Analysis of the Reasons for the Inaccurate Weighing of Samples in the Experiment
Sample weighing is an important part of the analysis experiment. The inaccurate weighing will lead to the failure of the whole experiment. If we always say that we are not on time, we need to analyze the reasons in detail. Generally speaking, the reasons for the inaccurate weighing of samples can be divided into four aspects: the scale is not calibrated, the environment and the physical factors of the sample influence and improper operation etc.
1. The balance is not calibrated before using
Before using a balance, it is necessary to confirm whether its correctness is qualified. The accuracy of the balance refers to the equal accuracy of the two arms of the balance beam and the equal and parallel accuracy of the three blades. When using the balance for the first time, it should be calibrated regularly. A continuously used balance, calibrated approximately once a week. The calibration shall be carried out according to the specified procedure, and the standard weight must be used for calibration.
2. The balance is not installed correctly
When installing the balance, the room with dust-proof, moisture-proof, shockproof, windproof, sunscreen and constant temperature shall be selected as the balance room. Secondly, the balance should be placed on a firm and reliable workbench, and the appropriate position should be selected. Before the installation of the balance, the balance shall be counted according to the packing list to see whether all parts are complete and intact, and all parts of the balance shall be carefully cleaned. During installation, the balance shall be assembled correctly according to the instructions of the balance. After installation, check whether the installation of each part is normal again, then check whether the power supply voltage meets the requirements of the balance, and open the balance to check whether it is normal.
3. Influence of physical factors on environment and samples
In the process of weighing with the balance, environmental and physical factors will interfere with the weighing results, such as temperature, sample volatilization, moisture absorption, magnetic force, static electricity, etc.
1) Influence of temperature change on balance
If the display value is found to drift in one direction during weighing, it may be the effect of temperature change. If there is a temperature difference between the sample and the surrounding environment, this temperature difference will result in a flow of air along the weighing container. The air flow along the outside of the container creates an upward force, which causes the weighing result to be wrong: under the dynamic buoyancy, the weight of the sample is lighter than the actual weight. This effect will not stop until the temperature equilibrium is formed. When the sample is taken out of the drying oven or refrigerator, it can only be weighed when the temperature of the sample is consistent with that of the laboratory or weighing room. The sample shall be placed in a peeled container with the smallest surface area. The weighing container shall be taken with tweezers instead of hands in the weighing chamber.
2) Influence of sample moisture absorption or volatilization on weighing results
If the indicated value continuously drifts in one direction during weighing, it is possible to measure volatile or hygroscopic samples. If the moisture absorption of the sample is strong, the weight will increase; if the sample is volatile, the weight will decrease. For hygroscopic or volatile samples, use a small neck container, cap or plug the container, use a clean and dry weighing container and keep the weighing plate free of dust, pollutants and water drops.
3) Influence of static electricity on weighing results of sample or container
If each weighing shows different weighing results or unstable values, or the repeatability of weighing results is poor, it can be considered that the weighing container or sample has static electricity. The influence of electrostatic phenomenon will make the weighing container show different weight each time, and the repeatability of the result is very poor. Materials with high insulation such as weighing containers made of glass and plastic are easy to carry static electricity. This phenomenon is mainly due to the stirring or friction of the sample or container in the process of handling. Once charged, the charge removal will be very slow, and the probability of static electricity of the sample or container in the dry air with relative humidity less than 40% will increase. Generally, the humidifier can be opened or the air conditioning system can be adjusted properly to increase the air humidity, the weighing container can be placed in a metal container for weighing, and measures such as trying to ground the balance can be taken to remove or shield the static electricity on the weighing sample.
4. Inaccurate weighing caused by improper operation of users
Before weighing, check whether the balance is normal, whether the balance is level, whether the weighing pan is clean, whether the coil index pan is at the "000" position, whether the coil is dislocated, whether the lifting lug is falling off, displaced, etc. Check and adjust the zero point of the empty pan of the balance, use the balance screw of the crossbeam to roughly adjust and use the adjustment bar of the projection screen to fine tune.
1) In the open state of the balance, take and place the weight and increase or decrease the weight.
In general, adding, subtracting, taking and putting the weight of the balance must be carried out when the balance is closed. During the weighing process, the balance is always half opened, and can not be fully opened until the last moment.
2) The user's operation is not stable when rotating the reading plate, resulting in the hanging code falling off or winding.
Use the motorized switch on the balance, such as opening and closing the balance, adding and subtracting the balance weight, etc., and it must rotate slowly and evenly. According to the said weight, slowly and steadily turn the weight knob with the middle taking rule, and use the simplest operation to add and subtract the weight.
3) When weighing, take and place the weighing materials directly by hand.
If the weighing object is directly taken and placed by hand during weighing, the sweat stains on the hand will stick to the weighing object, resulting in weighing error. Therefore, it is not allowed to directly touch the weighing object by hand during weighing. Generally, gloves can be worn and the weighing cup can be taken and placed by paper strip.
4) Wrong reading results in wrong weighing results.
After the reading weight is determined, turn on the balance knob fully, and then read after the scale stops stably. The mass of the weight is equal to the total weight plus the scale reading. The total weight shall be added from the largest to the smallest, and finally to the scale reading.
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