(A) Preheating the balance
Firstly, the electronic balance should be fully preheated before weighing. The basic working principle of an electronic balance is equilibrium, and once it is out of balance, an electromagnetic force is used to pull the balance back into balance. This electromagnetic force is generated by the current flowing through the coil in proportion to the mass of the object in the * magnet. The magnitude of the electromagnetic force F is proportional to the magnetic flux of the magnet, the current flowing through the coil I and the length of the coil L. When the balance is in the warm-up phase, it will gradually drop as the internal temperature rises and I decreases, resulting in a smaller F. The balance will lose its balance and the value will show a positive unidirectional drift. Only after sufficient preheating to bring the magnets into thermal equilibrium is this process of change completed and the balance is in balance. The zero/peel function is then used to return the display to zero and the balance is then in a truly operational state. Humidity sensor probes , , Stainless steel heaters PT100 sensors , , Cast aluminium heaters , Heating rings Fluid solenoid valves
In order to reduce the influence of electronic devices such as transformers, bridge rectifiers, three-terminal DC regulators and other heat-generating devices, the new generation of electronic balances has moved the main heat-generating device, the transformer, to the outside of the balance as a separate part. For micro or ultra-micro balances with an actual division of 1g or 01g, the weighing chamber is separated from the electronic section into two separate parts. The purpose of this is to reduce the thermal impact of thermal noise on the sensor, in order to facilitate more stable and accurate weighing. Analysis of factors affecting the accuracy of electronic balances
In addition, electronic balances are usually not similar to the power switch of household appliances, as long as the balance is powered, even if the display does not show, the balance has been in a preheated state. The ON/OFF key on the balance is simply a switch to switch the display on and off. Therefore often weighing balance does not need to unplug the power, especially the high accuracy balance, in the case of conditions permitting, long-term non-stop power can keep the balance always in a preheated state. Analysis of factors affecting the accuracy of electronic balances
For metrological verification or the actual index value d up to 001mg electronic balance, warm-up time of at least 5 hours; for d up to 0001mg micro and ultra micro electronic balance, it is necessary to preheat more than 24 hours. For the use of the balance, warm-up time can be reduced, when the display value no longer shows a unidirectional drift, back to zero / tare can be weighed. Analysis of factors affecting the accuracy of the electronic balance
(B) motion balance
After the electronic balance has warmed up, do not immediately weigh, but to briefly load, unload, to move the balance. This is often easily overlooked. The sensor of an electronic balance is usually an elastic support made up of 9 or 11 reeds, the balance sensor is the use of these reeds for force transfer, so that the balance is in balance and out of balance again in the process of balancing to complete the weighing. However, in the balance in the power preheating stage or longer stop weighing, the balance sensor is in a dormant state to stop working, of course, the reed is also in a dormant state, its recovery performance is not good, if this time for weighing, will inevitably cause the balance loaded back to zero poor, the value of the stability is not good, poor repeatability. This is like a person after a night’s sleep, muscles and bones completely in a relaxed state, if you get up in the morning, on a large amount of work or exercise, this person will certainly not be able to eat, but also out of good results, for this reason must be warm-up exercises, activities, he can achieve good work efficiency and results.
Movement balance method is the equivalent of the balance zui weighing weights or objects loaded onto the scale, and then unloaded, at this time and do not care whether the load value and back to zero, so repeatedly more than 10 times, so that the sensor reed from the dormant state gradually into working condition, the balance can be stable weighing.
For the measurement of calibration balance, this step is essential, otherwise it will not get good weighing results. This has been proven by a large number of facts. For the use of the balance, if the actual scale value d!01mg, can not be made or reduce the number of times to add, unload, back to zero / tare can be weighed. For d001mg semi-micro, micro and ultra-micro balances, it is necessary to carry out this step, otherwise no stable weighing results; if such balances have a longer period (more than 30 minutes) to stop working, before restarting weighing also need to carry out a short period of addition, unloading to exercise the balance.
(iii) Calibration of the balance
This is an important step in the correct use of the electronic balance is essential, any electronic balance is not calibrated, you will not get accurate weighing results. Some users think that the calibration of the balance is only the measurement department, once the balance is qualified, the use of the calibration is not necessary. In fact, the measurement department of the balance is the measurement of performance testing, mainly including the balance of the offset error (i.e., the error of the four corners), as well as the accuracy of each load point and repeatability testing, of course, before the measurement test must first calibrate the balance, otherwise the same can not get the correct calibration data. Before each use, the user must calibrate the balance for two reasons:
1 The balance weighs the mass of the object, not the weight. This means that any balance, no matter where it is placed on the earth or at what altitude it is placed, should give exactly the same result when weighing the same object, as the mass of the object is constant. However, the Earth has gravity and the weight of any object with mass is affected by the acceleration of gravity. For example, an object weighing exactly 2000000g on the ground floor becomes 19999937g on the fourth floor (10m height), obviously, due to the effect of gravity, we cannot get the same result when weighing with an electronic balance. This requires the application of calibration techniques.
We know that for the double-disc equal-arm mechanical balance, when the weight of the object on the left disc (gravity) and the weight of the weight on the right disc (gravity) balance due to the same place of gravitational acceleration value equal, the left and right eliminated, so the mass of the object is equal to the mass of the weight, the natural completion of the purpose of weighing the mass of the object. The difference is that the weight of the weight (gravity) is replaced by the electromagnetic force which balances the weight of the object (gravity). Similarly, as long as the gravitational force of the object is balanced by the electromagnetic force, the result of the weighing with the electronic balance is the mass of the object.
In order to obtain an accurate mass of an object, it is not enough to have an electronic balance; calibration techniques must also be applied. Now let us first look at the calibration process, to external calibration for example, such as AB204N balance, into the calibration process, in the balance on the display appears a flashing 2000000, this number is the balance calibration required weight value, at this time, the 2000000g weight placed on the weighing plate, the balance will experience imbalance to rebalance the process, when the display becomes a flashing 00000 When the display changes to a flashing 00000, the weight is removed and the whole calibration process is completed. It is of course the electromagnetic force that is used to balance the 2000000g. As the magnetic flux changes with fluctuations in temperature, it is obviously the current flowing through the coil that mainly changes the magnitude of the electromagnetic force, so in the calibration process the balance is actually looking for a balance current value corresponding to 2000000g. Now let’s go back to the example above, on the ground floor we calibrated the balance and found the corresponding equilibrium current, noted as I1, and the result was 200.0000g. If the balance is moved to the fourth floor (10m height) and the balance is not recalibrated, it is clear that as the magnitude of the electromagnetic force generated by I1 does not change, but the gravity becomes lighter due to the smaller g value of the acceleration of gravity, the balance will lose its balance and the result will be The balance would be out of balance and the weighing result would be 19999937g, which would be wrong. Therefore, when the balance is moved to the fourth floor, it is necessary to recalibrate the balance and find the corresponding balance current value again as I2.
work, we do not just weigh the calibration of 2000000g, but in the weighing range of the quality, note is in the balance of the zui small weighing and zui large weighing, that is, in the allowable error range of accurate weighing. According to the balance balance principle and the application of calibration technology, a corresponding unloaded balance current I0 is obtained when the balance is unloaded, and a corresponding calibration balance current Ical is also obtained when the balance is calibrated by the required calibration weights, as long as the balance is sufficiently linear, the balance will automatically find the balance current IB corresponding to the object in the line connected to I0 and Ical, and then through the The electronic part of the balance is then converted into the exact mass of the object, which is displayed on the balance’s display, and this is the whole process of applying calibration techniques to accurately weigh an object.
2 When the balance is placed in the same place and the same object is weighed, there seems to be no need to calibrate it before each weighing as there is no influence of gravity. This can lead to changes in the magnetic flux and current flowing through the coil, and changes in the electromagnetic force. This, in turn, can lead to inaccurate weighing results. Although the electronic balance has a temperature compensation function, it is only effective within a small temperature fluctuation range, the temperature exceeds a certain limit, it is unable to compensate. zui ultimately still rely on calibration techniques to ensure accurate weighing.
In order to ensure that the balance is not affected by changes and fluctuations in ambient temperature, METTLER TOLEDO has recently introduced the advanced calibration technology FACT (Fully Automatic Calibration Technology). It ensures that the balance is always accurate within the permitted error range. With this calibration technology, the balance checks its own accuracy at all times and is automatically calibrated once the permissible error range has been exceeded.
The majority of domestic users, the use of electronic balances are off-duty power off, on-duty power on, so in order to ensure that the balance preheats well under the premise that the use of the balance before the calibration is more necessary. Do not start weighing without calibrating the balance, which is not accurate weighing results. In the case of conditions permitting, we recommend that you do not turn off the balance power supply, so that it is maintained for a long time to keep the power preheating state, the balance will be stable and accurate work.
(iv) Using the balance
Only after completing the above three steps can the balance be used for stable and accurate weighing.
In summary, the correct use of the balance is divided into four steps, namely, preheating the balance, moving the balance, calibrating the balance, and using the balance.
II. Questions about the natural physical properties of the sample itself and the effects of change
(i) The temperature of the sample and the container
When the temperature of the sample and the container are not the same as the temperature of the weighing chamber of the balance, there is a difference in temperature and this difference results in a different direction of air flow along the outside of the weighing vessel.
In order to avoid the influence of the temperature of the sample and the container, it is important to keep the sample at the same temperature as the weighing chamber before weighing, and not to weigh samples taken directly from the desiccator or refrigerator; to avoid the influence of hand temperature on the container, use tweezers to hold the container; in particular, it is important to avoid, as far as possible, the entry of hands into the weighing chamber of the balance, which can cause an increase in the temperature of the weighing chamber, This can be achieved by using long-handled forceps to hold the container, or by using a long-handled sample spoon to add the sample; of course, attention should also be paid to the shape of the container used, which should have a smaller surface area to reduce the effect of temperature.
(ii) Hygroscopicity or volatility of the sample
When weighing some samples with hygroscopicity, the balance will slowly increase in value, and conversely, when weighing some samples with volatility, the balance will show smaller and smaller values. This phenomenon is also often misunderstood by some users, thought to be the balance itself drift caused, in fact, not.
(C) samples and containers of static electricity phenomenon
Weighing container once the static electricity, will lead to the balance display value for a long time unstable, can not be stable to show accurate weighing results, of course, repeatability is very poor. This static electricity is mainly generated by the friction between the powder or granular sample and the insulating container. In order to eliminate the static electricity of samples and containers, a simple way is to make good contact with the earth at the grounding end of the three-pronged power socket, especially in standard laboratories, a special grounding wire should be used, buried in the earth, and then the grounding wire will be introduced into the grounding end of the power socket, using a humidifier to increase the humidity of the air, the ideal relative air humidity of 45% to 60%. In the warm and dry winter, you can place a glass of water in the balance weighing room, 1 to 2 hours after taking out, to increase the humidity of the air in the weighing room; pay attention to the choice of different materials weighing containers, try to avoid using plastic containers, it is very easy to generate static electricity. Glass containers are better than plastic containers in this respect, and metal containers are better; use special devices to eliminate static electricity.
(iv) Magnetic samples and containers magnetised for weighing
Magnetised objects, as magnetic objects and stainless steel weighing plate attract each other, will produce excess force is mistaken for the balance is loaded, affecting the accurate weighing, resulting in weighing results dependent on its position on the scale, weighing results repeatability is very poor. This weighing phenomenon is easy to spot, but users often ignore the reasons for it, or simply do not consider it from this aspect, mistaking it for the balance’s own poor repeatability. To solve this impact of the zui basic idea is to increase the distance between the sample and the container and weighing plate, weaken the impact of magnetic force. zui commonly used is the use of hanging under the scale, increase it and weigh the distance between the pan inside the weighing chamber, can also be placed in the sample of non-magnetic supports such as high glass measuring cups, aluminum, copper bracket, increase the distance from the weighing plate. Where possible, the sample and the container can be demagnetised, and the sample can be placed in a good magnetically conductive alloy container so that the magnetic lines of force form a self-enclosure along the inner wall of the alloy container, reducing the influence of magnetism.
