Weighing is used in many applications to quantify the amount of matter that makes up objects. This information is then used to transport, record and process various items.

For analytical weighing, a chemical substance should be transferred to a tared container from a weighed piece of special glazed paper. Never add a solid directly to the balance pan or weighing paper.

Accuracy

The accuracy of a weighing process is how close a measurement result is to the actual value. It is also referred to as trueness or reproducibility.

Resolution is the number of intervals, increments or divisions (all referring to the same thing) that a weighing instrument can resolve a measurement into. This can be determined by the number of bits in the analog-to-digital converter of a particular balance.

Accuracy is a function of load cell capability and the weighing environment. A common cause of inaccurate weighing results is shock loading. This occurs when heavy material is dumped onto a scale at forces that exceed the load cells’ rated capacity. This can damage the weighing system and degrade its resolution.

Using high-capacity load cells and isolating the weighing system from vibration sources can help improve accuracy. Regular performance verification can also help ensure the reliability of weighing processes. This is done by comparing the weighing results to a calibrated standard.

Errors

A high-quality balance with proper calibration is crucial to weighing accuracy. But there are other factors that can cause errors during the weighing process. These include temperature changes, air currents, lack of thermal equilibrium, electromagnetic fields, and manipulative techniques by the user while weighing.

It is important to avoid these weighing errors. For example, it is recommended to use gloves during weighing to prevent the addition of fingerprints or oil from the hands to the weight reading. Moreover, the user should not lean on the scale to eliminate vibrations that may affect the weighing results.

Another way to avoid these weighing errors is by using the weighing-by-difference method, which eliminates the need for a container. This technique is used in pharmaceutical and biotech manufacturing to ensure that the correct amount of material is consumed in production. It is also an accurate, efficient, and clean method.

Calibration

The more a weighing device is used, the more frequently it needs to be calibrated. A manufacturer’s recommendation for calibration frequency should always be consulted first.

During hard calibration, the scale is tested at both zero and span using known test weights. Generally, 80-100% of the target load is used as the span weight to produce the most accurate results.

Eccentricity tests are also done to establish a relationship between the measurement technique and known values. The result is that the weighing instrument “teaches” itself to produce more accurate results when measuring unknown samples.

Tolerance assessments are a great way to ensure your equipment meets legacy quality assurance requirements such as Repeatability, Eccentricity and Error of Indication tolerances. Locally accredited calibration annexes give you clear statements on whether your weighing device meets your legal metrology regulations. You can even choose to have a custom annex that matches your weighing application. This helps you cut down on variations that can lead to defects in production.

Safety

Weighing is a critical component of the production process. Inaccurate weighing can waste raw materials, product, and human resources. It can also lead to image-damaging recalls and put the company’s reputation at risk.

Often, a weighing process takes place in hazardous areas that require special protection from explosive gasses and flammable liquids. Depending on the application, load cells and weighing scales may be positioned in tanks, vessels, mixers, or reactors. In such cases, Hardy Process Solutions recommends using intrinsically safe low-energy components in the hazardous area. This is achieved by combining intrinsically safe instruments with associated elements and special approved wiring in an intrinsically safe system.

Be sure to clean up spills around the balance immediately to prevent cross-contamination and erroneous readings. Also, do not touch standard weights with bare hands as hand grease can affect the readings. In addition, the weighing system must be placed in a stable environment, and the temperature should not fluctuate.