An analytical balance is one of the most precise weighing instruments in a laboratory. Its weighing accuracy typically reaches 0.1 mg or even 0.01 mg, which directly determines the accuracy of experimental data and analytical results.
Over time, analytical balances may develop weighing errors due to various factors. Therefore, regular and standardized calibration of analytical balances is a critical step in ensuring the accuracy of results.
This article aims to systematically outline the complete operating procedures for analytical balance calibration. It covers pre-calibration preparations, the calibration process, calibration intervals, and troubleshooting common issues, providing laboratory technicians with clear, actionable operational guidance.

Key Takeaways

  1. Calibration: The process of comparing the value indicated by a measuring instrument with the corresponding standard value (standard weight) under specified conditions to determine its deviation.
  2. Internal Calibration: A calibration operation performed using the balance’s built-in automatic calibration weights; it is completed with a single button press and requires no manual intervention.
  3. External Calibration: A calibration procedure performed using externally sourced, metrologically verified standard weights, which must be manually placed on the balance.
  4. Division Value d: The smallest unit of measurement on the balance, defined as the difference between two adjacent scale marks on the display.
  5. Maximum Capacity Max: The maximum weight the balance is designed to handle; exceeding this limit may cause permanent damage.

How to Calibrate an Analytical Balance

Preparation Before Calibration

  1. Temperature: The laboratory temperature should be stable between 15 °C and 25 °C. Temperature fluctuations during calibration should not exceed ±1 °C/h. Avoid placing the balance near air conditioning vents, windows, or heating equipment.
  2. Humidity: Relative humidity should be maintained between 30% and 70%. High humidity may cause a film of moisture to form on the surface of the weights, affecting weighing accuracy.
  3. Airflow: Avoid air convection; close doors, windows, fume hoods, and fans. It is recommended to use a draft shield for the balance.
  4. Vibration: The analytical balance should be placed on a sturdy, dedicated anti-vibration bench, away from sources of vibration such as centrifuges, shakers, and grinders.
  5. Light: Avoid direct sunlight; intense infrared radiation can cause uneven internal temperatures in the balance.
  6. Power Supply: The power supply voltage should be stable at 220 V ± 10%, with a frequency of 50 Hz ± 2%. It is recommended to use a voltage stabilizer or an uninterruptible power supply (UPS).
  7. Leveling: Check whether the bubble in the level indicator at the rear of the balance is centered within the circle. If it is off-center, adjust the two leveling feet at the base of the analytical balance until the bubble is centered. Proper leveling is essential for ensuring weighing accuracy.
  8. Cleaning and Inspection: Check the weighing pan, draft shield, and interior of the analytical balance for any residue, dust, or liquid. Gently wipe them with a soft-bristled brush or lint-free cloth dipped in anhydrous ethanol to ensure the weighing pan is clean and dry.
  9. Mechanical Inspection: Check that all parts of the balance are in good condition, that the draft shield door slides smoothly, and that the display is functioning properly.

Power-On Warm-Up

After the analytical balance is powered on, it must be allowed to warm up sufficiently so that the internal electronic components reach thermal equilibrium. The warm-up time varies depending on the balance model and accuracy class:

  1. 0.0001 g balance (d = 0.1 mg): Warm up for at least 30 minutes.
  2. 0.00001 g balance (d = 0.01 mg): A warm-up time of 60 to 120 minutes is recommended.
  3. High-precision microbalances: A warm-up time of 2 to 4 hours is recommended.
  4. Keep the balance powered on during the warm-up period; do not perform any weighing or calibration operations. Once the warm-up is complete, press the Tare/Zero key to zero the balance.

Prepare Standard Weights

External calibration must use standard weights that a metrology department has verified; the weight class should be no lower than the balance’s accuracy requirements:

  1. One-ten-thousandth balance: Use E2-class standard weights.
  2. One-hundred-thousandth-of-a-gram balance: Use E1 or E2 grade standard weights.
  3. One-thousandth-of-a-gram precision balance: Use F1 grade standard weights.

Standard weights should be handled using specialized tweezers or gloves to avoid contamination from sweat caused by direct hand contact. Store the weights in a desiccator before and after use.

Starting Analytical Balance Calibration

1. Internal Calibration

Balances equipped with an internal calibration function feature a motor-driven set of standard weights. They are easy to operate and suitable for quick calibration before daily use:

Procedure

  1. Ensure the balance has been fully warmed up and zeroed.
  2. Close the draft shield door and ensure the weighing pan is empty.
  3. Press the “Calibrate” (CAL) key on the balance, or select the “Internal Calibration” function from the menu.
  4. The balance automatically loads the built-in weights for calibration; during this process, the display typically shows “CAL” or “Calibrating.”
  5. Once calibration is complete, the analytical balance will display “CAL Done” or return to weighing mode. If an error code appears, calibration has failed, and the cause must be investigated.

The advantages of internal calibration include speed, standardized operation, and immunity to human error in weight placement; however, its accuracy is limited by the metrological status of the built-in weights. It is recommended to have the built-in weights externally verified annually.

2. External Calibration

External calibration uses verified external standard weights, offering controllable accuracy and traceability; it is a mandatory requirement of laboratory accreditation systems (such as ISO/IEC 17025).

Procedure

  1. Complete the warm-up, zero adjustment, and verification of environmental conditions.
  2. Select “External Calibration” or “Calibration with External Weight” from the balance menu.
  3. When the system prompts you to place the calibration weight, use tweezers to gently place the standard weight of the corresponding mass in the center of the weighing pan.
  4. Close the draft shield door and wait for the analytical balance to read and record the weight value automatically.
  5. The balance compares the measured value with the standard value of the weight, automatically calculates the correction factor, and saves it.
  6. Remove the weight, and the balance returns to weighing mode. Some balances require multi-point calibration (e.g., zero, 1/3 of full scale, 2/3 of full scale, full scale).

Calibration Performance Testing

After calibration is complete, a series of performance tests must be conducted to verify that the balance meets operational requirements. The following are the five core tests:

1. Repeatability

Repeatability reflects the consistency of the balance’s measurements of the same load under identical conditions.

Test Method

  • Select a weight of moderate mass (typically 50% to 100% of the full-scale capacity).
  • Place the weight in the center of the weighing pan, record the reading, and then remove it.
  • Repeat the above procedure at least 10 times (10 or 20 times is recommended).
  • Calculate the standard deviation (SD) and relative standard deviation (RSD) of the measured values.

Acceptance Criteria: For a 0.0001 g analytical balance, SD ≤ 0.1 mg; for a 0.00001 g analytical balance, SD ≤ 0.02 mg.

2. Eccentricity / Corner Load Test

The eccentricity test verifies the consistency of the balance’s response when the load is placed at different positions on the weighing pan.

Test Method:

  • Place standard weights equal to 1/3 of the capacity sequentially at five positions on the weighing pan: center, front, back, left, and right.
  • Record the reading once at each position.
  • Calculate the maximum difference between the reading at each position and the reading at the center position.

Acceptance criterion: The maximum difference must not exceed 1/2 of the permissible error for that load. For a balance with d = 0.1 mg, the eccentricity error should be ≤ 0.2 mg.

3. Linearity

The linearity test verifies the linear relationship between the balance’s measured values and the standard values across the full measurement range.
Perform a linear regression between the measured values at each point and the standard values, and calculate the correlation coefficient R². Acceptance criterion: R² ≥ 0.9999.

4. Sensitivity

Sensitivity reflects the balance’s ability to respond to changes in mass.

Test Method:

  • After zeroing the balance, place a standard weight close to full scale and record the reading.
  • Remove the weight and observe whether the zero point returns to zero.
  • Calculate the sensitivity deviation: Sensitivity deviation = (Measured value – Standard value) / Standard value × 100%.

Acceptance criterion: Sensitivity deviation ≤ 0.1%.

5. Minimum Weight / Minimum Sample Weight

Calculation formula: m_min = k × SD / R, where k is the coverage factor (typically 2 or 3), SD is the standard deviation of repeatability tests, and R is the required relative accuracy (e.g., 0.1%).

Analytical Balance Maintenance Recommendations

  1. Keep the analytical balance and its surroundings clean; wipe down the weighing pan after each use.
  2. When the balance will not be used for an extended period, unplug it and cover it with a dust cover.
  3. Have standard weights inspected regularly (recommended once a year) to ensure traceability of measurement values.
  4. Check the position of the leveling bubble regularly; the balance must be re-leveled every time it is moved.
  5. Maintain a usage log for the analytical balance, including the time of use, items weighed, and any abnormalities.
  6. Perform a thorough cleaning of the balance’s draft shield, guide rails, sensors, and other components every six months.