Weighing is an essential process in any laboratory workflow. Good weighing practices can reduce or eliminate errors in the measurement of mass.
A creased square of glazed paper can be used as a ‘tared’ container for measuring solids directly on analytical balance pans (the first reading is the weight of the empty container). Taring can eliminate error caused by a difference in initial readings by subtraction.
Workspace Preparation
In this phase, the area around the weighing system is cleared and prepared for the installation of equipment. This includes ensuring the system can be properly positioned, that no metal objects or ferromagnetic items are present in the vicinity and that any electrical shielding needed to prevent electromagnetic interference (EMI) is in place.
Process Weighing
This is when the weight of a product is critical to the production process and must be recorded at high levels of accuracy, such as within 0.02% and 2% of full scale capacity. Typically deployed in the STOCK, MAKE, PACK and SHIP manufacturing areas in plants, process weighing can help verify that a product is complete and accurate or that ingredients are correctly mixed.
This type of weighing also provides valuable statistical information that can be fed back into the source process, helping to reduce costs and improve quality. PCE can supply, install and maintain all types of industrial process weighing systems including hazardous area (ATEX) weighing applications for the mining, chemicals, oil and gas industries.
Equipment Preparation
In process weighing, a powder material is measured continuously during production. This technology is used in a wide range of industries: monitoring inventory levels in silos, discharging material by weight or rate, batch mixing of multiple ingredients, and so on.
The most important aspect of the equipment that will measure the mass of your product is the load cell (also called a sensor or transducer). It’s a piece of machined metal that bends with the force exerted on it, and the strain is sensed by sensors bonded at points on the cell. This measurement produces a proportional electrical signal that is recorded by the balance.
Moisture in the weighing system’s junction box can wick into cables to the individual load cells, reducing the capacitance between signal lines and causing electronic noise. This can lead to inaccurate weighing results. For this reason, it is recommended to seal the junction boxes and plug any unused holes. It is also best to test automatic bulk weighing systems to their maximum capacity when new, using a combination of test weights and bulk material.
Weighing
Weighing is a process that can be complicated and requires careful attention to detail. Having the right weighing instruments can make all the difference in production, and ensuring that they are installed correctly and regularly calibrated is essential for quality control.
When it comes to determining the correct mass of an object, the most accurate method is to weigh the item on a high-precision analytical balance that is kept in a clean room with windows closed to prevent air drafts from influencing the reading. Additionally, the weighing pan(s) should be placed inside a clear enclosure so that dust cannot collect and cause an error.
In many manufacturing applications, it is important to connect a weighing instrument to PLCs for data transfer and automation. COOPER Instruments & Systems offers a wide range of local displays with PLC outputs as well as DIN rail mount units to make integration easy. Contact us today for help with selection, installation and calibration of your weighing equipment.
Recording
Getting a clear, precise measurement is only one step in the process. Taking action based on the measurements is equally important.
For example, if a pharmaceutical product fails quality inspections due to inconsistent blending of ingredients or weighing errors, that wastes time, money and resources while compromising human health and safety. That’s why it’s critical to design a process that improves efficiency and accuracy.
Choosing quality components that are specially designed for your application will go a long way toward achieving the kind of system weighing accuracy you need. Look for load cells with impressive worst-case specifications and a weight controller that can ignore plant and processing mechanical noise to provide accurate, repeatable results. Moisture that enters a weighing system’s junction box can wick into cables to each cell and reduce the capacitance between signal lines, resulting in erratic readings. Avoid this by using a NEMA 4-rated junction box and plugging unused ports. Also, be sure to keep moisture away from sensitive calibration standards that may oxidize.