Correct weighing - the use of microbalances, maintenance

Introduction Weighing is one of the most common operations in the laboratory. Weighing techniques for microbalances, semi-microbalances, analytical balances and precision balances have been greatly developed. Usually, the weighing operation can be carried out without a specially designed weighing chamber.
The development of electronic technology has greatly simplified the operation of the balance, significantly shortened the weighing time, and has good adaptability, which can be directly integrated into the production process.
However, the inevitable risk in the weighing process is that sufficient measures cannot be taken to prevent the interference effects of environmental factors, usually including physical factors affecting microbalances, semi-microbalances and analytical balances: weighing results cannot be stabilized (eg slow evaporation) The external force (magnetic force, static electricity, etc.) acting on the weighing pan and the sample causes the weighing result to change.
The above description is intended to explain the important points to be aware of when using microbalances, semi-microbalances and analytical balances if very accurate weighing results are required.
After a brief description of the placement of the balance and the correct operation, we will explain in detail the interference factors affecting the weighing. You can detect the effects of these factors by slowly changing the value of the weighing on the display (drift).
Since the correct understanding of the technical parameters is also critical to the evaluation of the weighing results, the usual technical terms are explained at the end of this manual.

The accuracy and reproducibility of the weighing position of the balance are closely related to the placement of the balance. To ensure that your balance is always working at its best, follow these guidelines:
The weighing platform is stable (laboratory, experimental table, marble table). During the weighing process, your weighing platform cannot be deformed and the vibration is avoided as much as possible.
 diamagnetic (non-ferrous material).
 Antistatic charge (non-plastic or glass material).
安装 Install on the wall or on the ground. The weighing platform should be standing on the ground or fixed to the wall. If it is fixed at both places, the vibration from the wall and the ground will be transferred to the weighing platform at the same time.
保留 Reserve a location for the balance. The balance placement and weighing platform must be strong enough to ensure that the balance display does not change when someone leans against the weighing platform or near the weighing position. Do not place a cushion such as a writing pad under the balance. It is best to place the balance directly above the stand of the weighing platform, as this area is subject to minimal vibration.

Weighing chamber  No vibration  No air flow Place the weighing platform in the corner of the room. These areas are the areas of the building that are least affected by vibration. It is best to use sliding doors at the entrance to the room to reduce the impact of the door opening and closing.

Temperature  Keep the temperature of the weighing chamber as constant as possible. The weighing result will be affected by the temperature! (Typical temperature drift value: 1-2 ppm/ ̊C)
请勿 Do not weigh near the radiator or window.
The METTLER TOLEDO balance with built-in “Full Automated Calibration Technology (FACT)” compensates for the effects of temperature drift symmetry results. Therefore, always turn on the "FACT" function.

Air humidity 相对 Relative humidity (% RH) must be 45% to 60%. Do not operate the balance when the relative humidity is less than 20% RH or greater than 80% RH.
For microbalances, the relative humidity of the weighed environment should be kept as constant as possible.
Light  If possible, place the balance near a windowless wall. Direct sunlight (temperature) can affect the weighing result.
放置 The balance should be placed at a distance from the lighting unit to avoid heat radiation. Use less illuminating equipment that generates heat radiation, such as fluorescent lights.

Airflow 请勿 Do not place the balance in a stream of air such as a computer or a large laboratory unit such as an air conditioner or fan.
请将 Keep the balance at a distance from the radiator. In addition to the potential temperature drift, strong airflow also affects the balance weighing.
请勿 Do not place the balance near the door.
请勿 Do not place the balance in a crowded place. The flow of people creates a flow of air at the weighing position.

The balance operation micro-balance, semi-micro balance, analytical balance and precision balance are high-precision measuring instruments. The following will help you get reliable weighing results.

Balance start  Do not disconnect the balance from the power supply and keep it energized at all times. This will maintain the temperature balance inside the balance.
使用 When you turn off the display, use the on/off key (for older models, the tare key). The balance is now in standby. The electronic components are always energized and do not require preheating.
Adjusting the level of the balance 调节 The adjustment method of the balance level is to adjust the level of the foot using the level of the balance and check if the bubble is in the middle of the level indicator. After that, you must correct the balance. Please refer to the operating instructions of the balance for the specific operation procedure.
Tip: We recommend that when the balance is connected to the power supply for the first time, depending on the balance model, the balance should be warmed up at different times. The warm-up time is:
● The preheating time of the microbalance is at least 12 hours. ● The warm-up time of the semi-microbalance and the analytical balance is about 6 hours. ● The warm-up time of the precision balance is about 3 hours. In addition to these principles, the instructions should be followed in the operating instructions. The shortest time to proceed.
Note: In order to ensure and record that the balance is correctly adjusted to the level, for example, in accordance with GxP1), we recommend the use of the Excellence Series XP balance with built-in “Level Control”.

Correction 定期 Regularly correct the indication error of the balance, in particular ● When using the balance for the first time ● When changing the balance position ● After adjusting the horizontal position of the balance ● Read the weighing value when there is a large change in temperature, air humidity or air pressure 提示 Note: When After the temperature changes, if the balance is fully automated, it is important for you to use a balance with “Full Automated Calibration Technology (FACT)”. In addition, it allows you to extend the time interval between daily tests.
Read the weighing value 前 Before starting the weighing, confirm that the balance display value is at the zero position. If necessary, press the tare button to avoid zero error.
读数 Only read when the small circle at the top left of the balance display disappears. The weighing result is released by the stability detector.
Tip: Beyond the Series XP balances have advanced stability detectors. The XP series balances display unstable weighing values ​​in blue. Once stabilized, the displayed value immediately turns black and the small circle at the top left disappears. This will allow you to read stable weighing results more quickly, safely and reliably.

Weighing  Always place the weighing sample in the center of the weighing pan to avoid eccentricity errors.
 When using micro and semi-micro balances, after a relatively long period of time (more than 30 minutes), load briefly once to avoid “initial weighing effects”.

Weighing the container  Use the weighing container as small as possible.
避免 Avoid using plastic weighing containers when the air humidity is below 30%~40%. The use of plastic containers increases the risk of static electricity. Materials with high insulation properties (eg glass and plastic) carry an electrostatic charge. This can lead to serious misrepresentation of weighing results. Therefore, you need to take appropriate corrective actions (see Statics for details).
称 The temperature of the weighing container and sample should be the same as the ambient temperature. The temperature difference produces a gas flow that causes the weighing result to be inaccurate (see temperature). After the weighing container has been taken out of the oven or dishwasher, it needs to be cooled and placed on the balance for weighing.
 If possible, do not place the weighing container directly into the draft shield by hand. This changes the temperature and humidity inside the draft shield and the weighing container, so that the symmetrical amount process has a negative impact.
Tip: Various easy weighing parts (ErgoClip) provide the best weighing conditions to ensure safe weighing without errors

Windshield  Open the draft shield as far as possible at a greater distance. This will keep the inside of the draft shield stable and does not affect the weighing result.
调节 Adjust the balance's configurable automatic door windshield (eg, Beyond Series XP Balances) to open the draft shield to the minimum position.
Note: In order to ensure simple and accurate weighing even under the most severe conditions, we recommend using specific options with the Excellence XP/XS balance. Even in the case of poor sample conditions for high-accuracy micro-sample weighing, the best results are obtained. For example, the “MinWeigh Door” is ideal for weighing in a safety cabinet. However, the minimum weighing damper has advantages for "normal" weighing conditions, reducing the repeatability error of weighing by about half!
With the specially designed "SmartGrid", even if the windshield of the readable analytical balance of 0.1 mg remains open during the weighing process, a stable weighing result can be obtained quickly.

Balance Maintenance 保持 Keep the weighing room and weighing pan clean.
 Use a clean weighing container.
清洁 The balance can be cleaned with a normal glass cleaning solution.
不得 Do not use a cloth containing fusel oil.
不得 Do not brush contaminants into the gaps and openings. +
拆 Remove all removable parts, such as the weighing pan, before cleaning.
Note: All windshield glass that goes beyond the Series XP/XS Analytical Balance can be removed for cleaning.

Physical factors If the weighing result is unstable, the weighing result will drift slowly in one direction or directly display the wrong value, which is often caused by poor physical influence. The most common reasons are:
 The effect of weighing the sample  The effect of the surrounding environment on the balance 称 Weighing or volatilizing the sample  Sample or container with electrostatic charge 磁性 Magnetic sample or container In the next section, we will detail these effects and cause these effects Factors and corrective actions.

Temperature problem:
Possible reasons for the display value to drift in one direction:
The balance is not enough for the warm-up time.
There is a temperature difference between the sample and the surrounding environment, resulting in airflow in the weighing container. The air flowing along the sides of the container produces an upward or downward force, resulting in an incorrect weighing result. This effect is called dynamic buoyancy. This effect does not disappear until the temperature balance is reached. The principle is as follows: cold objects display heavier, warm objects show lighter. This effect can lead to a variety of problems, especially leading to differences in the weighing results of microbalances, semi-microbalances and ultra-microbalances.
Example:
You can test dynamic buoyancy by weighing a cone or similar flask and recording the mass. Hold the flask in your hand for about a minute and then weigh it again. Since the temperature of the flask rises and a temperature difference occurs, the weighing result of the flask is lighter than the original. (The sweat in your hand has nothing to do with this effect. Otherwise you should get a heavier weighing result).
Corrective Action:
请勿 Do not weigh directly from the sample taken from the drying oven or the refrigerator. The temperature of the sample is the same as the temperature in the laboratory or weighing chamber. Use a tweezers to remove the peeling container. 请勿 ​​Do not put your hand in the weighing chamber.  Select a small surface area. Peeled container

Moisture/volatilization problem:
The displayed value continues to drift in one direction.
Possible Causes:
You measure the loss of volatile matter (eg, evaporation of water) or the increased mass of the hygroscopic sample (atmospheric humidification).
Example:
You can replicate this effect using alcohol or silica gel.
Corrective Action:
Use a clean and dry peeling container and avoid dust or water droplets from the weighing pan. Use a container with a narrow neck and install a lid or stopper. Do not use cork or cardboard for flasks with a round base. This will increase or deplete a lot of water. It is recommended to use a round bottom flask that is superior to the XP/XS balance.

Static problem:
Each weighing shows a different weighing result. The display value is unstable.
The repeatability of the weighing result is poor.
Possible Causes:
The peeled container or sample has been charged with static electricity. Low conductivity materials such as glass, plastic, powder or particulate matter cannot or can only remove electrostatic charges very slowly (several hours). This discharge phenomenon is mainly caused by agitation or friction during handling or transport of containers or materials. Dry air below 40% relative humidity increases the risk of this problem.
The weighing error is generated by an electrostatic force acting between the sample and the environment. Microbalances, semi-microbalances and analytical balances are affected by electrostatic charges during operation, causing the above errors.
Example:
A clean glass or plastic container that is slightly rubbed by the plush clearly shows this effect.
Corrective Action:
 Increasing atmospheric humidity Electrostatic discharge is a particularly obvious problem in winter heating rooms. In an air-conditioned room, it is helpful to set the air conditioner to increase the humidity (relative humidity 45% to 60%).
屏蔽 Shield the electrostatic force and place the peeled container in a metal container.
 It is not suitable to use static electricity from other peeled containers and glass. Metal is a good material.
 Use an antistatic gun.
However, products that are available for purchase in the market are not valid in all cases.
 Use the destaticizer supplied by METTLER TOLEDO.
Note: The balance, including the weighing pan, must be grounded at all times. All METTLER TOLEDO's daily average automatic grounding tips with three-pin power plug: "ErgoClip Basket" can effectively avoid the effects of static electricity, therefore, avoid the static electricity of glass containers such as test tubes. influences.

Magnetic problem:
The weighing result of the sample depends on its position on the weighing pan. The repeatability of the weighing result is poor. However, the displayed value has remained stable.
Possible Causes:
You are weighing a magnetic material. Magnetic and magnetically conductive objects exert mutual attraction. The other forces generated are misinterpreted as loads. Almost all objects made of iron (steel) are highly magnetically (ferromagnetic) to the magnetic force.
Corrective Action:
If possible, place the sample in a container made of a material such as a Mu magnetic alloy film to eliminate the magnetic force. Since the magnetic force decreases with increasing distance, the sample can be moved further away from the weighing pan by using a non-magnetic bracket (eg, beaker, aluminum bracket). In addition, the same effect can be achieved by weighing the lower hook. Most METTLER TOLEDO microbalances, semi-microbalances, analytical balances and precision balances have a lower weighing function.
Tip: If you want to use a precision balance to measure magnets with normal and large sizes, we recommend using the “MPS weighing pan” (antimagnetic weighing pan). For analytical balances, we recommend using a triangular bracket that will increase the distance between the magnet and the weighing pan. For the balances in the Excellence XP/XS series, we offer a dedicated “Easy Weighing Weigher” for this purpose.

Static buoyancy effect:
Samples weighed in air and vacuum did not have the same quality.
Cause: "The buoyancy of an object immersed in a liquid is equal to the weight of the liquid discharged by the object being read" (Archimed's principle). This principle explains why the vessel floats, the balloon lifts off, and the quality of the sample is affected by atmospheric pressure. The medium surrounding the sample we weighed was air. The density of the air is approximately 1.2 kg/m3 (depending on temperature and atmospheric pressure). Therefore, the buoyancy of the sample (object) is 1.2 kg/m3.
Example:
If we place a 100g calibration weight in a beaker on one end of a lever-type double-disc balance, then add water to the same beaker on the other weighing pan until the lever is in equilibrium, in the air Both weighing samples have the same mass. Then if we close the lever balance with a bell jar and create a vacuum inside it, the lever will tilt towards one side of the water, because the water will replace more air because of the larger volume, so Withstand greater buoyancy. There is no buoyancy in the vacuum. Therefore, in the vacuum, the water quality on the right side exceeds 100 g.
Corrective Action:
The indication error of the balance is corrected using a standard weight with a density of 8.0 g/cm3. Air buoyancy errors occur when weighing samples with different densities. When performing weighing that requires high measurement accuracy, the corresponding display quality should be corrected.
When weighing on different dates (differential weighing, comparative weighing), check atmospheric pressure, atmospheric humidity and temperature and calculate the air buoyancy correction value as follows:

1. Calculate the air density ρ air density (unit: kg/m3)
P Atmospheric pressure (unit: hPa (= mbar)) (using weighing station pressure)
h Relative air humidity (unit: %)
t temperature (unit: ÌŠC)

m quality
a Air density (unit: kg/m3)
ρ sample density
c Conventional object density (8000kg/m3)
W Weighing value (balance display value)

Gravity effect:
When the height of the weighing changes, the displayed weighing values ​​are different. For example, when the height is increased by 10 meters when weighing (for example, moving from the first floor of the building to the fourth floor), the displayed value will change.
the reason:
To determine the mass of an object, the balance measures the force between the Earth and the sample (ie, attraction, also known as gravity). This force is mainly determined by the latitude and altitude of the location (distance from the center of the Earth).
The theorem is as follows:
1. The farther the weight is from the center of the earth, the smaller the gravity acting on it. The weight decreases as the distance increases.