Weighing is used to quantify the matter in various objects so that their properties can be measured and recorded. This is useful in a variety of fields such as medicine, transportation or industry.
When using direct weighing, the balance is zeroed (reads zero with nothing on the pan) and the substance to be weighed out is added. Subtraction of the two readings gives the mass of the substance.
For most laboratory applications analytical balance precision and accuracy are key. These include the formulation of chemicals, density determination, purity and quality assurance testing as well as solution preparation for chromatography.
Weighing of solids is usually done directly on the weighing pan of the balance by opening the chamber doors and placing a container with the weighed material on the weighing platform. Care must be taken to ensure that the container is clean and the tare button (often indicated by a red square) is pressed. The resulting mass is recorded on a piece of weighing paper and the difference in the readings is calculated.
The weighing process is more reliable if the balance is situated in a draft-free location on a solid bench and the temperature is controlled. It is also advisable to calibrate the balance regularly with the supplied calibration weights to maintain its accuracy. This is especially important for older balances. It is not a good idea to place a high-precision balance next to a heat source as this will interfere with its operation.
The calibration phase involves comparing the measurement instrument to an accurate standard. The purpose is to ensure that the measuring instrument provides measurements that match those of a reference standard over an extended period of time and under varying environmental conditions.
This can be achieved by establishing traceability to national standards (such as the National Institute of Standards and Technology in the US) or international standards maintained by private firms that offer metrology services. Traceability is also achieved by using a calibrated check weight that is weighed regularly and stored with the balance.
The calibration process is one of several ways a factory can reduce defect rates and increase quality. However, it is important to keep in mind that a company’s calibration strategy should not become more important than the actual production processes itself.
Many chemicals are corrosive and very fine powders may pick up static charge that causes the particles to fly off the balance pan during the weighing process. To prevent this, an antistatic device is used to dissipate the static charge.
This is especially important with analytical or top loading balances. In fact, chemical substances should NEVER be added directly to the weighing pan on these types of instruments. Instead they should be added to a tared container that holds the substance, such as a volumetric flask, beaker or conical flask. This process is called transferring the solid to the balance.
The weight of the empty tared container will then be subtracted from the mass reading on the display to give you just the mass of the chemical. The process is often simplified by taring the balance (setting it to zero) and then adding the weighing container plus the solid. This method is called weighing by difference.
Whether it’s an ingredient in a batch of food that will be sold or the tare weight recorded in a resident’s room, accurate measurement is essential in a variety of applications. Having the right balance and recording software is the key to providing reliable and consistent results.
For example, a volatile sample like white phosphorous needs to be kept in a confined space. Exposure to atmospheric changes such as temperature and wind pressure can change its chemical state and cause it to combust. In such cases, direct weighing is recommended as the safest option for measuring such substances.
Process weighing systems allow for pre-programmed set points to be entered into a balance, automating the weighing and reducing human error in the process. Additionally, a weighing system can record the weights of each container and provide a traceable audit trail to ensure that the correct tare vessel was used for each weighing event. This is particularly important in forensic laboratories, where the outcome of an analytical process can hinge on the accuracy of a weighing process and the resulting evidence can be challenged in court.