Body mass. Measurement masses body on the scales stage 4 - Frontal experiment " Measurement masses body on the scales." Purpose: mastering the rules for weighing bodies on the scales.
laboratory Job №3.

The purpose of the lesson: teach students how to use a balance scale and use it to determine body weight.

I. Checking the assimilation of the studied material.

1) Frontal survey:

1. What property of bodies did you learn about in the previous lesson?
2. What is the inertia of bodies?
3. What physical quantity characterizes the inertia of bodies?
4. Which of the two interacting bodies is more (less) inert?
5. What is the basic unit of mass in the SI system?
6. What are the derived units of mass?

2) Convert to SI:

1. 200 g = ? (0.2 kg)
2. 4500 mg = ? (0.0045 kg)
3. 0.5 t = ? (500 kg)
4. 3.5 q = ? (350 kg)
5. 2500 g = ? (2.5 kg)
6. 0.07 t = ? (70 kg)
7. 20 q = ? (2000 kg)

3) Solve the problems:

1. The weight was lowered into a vessel filled with water. Has the weight of the weight changed?
2. The air under the pump piston is compressed. Has the mass of air changed?
3. Does the mass of a substance change when water turns into ice or steam?

II. Learning new material.

What methods of determining mass do you know?

Fixing: Calculate the mass of trolley #2 if its modulus of speed is 4 times greater than the modulus of speed of trolley #1.

What types of weights do you know?

Study the device and principle of operation of a balance scale (working with A.V. Peryshkin's textbook "Physics Grade 7", p. 47).

What is the main part of the balance?

What is the balance condition for the weights?

How to determine body weight using scales?

What rules should be followed when determining body weight?

Weighing rules:

1. Balance the balance using strips of paper, cardboard, etc.
2. Place the body to be weighed on the left scale pan and the weights on the right pan.
3. Having balanced the body, calculate the total mass of the weights lying on the scales.

Review the set of weights and weights for laboratory balances and answer the questions.

1. Why are there identical weights in a set of weights?
2. Why is it desirable to start weighing with a larger weight?
3. Why should small weights and weights be taken only with tweezers?
4. Why are there no 30 g, 3 g weights and 300 mg, 30 mg weights in the kit?
5. What can replace weights 4 g, 40 g and weights 400 mg, 40 mg?

III. Performing laboratory work.

Progress.

1) Read the weighing rules carefully.

2) Adhering to the rules, measure the mass of the bodies given to you.

3) Record the measurement results in the table.

4) Individual tasks.

Option 1. Measure the mass of sand in the box.

Option 2. Measure the mass of the liquid in the glass.

Weight of an empty glass, g	Weight of a glass of water, g	Mass of water, g

5) Write down the conclusion: what you measured and what you learned.

IV. Homework.

Propose and describe a method for determining the mass of one drop of water using a laboratory balance.

Physics lesson in the 7th grade on the topic "Measuring body weight on balance scales"

Lesson Objectives: the formation of the ability to compare the masses of two bodies, measure body weight on a balance scale, express one's thoughts and describe actions in oral and written speech.

I have six servants,
Agile, remote.
And all that I see around -
I know everything from them.
They are at my behest
Are in need.
They are called: How and Why,
Who, What, When and Where.
R. Kipling (translated by S.Ya. Marshak)

During the classes

1. Organizing time.
2. Topic update.

In the last lesson, we got acquainted with the physical quantity - body mass.

What property of a body is characterized by its mass? (Inertia).

What is the inertia of bodies? (The less the speed of the body changes during the interaction, the more inert it is).

What is the base unit of mass? (kg).

III. Message about the topic and purpose of the lesson.

Theme of the lesson: "Measuring body weight on a balance scale." Purpose of the lesson: to learn how to measure body weight.

IV. Rationale for the relevance of the topic.

Why measure body weight? To answer this question, we conducted a study, which was designed in the form of a presentation. Student performances:

Find out people of what professions need to measure body weight.

With the help of what scales they do it.

Collect Interesting Facts measurements of the mass of bodies.

To complete the task, we split into groups.

The first group will study literature. The guys in this group should find interesting facts related to the mass.

The second group will go to shops and other outlets.

The third group will find out in the hospital and polyclinic what is the significance of the measurement of body weight for the doctor and his patient.

The guys of the fourth group will visit the post office and find out how accurately they weigh parcels and parcels at the post office.

This is interesting

The mass of the Sun is 2 x 10 30 kg. The mass of the Earth is 6 x 10 24 kg.

The largest hailstorm hit Bangladesh in 1986. The weight of hailstones reached 1 kg.

This summer, an English farmer grew a bulb that weighs 7 kg.

The largest animal on earth is the whale, which can weigh 150 tons.

Elephants are the largest modern land mammals. Their weight is up to 7.5 tons.

A small bird, the kinglet, weighs only 5–7 g, and some nectaries - 3-4 g.

Frog - bull weighs up to 600 g.

Bamboo bear - panda, weighs 150 kg, a day he eats 10 - 20 kg of bamboo.

In the clinic, we visited the office of a healthy child.

Monthly weighing of children from birth to a year, allows doctors to conclude: is the baby healthy, is it developing correctly.

There are scales for adults in the clinic and in the hospital. We found them at the endocrinologist's and in the emergency room. Overweight people are more likely to suffer from some kind of disease.

In the "Soyuz" store, we properly examined the scales. We found out that they were manufactured in 1999. We made a test purchase. In the "Visit" store, we saw more modern and convenient scales.

At the post office, scales are necessary for weighing parcels, parcels, registered letters. If the postal worker does not correctly measure the weight of the parcel, then he may be in trouble later.

Concluded:

• body weight measurement, important point for professionals in many professions.

Teacher: Measuring body weight is a very important point in physics lessons.

V. Learning new material

The explanation corresponds to paragraph 20, according to the textbook by A.V. Peryshkin "Physics - 7". And also students get acquainted with the rules of weighing.

VI. Lab #3

"Measurement of body weight on a balance scale".

Goal of the work- learn how to use lever scales and use them to determine the mass of bodies.

Devices and materials: scales with weights, several small bodies of different masses.

Instructions for work

1. Read the weighing rules carefully.
2. Adhering to the rules, measure the mass of the bodies given to you.
3. Record the measurement results in a table.

VIII. Individual task.

Task number 1

Measure the mass of sand in the box.

Task number 2

Measure the mass of the liquid in the bubble

IX. The discussion of the results. Summarizing.

X. Homework.

Interview mom, grandmother: In what situations do you have to measure body weight at home, how do they do it?

laboratory Job № 4 Measurement masses body on the scales. 9. (ppt)festival.1september.ru/articles/507110/pril2.ppt Digital educational resources Electronic educational edition « Laboratory work in Physics, issued by Drofa LLC for grades 7-11.
Instruction Worksheet laboratory Job № 7.3 Measurement masses body on a balance scale Purpose work...10. Measurement masses body on the scales. Grade 7 What can replace weights 4 g, 40 g and weights 400 mg, 40 mg? III. Performance laboratory work.
Suggest and describe a way to determine masses one drop of water with laboratory scales.

Laboratory scales differ in purpose, design, weighing range and other characteristics.

Weighing methods are divided into two fundamentally various groups- the method of comparison with the measure and the method of direct evaluation. According to the method of comparison with the measure, the mass of the load is taken equal to the mass of the weights compared with it (simple weighing) or calculated as the sum of the weights of the weights and the readings of the scales (exact weighing). The method of direct assessment consists in determining the mass of the load using the reading device of the scales without the use of weights.

In most modern laboratory scales, a differential weighing method is used, in which most of the measured body mass (over 99%) is balanced by weights or a counterweight (zero method), and the remaining small difference between the mass of the weighed body and the mass of the weights is measured by the angle of deviation of the rocker from the initial position balance (direct method) using reference scales.

Laboratory scales are characterized by a number of parameters. The main ones are the following.

1. Ultimately permissible load, in the range of which the error of indications is within the established limits. It is impossible to go beyond the maximum permissible load for which this scale model is designed. Too much load may cause permanent deformations in the rocker, resulting in damage to the balance.

2. Permissible error of indications - the maximum difference between the actual value of the mass of the weighed load and the indications of the scales. The error value characterizes the correctness of the weighing results under standard conditions and cannot be less than the non-excluded errors of weights used in weighing and certification of scales.

3. Permissible variation (inconstancy) - indications - the maximum permissible difference in the readings of the scales when repeatedly weighing the same load under standard conditions using the same weights. The variation value characterizes the reproducibility of the weighing result and, to a large extent, the accuracy of the weighing.

4. Sensitivity - the limiting ratio of the increment of the deviation of the scale indicator to the increment of the measured value. Sensitivity is determined by the number of divisions of the scale by which the arrow of the balance deviates when a weight of 1 mg is placed on one of the scales. Express sensitivity in scale divisions per milligram or reciprocal.

With an increase in the load on the cups, the sensitivity of the scales decreases, i.e. the greater the mass of the weighed object, the weaker the scales react to changes in mass.

5. Division price - the value of division of reading devices. Often the division value is consistent with the value of the permissible error or the variation in the readings of the scales.

6. Performance - the possible productivity of work on the scales, i.e. the possible number of weighings per unit of time.

Weight classification

By purpose, laboratory scales are divided into technical (general laboratory), analytical and special, and weights - into weights for general use and special.

The largest weighing limits of technical scales are in the range of 20 g - 50 kg. The most common scales have a load of 0.2-5 kg, with a division value of 0.05-0.1 g.

Analytical balances are used for macro- and microchemical analyzes when weighing the highest and highest accuracy. Analytical balances are divided into the following groups depending on the maximum permissible load and scale division:

Special scales are used to determine values ​​that depend on the mass (weight moisture meters, scales for measuring magnetic susceptibility, etc.).

Scales of the analytical group belong to the 1st and 2nd accuracy class, technical scales - to the 3rd and 4th classes. The average reduced weighing error for scales of the 1st class is 0.0001%; 2 classes - 0.0005%; 3 classes - 0.001%; 4 classes - 0.01%.

Weights for general laboratory use are divided into four classes. Weights of classes 1 and 2 are intended mainly for analytical balances, classes 3 and 4 - for technical ones.

According to the nature of the movement of the movable system, the scales are divided into leverless and lever. In a non-lever scale, the movable system moves reciprocating vertically, so weights cannot be used to balance the weighed load. When using leverless balances, only the method of directly evaluating the weighing results is suitable.

Lever scales are characterized by the rotation of the movable system around a fixed or conditionally fixed axis. They are weightless (with overhead or built-in weights) and weightless. Scales with built-in weights are more productive and more convenient, but it is difficult to control the actual values ​​of the mass of weights in them.

Lever scales differ in the type of weight arm supports and hangers. The most common rigid support is a cushion, on which a prism rolls with a sharp edge. Scales with such supports are called prismatic. Prism balances are divided into equal-arm, two-prism (single-cup) and quadrant.

An equal-arm scale is basically a lever of the first kind, in which the distances from the application of forces to the fulcrum are equal (Fig. 71). If a load having mass M1 is placed on the left pan of the balance, then to return the arrow P to its original position, it will be necessary to place a certain number of weights (with a known mass) on the right pan. When equilibrium is established, the moments of forces acting on the left and right parts of the rocker at the points on which the cups rest, at a distance l1 and l2 from these points to the fulcrum, will be equal to: F1l1 = F2l2.

Since l1 = l2, then, consequently, when equilibrium is reached, F1 = F2. The emergence of the forces F1 and F2 is associated with the attraction of the bodies on the scales by the Earth. Force F1 determines the attraction to the Earth of a body with mass M1, i.e. its weight. The unit of weight is newton (N). newton equal to strength giving a body of mass 1 kg an acceleration of 1 m/s2 in the direction of the force. The weight of the body G is related to its mass by the relations: G = Mg, where M is the mass of the body, and g is the acceleration of free fall. The unit of mass is the kilogram (kg).

From the foregoing, it follows that scales are instruments for determining mass, not weight.

Equal-arm balances are shown in fig. 72. The equilibrium position of unloaded scales is called the zero point, loaded - the equilibrium point.

To protect the ribs of the rocker prisms from damage and rapid wear, all moving parts of the balance can be raised and the ribs of the prisms separated from the plates with which they come into contact. A device that serves to raise the rocker arm and earrings is called the arrester (isolyra). When the scales are not in use and when weighed objects and weights are placed on the cups, the scales must be locked.

Until recently, V-shaped depressions of the rater scale (Fig. 73) were applied to the balance arm of an equal-arm analytical balance at the same distance from each other, into which a 10 or 5 mg weight-rater was installed with a special device. By moving the rider along the rocker, it was possible to determine the mass with an accuracy of tenths of a milligram.

In modern prism balances there are vibration dampers of the balance pointer - dampers. In damper balances, the scale division against which the arrow stops is taken as the zero point and the equilibrium point. For balances that do not have dampers, these points are determined by the swing method. This method is based on measuring 3-5 successive needle deviations. The first 2-3 fluctuations after turning on the scales are not taken into account, and the next 5 deviations of the arrow in one direction and the other are recorded with an accuracy of tenths on the scale. The zero point is calculated, for example, as follows.

Deviations to the left: -3.4 and -2.8; mean value -3.1.
Deviation to the right: +4.0, +3.5 and 3.0, average value +3.5.
Let's find the sum of deviations: +3.5 + (-3.1) = 0.4.
Let's find the zero point: +0.4: 2 = +0.2.

The accuracy of damper balances is of the same order as the accuracy of conventional balances.

Two-prism (single-cup) scales are shown in fig. 74. In the initial position, all built-in weights are loaded on the suspension and the lever is balanced by a counterweight. Having placed a load on the load-receiving cup with the help of a special gyre-laying mechanism, such a number of built-in weights are removed from the rail so that their total mass approximately corresponds to the mass of the load. The difference between the mass of the load and the mass of the removed weights is determined by the readings of the reading device. Two-prism one-cup balances are mainly used as analytical balances. The advantages of this design of the scales are that the work is always carried out at a constant load on the rocker, and in this case, both the sensitivity of the scales and the accuracy of weighing are constant.

Quadrant scales or scales with an upper location of the load-receiving bowl (Fig. 75) are a kind of two-prism ones.

Lever scales with supports on elastically deformable elements are produced for weighing not large masses. These include torsion balances and spring lever ultra-microbalances.

General laboratory balance scales

General laboratory equal-arm scales - technical scales mainly of 3 and 4 accuracy classes - are used for weighing relatively large masses. They can be enclosed in a glazed showcase and equipped with a weight mechanism with built-in weights, or they can be hung on a stand fixed on a stand, without a weight mechanism. The simplest type of two-cup equal-arm scale is the manual or pharmacy scale.

Technochemical scales of the VLT-200g (T-200) and VLT-1kg (T-1000) types are shown in fig. 76. When weighing by turning the handle of the arrester, the scales are brought into working position. Permissible error for scales VLT-200g ± 60 mg, for VLT-1kg ± 200 mg.

More advanced technochemical scales of the VLR-1kg type consist of an equal-armed rocker arm with an arrow, a column with a support cushion, an insulating device, and two load-receiving cups suspended on the end prisms of the rocker arm. The scales are equipped with an oil vibration damper of the rocker arm and a device for mechanical weighting of built-in weights (from 10 to 990 mg).

Before weighing, make sure that the balance is properly leveled by level. If necessary, with the help of screw legs, the scales are installed strictly horizontally. Then you need to check the deviation of the arrow and achieve its complete alignment with the control stroke of the scale dial.

IN last years Equal-arm technical two-cup scales of the VLT type were significantly modernized and a series of new models of VLR type balances was launched, 2 accuracy classes (with an error of ± 10 mg), with a load capacity of 1, 10, 20 and 50 kg, with a scale division of 10 mg.

VLR scales are placed in a glass case with doors on two sides. At the upper end of the column there is a pillow, on which the middle prism of the rocker rests with an edge. An oil dampener is fixed at the base of the column. At the ends of the rocker, prisms are fixed in special saddles, on which earrings with load-receiving cups are hung. Ring weights (from 100 to 900 mg and from 10 to 90 mg) associated with the large and small limbus are hung and removed on the bar fastened to the right earring using a weight mechanism.

An arrow is fixed in the middle of the rocker, and a scale is located at the bottom of the column, by which the balance of the scales is checked. An insulating device (catch) is mounted under the base of the scales. It is necessary to open and close the cage carefully, by smooth rotation of the handwheel at the moment when the scale pointer passes the zero division of the scale.

General laboratory quadrant balance

In recent years, quadrant scales have become widespread, which favorably differ in speed of action. This is a two-prism balance with an upper cup position. The vibration damper is magnetic. Available optical device and a screen on which the weighing results are counted. The application and removal of weights is carried out by a handle located on the metal case of the balance. The scales are connected to the alternating current mains through a built-in transformer mounted under the showcase of the scales.

Quadrant scales are designed to determine the mass of various substances and materials during laboratory technical analyzes and preparative work.

The principle of operation of the scales is based on balancing the moment of forces created by the measured mass, quadrant deviation and built-in weights.

Currently, six modifications of laboratory quadrant scales of the 4th class VLKT and VLK are produced with weighing limits from 160 to 10000 g.

VLKT scales (Fig. 77) have a tare compensation mechanism that allows you to increase the weighing performance and is designed to set the scale to zero after placing the tare on the weighing pan.

The value of the measured body weight, located on the scales, is found by summing the readings on the optical scale and on the counter. The number of hundreds or thousands of grams is counted by the counter, in the window of which the numbers 0, 1, 2, 3 and 4 appear, depending on the mass of the weights removed from the suspension.

Weighing on technical scales

Scales are installed on strong stable tables in the working room of the laboratory strictly vertically along the plumb line. Before weighing, they check whether the scales are correctly installed, after which they lower the rocker with a cage and observe the fluctuations of the arrow along the lower scale. If the arrow deviates from zero by the same number of divisions to the right and left, the scales can be used. Otherwise, the balance of the scales is achieved using the balancing nuts of the rocker arm.

The mass to be weighed is placed on the left platform of the balance, the weights of the gram set are placed on the right, and the weights of the milligram set are hung by the weight mechanism.

The mass of a substance is best determined by the method of double weighing, which consists in the following: the object to be weighed is placed on the left scale pan, and the weights are placed on the right pan until the scale pointer is set to zero on the scale. After that, the object to be weighed is transferred to the right cup, and the weights to the left. If one of the cups outweighs the other, then by adding or removing weights, the balance point is again established. The actual weight of the weighed object is equal to the arithmetic mean of the results of these two weighings. At the end of the weighing, the object to be weighed is removed from the scales, the weights and weights are removed, placing them in the case in the prescribed manner.

Analytical balance

Even more than technical and technochemical balances, the scales of the analytical group have undergone modernization in recent years. At the same time, equal-arm balances without dampers are still successfully used in many chemical laboratories - balances of periodic oscillation that are not equipped with built-in weights. The peculiarity of work on the scales of periodic swing is to determine their zero point. The rocker, freed from the arrester, begins to make gradually damped oscillations. The zero point and the equilibrium point are determined by the method of multiple deflections of the rocker arrow. Before determining the zero point, the rider must be removed from the rocker if the zero is in the center of the rocker, or set to zero if the zero is at the left end of the rocker.

To determine the sensitivity of the balance, set the equilibrium point at various loads. To do this, after establishing the zero point, a rider is placed on the rocker (with a caged balance) so that it shows 1 mg, the arrester is lowered and the equilibrium point is determined.

For example, if the zero point of the scales is +0.2 divisions, and the equilibrium point with a load of 1 mg on the right cup is +3.8 divisions, then the sensitivity of the scales is found by placing 5, 10, 20, 30, 40 consecutively on both cups. , 50 and 100 g. The results are plotted on a graph.

When using rater damper balances, the determination of mass using built-in or overhead weights is carried out only up to 10 or 5 mg (i.e., up to the mass of the rater). Further balancing is carried out with the help of a rater, which is set only to the nearest whole milligram to equilibrium. If a weighing accuracy greater than 0.1 mg is not required, tenths of a milligram are found by moving the rater along the rocker. If more accurate weighing is required, the rater is set as in the previous case, and tenths and hundredths of a milligram are found by the difference between the zero point and the equilibrium point found based on the previously determined sensitivity of the scales for a given load.

Let, for example, the zero point of the weights be equal to +0.5; the equilibrium point of the scales with a load of 14.3300 g on the right cup and the rater at a division of 3 mg is +2.0; the sensitivity of the balance at a load of 14.5 g is 4 divisions per 1 mg. Obviously, the object being weighed is not completely balanced. If the rater is moved to a division of 4 mg, then the equilibrium point will move 4 divisions to the left, i.e., it will be equal to -2.0. In order for the equilibrium point to coincide with the zero point (+0.5), the rider must be moved by (2.0 - 0.5) / 4.0 = 0.38 divisions, i.e. by 0.38 mg. Consequently, the mass of the object being weighed will be equal to 14.3300 g (on the scale pan) + 0.00038 g (reiter reading) - 14.33038 g.

In many laboratories, two-cup equal-arm laboratory analytical balances VLA-200 g-M (AD-200) are used with the following main characteristics: the maximum allowable load is 200 g; measurement range on the optical scale ±10 mg; the error due to the unequal arm of the rocker is not more than 2 mg. Kettlebells are controlled by means of limbs. When the small limb is rotated, tens of milligrams are hung or removed, while the large limb is rotated - hundreds of milligrams. The limbs rotate independently of each other. Switching the scales on and off is done by a handle, put on the cage roller, placed on the front wall of the base.

At present, the industry mainly produces two-cup equal-arm balances of the VLR type, for example, balances of accuracy class VLR-200g and VLR-20g. Balances VLR-20g, replacing the semi-microanalytical balances VLM-20g-M, are characterized by high sensitivity and smaller overall dimensions. On the basis of scales VLR-200g with an electronic prefix are produced electronic balance VLE-200g.

The technical data of VLR-200g scales (Fig. 78) and VLR-20g scales are given below:

When using scales of the VLR-200g type, first of all, turn on the illuminator in the network, after which, without opening the doors of the scale cabinet, carefully turn the arrester disk to failure. The light bulb automatically lighting up at the same time illuminates on the screen of the weighograph an enlarged image of a microscale attached to the balance pointer. If the balance is not loaded, the zero of the scale must exactly match the vertical line on the screen (mark). Otherwise, the match is achieved by turning the adjusting screw located on the outside of the lower board of the scales above the arrester disk. Then the load is placed on the left pan of the scales, and on the right - gram weights from the set of weights to the scales; in this case, the mass of the number of whole grams is found. Close the cabinet door; turning a small limb with tenths of a gram, they combine a fixed pointer with various numbers of the disk. At each turn of the disk, it is necessary to first cock the scales. Having established the number of tenths of a gram, they find the hundredths of a gram using a large limb. Next, the arrester disk is turned to failure and, after the oscillation of the rocker arrow stops, the position of the vertical line is read on the scale on the screen. Large divisions of this scale, corresponding to milligrams, are indicated by numbers with a “+” or “-” sign. Plus shows that the value of the count made must be added to the mass of the weights placed on the scales, and the minus must be subtracted.

After the weighing is completed, the result is recorded, the weighed object and weights are removed from the scales. To release the rocker from the built-in weights, rotate the disk handles until the fixed pointer is aligned with the zero division of both disks.

In addition to equal-arm analytical balances of the VLR type, the industry produces one-arm balances of the 2nd class of the VLDP-100g type (Fig. 79). The principle of weighing on a double-prism balance is based on balancing the moment created by the load and the moment obtained when the built-in weights are removed from the suspension. The balance rocker is an unequal lever; on short shoulder a saddle with a load-receiving prism is fixed, and a reading scale on the long one. An earring rests on the load-receiving prism of the rocker with a pillow, to which a bar is rigidly attached for applying built-in weights. To remove and apply the built-in weights, a weight mechanism is used. Simultaneously with the removal of the weights, the value of their mass (in g) is displayed in the three left windows of the screen. With accurate weighing, the rocker is calmed with an air damper; with preliminary - oil. The handle for entering the scales into the working position is located on the left side of the scales. Preliminary weighing is carried out by turning the handle away from the operator, accurate - on the operator. The zero position of the scale during preliminary weighing is adjusted by the handle located on the right side of the balance, at the top; at exact weighing - the handle below. The pre-weighing mechanism is designed to determine the mass of built-in weights. To read off the scale on the screen there is a reference mark in the form of two parallel strokes.

The result of weighing is determined by the sum of the readings of the reference scale, the counters of the weight mechanism and the dividing device. Weighing range from 0 to 100 mg. The price of the smallest division of the scale is 0.05 mg. Weighing error ±0.065 mg.

Analytical balance installation

The installation of an analytical balance begins with the choice of premises and the organization of the workplace of the chemist. The room for installing scales of classes 1 and 2 should consist of a weight room and a preparation room. One of the conditions for a weighing room is its complete isolation from adjacent laboratory rooms.

For the weight room, choose a light, dry room. It is desirable that it be located on the first floor, facing north. The weighing room must be maintained at a constant temperature of about 20°C. The balance must be protected from heat and air currents, as well as dampness, dust, harmful gases and shocks. To reduce the influence of air and heat flows, it is recommended to close windows and doors with thick curtains. Windows should be double-glazed and sealed; Windows and vents must not be opened. It is recommended to ventilate the weighing room with a fan, and only when weighing is not being carried out. It is recommended to cover the floor with linoleum, which is easily cleaned of dust and is a poor conductor of heat.

The balance must be installed in horizontal position on especially strong pedestals, protecting the scales from any concussions. It is not recommended to move the scale from place to place.

Analytical balances with a maximum load of 100 g or more are recommended to be installed on a cantilever table, consisting of a concrete slab, lying freely on shock-absorbing rubber or foam pads in a table harness resting on two metal brackets attached to the main wall.

It is advisable to install semi-microanalytical balances on a table with massive legs. The table consists of a massive cover, in the frame of which felt, a reinforced concrete mosaic slab and linoleum are embedded.

The lamps in the weighing room should sufficiently illuminate the scale of the scales and, at the same time, not heat the rocker arms. It is best to install fluorescent lamps.

In the weighing room, a table with the basic rules for handling scales should be posted.

The cleanliness of the weighing room must be carefully monitored. After weighing, it is recommended to cover the scales with covers.

Do not place anything on the console table or shelf on the brackets where the balance is installed. To the left of the table (shelf) it is advisable to have a mobile table for a desiccator with a weighed substance and for making records.

Rules for using analytical balances

1. The load on the weighing pans should not exceed the maximum load for this type of scale. They weigh only while sitting against the scales, leaning their hands on the table top. The object to be weighed is taken with tweezers, tongs or clean paper and placed in the middle of the left cup. Chemical substances are weighed in glassware (bottle, ampoule). Do not place chemicals directly on the weighing pan or weigh on a piece of paper.

2. The object to be weighed must be at the same temperature as the balance. Therefore, before weighing, the substance should be kept in a desiccator near the balance for 20-30 minutes. If, when weighing over the scales, the lamp is turned on, then this must be done 10-15 minutes before the start of work.

3. Add or subtract the weighed substance only outside the weighing cabinet. If the substance to be weighed is spilled on the weighing pan or on the bottom of the cabinet, immediately sweep it up with a brush.

4. The weights should be placed on the right scale pan in such a way that they are in the center of the pan. Take weights with tweezers with bone (plastic) tips.

5. When the weighed substance or weights are placed on or removed from the weighing pan, the weighing instrument must be locked.

6. Before each weighing, check and, if necessary, set their zero point. While observing the deviation of the scale pointer, the doors of the cabinet must be closed.

7. When balancing a weighed object, start with large weights and then move on to smaller ones.

You should always use the smallest number of weights, for example, take a weight of 2 g, and not two weights of 1 g each. On the scales, the weights should lie in a certain order; small weights should not be stacked on top of each other. Large weights should be placed in the center of the cup so that it does not swing.

Weighing errors and their elimination

Errors in accurate weighing can occur from various reasons: from the imbalance of the scales; from being weighed in the air and not in the void; from changes in the mass of bodies in the process of weighing due to fluctuations in temperature, humidity and air pressure; from inaccurate values ​​of masses of weights; from instrumental errors.

Errors due to the imbalance of the scales mostly occur with the method of simple weighing on the scales of periodic oscillation. However, corrections for uneven shoulders are not always required. So, when determining the percentage composition of a substance (in % (mass.)) When the weighing of the analyte and its weight form is carried out on the same scales and when the weighed substances are placed on the same cup, the relative error for both weighings will be approximately the same. But when it is required to determine the absolute mass of an object with an accuracy exceeding 0.1 mg, one has to resort to weighing methods that exclude corrections for uneven arms, for example, the substitution method.

The Borda substitution method is as follows. The mass to be measured is placed on the right scale pan and balanced with any tare weight on the left pan. Determine the equilibrium position E1. Then, the measured mass is removed from the right cup, without removing the container from the left one, and instead of the removed mass, weights are applied in such an amount that it is possible to read on the scale, and the equilibrium position E2 is determined. The measurement result is equal to the weight of the applied weights plus the reading on the scale and is determined by the formula (E1 - E2)S, where S is the sensitivity of the balance.

The substitution method proposed by D. I. Mendeleev consists in placing weights on one of the cups in an amount corresponding to the maximum load of the scales, and balancing the scales with a tare load. The body to be weighed is placed on a cup with weights and such a number of weights are removed so that the scales come to the initial equilibrium position. The value of the mass of the weighed body is determined as the algebraic sum of the mass of the weights taken from the cup and the readings on the scale of the scales. This method is the basis of the principle of operation of two-prism single-arm balances.

Errors caused by weighing in air follow from the well-known physical law that each body immersed in a liquid (gas) loses as much in its weight as the liquid (gas) displaced by it weighs. All bodies, therefore, weigh less in air than in a vacuum. Ordinary weighing in air would lead to the correct result if the weights lost in their mass as much as the weighed body loses. However, analytical weights are usually made of stainless steel (p = 8.0 g/cm3) or brass (p = 8.4 g/cm3), while milligram weights are made of aluminum (p = 2.7 g/cm3). If the density of the weighed body is less than the density of the weights, then the body displaces more air than the weights and, therefore, it weighs less in air than in a vacuum. The error value usually does not exceed 0.04-0.05%.

Errors caused by a change in the mass of bodies during the weighing process can occur due to absorption or loss of moisture, evaporation of volatile substances, temperature changes, inattention and inaccuracy of the experimenter. These errors can be eliminated by weighing the substances by difference in small volume hermetically sealed glassware. When weighing by difference, the position of the zero point can be ignored.

The mass errors of weights depend on the degree of accuracy of fitting their mass to the nominal value, certification errors and on irreversible changes in mass in the period between verifications, mainly due to corrosion. Errors associated with the inaccuracy of the masses of the weights used can be eliminated by comparing them with the mass of exemplary weights on the scales on which they will be used.

Micro and ultra microbalances

For especially accurate measurements small masses when carrying out physical and chemical researches and microanalyses use exact lever and without lever scales of various designs.

Lever spring scales are produced with a maximum load of 20 to 100 mg, with a division value of 10 v minus 7 - 10 v minus 5 mg (VLU-20mg and VLU-100mg). The principle of operation of these scales is based on balancing the moment created by the measured mass by twisting the quartz stretch. By design, these are stretched scales with an equal-arm yoke and a zero weighing method. The rocker is placed in a special container that protects it from the action of air currents and at the same time serves as a heat distributor. A cup with a weighed load is taken out of the rocker column into the side compartment of the showcase by a manipulator, which is interlocked with the column opening and closing mechanism. The reading of the measurement results is made on the scale of the measuring limb, the division value of which is 0.00032 mg (VLU-20mg) and 0.0005 mg (VLU-100mg). Calming time of rocker oscillations is about 1.5 min.

Microanalytical balances VLM-1g are designed for weighing precious stones and metals, as well as for various substances in microchemical analyzes of increased accuracy. Scales have equal arms with two pendants and cups. Full mechanical weight-lifting is carried out by two weight-bearing mechanisms. Scales are supplied with the mechanism of removal of the left cup. Measurement range on the optical scale ±1 mg. The division price of the optical scale is 0.01 mg. Weighing error ±0.07 mg.

To quickly determine the mass of very small amounts of substances, torsion (spring) scales are often used (Fig. 80). They differ from quadrant ones in that the load-receiving cup in them is enclosed in a showcase and equipped with a locking device. Torsion scales are available with various weighing limits. In laboratory practice, the BT-500 model is often used. The maximum allowable load of the scale is 500 mg, and the smallest is 10 mg. The absolute error of indications in any mark of the scale is no more than ± 1 mg.

The measuring element in a torsion balance is a spring, the tension of which, when twisted, balances the weighed sample. The angle of twisting of the spring is proportional to the mass of the weighed sample, therefore the scale of the scale is graduated in units of mass.

When using VT-500 torsion scales, they are set at level 1 by means of support screws 2, after which the rocker arm 3 is released by moving the fixing lever 4 to the right. The mass indicator 5 is set to zero using the tension lever 6. In this position of the balance, the balance indicator 7 overlaps the balance line or it is brought into this position by the calibration head located on the reverse side of the balance in the center. Then the rocker is fixed by moving the fixing lever to the left to failure, and proceed to weighing. To do this, open the safety cover 8, hang the weighed load on the hook of the rocker arm 9 and close the cover again. The rocker is released by moving lever 4 to the right. By turning the lever 6 to the left, the pointer 5 is moved until the pointer 7 is set exactly on the balance line. In this position, pointer 5 shows on the scale the value of the mass of the measured load. After weighing, the rocker is fixed by moving the lever 4 to the left, open the cover 8, remove the weight from the hook and close the cover. Lever 6 is moved to the right, pointer 5 is set to zero - and the balance is ready for the next weighing.

Tariff scales

In addition to manual scales, tare scales VKT-1000, T-1000, as well as desktop ordinary scales (NO) are used in the pharmaceutical production of medicines.

Tare scales VKT-1000, T-1000 - general-purpose scales of class II, which are used for weighing liquid, viscous and bulk substances. Their designs do not have a fundamental difference from manual scales. These are also lever scales with a rectilinear equal-armed rocker arm, but with a higher carrying capacity. Equal-arm rocker with one support, two load-receiving prisms rests on a pillow located on the column. Earrings with removable cup holders and cups are suspended on the tips of the load-receiving prisms. Balance regulators are located at the ends of the rocker arm. The arrow is rigidly attached with a rocker arm and a reference scale. Scales are installed on a plumb line.

hand scales

In pharmacy practice, hand-held scales of the BP and VSM brands are used to weigh small amounts of medicines. The industry produces hand scales of 4 standard sizes (Table 1).

Brands and sizes of hand scales

Weighing on hand scales

Hand balances are used almost exclusively for weighing bulk solids in quantities from 0.05 to 100 g.

1. In non-working condition, hand-held scales are kept on the hook of a special tripod, less often in a closed box. When the scales are placed on the tripod hook, one scale pan is placed in another pan and the scales are suspended by the cords of the lower pan in such a way that the tripod hook passes between the vertically hanging rocker arm and the cords of the lower pan. To avoid unnecessary wear of the prism tips, do not hang the balance by the ring of the holder.

2. Before starting work, make sure that the scales are balanced: rocking of the rocker is stopped not by clamping the arrow inside the clip, but by bringing the horizontally located palm of the right hand under the cups or lowering it until the bottoms touch the table surface.

3. When weighing on hand scales, they are held in the left hand on weight. It is usually recommended to take the scales with the thumb and forefinger of the left hand in such a way that the clip hangs strictly vertically and is completely free. The middle and ring fingers of the left hand are placed on both sides of the clip, without touching it, but having the ability to limit the movement of the balance needle and, if necessary, stop its vibrations. Before weighing, a sheet of white paper is placed under the right scale pan.

4. After making sure that the scales are balanced and their cups are clean, the required number of weights is placed on the left cup, and the substance to be weighed is placed on the right cup (a small weight is taken only with tweezers, protecting the scale cords from contamination with powder). Nameless or thumb the left hand supports the arrow of the scales, not allowing it to deviate far to the left from the plane of the clip and feel the approach of balance. At the end of weighing, the scales are wiped with a gauze cloth.

5. To weigh out toxic substances, a separate weight is used.

Weighing on tare scales

Taring scales are used for weighing bulk, liquid and viscous substances in quantities allowed by maximum loads (from 50 g to 1 kg). Before starting work, you should install the scales on a plumb line and balance the scales (using the balance regulators); the needle must be at zero or deviate from it by equal distances. Weights and weights are recommended to be placed in the middle of the cups. The total mass of the weights is calculated carefully, preferably twice - when placing them on the weighing pan and when removing them from it. Weighing of bulk materials is carried out directly into a paper bag or box with an appropriate label, sometimes onto a sheet of writing, waxed or parchment paper. The prepared package in the open form is placed on the right pan of the scales. The same bag or sheet of paper and weights are placed on the left cup as containers. From a stanglas that is held in right hand, a powdered substance is carefully poured into the bag, and the rod is slightly swayed along the longitudinal axis; with the index finger of the right hand, lightly tap on the neck of the stem. Touching the cup with the load with the index finger of the left hand, they try to feel the moment of balance.

Caring for scales and weights

1. Before starting weighing, check the condition of the balance, brush off the dust from the cups and set the zero point (see below).

2. The object to be weighed and the weights can be placed on and removed from the weighing pans only when the cage is closed, i.e. when the arrester is lowered. When the cage is open, the balance must not be touched. The arrester disk should be turned slowly, smoothly and carefully.

3. Do not move the scales from their place.

4. Do not overload the scales above the allowable limit (100 g), as this can lead to damage to the scales. In case of doubt, the object must be weighed on a laboratory chemical balance and, making sure that its mass does not exceed the limit, proceed to weighing it on an analytical balance.

5. Do not place wet and dirty objects on the scales. Do not spill or spill anything inside the scale cabinet. Do not weigh hot objects, as well as very cold ones. The object to be weighed must be at the temperature of the room in which the balance is located.

6. Do not place the item to be weighed directly on the weighing pan. Do not weigh on a sheet of paper. Weighing is carried out on a watch glass, in a bottle, in a crucible or in a small glass. Hygroscopic substances, as well as liquids that emit caustic vapors, are weighed in a closed weighing bottle.

7. When weighing, it is allowed to use only the side doors of the weighing cabinet, the front wall of the cabinet must be closed.

8. Weights when weighing should be taken only with tweezers. Weights can only be placed on the scale pan or in a case, in their nest.

9. When not in use, the balance must always be caged (cage lowered). You can not lean on the table on which the scales stand, interfere with the person working on the scales with extraneous conversations. In case of any malfunction of the scales, you must contact the supervisor.

The purpose of the laboratory work is to study the methodology for using lever-type scales and mastering the practical skills of measuring mass. The principle of operation and the device of lever scales allow weighing with sufficient accuracy. The basis of a simple and reliable device is the use of an equal-arm lever. People have been using this reliable device for thousands of years.

The preparatory stage and the process of weighing with a balanced instrument

Before measuring body weight on a balance scale, they need to be balanced. As a result of the balancing procedure, it is necessary to ensure the most accurate coincidence of the instrument pointer with the central risk. To fine-tune the measuring device, you can use pieces of cardboard - they are added to the weighing pan, which turns out to be lighter.

Important. The weighing process involves balancing the weighed object with other bodies (loads), the mass of which is indicated on their surface.

When equilibrium occurs, the weight of the body under study and the sum of the masses of the weights will coincide in magnitude. Now it remains to compare the masses of the bodies that are placed on one and the other scales. Thus, after calculating the total mass of all weights, you can find out the mass of the weighed body.

Weighing by taring on a detuned balance

Taring is a special weighing method. For its use, in addition to weights, a jar and shot (dry sand) should be used. On one cup at the initial moment of measurement there will be a body (object) that needs to be weighed. A jar is placed on the second to equalize the mass with the object, into which shot and (or) dry sand are poured.

After equilibrium is reached, the object to be measured is removed, and weights are put in its place, the selection of which also achieves equilibrium. The masses of the object and weights are equal.

The advantage of using the taring method is the possibility of fairly accurate weighing on a detuned balance. About how accurate is this method, can be judged by the fact that after reaching equilibrium, adding even a small piece of cardboard to one or another scale pan will lead to an imbalance in the balanced system of weights. It can be recommended to check the accuracy of measurement readings using lever-type scales in one and the second way.

Weighing rules

There are a number of rules, the observance of which will allow you to measure body weight on the scales with the required accuracy.

It is important to properly balance the measuring device - it must be placed on a hard and level base area. It is advisable to put the weights on the scales on the right, and the object to be measured (body) on the left.

Placement of weights and measured bodies should be carried out with care, avoiding falls and bumps. It should be borne in mind that among the characteristics of this type of measuring instruments, an important indicator is the ultimate load.

When weighing, objects (bodies) whose weight exceeds the maximum allowable value of the indicator should not be placed on the scales. Heavy bodies should be weighed with devices with the appropriate characteristics.

It is highly undesirable to place hot, dirty and wet objects on the weighing pan. Do not pour certain liquids, place loose substances. If it is necessary to measure their mass, one or another container with a known net mass can be used. Accuracy in carrying out the weighing process will increase the life of the balance.

Important. In accordance with the rules, tweezers should be used if small weights or smaller weights are to be used.

With weights work as follows:

1. A weight is placed on the right cup, the weight of which is greater than the weighed object placed on the left cup. After making sure that she pulls the left cup with the object, they put it into the case.
2. Take the next one at face value. If it is less than the mass of the weighed object, we leave it in the cup.
3. The next weight or weight is selected in exactly the same way. If you act according to a similar algorithm, you will always have enough small weights for weighing.
4. Each weight in the case has its own place, after the completion of the measuring process, they should all be in their places.

To fully assimilate the information material, it is necessary to consolidate it in practice. Take any metal bar, the weight of which does not exceed the permissible measurement limit, or other objects and try to measure their mass as accurately as possible. Write down the result, for example, in the form: 20g + 10g + 2g + 1g + 500mg + 10mg = 33.51 g.

Conclusion

The mass of the body, when measured with a lever-type balance, can be determined with great accuracy.