Load Cell and Weigh Module Resource Center

Aluminum load cells are used primarily in single-point, low-capacity applications. When compared to alloy steel cells of comparable capacities, aluminum load cells have relatively thick web sections. This is necessary to provide the proper amount of deflection in the element at capacity. Machining costs are usually lower on aluminum elements.

Load cells manufactured from alloy steel elements are some of the most popular cells used today. The cost-to-performance ratio is better for alloy steel elements compared to either aluminum or stainless steel designs. This type of steel can be consistently manufactured to specifications, which means that minor load cell design changes don’t have to be made every time a new lot or steel vendor is selected.

Stainless steel load cells are made from the best overall performance qualities that allow for a cell to perform at peak levels. They can be fitted with hermetically sealed web cavities, making them an ideal choice for corrosive, high-moisture applications. This is the best material to use in any application that requires extra protection because of the metal’s environmental resistance properties.  

Multi-layer coated (MLC) steel load cells undergo a two-step finishing process that protects them from the damaging effects of corrosive environments, while also easily distinguishing them from stainless steel load cells.

What harsh conditions does a load cell have to withstand?

Load cells are critical components of all weighing systems. In some applications, a load cell may be exposed to a hostile environment with corrosive chemicals, heavy dust, high temperatures or excessive moisture from washing down equipment with large amounts of liquid. The load cell could also be exposed to high vibration, unequal loads or other harsh operating conditions. These circumstances can lead to weighing errors or even damage the load cell if the wrong one has been selected. 

To choose the right load cell for a demanding application, you should have a solid understanding of your environment and operating conditions, as well as what load cell features are best for handling them. Load cells are critical components in all weighing systems where they sense the weight of material in weigh hoppers, other vessels or processing equipment. In some applications, a load cell may be exposed to a hostile environment with corrosive chemicals, heavy dust, high temperatures or excessive moisture from washing down equipment with large amounts of liquid. Or the load cell may be exposed to high vibration, unequal loads or other harsh operating conditions. Such conditions can lead to weighing errors or can even damage the load cell if it hasn’t been chosen correctly. 

What makes an application 'tough'?

Take a close look at the environment surrounding your weighing system and what operating conditions the system must work in.

• Will the area be extremely dusty?
• Will the weighing system be exposed to temperatures higher than 150⁰F?
• What are the chemical properties of the material being weighed?
• Will the system be washed down with water or another cleaning liquid? If cleaning chemicals are to be used to wash down the equipment, what are their characteristics?
• Will your washdown method expose the load cell to excessive moisture? Will the liquid be sprayed at high pressure? Will the load cell ever be immersed in liquid during washdown?
• Will the load cells be loaded unequally because of material buildup or other conditions?
• Will the system be subject to shock loading (sudden large loads)?
• Will the weighing system’s dead load (the vessel or equipment containing the material) be large in proportion to the live load (the material)?

It may be necessary to trim the load cell outputs as a first step before starting the calibration process. Trimming is performed at the junction box to equalize the weight reading from all cells in a system. This ensures the scale weighs correctly regardless of where the load is applied to the scale.

Trimming is necessary if:

1. It is a Legal for Trade weighing application
2. The location of the center of gravity of the contents is not fixed, e.g., powder material which may accumulate on one side*
3. A high-accuracy weighing system is required*

*Assume the vessel’s center of gravity (see 2 and 3 above) rises along the same vertical line as the vessel is filled. Each load cell is always subjected to the same percentage of the weight. 

Trimming is not necessary if:

1. Matched output load cells are used (as in Paramount weigh modules)
2. Weighing self-leveling materials (liquids)
3. The vessel is partially supported on flexures

Trimming involves placing the same weight over each load cell in turn, and adjusting the corresponding trim pot in the junction box until the indicator reads the same for all cells. To further illustrate load cell trimming, please review the following examples of signal trim and excitation trimming procedures. 

Many weighing systems use multiple load cells and therefore require a summing junction box to tie or "sum" load cell signals together. This allows a digital weight indicator to read a single "system" signal. The summing process actually wires multiple load cells so all their signal lines and excitation lines are in parallel, providing instantaneous electronic summing of the signals. 

Load cell summing is necessary because:

• Weight distribution in multiple load cell systems is not equal at each load cell. The vessel loading process, presence of agitators and the characteristics of the material affect how weight is distributed on the load cells.
• It is virtually impossible to make each load cell exactly alike. Load cell manufacturing process tolerances allow for some variance in individual cell specifications. This variance, if unchecked, would not allow for the kinds of accuracy required in modern process applications.

There are two summing methods: Excitation trim and signal trim.

Excitation Trim

This is the oldest method of trimming the output from a strain gauge load cell. Excitation trimming adds series resistance to the excitation circuit of the load cell, thereby reducing the excitation voltage at the cell. The load cell with the lowest millivolt per volt output receives the full excitation voltage. All other load cells in the system with a higher millivolt per volt output receive proportionally smaller excitation voltages. This results in matched full load outputs for all load cells in the system.

Signal Trim

Signal trimming is the most common and popular type of trimming because it is the easiest to use. Compatible with virtually all indicators and relatively immune to temperature and vibration problems, signal trimming is gaining popularity for all installations. It involves adding a relatively high parallel resistance between the signal leads of each load cell. The added parallel resistance creates a "leakage path" that shunts some of the available load cell signal away from the indicator. The larger this parallel resistance, the more signal available to the indicator from the load cell. Conversely, the smaller this parallel resistance, the less signal available to the indicator from the load cell.