How to choose a Load Cell?

Below is a practical guide for the selection of a load cell. It has to be taken into account that there may be other technical circumstances or requirements, and it must take as an orientation that may be valid for most cases. This guide is only suitable for systems totally supported on load cells and systems with evenly distributed loads without great asymmetries and is not suitable for systems where the power is transmitted to cells by means of levers, systems with great asymmetries in the distribution of loads or systems with rolling loads. In order to choose or recommend a load cell, basically, the following questions should be answered:

1st: What load is going to be applied on the load cell.
2nd: What environment is it going to work in.
3rd: Other considerations.

The load to be applied on the cell will give an orientation about the Nominal Capacity of the cell necessary for the load cell. With this, we can restrict the number of possible models from which to choose. The working environment, together with other considerations and the Nominal Capacity will help us to choose the model.

Selection of the Nominal Capacity:

The aim is to estimate the real load on each supporting point in all operating circumstances and life of the system, including extreme situations, and choose a load cell with a suitable capacity and enough safety margins.

The capacity of a load cell is determined in the following way:
– Dead Load: Estimate the dead load of the structure, tank or silo, including all its elements: pipes, pumps, motors, agitators, insulators, heating fluids and accessories.
– Product Weight: The capacity and maximum range of the scales or the weight of the product must be known.
– Gross Weight: It is the addition of the Dead Load plus the Product Weight.
– Number of Supports N: It is the amount of supports on which the weighing structure, tank or scales is supported. It usually has from 3 to 6 supports.
– The theoretical load per support is the result of dividing the Gross Weight into the Number of Supports.
– Select a load cell with a nominal capacity higher than the theoretical load per support according to:

Cell Nominal Capacity = k x Gross Weight / N

Where k has a value between 1,25 and 2,5 , as safety coefficient to increase the capacity of the cells between 25% and 150% of the theoretical value, according to the presence of static or dynamic loads, vibrations, asymmetries, effect of the wind, impacts or rolling loads. A good choice for static loads in indoor tanks is to use k= 1,5 and round up a nominal capacity of a commercial cell.

Examples of common applications:
– 3 supports interior tank k = 1,3
– 4 supports interior tank k = 1,5
– Tank with agitation (moderate) k = 1,7
– 4 cell platform k = 1,8
– Bridge scales for weighing trucks of 6 or 8 cells k= 2

Note: When the Dead Load is over 50% of the Gross Weight, it is recommended to increase the safety margin to k=2, as it is usually due to large motors, accessories or heating systems and very probably, there exist non-centred and not uniform loads on the supporting points.
Note: Alter instalation it is important to check the load distribution for each bearing point. Generally, the load cells may be over-dimensioned up to over twice the weight of the product without any loss of accuracy. It is very common in scales and the only thing to bear in mind is that the sensitivity of the electronic indicator used or the micro-volts per division is enough.

Environmental issues:
It is very common that there exist various models of load cells of the same nominal capacity and so, the most suitable for the concrete environmental working conditions should be chosen:
– For corrosive environments or in presence of permanent humidity, it is recommended the stainless steel load cells, instead of aluminium or nickel plated steel.
– The degree of environmental protection increases with the choice of hermetically sealed load cells with a welded capsule.
– For potentially explosive environments, there also exist specific load cells.
– Verify the need of additional safety elements for those areas with special requirements against earthquakes or strong winds.

Final check:
Finally, try to answer the following questions and correct the nominal capacity of the cell if necessary:
– Is the value of the Dead Load exact?
– Can the load be distributed non-uniformly?
– Are there any agitations or impacts?
– Is it possible that the tank has a superior capacity and it may overflow exceeding thus the estimated Product Weight?
– Is there the possibility of strong winds or earthquakes in the area?
– Can a vehicle impact on or overload the system?
– Can you asure a good leveling for obtaining a good load distribution for each bearing point after the instalation?