Calibration Criteria for Analytical Balance

Weighing is a basic operation in any analytical laboratory. It is possible to run a laboratory without modern analytical instruments but it is not possible to carry out analysis without a weighing balance.

Weighing is an essential requirement for preparing sample solution, reagents, titrants, precipitates,etc. The accuracy of your results will largely depend on the accuracy of your analytical balance. Precautions for correct weighing of samples discussed the need for analytical balance calibration amongst other factors.

Calibration against certified standard weights will give you the confidence on your weighings. Regular calibration will guarantee the correctness of your results along with those precautions you should take in preparing your samples.

Balance calibration validates the weight readings provided a regular calibration schedule is followed. Recommended balance calibration schedules are discussed in this article.

But before moving on to that, you first need to know how an analytical balance functions and what can happen if you do not go for a regular balance calibration.

Let’s begin with the details! 

Understanding Analytical Balances

Balances are found at almost every place that requires precise measurements of weights. But it is crucial to understand that not all balances work similarly. That is why an analytical balance is explicitly designed to be used in laboratories. It helps in measuring very small quantities, ones that are in the range of milligrams and sub-milligrams. Typically, it consists of the following parts:

• A transparent cell with doors
• Measuring pan inside the enclosure 
• Digital display for the weight

The transparent cell here helps keep air and dust particles away from the balance so that it shows accurate measurements of the weight. Further, the digital display makes it easy to read the exact mass of the component. Therefore, these equipment carry little to no chances of inaccurate measurements after proper analytical balance calibration. 

The most significant feature of an analytical balance is that it can show you even the very slightest weight changes, ranging in tenths of milligrams. It is able to do so because it is much more sensitive towards weight alterations as compared to any other balance. You can also use certain accessories with an analytical balance to increase its efficiency towards different kinds of tasks. 

Importance of Balance Calibration

When we do most of our measurements in tenths of milligrams, precision becomes a must-have property. For this very reason, analytical balance validation becomes crucial. It helps in keeping the results as accurate as possible. So you will not end up creating a more significant problem by adding wrong quantities of any substance in any laboratory experiment. 

One factor you must remember here is that balance calibration is not a one-time task. An analytical balance verification needs to be done on a regular basis so that all problems that occur to the equipment can get resolved on time. This way, you can receive correct results on every measurement of weight. 

Aspects Covered in Analytical Balance Validation

While the process of balance calibration may not be complicated, it still includes checking a number of aspects, such as:

• Performance: A performance check means examining the parameters of performance qualification of analytical balances. In short, the equipment has to be checked for overall performance in measuring different weights. 
• Drift: Here, the display of the equipment is checked for any drifts in a specific direction. There can be several reasons for this to happen. However, it has to be resolved on time to avoid any inaccuracies in the results. 
• Measurement Uncertainty: Finally, an analytical balance verification is done to examine any uncertainties in the measurement of different masses. 

All these aspects help in conducting a complete check-up of the analytical balance. Thus, you can ensure that your results are precise and correct, and you can rely on them. 

Now, let’s see how you can go for a regular analytical balance calibration.

Self Calibration

Self calibration is a simple exercise which takes less than a minute and should be done every time before using the balance

Do the self-calibration by pressing ‘cal’ key. Balance will go into automatic self-calibration mode and following display will appear:

After self-calibration is over display will show 0.000 or 0.0000 or 0.00000 g. The balance is then ready for use.

After performing self calibration calibrates the balance using standard weights of 0.02 gm, 0.05 gm, 0.2 gm, 1.0 gm, 10 gm and 20 gm to cover the average range of weighings that you’re likely to perform during the day.

Acceptance criteria: The observed weight should be within +/- 0.1% of the value of standard weights.

Weekly Calibration

In addition to self calibration eccentricity calibration is carried out.

Eccentricity refers to the recorded differences in readings when the same weight is placed at different positions of the pan e.g. center, top, bottom, left or right.

The standard weights taken for calibration are 0.01 gm, 0.1 gm, 5gm and 50 gm.

Acceptance criteria: the RSD of eccentricity for observed readings should not exceed 0.6%

Monthly Calibration

Monthly calibration needs to be carried out as per weekly calibration schedule but covering a wider range of standard weights:

0.01 gm, 0.02 gm, 0.05 gm, 0.10 gm, 0.20 gm, 0.50 gm, 1.00 gm, 2.00 gm, 5.00 gm, 10.00 gm, 20 gm, 50gm, and 100gm.

Acceptance criteria: the observed weight should be within +/ – 0.1% of the actual value

It is true that calibration takes your valuable time but the accuracy and consistency of your readings is guaranteed every time provided you use and handle your balance with the required care.