The latest upgraded balance use notes and basic principles

The balance is an equal arm lever. A balance is an instrument that measures the quality of an object.

The invention of the balance was very early. According to the paper cursive, as early as 1500 BC, the Egyptians had already used the balance. Others said that this time was earlier, about 5,000 BC. Although the balance of ancient Egypt is very rough, it has the outline of a modern balance and has become the prototype of a modern balance.

basic introduction

Balance, a scale. Two arms are formed by the fulcrum (axis) supporting the balance beam at the center of the beam, and each of the arms is hung with a disk, one of which has an object of known quality, and the other of which is placed with the object to be measured, and is fixed at The deflection of the pointer on the beam without swinging and pointing to the center scale indicates the mass of the object to be tested. The balance is an equal arm lever. A balance is an instrument that measures the quality of an object. It is made according to the principle of leverage. There is a small plate at each end of the lever, one end is placed with a weight, the other end is placed with an object to be weighed, and the center of the lever is equipped with a pointer. When the two ends are balanced, the masses (weight) of the two ends are equal. These reasons are already commonplace for those who have studied physics. Modern balances are becoming more sophisticated, more sensitive, and more diverse. We all know that there are ordinary balances, analytical balances, constant analytical balances, microanalytical balances, semi-micro analytical balances, and so on. It should be noted that the balance does not develop into what it is today, it still has a history of development!

The invention of the balance was very early. It is rich in aquatic plants in the Nile Delta of Egypt. It is very similar to the reeds growing in the watery areas of China. The stems are peeled off and sliced â€‹â€‹layer by layer. It can be written. This kind of thing is called papyrus. The paper in the texts of many European countries evolved from the Latin of papyrus. The book written in papyrus is a paper cursive, which has become an important historical document of ancient Egypt. The situation in ancient Egypt that we now know, especially the historical development of science and technology, is mostly derived from the records in the paper cursive. According to the paper cursive, as early as 1500 BC, the Egyptians had already used the balance. Others said that this time was earlier, about 5,000 BC. Although the balance of ancient Egypt is very rough, it has the outline of a modern balance and has become the prototype of a modern balance.

Ancient balance

According to the paper cursive, as early as 1500 BC, the Egyptians had already used the balance. Others said that this time was earlier, about 5,000 BC. Although the balance of ancient Egypt is very rough, it has the outline of a modern balance and has become the prototype of a modern balance.

An ancient weighing instrument was produced earlier, and by the late Spring and Autumn Period, the manufacturing techniques of balances and weights were quite sophisticated. The bamboo piece is used as the beam, the wire is the button, and a copper plate is hung on both ends. After the balance is heavy, the balance is changed to "é“¨", and the balance is used when weighing small objects.

Modern balance

definition

The balance is used to weigh the mass of the object. In the narrow sense, it is also called the pallet balance (multiple pallet balances in the laboratory).

The commonly used precision is not high, and is composed of a tray, a pointer, a beam, a ruler, a swimming code, a weight, a balance nut, an indexing plate, and the like. The division value is generally 0.1 or 0.2 grams.

An instrument. A fulcrum (axis) supports the balance beam at the center of the beam to form two arms. Each of the arms is hung on a disk, one of which has a known weight and the other object that is placed in the disk, fixed in The pointer on the beam does not swing and the deflection when pointing to the center scale is the weight of the object to be weighed.

Precautions for use

1. Place it in a horizontal place.

2. Before use, balance the balance to the left and right (the travel code must be "0" balance nut to the opposite direction, use the mouth: left high, left to adjust).

3. The weight can't be taken by hand with a pair of tweezers. Do not wet or stain the weight (this will make the weight corrosion and rust, the weight of the weight becomes larger, the measurement result is not accurate), and the travel code is also Use the dice to move.

4. The mass of the measured object should not exceed the balance of the balance or the minimum scale of the balance.

5. Wet objects and chemicals cannot be placed directly in the balance pan.

6. Pay attention to the left object right code when weighing (the value of the swimming code is based on the left-aligned tick mark).

7. After weighing, you should reset the game code to zero, and use the tweezers to put it back into the code box.

Composition of the balance

The common signage balance is mainly composed of a column, a beam, a hanging system, a base and a brake device.

Balance composition diagram

The column is vertically fixed to the base to support the beam. The lower part of the column is equipped with an indexing plate, and the top is equipped with a bracket to support the beam when the balance is not working. A middle knife is placed in the middle of the beam. When the balance is working, the middle knife rests on the knife holder connected to the top of the lifting rod as a fulcrum. There are two side knives on both sides of the middle knives, which serve as the key points and strength points to bear and transmit the load. The bottom of the lower beam of the middle knife is equipped with a pointer, and the center of gravity of the pointer is fixed to adjust the position of the center of gravity of the beam, which adjusts the sensitivity of the balance.

The indexing ruler is engraved on the top of the beam, and there is a moving game code on the ruler. A balance nut for adjusting the balance of the balance of the balance is also installed at both ends of the beam.

The hanging system includes a small lifting ring, a hanging frame and a weighing pan. The hanging frame is hung on the small lifting hook, and the two side knives respectively bear the weighing weight of the weighing pan and the weight of the weighing object through the small lifting ring.

The base is equipped with two spiral adjustment feet for adjusting the level of the balance, and a level is placed on the base to display the level of the balance. The level of adjustment is to cause weighing errors in order to avoid the balance being level.

The brake device is mainly composed of a switch knob, a switch shaft and an eccentric cam (or a connecting rod). Turning the knob to deflect the cam (or eccentric link) by a certain angle, the lifting rod in the column can be moved up and down, and the beam can be lifted or dropped by the middle knife to open or close the balance.

Classification of balances

There are narrow and broad points. The narrow balance refers to the double-disc equal-arm mechanical balance, which is an instrument that uses the equal-arm balance principle to compare the measured object with the corresponding weight to determine the mass of the measured object. The broad range of balances includes double-disc equal-arm mechanical balances, single-disc unequal-arm mechanical balances and electronic balances.

Double-disc equal-arm mechanical balances are generally classified into three types: ordinary signage balances, differential signage balances and plate balances. It can also be divided into four types: verification balance, analytical balance, precision balance and ordinary balance.

The balance is a balance that is used by the metrology department, the commodity inspection department or other relevant departments or factories to check or calibrate the weight.

Analytical balances are high-accuracy balances for chemical analysis and accurate measurement of matter. In most cases, the minimum division value of such balances is less than 10-5 of the maximum weighing. The analytical balance can be divided into a constant balance (weighing and minimum division values â€‹â€‹of 100~200g and 0.01~1mg respectively), a semi-microbalance (30~100g and 1~10g), and a microbalance according to the measurement range and the minimum division value ( 3~30g and 0.1~1g) and ultra-micro balance (3~5g and below 0.1g).

Precision balances are widely used in the precision measurement of various materials, and the minimum division value is usually 10-5~10-4 of the maximum weighing.

Ordinary balances are used as a general measure of matter. The minimum division value is equal to or greater than 10-4 of the maximum weight.

Special balance

Differential signage balance

The structure is similar to the ordinary signage balance, the difference is: 1 The lower end of the beam pointer is equipped with a differential scale card. The lower end of the column is equipped with an electro-optic system for amplifying and displaying a differential reading on the projection screen. 3 The hanging system adds a sleeve type air damper. When weighing, the beam can be quickly stopped to swing, which is convenient for accurate reading. 4 On the outer frame of the balance, a cam lever type or other form of partial range mechanical overweight (usually 10~999mg) or a full-scale mechanical overweight device is installed instead of manual addition. The minimum scale value of differential signage balances is generally above 0.1mg, and the accuracy is higher than that of ordinary signage balances.

Shelf balance

A double pallet balance. The weighing pan is placed on the shelf above the knife on both sides of the beam, and the center of gravity of the weighing pan and the pallet frame is higher than the pivot point of the beam. When the weight or the object is in the front and rear position of the weighing pan, it will cause the weighing pan and the cross member to tilt forward and backward. When it is in the left and right position of the weighing pan, it will cause the weighing pan to tilt left and right. In order to overcome this shortcoming, the platform balance adopted measures of lengthening the middle knife, side knife and widening the knife holder, and adopted the Roberval mechanism in the structural design. In the Robwell mechanism, the rods AB, A'B' are hingedly connected to the longitudinal rods AA', BB', and the strut EE' to form two equal parallelograms AA'E'E and EE'B'B. When the equal-sized forces P, P' act on the left and right cross arms, respectively, the strut can be horizontally balanced even if the acting position is asymmetrical. Regardless of how the AB is tilted, AA', BB' are parallel to the pillar EE'. From the left side of EE', when the force P of the distance d from the longitudinal rod AA' is applied to the cross arm, a force equal to the size of P and the same direction acts on the points A and A'; At the same time, a torque having a value of PÂ·d acts on the longitudinal rod AA', thereby pulling the lever to the left at point A and pushing the lever to the right at point A'. However, since the lever is limited by the EE' point, the equal-reverse and opposite-direction reaction forces f, f' will be generated on A and A', respectively, thereby forming a reverse torque fÂ·s equal to PÂ·d (f 'Â·s), the result PÂ·d torque is balanced by fÂ·s(fâ€²Â·s). Finally, on A, A' only the force equal to P acts, regardless of the acting position d of P on the cross arm. This situation is also identical on the right side of EE'.

Resistance balance

A special resistance box is hung on the beam to stop the swing of the balance.

Single disc unequal arm mechanical balance

It is also designed with the principle of leverage balance. When working, after adding the object to be measured, subtract the weight on the suspension system so that the beam is always fully loaded. The weight of the reduced weight plus the value of the differential card reading is the mass of the object being measured.

Electro-optical resistance balance

Using the cursor principle, the position of the pointer can be read more accurately

Dish balance

The weighing pan has lower sensitivity on the upper side.

Electronic balance

It is a combination of sensing technology, analog electronics, digital electronics and microprocessor technology. It has automatic calibration, automatic display, taring, automatic data output, automatic fault tracing, overload protection and many other functions. Electronic balances usually use electromagnetic force sensors (see load cells) to form a closed-loop automatic adjustment system with high accuracy and good stability. The working principle of the electronic balance is shown in Figure 5. When the weighing object is added to the weighing pan, the position detector signal of the sensor changes, and the current in the sensor coil is increased by the feedback of the amplifier, and the current generates a feedback force in the constant magnetic field to balance the applied load; At the same time, the voltage value of the current on the measuring resistor Rm is sent to the microprocessor through the filter and the analog/digital converter for data processing, and finally the display quality value is automatically displayed by the display.

Fundamental

The balance is a commonly used instrument in the laboratory. A balance is an instrument that measures the quality of an object. It is made according to the principle of leverage. There is a small plate at each end of the lever, one end is placed with a weight, the other end is placed with an object to be weighed, and the center of the lever is equipped with a pointer. When the two ends are balanced, the masses (weight) of the two ends are equal. These reasons are already commonplace for those who have studied physics. Modern balances are becoming more sophisticated, more sensitive, and more diverse. We all know that there are ordinary balances, analytical balances, constant analytical balances, microanalytical balances, semi-micro analytical balances, and so on. It should be noted that the balance does not develop into what it is today, it still has a history of development!

Schematic diagram of the balance

The invention of the balance was very early. It is rich in aquatic plants in the Nile Delta of Egypt. It is very similar to the reeds growing in the watery areas of China. The stems are peeled off and sliced â€‹â€‹layer by layer. It can be written. This kind of thing is called papyrus. The paper in the texts of many European countries evolved from the Latin of papyrus. The book written in papyrus is a paper cursive, which has become an important historical document of ancient Egypt. The situation in ancient Egypt that we now know, especially the historical development of science and technology, is mostly derived from the records in the paper cursive. Of course, the text on the paper cursive is not a modern text, but a hieroglyphic text that has been read by many experts. According to the paper cursive, as early as 1500 BC, the Egyptians had already used the balance, and others said that it was still early, about 5,000 BC. Although the balance of ancient Egypt is very rough, it has the outline of a modern balance and has become the prototype of modern balance. The picture below shows the balance used by ancient Egyptians.

As can be seen from the figure, the balance is drilled with a hole in the middle of a pen, across a stick, and a plate is hung on the ends of the stick. This kind of balance has been used for a long time, until about 500 BC, Rome's "rod" appeared, the rod said to move to the position of the weighing to maintain the balance with the weight of the item, in fact, the end of the balance (Putting the code end) from fixed to active, the advantage is that you only need to match a scale, and the weight of the balance is several. The pole is also used to hang a plate with a rope, and then hang a rope with a rope. The basic form is the same as the balance except that one end is movable.

In the process of using the balance and the rod, people feel that it is very troublesome to hang a plate with a rope, which is very inconvenient to use. So, some people want to get rid of this hateful rope. In the middle of the 17th century, the French mathematician Lobel Barr invented the swinging pallet balance. The invention of the pallet balance is considered to be a major improvement on the ancient hanging balance. So far, the pallet balance is still being widely used. The picture below shows a common type of pallet balance in the laboratory, which is much better than the 17th century pallet balance. In the figure, 1 is a balance beam, and each end supports a weighing plate 2. These two parts constitute the skeleton of the pallet balance, which embodies the basic design principle of the pallet balance. See the following diagram: When the beam 1 is balanced, the torque is equal, F1L1=F2L2, F1=m1g, F2=m2g, L1=L2,âˆ´ m1gL1=m2gL2, m1=m2, that is to say, the mass of the weighed material is known from the mass of the known weight. In the figure below, 6 is the pointer, 7 is the dial, the pointer is aligned with the center of the dial to indicate that the two ends are balanced, 4 is the travel yard scale, 8 is the travel code, and 5 is the zero adjustment nut. There is also a weight for the balance.

It should be pointed out that the invention of the pallet balance did not cause the hanging balance to withdraw from the historical stage. On the contrary, the hanging balance not only continued to be used by people, especially the scientists continued to use it, and it was continuously improved in use, and the precision balance widely used in modern times. Most of them are hanging type, and the tray balance is used more in daily production and life. In scientific experiments, it is mostly used in weighing with less precise requirements.

History of balance development

The British chemist Black, who used the balance earlier in chemistry experiments, lived and worked in the 18th century, when it was the time when chemistry constantly discovered gas and began to establish theory. Black attaches great importance to experiments in chemical research, and is the first person to apply quantitative methods to study gases. Quantitative research requires weighing, and weighing is inseparable from the balance. Historical data shows that Black did use the balance, and his used balance is still preserved in the Royal Museum of Edinburgh. The picture below is the balance that Black used.

The weighing plate is hung with a string, and the beam is hung on another weighing hook. Black used this balance to conduct chemical experiments and found out that the use of this balance not only established quantitative methods in chemical experiments, but also important for the further development and improvement of the balance.

After Black, the British chemist Henry Cavendish also conducted a precise quantitative experiment. It is said that he also designed and manufactured the balance. It is difficult to know exactly when and how he designed because the life of Cavendish himself is unknown. , making the balance. Later, the balance used by Cavendish was exhibited at the Royal Academy of Sciences. The balance is placed in the cabinet, and the scale of the balance is not clear from the outside, see the picture below:

Lion balance

In the 18th century France, the balance was popular, and many small shops sold the balance. What we see in the picture below is the style of the French balance of the 18th century, perhaps a style in the balance at that time.

At the end of the 18th century, the United Kingdom had also created a balance with a steel blade in the center of the beam. Putting it in the agate plate greatly improves the precision and sensitivity. At that time, some balance designers and manufacturers appeared. However, it is said that the price of the balance is relatively expensive, and it must be booked first. Unlike France, although the balance is not so precise, it is relatively easy to buy. The picture below shows a balance made by Federer's Royal Society and exhibited at the Science Museum in London.

In the UK, the use of balances is not limited to chemists and scientists. Balances are commonly used in general medicine stores. Dalton, the proponent of chemical atomism, often uses balances in scientific research because of Daltonâ€™s important position in the history of chemistry and his The influence of the British scientific community made him a famous figure and became a famous chemist. The balance he used was said to have become an extraordinary thing. Later he was owned by the Manchester College of Literature and Philosophy and collected for the association. Dalton was a member of this society.

In the 1920s, an instrument designer in London called Robinson, he began to design and manufacture analytical balances, not only in the United Kingdom, but even in the United States for a period of time. Robinson used a hollow material as a beam to make the beam a triangle with a pointer in the middle of the vertical beam.

The balance with the scale beam and the travel code was also born in the 19th century (in the case of the pallet when you introduced the balance, the scale and the travel code are marked on the beam in the picture), who is the inventor of this balance has different opinions, There is also controversy. However, an international exposition was held in London in 1851, and some major balance manufacturers from the UK and continental European countries participated. After the deliberation of the expo, the highest prize was awarded to Oye, in recognition of his presence on the scale beam and The contribution made by the manufacture of the sliding device (ie the swimming code) balance, it seems that the right to invent the invention of Oye has been internationally confirmed.

For a long time, a theory was popular in the balance manufacturing industry, which believed that the longer the beam of the balance, the more sensitive the balance. But there is a person named Boer Bunge who is not restricted by this theory. In 1866, he designed and built a short beam analytical balance. Later, in the theory of balance design, he also made achievements. The picture below shows the earliest short beam balance made by Bunge in 1866.

Special coin balance in the late Roman era

The improvement of the balance, in addition to the beam, contact points, travel codes, scales, etc., is also manifested in other aspects. For example, in the first half of the 19th century, knob balances have emerged. What are the advantages of the knob balance? Originally, the early balance, the beam was placed on the vertical column, and it was the same when used and when not in use. Later, the analytical balance appeared. The edge of the analytical balance was made of agate. In order to reduce the damage of the knife edge, when the balance is not used, the balance beam is placed on a shelf, so that the knife edge is no longer stressed, and the knife edge is placed on the support bowl. . These are all controlled by the knob device. Modern analytical balances have a knob device. The picture below shows a knob balance made in 1833.

With the development of science and the advancement of technology, the design and manufacture of the balance has made great progress. It is through the unremitting efforts of generations and generations that through the accumulation and improvement of technology, we have today's various modern balances. Today, in chemical laboratories, the commonly used balances are: pallet balances, as mentioned before, for precision requirements or to determine the approximate mass of the material, weigh 100 grams, 200 grams, 500 grams or even 1000 grams. Analytical balance (constant analytical balance, microanalytical balance and semi-micro analytical balance); electro-optical balance with air damping device or electromagnetic damping device, so that the balance has high sensitivity and can quickly prevent the beam from shaking, the electro-optical balance from the appearance If you can't see the weight, you can see the weighing plate on which the object to be tested is placed. The weight of the weight is operated by the rotary dial. The value of the weighing can be read directly by the projection scale.

Instructions

1. Place it in a horizontal place. The game code should be zeroed.

2. Adjust the balance nut (nut on both ends of the balance) to adjust the zero point until the pointer is aligned with the center mark.

3. Place the weighing object on the left tray and the weight on the right tray (left object right code). According to the traits of the weighing object, it should be placed on glassware or clean paper. The quality of the glassware or paper should be weighed on the same balance beforehand, and then weigh the substance to be weighed.

4. Add the weight from the estimated maximum value of the weighing object and gradually reduce it. The pallet balance can only be weighed to 0.1 grams. Add or subtract weights and move the cursor on the ruler until the pointer is aligned with the center tick.

5. Objects that are too cold and too hot should not be weighed on the balance. It should be weighed in the desiccator until it is at room temperature (or weighed in a special vessel).

6. Object quality = weight weight + degree displayed by the travel code

7. The weight must be handled with a pair of tweezers. The removed weight should be placed in the weight box. After weighing, the code should be moved back to zero.

8. When weighing dry solid medicine, place one sheet of the same quality on each of the two trays, and then weigh the medicine on the paper.

9. Drugs that are deliquescent must be weighed on glassware (eg, small beakers, watch glasses).

10. If the weight is rusted, the measurement result is too small; if the weight is worn, the measurement result is too large.

Use attention

1. Move the travel code to the zero mark line in advance and adjust the balance nut. When adjusting the balance nut, it is best to use the tweezers to balance the balance.

2. Put the weight on the right and place the object on the left.

3. The weight cannot be taken by hand, and it should be taken with a pair of tweezers. The travel code cannot be moved by hand when using the balance.

4. Objects that are too cold and too hot should not be weighed on the balance. It should be weighed in the desiccator and then placed at room temperature.

5. Plus weights should be large to small, saving time.

6. Do not touch the balance nut during the weighing process.

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