A weighing process involves measuring the amount of material in an object. In most cases, this is done using a balance.
A typical balance consists of a pivoted horizontal lever with arms of equal length – the beam. It can determine mass by placing the unknown substance in one pan & standard masses in the other pan until the system achieves equilibrium.
Measurement
From food packaging to production line weighing, accurate measurements help make business processes efficient. But how can you ensure your weighing equipment delivers the results you need?
Vibration:
The vibration transmitted through process equipment or around the weighing system can disturb the load cells. This can cause the weighing system instrumentation to produce inaccurate readings or even fail altogether. Ensure your weighing system is isolated from vibration sources when possible, or select a weighing system with sensor instrumentation that eliminates vibration effects.
The sensitivity of an optical balance is determined by comparison (or substitution) weighing with standard masses that are calibrated in advance. The sensitivity weights should be selected to minimize the influence of air buoyancy on the measurement and their handling must be carefully controlled to prevent grease or oily films from contaminating the mass. This will affect the ability to correctly measure an unknown object and will lead to poor repeatability. To correct for this, a cornerload test should be performed periodically.
Accuracy
Weighing accuracy is the closeness of a measurement to a known value. It is affected by four factors:
The major weighing component in all electronic weighing systems is the load cell (also called a load sensor or transducer). This piece of machined metal bends under weight, and its bending is sensed by strain gauges bonded to the cell at points that correspond to load areas. The gauges output an electrical signal that the weighing system’s controller interprets as the weight reading.
Any change to the signal or any interference in the weighing process can throw off the weight results. This can include vibration from equipment placed near the scale, electromagnetic fields from nearby power lines or radio signals, and temperature changes.
Control
A number of things can affect weighing accuracy. These include shock loading (such as dropping heavy materials on a scale that exceeds its max rated capacity) and wind loading (small amounts of air movement caused by things such as air ducts or air conditioning). Vibrations can also cause strain to sensitive load cells, which send an electrical signal that can be misinterpreted by the weight controller.
The response time of a load cell is important when an application involves rapid readings such as a high-speed checkweighing or rotary filling machine. Load cell responses can be influenced by temperature, wire resistance, electromagnetic interference and moisture.
Large temperature changes can impact a load cell’s sensitivity, changing its output range and requiring new calibrations. If a weighing system is designed to rezero (zero out) the weight of a container before each weighing cycle, this problem is less severe. Other random fluctuations can be digitally averaged by a weight controller.
Safety
When weighing hazardous chemicals, safety is an important factor. It is difficult to prevent accidental minor or major spills, but weighing systems can be designed to mitigate exposure risks by using a fume hood or specialised isolator.
When direct weighing a substance, the balance is first zeroed by placing a piece of clean weighing paper on the pan. Then the mass of the weighing substance is displayed by subtracting the first reading from the second. A process called taring is often used to minimise weighing errors.
The precision of a balance is also influenced by the environmental pressure around the system. Pressure differentials can affect the accuracy of a balance, so weighing equipment should be placed in an area with consistent ambient air pressure. Also, vibration can cause issues with accuracy. This is because vibration can make a weight signal inaccurate by causing the weigher to recognise the movement of nearby equipment as an actual weighing load.