Weighing is a process that relies on many variables. To ensure accuracy, it is vital to understand the fundamentals.
A load cell is a precision piece of metal with micro-sized strain gauges that sense the smallest movement when weighed. These gauges then translate the mechanical force into electronic signals that are read by a weight indicator.
There are several things that can affect the performance of a weighing system. This includes the weighing itself, as well as the surrounding area. If you have any questions about the accuracy of your weighing systems, consult with your service provider for more information.
A balance or scale is a mechanical instrument that measures the mass, force exertion, tension, and resistance of an object without using power. It consists of a pivoted horizontal lever with arms of equal length – the beam – supporting a weighing pan. The unknown mass is placed in one pan, and standard masses are added to the other until the weighing pans are as close to equilibrium as possible.
Several factors can cause inaccuracies in weighing systems including dirty scales, moisture, and wind. Moisture can cause problems because it can interfere with the electrical signals sent from the load cell to the digital indicator. Wind can also cause issues because it creates pressure on areas of the scale that aren’t carrying weight, causing the weighing system to display inaccurate weight results.
Although the terms mass and weight are often used interchangeably, they have different meanings. Mass is a measurement of the inertia of an object, while weight is a measure of the force of gravity on the object. A spring scale measures weight by seeing how much the object pushes on a spring inside. Analytical balances, however, directly measure an object’s mass by comparing it to known reference standards.
Large temperature changes can cause a weigh vessel to expand and contract, which leads to incorrect weighing results. Make sure that the load cells and mounting hardware can handle large temperature changes and that the weigh vessel is isolated from other equipment to prevent vibrations and swaying.
Ensure that the weights are clean before use and are not exposed to moisture or ice. Moisture and ice can cause electrical interference. This causes the controller to display a different result from the actual value. In addition, ice and frost can affect the nonrepeatability specification of a balance.
It doesn’t matter if your company is measuring lengths for windows, weighing produce at a grocery store, pumping gasoline, monitoring steam pressure at a power plant or torqueing lug nuts on a car, accurate measurements are critical to your operations. That’s where calibration comes in.
When you perform a calibration, your instrument is compared to a known standard. The accuracy specifications of the reference instrument are determined, and any variations in measurement uncertainty due to the balance, environment and operator are characterized. This data is incorporated into the calibration certificate and reflected in your weighing results.
To reduce your risk, it is best to look for an accredited calibration company that has Certified Weighing Technicians (CWT). This means the technicians have completed a formal calibration training program based on OIML R76 or NIST Handbook 44 (US only), and have years of experience performing quality calibrations. They also have superior documentation practices and understand your process needs.
The proper storage of your weighing equipment is critical for accurate measurements. It is best to store a balance in a vented enclosure that has balanced exhaust fans and a massive enough work top to yield low vibration levels for stable measurements.
It is also important to keep in mind that the immediate environmental conditions can affect your weighing process. For example, fluctuations in temperature may cause weights to expand or contract. This can result in inaccurate readings if not taken into consideration.
When using an analytical balance, it is a good idea to add chemical substances directly to the tared container that will hold them, NEVER directly to the pan. Adding chemical substances directly to the pan can lead to erroneous mass measurement due to temperature differences or hysteresis effects. For more information on solving difficult in-process weighing applications, please contact COOPER Instruments & Systems sales engineers. They can help you select the appropriate weigh modules for converting your tank or hopper into a weighing system or turning a conveyor section into a check-weigh station.