Repeatability is one of the core indicators for measuring the performance of semi-micro balances, which reflects how consistent the balance can weigh the same load multiple times under the same measurement conditions. In many fields, such as environmental monitoring, materials science, food analysis, and chemical synthesis, small deviations in weighing results can have a significant impact on subsequent experiments or production. Therefore, establishing a scientific and standardized repeatable test method, and correctly understanding and using supporting weights are the basis for ensuring the reliability of weighing data.
Definition and calculation of repetitiveness
According to internationally accepted metrology standards, the repeatability of a balance is usually characterized by standard deviation or extreme deviation. For semi-microbalances, standard deviation is recommended for evaluation, as it is more sensitive to the discreteness of the measurements. The calculation formula is as follows:
s = √[ Σ(x_i - x̄)² / (n-1) ]
where s is the standard deviation, x_i is the single measurement, and x̄ is the arithmetic mean of n measurements. During the test, a single weight close to the maximum scale of the balance is usually selected for multiple repeated weighing, and the number of times n is generally not less than 10 times. The calculated standard deviation s should be less than or equal to the absolute value of the allowable error of the balance under this load.
Selection of weights for testing
The choice of test weight directly affects the validity of repeatable test results. Weights used for repeatability testing should meet the following basic requirements:
First, the nominal value of the weight should be close to the maximum weighing capacity of the balance to evaluate the balance performance under typical workloads. Secondly, the weight must have sufficient mass stability, and its material should be corrosion-resistant, magnetic weak, and smooth on the surface to reduce the error caused by adsorption. Finally, and most importantly, the weight must be verified or calibrated and hold a valid metrology certificate, and its mass tolerance should be much less than the repeatability requirements of the balance under this load, usually requiring the uncertainty of the weight to be no more than one-third of the balance's repeatability tolerance.
Operation process
A standardized test process is a prerequisite for reliable, repeatable data. It is recommended to do it under the following controlled conditions:
1. Environmental Preparation: The test should be conducted in a stable environment with no vibration, no strong airflow, and low temperature fluctuations. The balance should be fully preheated and leveled and self-calibrated.
2. Prepressure and Cleaning: Use the test weight to preload and weigh the balance several times to bring the sensor to a stable state. Use special tools to pick and place weights, avoid direct contact with hands, and prevent sweat or dirt from affecting the quality.
3. Weighing cycle: Place the weight in the center of the scale and record the result after the displayed value is stable. Then remove the weight and wait for the balance value to completely return to zero. This complete cycle of weighing, unloading, and zeroing is repeated until the preset number of measurements (e.g. 10) is completed. There should be appropriate time intervals between each cycle.
4. Data processing: Record all weighing values and calculate the standard deviation according to the aforementioned formula. It is recommended to record the ambient temperature and humidity during the test at the same time for reference in the analysis of the results.
Influencing factors
In real-world testing, a variety of factors can cause repeatable results to deviate from expectations. Operators need to pay attention to the following points:
Static electricity and humidity: In dry environments, weights or sample containers are prone to static electricity, leading to drift or poor repeatability of weighing results. It can be mitigated by controlling ambient humidity or using anti-static devices.
Air buoyancy correction: For high-precision semi-micro weighing, the influence of air buoyancy cannot be ignored. The test report should indicate whether buoyancy correction has been made, as well as the air density parameters used.
Weight picking and placing method: Irregular picking and placing will introduce mechanical shock or temperature transmission, affecting the stability of the sensor. Tweezers or gloves should be used and the weights should be temperature balanced in a laboratory environment.
Installation and maintenance of the balance: The balance should be installed on a stable anti-vibration table and regularly maintained and verified by professionals to ensure its long-term stability.
Maintenance of weights
As a benchmark for testing, proper custody of weights is crucial. Special weight management procedures should be established:
Store in a special box that is dry, clean and free of corrosive gas; avoid contact with any objects that may scratch or contaminate its surface; Regularly send it to the legal measurement agency for verification or calibration to confirm that its quality value has not changed from the deviation; Establish usage records and track their circulation and status.
Summary
Repeatability testing of semi-micro balances is a systematic metrology work that relies on rigorous methods, qualified weights, stable environments and standardized operation. By regularly performing standardized repeatability tests, users can effectively monitor the performance status of the balance and identify potential issues in a timely manner, thereby providing a solid and reliable data foundation for scientific research and quality control. Understanding and controlling every detail of the testing process is key to achieving accurate weighing.
References
OIML R 76-1, non-automatic scale.
JJG 1036-2008, Electronic Balance Verification Procedures.
EURAMET cg-18, Calibration Guide: Calibration of Non-Automatic Scales.