Weighing is critical in several industries to ensure products meet quality standards, ensuring safety and hygiene for consumers. It also helps businesses minimize costs associated with shipping or storage.
Using automated weighing systems can reduce human error significantly, especially in food and pharmaceutical manufacturing. However, ensuring that these systems perform correctly requires the right installation and calibration, along with regular maintenance.
Scales
Scales use a variety of mechanisms, but most employ a spring that either stretches (like the one in the produce department scale at a grocery store) or compresses, based on Hooke’s Law. A rack and pinion mechanism converts this spring displacement into a dial reading.
Most commercial weighing instruments use this method of measurement. They’re designed for trade-approved tare weights that are useful for shipping and logistics purposes. It’s also helpful for scientists to use standardized weighing units so that they can communicate with each other globally without having to translate from one unit to another.
Before unpacking a precision analytical balance, select a firm level surface remote from extreme heat, humidity changes, drafts, dust, and vibration. Then follow the instructions to calibrate it for local effects of gravity. Recording the weight measurement directly into your lab notebook will avoid errors caused by transcription. This is especially important for corrosive or volatile samples that can react to atmospheric changes in temperature and wind pressure.
Load Cells
Load cells are small devices that measure mechanical stress, also called strain. They are used in industrial weighing applications such as truck scales, cranes and railways. Load cells flex slightly when force is applied to them, changing their electrical resistance and giving a signal that’s proportional to the mechanical stress. These signals are then converted to weight measurements by a weighing instrument or weight controller.
There are several factors that affect the accuracy of a load cell, including nonlinearity, hysteresis, nonrepeatability, and temperature effects on rated output. Nonlinearity is the deviation of a load cell’s calibration curve from a straight line, starting at zero load and ending at the cell’s maximum rated capacity. Hysteresis is the difference in a load cell’s output reading after increasing from zero and decreasing from its maximum rated load. Temperature effects on rated output are the variations in calculated load caused by changes in environmental conditions. The last specification, creep, is the change in a load cell’s output over a long period of time.
Calibration
Unreliable weight measurements can lead to excess use of materials, slowing production and increasing costs. Inaccurate results in a research lab may result in multiple trials that consume time and resources and lead to unnecessary delays, while in manufacturing, inaccurate readings can cause material waste and reduce profits.
The process of calibration involves comparison of a device against a standard to determine how well it is performing and the amount of room for error. The process is often confused with adjustment of a device or instrument, however formally calibration does not include this and should always be performed before any measurement.
Calibration is essential to any balance or scale. The frequency of calibration depends on the intensity of use and manufacturer recommendations, but routine testing is recommended to ensure optimal performance. Regular calibration can help reduce uncertainty and support improved weighing processes.
Installation
Weighing systems improve quality control and efficiency, helping businesses meet customer demands. They also reduce waste and help ensure that raw materials are used to their full potential. However, they can have a negative impact on operations if the system is not properly maintained or set up.
A common issue is the build-up of substance or moisture in load cells. Moisture can cause the cell to bend and not return an accurate measurement. This is known as creep and is a problem that can be caused by the environment or simply from use over time.
Another important factor is limiting friction in the structure being weighed. This can be achieved by using anti-static pads and carefully placing weights on the balance. It is also recommended that all weighing equipment be operated gently to limit shock, as excessive shock can damage the instrument and lead to inaccurate measurements. Additionally, ensuring the balance is located in an area with a controlled environment can limit interference from RFI/EMI, such as radio, lightning and power lines.