a pointer or indicator . . . which naturally must be in front of the index scale to avoid rubbing on . . . readings will vary slightly with the direction of the line of sig! A reading taken with one eye may differ appreciably from the reading with the other eye, provided the head of the operator remains in t} same position.

To obtain correct results, it is essential that all readings for any one weighing be taken with the line of sight in the same direction. One of the simplest ways to accomplish this is to place a mark c the balance door, or window, and always take readings with the e, located so the zero on index scale appears directly behind this mar

TEMPERATURE EQUALITY

The temperature of the object being weighed, and of the weigh must be approximately the same as that of the balance. Different objects may vary greatly in the length of time required to effect a temperature change. For this reason, procedures for insuring equality of temperature should be worked out in advance of weighing operator

EQUALITY OF RELATIVE HUMIDITY

Objects should not be taken into a room of extremely hi relative humidity for immediate weighing. Generally, however, procedures in regard to humidity should be controlled chiefly with regard to the effect upon the object itself. Special caution should be observe with hygroscopic materials, as high relative humidity may cause an increase in weight.


DAILY ROUTINE

26

DAILY ROUTINE
in care of Analytical Balances

BEFORE WE1GHING

1. Inspect the balance for cleanliness. If necessary, using a camel hair brush, dust the pans, etc., taking special care not to jar the balance.

2. Determine zero point before "adjusting the zero." Keep a record of the zero point before adjustments are made, so that you may have an indication of the general constancy of the balance.

3. Move rider, chain, or other device equivalent to adding 1.0 or 2.0 mg. on the right pan so as to determine whether the beam swings freely on release.

4. If the balance is not damped, let the beam swing 3 or 4 times over about half of the length of the index scale. Note the uniformity of "falling off," or whether there is any overshooting.

5. If the balance is damped, release and let it come apparently to rest. Note readings 3 or 4 times about 5 seconds apart. Watch for drift or any irregular change in reading, which might indicate any unsatisfactory weighing conditions.

6. Redetermine zero point as a check on the constancy of the balance.

AT COMPLETION OF WEIGHING PERIOD

1. Determine zero point (for comparison with other read-ings ).

2. Dust and clean balance thoroughly, if necessary.

3. Close balance case tightly, and see that all arrestments are in proper position.

4. Remove weights and other material from pans.

5. If there is a special dust cover, outer case or other extra protection, see that it is used.


 

CHECKING


27

CHECKING ANALYTICAL BALANCES

Only those thoroughly familiar with the operation of an analytic balance should attempt to perform the various tests or checks that follow. No one should make any adjustments, particularly on a new balance, until a thorough examination has indicated the need.

Many fine adjustments can best be performed by the manufacturer or an accredited repair organization. There are, however, certain checks that can be made by an experienced operator which will often point to the cause of inaccuracies. The following is the procedure for testing the functionability and accuracy of an analytical balance.

GENERAL OBSERVATIONS

1. Check smoothness and positive operation of all control knob and handles.

2. Observe gap between knife edges and bearing surfaces when the beam is arrested. The gap can best be shown by providing a white background; a piece of white paper held in back of the beam will serve the purpose. A small ribbon of light should be seen between the agate bearing plane and the knife edge. The amount of light is a matter of adjustment that should be performed by the manufacturer or an a credited repair organization.

3. Release the beam and observe the relative position of the pointer before and after release. Little or no shift in either direction should be evident. If an appreciable shift is note improper positioning of the pan arrest pads is indicated. I correct this condition, first make sure that the bows an stirrups are properly positioned. Then check the pan arrest making certain that the pads contact both bow pa simultaneously upon arrestment. If not, one pad should be raised or lowered as required.

If upon releasing the beam, the pointer swings off the index major zero adjustment must be made through the star-shaped adjusting nut at the end of the beam. If less than a fu index differential is noted, adjustment can be made through adding or removing chain on the chain-weighing device


CHECKING

28

the balance is so equipped. After such adjustment the vernier on the chain weighing device should be reset at zero.

If only a one or two index division shift is indicated on release-the shift may be compensated for by shifting the index scale, (on balances equipped with an "Autodex" or similar device).

Zero adjustment on balances without a chain-weighing or 'Autodex' device can only be accomplished through the beam adjusting nut.

After adjustment is made, continue to observe any balance shifts which occur for no apparent reason. Such shifts may well result from temperature or humidity changes.

CHECKING THE SENSITIVITY

The rated sensitivity of an analytical balance is that amount of weight which will change the sum of the turning points of the pointer one quarter of one division on the index scale. To check the sensi-tivity rating proceed as follows:

1. Load pan with a weight sufficient to cause the pointer to deflect at least ten index division. (For standard analytical balance use 2 mg.)

2. Note the deflection and the turning points caused by this 'sensitivity weight.' ~-

3. Place the same 'sensitivity weight' on the other pan and note the deflection and turning points.

4. Compute the average deflection obtained with the 'sensitivity weight.' Note whether the deflection is even on both sides, if not, adjust the beam as described under General Observations, point 3.

Note the amount of loss in succeeding swings due to friction, buoyancy of air, etc. Losses due to these effects should not be greater than 1:10 in each succeeding swing. (This applies only to free swinging balances.)

5. Apply full rated load to each pan, using a pair of matched weights if possible. If weights are not matched bring the balance to zero by adding weights to the lighter pan. Check the deflections with the same 'sensitivity weight' as in point


 

CHECKING


29

four (4); note the difference in the readings between the loaded and unloaded observations. The reading obtained in this loaded condition should be within 85% to 100% of the reading obtained in the unloaded condition.

CHECKING THE SENSITIVITY REClPROCAL

If a check on sensitivity reciprocal is required it may be
determined by finding the amount of weight on either pan
needed to change the position of equilibrium one division on
the index scale (at capacity or lesser load). For example, a
sensitivity of 1/20 mg. implies a sensitivity reciprocal of 0.4 mg.

CHECKING THE WEIGHING CHAIN

1. Adjust the equilibrium of the balance to zero.

2. Set vernier at zero, place an accurate 100 mg. weight on the
left pan.

3. Move the chain device to 100 mg. and check for shift in
rest point.

4. Move chain device the required distance from 100 mg. to
restore balance of beam. Note the amount of movement
necessary to restore balance. Check chain at 50 mg. and also
at other points if desired.

In checking the weighing-chain device make sure that the
chain hangs freely with no knots or restrictions. It is often
necessary to comb the chain straight by running it very lightly
between the thumb and forefinger before assembling.

In any check the zero setting and the setting to be checked
should be approached from the same direction (e.g. counter-
clockwise, down, etc.)

In case the chain is not within 0.1 mg. at any point, the
knife edge, attached to the balance beam which supports the chain, must be adjusted laterally. Movement of the knife edge to the right or left is dependent upon whether the chain is light
or heavy.

CHECKING CONCENTRICITY (on notched beam balances)

1. Balance the beam system with the rider in the zero beam
notch, and release the balance several times noting any deflection of the pointer (deflection must be within one- quarter index division or closer).


CHECKING

30

Raise the rider with the rider lifter and move the rider rod laterally about l/32". Then release the rider into the same notch. Note that it will roll against the side of the notch before coming to rest at the bottom.

Note the pointer deflection and any change from the original reading.

Continue to roll the rider in this manner and note any changes in reading until you have a range of variation. This variation should not be in excess of one~uarter index divi-sion on an analytical balance.

CHECKING THE BEAM NOTCHES

1. Obtain a positive zero point with the rider in the zero notch.

2. Shift rider into 0.1 notch being careful not to roll rider.

3. Place an accurate 100 mg. weight on left pan and note difference between this reading and previous reading at zero.

4. Check each succeeding notch in the same manner. The total error caused by rider eccentricity and notch inaccuracy should be within 0.1 mg.

CHECKING THE DAMPER

1. Balance the beam and engage the damper, noting any shift in the rest point.

2. Apply a sufficient load to shift the rest point a full index scale ( 10 to 12 divisions) and observe function of damper.

Magnetic dampers function at maximum speed when the pointer overshoots slightly on the first swing. Since the strength of damping must be varied each time the load is changed it is often advisable to have the vane well into the magnet for zero load, so that when additional weight is added there will be three or more turning points on the swing.

When a balance is to be used repetitively at the same load set the damper for optimum effect at this load. Avoid having a balance damped so strongly that the pointer creeps to a stop without once changing direction of swing, which makes it difficult to determine an actual rest point.


 

WHAT TO DO


31

WHAT TO DO IF BALANCE

IS NOT OPERATING PROPERLY

If, after careful checking, the balance does not appear to conform to its rated sensitivity reciprocal, something may be impairing the operation of the delicate mechanisms. Before contacting the manu-facturer, again make sure that inaccuracy in weighing is not due to improper handling, and that variations are not caused by surrounding conditions.

If it is then definitely determined that the balance cannot be repaired in the field, shipment to the manufacturer for repair is in order. The manufacturer will have furnished complete packing instruc-tions. If no specific information is available, the following general precautions should be observed:

Remove the beam, stirrups, bow assemblies, and all loose parts. Wrap and pack each item separately, taking particular care to protect the beam and delicate knife edge. Place the balance case and parts in a wooden case or strong corrugated box, making certain that the unit is substantially cushioned all around with excelsior. The parts should be packed in the original beam box. The beam, especially should be properly placed and cushioned to avoid damage in transit. Then ship the complete package, insured, to the manufac-turer or his designated repair agent. Write a separate covering letter describing broken parts, and include any other information which may be helpful in determining why the balance is not operating properly.


MEMORANDA

32

 
 

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Last modified 13 September 2000