Analytical Balances are a specialized type of weighing scale that are designed to measure the masses of lighter objects. The weighing capacities of Analytical Balances range up to 300g and they have display resolutions down to 0.001mg.
The high sensitivity of these balances creates unique challenges during the calibration process. The proper assessment of those challenges ensures that the Analytical Balance will give the correct measurement results.
Analytical Balances must be calibrated at the site where the balance will be used. Depending on the accuracy and resolution of the balance, ANSI/ASTM Class 00, Class 0, or Class 1 certified calibration weights will need to be used to obtain the accuracies required for the calibration of most Analytical Balances.
Factors that Affect the Calibration of Analytical Balances
Analytical Balances are influenced by a number of environmental factors that can have a significant effect on the calibration of the balance. The most common influences are:
- Temperature and Humidity – Changes in the room environment can cause noticeable changes in the measurement values. The environmental conditions should be controlled during the calibration of the Analytical Balance.
- Vibration – The biggest source of vibration that affects Analytical Balances is foot traffic in the area where the balance is located. The calibration should be conducted at a time when foot traffic is at a minimum.
- Air Currents – Air currents passing across the weighing pan can change the indicated weight or make the balance indication fluctuate and become unstable. Weighing should always be performed with the doors to the draft shield closed.
- Barometric Pressure – Changes in air pressure can affect the accuracy of the calibration measurements. The calibration should not be performed on days with expected extreme pressure variations.
- Gravitational Acceleration – Every location has a different acceleration of gravity depending on the location and altitude. It is very important to calibrate any balance after a change in location.
Analytical Balance Calibration Procedure
The balance needs to be leveled prior to beginning the calibration. Ensure that the bubble of the level on the balance is centered.
Before beginning the calibration, close the doors of the balance and allow the reading to stabilize. Zero the balance by pressing the ‘Zero’ or ‘Tare’ button.
The calibration weights should only be handled with tweezers or gloves. The oils on the fingers and any temperature variations while holding the weights can change the weight value.
For the linearity test, multiple points through the measurement range of the instrument are checked. Typically, 5 to 10 different test points are checked. The test points are selected so that they are distributed equally throughout the range of the balance. With multi-range balances, there should be at least 3 test points checked on each range of the balance. The values should be within the stated linearity specification for the balance.
The eccentricity test is performed to determine the variation in the measurements depending on the location where the weight is placed on the weighing pan. The test weight used in the eccentricity test is a single weight of approximately 50% of the maximum load of the balance.
First, the weight is placed in the center of the weighing pan and the balance is tared. The weight is then placed in four different sectors of the weighing pan, equidistant between the center and edge of the weighing pan. All of the values are recorded to determine the maximum difference from the center of the weighing pan. The value should remain within the stated linearity specification for the balance.
The repeatability test is performed by placing the same calibration weight on the weighing pan between 5 and 10 times in a row. The repeatability measurements should be performed with the same placement on the weighing pan. The weight used should be between 50 % and the maximum load of the balance.
A standard deviation calculation of the recorded values is performed to determine the repeatability of the balance. The value should be within the stated repeatability specification for the balance.
Adjustment Procedure for Analytical Balances
Adjustments to Analytical Balances are typically done by aligning the zero and full-scale values of the balance through a calibration menu in the display of the balance. Most balances have a preset ‘calibration mass’ value at or near full-scale that needs to be used for the adjustment process. Some balances have an internal calibration mass that can be used to “auto-calibrate” the balance without needing an external weight applied. Once the adjustment has been completed, the entire calibration procedure needs to be performed to ensure that the adjusted values are within the stated specifications.