Why arrest torsion balance

Checking the rider concentricity. Checking the beam notches. Checking the damper. 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 At the end of the Civil War, manufacturing was resumed until, in , the two sons established the business under the name, Becker Bros.

Christian Becker, as it is known today, was established in 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. For precision weighing the rider balance is one of the many refinements employed. The principal elements. Thus, delicate weighing procedures are not endangered by external factors. The notched beam permits the adding of additional weight up to one gram, in mg.

All weight adjustments from 0. This can be done while the beam is swinging. It results in an important saving of time. 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. 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. 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:. Before you start to assemble the balance, make sure you know where parts go and how to put them in place.

Be extremely careful of the knife edges. Remember , an analytical balance is a precision instrument, be gentle and cautious in all assembly procedure. 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. 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.

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.

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. 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.

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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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.

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. 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. 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:.

Sensi-tivity is sometimes also expressed in terms of the change in the equilibrium point caused by a 1 mg. 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. 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. 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.

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. 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. 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.

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. 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.

A computed equilibrium position is calculated from the plus and minus readings at the turning point, or extreme end of each swing. The balance is always locked with internal shipping arrest posts B before shipping to avoid breakage. Figure 1 illustrates the locked position, that is, with shipping arrests B restraining the movement of the upper and lower balance beams. Figure 2 shows the unlocked position with the shipping arrests B moved away from the beams and secured in position for possible future use.

Lay the balance on its back and loosen the two knurled shipping arrest knobs A under the base shown in Figure 1.

Do not touch the hex head screws in the center of the knobs. Merely loosen the knobs slightly. Push them as far as possible toward the ends of the balance and tighten again as shown in Figure 2. Place the pan holders on the top of the trusses visible through the grommet openings in the top of the balance case.

Place pans on pan holders. If balance has a scoop, it should be placed on the left-hand pan holder. Detach the weight access plate from the left side of the balance case by removing the two slotted screws. The weights and weight applicator are contained in a small box shipped with balance and packed under the lid. The denominations of the weights can readily be determined by comparison with the outline drawings on the card included in weight box.

Instructions for aligning the weights on the plastic weight rack are noted on the inside of the weight access plate. When installing weights, be sure that metal lifter arms are below the plastic weight rack. After all weights are in place, rotate the weight loader dial to be certain the weights are seated properly on the weight rack. Replace access door. Level balance front to back. This is accomplished by turning both leveling screws in the same direction at the same time and in equal amounts.

This will raise or lower the balance as required to bring the bubble in the level on the lower right side or the case to the center of the level vial. Release the balance by turning the small knob in the lower, front center of the base counter-clockwise.

Bring balance into equilibrium by means of leveling screws as follows: If indicator moves to the right, rotate the left leveling screw counter clockwise and the right leveling screw clockwise.