Accuracy in the Weighing Process

Weighing is a key step in the food production process. Properly calibrated and operated balances can ensure product recipe specifications are met, quality compliance is maintained and waste is minimized.

Weighing by difference is a method that can be used for products that do not require pinpoint accuracy. It is a convenient and efficient technique for reducing defect waste in your production process.

Weighing Equipment

The quality of weighing equipment is a primary factor in determining system accuracy. Choosing quality components specifically designed for your application can help to eliminate errors caused by mechanical forces and environmental factors.

The load cell is the heart of any weighing system. It takes a mechanical force (your weight pushing down on the scale) and turns it into an electrical signal that is measured by strain gauges bonded to the load cell. Choosing a top-quality load cell with an impressive worst-case specification will go a long way toward improving your overall weighing accuracy.

Other factors can affect your weighing accuracy, including shock loading and vibrations. Dumping heavy material on a weighing system can cause it to overload, while sensitive load cells may interpret vibrations as extra force. Installing a feeder to control the flow of materials into your weigh vessel can help reduce this type of error. Large temperature changes can also cause a weighing system to expand and contract, which can affect accuracy.

Weighing Procedures

The weighing procedures used in a laboratory can greatly affect the accuracy of a measurement. Depending on the precision needs of the application and the characteristics of the substances being measured, the weighing method selected will vary.

For example, if you are weighing fine powders, it is important to use an antistatic device in order to minimize the dust particles that can cause errors. The same is true for process weighing applications such as level or inventory measuring and dispensing, bagging, or batch blending of various materials.

In addition, regular routine testing between scheduled calibrations can help reduce weighing errors. These tests should include sensitivity, linearity and eccentricity tests using calibrated test weights. Routine testing also helps to identify if the balance or scale is nearing tolerance or warning limits, so corrective action can be taken as needed.

Calibration Procedures

As any instrument, scales and balances require calibration. A properly calibrated weighing instrument will display an accurate zero when not under load and produce output results that are within the calibration tolerance limits set by the manufacturer.

A proper calibration is performed by a certified technician who uses known weights to adjust the weighing instrument. The scales should then be tested under varying loads and conditions to determine the calibration tolerance.

All instruments can have repeatability issues, meaning that if the same load is measured multiple times the result is not always exactly the same. To test for this, an eccentricity test can be done.

When selecting a calibration company it is important to choose one that has highly-trained technicians with years of experience performing expert calibrations. The technician should also be NIST H-44 certified and have a clear understanding of your specific process needs. The technician should also have superior documentation practices and attention to detail.

Error Prevention

Many factors can interfere with the measurement signal and distort weight results. For example, vibrations, temperature changes and draft can lead to inconsistent measurements. In addition, pressure differences can cause the load cell to interpret additional force as weight and cause the reading to change. Finally, the weighing system may be subject to interference from electromagnetic fields (EMI and RFI), which can cause noise that throws off the measurement.

Choose a location for your balance that is shielded from vibrations, draft and other environmental conditions that can distort the readings. Similarly, make sure that the balance is not positioned over air conditioning ducts or larger laboratory equipment, as this can lead to a variety of distortions.

Use clean, lint-free gloves when handling the weighing container and test weights to avoid transfer of sweat and oils that can alter the reading. Choose a tare container that is small in size and made of a metal that will not react with the sample material.

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