Several factors can affect the accuracy of a weighing measurement. Selecting a top-quality load cell is the first step to ensuring accurate mass measurements.
Keep the weighing weights free of moisture and corrosive gases. Also, avoid letting them rub against each other or hard surfaces. Doing so can cause them to change their mass, resulting in inaccurate weighing readings.
Accuracy
A weighing instrument’s measurement uncertainty is the sum of four significant components — repeatability, eccentricity, nonlinearity and hysteresis. These parameters are evaluated during a balance calibration and documented on the calibration certificate.
A quality load cell (also known as a load sensor) is the backbone of your weighing system. A machined piece of metal, it bends with the weight’s mechanical force and converts those movements to proportional electrical signals based on its elasticity.
Depending on the location of your weighing system, it may be subjected to temperature changes and static charge that can affect its accuracy. Moisture that enters the weighing system’s junction box can also cause problems by wicking into signal lines between each load cell and degrading their accuracy. Using a hermetically sealed, NEMA 4-rated junction box and plugging any unused holes in the box can help improve weighing accuracy. Also consider using a weigh platform with a waterproof evaporative cooling system to reduce the effect of ambient temperature changes and moisture.
Safety
Weighing is a safe measurement method, especially when the system is properly designed & operated. It is also a widely accepted method of quantifying food, making it an ideal ingredient-intake monitoring tool in manufacturing facilities.
A weighing system is composed of load cells that support (or suspend) a weigh vessel or platform, a junction box that sums the individual signals from each cell, and a weight controller that converts the sum signal into a weight reading. Many factors can influence the weighing system performance, including shock loading (forces greater than the load cell’s rated capacity), vibration, large temperature changes, and pressure differentials.
It is important to follow calibration procedures for your weighing equipment in order to maintain accuracy. Consider using a risk-based testing schedule for your balances, as it will reduce unnecessary testing and lead to time, cost & energy savings. It is also crucial to keep your weighing equipment in an environment that is free from moisture, corrosive chemicals, and dust.
Calibration
Weighing instruments may drift over time unless you regularly calibrate them. This reduces the error from environmental changes and ensures you always get accurate results. It’s important to have a calibration plan that is clear and that all users understand. The calibration plan should also be traceable to a primary standard.
External calibration is a manual process and requires trade approved calibration weights that are carefully purchased, preserved and managed. It’s also important to record the reading from each standard stamped weight used in the calibration process so that a linearity curve can be generated and a correlation factor identified.
For a deadweight calibration, the vessel is evenly loaded to about 10% of its live load capacity using standard weights and the weight indication is recorded. The weights are then removed and process material is added until the weight indicator registers the same weight as the calibration weights. This procedure is known as comparison weighing or substitution weighing.
Maintenance
Having an effective maintenance plan in place helps prevent system breakdowns, reduces repair costs, and optimizes the return on your equipment investment. The first step to a successful maintenance plan is to identify potential safety hazards. This includes a thorough risk assessment and implementing proper lockout/tagout procedures.
When performing a maintenance check on an industrial platform scale, start by visually inspecting the overall condition of the unit. Any issues found should be recorded. Next, the junction box should be disconnected from its power source (i.e. shut off the circuit breaker or unplug the cables). Finally, the scale should be isolated with barricades and signage placed to divert traffic away from the PM area.
Once the load cell is disconnected, its input and output resistance should be measured and compared with the tolerance range specified on its datasheet. Also, the cables connected to the junction box should be checked for damage and strained connections. Lastly, the heavy section test using certified weights should be performed and adjusted as needed to bring the scale up to its certified NIST Hand Book 44 standards.