Correct weighing method for microbalances

Correct weighing method for microbalances

Weighing is one of the most common operations in the experiment. Today's micro, semi-micro and analytical balances (readability 0.1 ug-0.1 mg) are perfect, so the weighing process is generally not required to be done in a special weighing chamber. Advances in electronic technology have greatly simplified the process and significantly shortened the weighing time. Similarly, the adaptability of the balance has increased, and now it can even be applied directly to the production site. However, these advances also impose certain requirements on the use environment. These effects mainly refer to the physical changes of the sample itself measured by trace, semi-micro and analytical balances, such as slow evaporation, weight change caused by moisture absorption, or magnetic force, static electricity, and force on the sample and the weighing plate. And all these changes reflect the weight change on the balance. This paper aims to introduce the correct use of trace, semi-micro and analytical balances in the following aspects to obtain high quality weighing results.

Placement of the balance

The accuracy and reliability of the weighing results are closely linked to the placement of the balance. Therefore, the following factors should be carefully considered when deciding to analyze the placement of the balance:
First of all, there should be a fixed workbench dedicated to the balance, placed on the floor at the corner of the room or fixed to the wall (but not both ways to prevent vibration from transmitting at the same time), the workbench should not be tilted during work, and should be as It is less oscillated and requires diamagnetic and antistatic protection.

Secondly, the studio can only have one entrance and exit to prevent the influence of air convection symmetry. The less the window should be, the better the risk of direct sunlight is avoided. At the same time, the degree of oscillation and vibration of the studio should be higher.

Third, the room temperature should be as stable as possible to prevent temperature drift, and weighing should be avoided near the source. The relative humidity of the air should be controlled between 45% and 607% (it is recommended to continuously monitor the microbalance to correct the changes if necessary).

Fourth, the balance should be weighed at the wall without windows to avoid the sun's shovel. The lighting device should also be kept at a certain distance from the workbench to avoid the interference of heat radiation.

Finally, pay attention to the effect of air on the balance. Avoid weighing in the vicinity of air conditioners or fans. Avoid weighing near the source of heat (causing temperature drift) to prevent the influence of strong airflow. Weighing by the door.

Balance method of operation

Micro, semi-micro and analytical balances are high-precision measuring instruments, and mastering the correct method of operation will help to quickly obtain reliable weighing results. The following are the basic methods and precautions for balance operation:

First check if the bubble is in the center of the level and adjust the leveling screw if necessary until the balance is level. Then calibrate the balance. The balance needs to be calibrated before the balance is first operated, or when the balance is replaced, the balance is leveled, and significant temperature, humidity or air pressure changes occur around it.

In the third step, check whether the windshield is closed. If it is closed, it can be turned on: continuously energize the balance and turn it on, and the temperature balance can be formed inside the balance; use the peeling door to close the balance. The balance will therefore be in standby mode and the internal electronics will still be energized, thus omitting the warm-up process.

Fourth, the weighing container should be as small as possible, and avoid using plastic weighing containers. When the air humidity is 30-40% lower, do not use the glass mouth to avoid static electricity. The sample in the weighing container and its interior should be kept consistent with the ambient temperature, as temperature differences can cause airflow and cause changes in the wet film on the weighing container and sample surface. At the same time, the weighing container cannot be placed directly into the balance weighing chamber by hand, otherwise the temperature and humidity of the weighing chamber and the weighing container will be changed. There will also be a reaction to the symmetry process.

Fifth, the weighing sample should be placed in the center of the weighing pan to prevent four corner errors. For micro and semi-microbalances, the weighing pan needs to be briefly loaded (initial weighing) after a weighing pause of more than 30 minutes. The weighing sample can be removed from the weighing pan after the weighing process is completely completed, so as to avoid the change of the weighing chamber temperature and the air humidity caused by the weighing sample.

Sixth, before the start of weighing, it should be checked whether the display is in the zero state. If necessary, it is necessary to perform peeling to return to zero to prevent zero error. Then adjust the automatic stabilization detector until it is satisfied. If the stability detector's mark is dark (= weighing result display), the weighing value is read immediately.

Seventh, the weighing chamber and the weighing pan should be kept clean, and only the clean weighing container can be used for weighing.

Physical influence factors of symmetric heavy results

If the weight display is unstable, the weighing result is slowly increasing or decreasing, or more simply, the wrong value is displayed, most of them are caused by the following physical factors: The most common reasons are: 1. Symmetrical heavy sample Incorrect transfer 2. Improper placement of the balance 3. Weighing and loss of sample and water; 4. Weighing the static phenomenon of the sample and container; 5. Weighing the magnetization of the sample and the container.

The following will list these factors, explain their causes and introduce possible corrections.

First, the effect of temperature will cause the weight value of the weighed sample to change frequently in one direction. This is because there is a temperature difference between the weigh sample and the surrounding environment, which causes a flow of gas along the weigh container. The air flows along the outside of the container to create an upward force that causes the weighing result to be erroneous, ie the sample weighs less than it actually is (dynamic buoyancy). This effect is not terminated until a temperature equilibrium is formed.

The changes inside the wet film that wrap each sample are hidden by dynamic buoyancy. The wet film changes with temperature, ie the cold object appears to be heavier and the hot object is lighter. In particular, this effect (regulation temperature) should be closely monitored when using a microbalance for differential weighing. In order to avoid the influence of temperature, the following correction methods can be used: 1. Weighing the sample without taking it directly from the dryer or the refrigerator; 2. Keeping the weighing sample with the same temperature as the laboratory or balance weighing chamber; 3. Using the tweezers Take the sample; 4, do not put the balance in the balance weighing chamber; 5. Use a sample container with a small surface area.

Second, the moisture absorption/evaporation effect will slowly increase or decrease the weight of the weighed sample. The increase in weight is due to the fact that the sample is more hygroscopic, and the decrease is due to the volatile matter being measured or the water being evaporated. For the effect of moisture absorption / evaporation, the following correction methods can be used: 1. Use a cleaned dry weighing container and keep the disc free of dust, contaminants and water droplets; 2. Use a thin neck container; 3. Cover the container or On the plug.

Third, the effect of the electrostatic phenomenon will cause the weighing containers to display different weights each time the weighing is performed, and the repeatability of the results is poor. Materials with high insulation (such as most glass or plastic weighing containers) are susceptible to electrostaticization. The static electricity on the weighing sample must be removed or isolated. It is possible to increase the humidity of the air by means of a humidifier or by adjusting the air conditioning system appropriately. In winter, you should pay special attention to this in a heated room (ideal relative air humidity is 45--60%); 2. Use a barrier to isolate the electrostatic force (put the weighing container in the metal container); 3. Use Industrial use that can be purchased

Antistatic gun, but it is not valid for all materials; 4. Ground the balance (then the grounding of the weighing pan).

Fourth, the effect of magnetization makes the weight of the weighed sample dependent on its position on the weighing pan, and the repeatability of the weighing result is poor. This is because the sample being weighed has been magnetized, and it attracts the iron to produce a surplus force that is mistaken for the load to be loaded. For the anti-magnetic effect, the following correction methods can be used: 1. If possible, de-magnetize the iron or iron alloy magnetic weighing sample (iron, steel, nickel, etc.); 2. The sample can be placed on a non-magnetic support ( Such as: a large number of cups, aluminum alloy brackets) in order to increase the distance between the weighing pan; 3 use the hanger: 4. Screen magnetic force (put the weighing sample in the magnetic alloy container).

Fifth, the effect of static buoyancy will make the weight of the weighed sample in air different from other vacuums. The medium surrounding the sample was air, and the air density was about 1.2 kg/m3 (depending on temperature and pressure), so the buoyancy of the sample was also 1.2 kg/m3. In order to avoid the influence of static buoyancy, the following correction method can be used. 1. When weighing with high precision measurement requirements (mainly with a microbalance), the displayed weight needs to be corrected accordingly, because the samples of different densities are weighed. On average, the calibration brick code with a density of 8.0g/cm3 is used for calibration, so there will be buoyancy error; 2. If the weighing process cannot be completed in one day (eg differential weighing, comparative weighing), the day must be detected. The air pressure, atmospheric humidity and temperature are calculated to calculate the buoyancy value for correction.

Finally, talk about the impact of gravity. When the weighing is carried out 10 meters from the ground (eg on the fourth floor of the building), the weight display will change. This is because: The balance also measures gravity when measuring the weight of the object, ie the attraction between the earth and the weighing sample. This force is mainly determined by the altitude at which the object is located (the distance from the center of the Earth). The farther the weight is from the center of the Earth, the smaller the gravity acting on it, so the Earth's gravity decreases with increasing distance. The closer the position of the weight is to the equator, the greater the centrifugal acceleration caused by the rotation of the Earth opposite to the direction of gravity of the Earth, and the smallest when it is at the equator. The change in weight is based on the strength of gravity. In order to overcome the effects of gravity symmetry results, the balance needs to be recalibrated after moving.