How to choose a load cell? The factors to assess

• to the environmental conditions in which the system will be working (WHERE will we be weighing?)
• to the type of application in itself: the characteristics of the structure to be weighed and its operation, as well as the subject and purpose of the weighing itself (HOW, WHAT and WHY will we be weighing?)

Analyzing these discriminating factors allows us to better orient ourselves among the different types of available load cells and among the many specifications the load cell must have.

We will thus know how to select the most appropriate standard load cell or to provide the necessary details for a custom solution.

→ Read also The main types of load cells
→ Read also How to choose a load cell? Technical features guide

Contents:

1. WHERE DO WE WEIGH? Assessing the environmental conditions of the weighing system
2. HOW DO WE WEIGH? Assessing the application and type of plant in which the weighing system is installed.
3. WHAT DO WE WEIGH? Assessing the object to be weighed.
4. WHY DO WE WEIGH? Assessing the reason for weighing.

1. WHERE DO WE WEIGH?
Assessing the environmental conditions of the weighing system.

The protection rating is indicated by the (Ingress Protection) IP code followed by 2 digits.
The first one indicates the protection rating of the enclosures of electrical equipment against the ingress of solid objects (for example, dust) and can range from 0 to 6.
The second digit instead indicates the level of protection against the ingress of liquids and can range from 0 to 9, with specific digits for resistance against jets of water under greater or lesser pressure or against immersions of different duration and depth.

LAUMAS load cells, for example, provide IP65 and IP69K protection ratings against jets of water and IP67 and IP68 against immersions.

The IP code, which describes the protection rating of the enclosures of electrical equipment.

❖ All these conditions also affect the choice of the construction material of the load cell.

For example, if the application is in a dusty environment, but indoors, aluminium or a non-stainless steel can be sufficient.
Whereas, if it is outdoors or exposed to liquids or aggressive agents such as chemicals, it will be necessary to consider stainless steel 420 as a basic requirement, to then reach stainless steel 17- 4 PH that is the one most resistant to corrosion.

❖ Another element of the data sheet to bear in mind is the working temperature range, that is the operating temperature range within which the load cell can in any case work without breaking, but it does not guarantee its methodological characteristics.

→ Read also How to choose a load cell? Technical features guide to find out more about: protection rating, construction materials, thermal compensation and working temperature range.

❖ If we have to install the load cell in an area classified as a place with a risk of explosion due to the presence of mixtures of gas, vapors or flammable mists or combustible powders, targeted certification will be essential.

The most suitable certification will depend both on the classification of the hazardous area and on the market where the application is installed.

The load cell will then have to be certified on the basis of the regulations in effect in the country of reference, for example:

→ See all our products for explosive atmospheres       

→ Watch the tutorial Constraints against horizontal forces and tilting

2. HOW DO WE WEIGH?
Assessing the specific application of the weighing system.

An analysis of the type of application in which we will need to install the weighing system gives us precise information on the load cell to use.

Certain applications are so specific as to require only one particular model of load cell.

This is the case, for example, of load cells for wire rope measuring when they work as load limiters in lifting systems or that of anchor load cells applied to measure and monitor the tensioning force of tie-beams in civil works.

Another example is the load cells for foot brake, specifically conceived to measure the force exerted by the foot on a car’s brake pedal.

→ Read also The main types of load cells

Application of an anchor load cell.

❖ Sometimes, in certain sectors such as the food or chemical-pharmaceutical sector, it is necessary or anyway desirable for the load cell to have a specific certification, which attests to it being perfectly sanitizable.
The international certifications to be referred to are the 3-A Sanitary Standard or EHEDG.

The LAUMAS hygienic load cell for foot, model FLC3A.

❖ If there are problems of space in the application to which the load cell has to comply with, the more or less compact dimensions of the sensor can be a discriminating factor to consider.

❖ It is also important to make provision for a proper installation with the aid of appropriate accessories mounting kits, which act as anti-tilt constraints and constraints against sideways movements.

3. WHAT DO WE WEIGH?
Assessing the object of the weighing system.

❖ The object to be weighed also gives us information about the capacity the load cell will need to have.

Factors such as the dimensions and weight of the goods, as well as whether it is a static or dynamic object, together with the weight of the structure itself and the possibility that parasitic forces might come into play, help define the maximum load contemplated during the measurement.
It will be necessary to make sure that the load cell has a sufficient capacity to handle it.

FOCUS:

As a rule it is always recommended to oversize the load cells in relation to the maximum load to be applied to the weighing system, choosing them with a rated capacity at least 25 – 30% higher. For some applications it could be necessary to oversize them by as much as 100% or more.

→ Read also How to choose a load cell? Technical features guide to find out more about the capacity of load cells.

❖ The type of goods to weigh can also affect the classification of an area as potentially explosive atmosphere: this is the case of weighing in petrochemical plants or generally plants with explosive substances, including powders and flours.
In these cases, too, we will take into consideration load cells with the most appropriate certifications (ATEX, IECEx, Hazloc, EAC Ex, Ex NEPSI etc.)

❖ The object to be weighed also affects the analysis of the degree of accuracy that must or may be obtained.

A medical scale that has to batch the active ingredients of a pharmaceutical product, for example, needs greater accuracy than an application weighing large quantities of building material.

In general, low-value goods (such as sand or gravel) do not need particularly precise weighing, while in applications for pharmaceuticals and for more significant products the weighing system needs a high level of accuracy.
In any case, for medical or legal applications or ones that entail calculating a price, it will be necessary for the load cells to be certified in accordance with obligatory regulations that establish the required degree of accuracy, even stringently.

In the following section we will be analyzing the main parameters to be taken into consideration for defining the degree of accuracy of a load cell: OIML R 60 certification and OIML accuracy class (also called “precision class”), combined error, resolution and the effect of temperature changes.

4. WHY DO WE WEIGH?
Assessing the purpose of the weighing system.

The weighing system can have the ultimate purpose of batching, load limitation for reasons of safety, checking quality or counting pieces of a production (piece-counting function), in-line weight checking system, statistical check on prepackaged goods or to establish a price.

Other times the purpose of the system may be to measure and monitor the force and not the weight, for example in the geotechnical sphere or in a material test machine.

In-line weight checking system.

❖ Other characteristics to be considered, especially when the application requires a high level of quality of the measurement result, are:

• OIML accuracy class / legal divisions.
  This is the classification, depending on the OIML R 60 certification, that describes the methodological characteristics of the load cell and indicates their degree of accuracy and precision.
  
  The different classes are marked by a letter expressing their level of accuracy (A, B, C and D) and by a number specifying how many legal divisions the load cell is able to measure in units of thousands.
  Legal divisions are the smallest certain weight changes, stable under all conditions of measurement, that the load cell is able to measure.
  
  More legal divisions means greater measurement accuracy since they permit detecting smaller weight changes.
  For example, the C3 accuracy class indicates that it is a load cell that can be installed in weighing systems capable of measuring weight changes of down to 1/3,000 of the system's maximum capacity.
  
  Class D1 therefore denotes less accuracy, acceptable for example for a load cell used for weighing construction material, while from class C3 up to C6 and beyond there are highly accurate load cells, suited for precision scales, checkweighers, approved weighing systems, bench scales and weighbridges.
  
  
• Combined error.
  Expressed as a percentage of the full scale, this value represents the maximum error that can be expected of a load cell.
  The characteristics of linearity, hysteresis and temperature error are verified in OIML tests in a single set of tests that give an overall limit to the sum of the three errors measured.
  This value is the combined error. The lower it is, the more it indicates that the load cell is accurate and precise.
  
  
• Minimum verification interval (V min).
  This is the minimum verification interval into which the load cell measurement range can be divided.
  It is obtained from the ratio between the maximum capacity of the load cell and the value Y (or relative V min) established by the OIML tests.
  It actually describes the resolution of the load cell, that is the minimum increase in weight that the load cell is able to measure.
  This value is essential when the load cell has to be used in a weighing system approved for legal for trade use.
  
  
• Effect of temperature changes.
  The effect of temperature on the zero and the effect of temperature on the full scale indicate exactly the maximum error that an increase or decrease in temperature causes to the reading of the load cell, that is by how much the zero or full scale value can differ from the actual value as a percentage of the full scale for each degree centigrade in temperature change.
  

→ Read also How to choose a load cell? Technical features guide to analyze these parameters

❖ In addition to all the discriminating factors analyzed so far, in order to choose the most appropriate load cell you also need to consider:

• the economic factor: load cells vary considerably in terms of cost, depending on the specifications, therefore it is necessary to bear in mind your budget too.
  
  
• A careful study of how the load cell will need to be installed.
  Planning and making a correct installation of the load cell is decisive in order to obtain the best performance from the sensor and ensure its safe and long-lasting use.

→ Read also How to install a weighing system correctly or watch the tutorial:

LAUMAS assists its customers in this activity of preliminary analysis, offering all its experience.

We can help you in the choice of the perfect load cells for every specific requirement and advise you with regard to the most suitable assembly for their application.