Load cells must be protected from dust, moisture and electromagnetic interference.

• Dust and moisture can be expelled by the use of silicone sealants, or better still fully welded and hermetically sealed, plasma welded metal bellows or covers/shields; high quality cable glands are also very important.
• Electromagnetic interference can be eliminated with the use of shielded cable and high quality cable glands.

Thames Side can offer you a comprehensive range of load cells, consisting of all the following types:

• Single point/off-centre/platform – load cells that are placed underneath the centre of a plane surface, they are capable of correctly measuring the force applied at any point on the surface. They are usually employed in all single cell platforms and small weighing scales.
• Bending beams – load cells that are commonly used in small weighing systems, dosing machines, bagging and filling machines, big-bag (FIBC) filling and emptying systems, small multi-cell platform scales
• Shear beams – load cells that are commonly used in larger, multi-cell platforms. They are suitable for weighing small and medium-sized hoppers, tanks or silos and transport (conveyor) systems with belts, rollers, etc.
• Double shear beams – robust load cells that are typically used with a corresponding mechanical accessory to create a complete weighing assembly for larger tanks, silos and reactors, especially those installed outside or in demanding applications
• Low profile compression – load cells that are robust and simple to install, together with a corresponding mechanical accessory to create a complete weighing assembly for weighing silos, tanks, hoppers, mixers and reactors
• Compression columns (rocker columns) – load cells that are typically used in weighbridges, or even in a weighing assembly for silos of very large capacity installed outside (or in remote areas with a risk of lightning damage)
• S-type (tension) – load cells used to create suspended weighing systems, concrete/asphalt batching plants; also for installation onto tie rods to transform old mechanical scales into electronic scales
• S-type (tension and compression) – for materials testing machines, special test rigs and fixtures, including field testing and R&D etc.
• Pin type (load pins) – loading pins used in in lifting systems with ropes or pulleys, or load monitoring systems on cranes/rigging systems. They are usually custom made according to the customer’s specifications.
• Pre-amplified type – load cells with an internal signal amplifier that provides a pre-amplified analogue output (usually in the form of a 0-10V or 4-20mA signal). These load cells can be any of the above types.

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 2,2 , as safety coefficient to increase the capacity of

the cells between 25% and 120% 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‐centered and not uniform loads on the supporting

points.

Note: Alter installation 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 aluminum 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 assure a good leveling for obtaining a good load distribution for each

bearing point after the installation?

When the scales do not repeat the value of the weight, experience tells that it is almost certain

that there exists a mechanical problem, of rigid anchoring, of flexion in the structure, of unstable

bases or frictions. A weighing system must have a firm base, a rigid weighing structure and a

certain freedom of movements in its joints. If this is not so, it will not repeat and, as a

consequence, perhaps it will not return to zero either.

Identify and separate clearly the weighing area and the rest of the structure. The weighing area

supports itself exclusively on the load cells and should be able to go up and down freely. There

must not be another contact between both areas except the load cells, without any friction.

The cells are almost rigid, but they have a necessary flexibility. They are deformations that can

hardly be seen, therefore, check the following:

•  That the load is applied on the right area of the cell. Normally, the load is placed on a dimple or

load groove or around the load drill and must not be placed on the general surface of the

cell (back or areas sensitive to deformation or parts with silicone, etc.).

•  That the assembly accessories are suitable and are properly assembled.
•  That the bases of the load cells are firm, without movement or flexion.
•  That the area around the load cells is clean and there is no accumulated dirt.
•  That there are no excessive flexion in the weighing area (weak weighing platforms, bent tank

legs).

• That the pipes connecting the tanks are flexible.
• That the lift‐off or overload bolts do not create a friction or block it from moving.
• That no rigid anchoring has been used in both extremes of a cell. It is not advisable to place the

load on the cell and fasten it with rigid bolts. There must be a joint, silent block flange,

ball support, enough space between them, etc.

• That the load arrives in the orientation, for which the load cell is designed, with no lateral

forces in other directions or torsion moments.

Try to disconnect momentarily the joints or feed pipes of the tank in order to verify once more the

repeatability of the scales.

Verify the correct adjustment of the corners by loading with the same weight on different points

of the scale, platform or tank. Adjust it by means of a summing box with fine tuning if necessary.

14/03/09

The output signal without load or zero level from factory is usually of a 2% value of the Full Scale

(F.S). So, for a cell of 2mV/V of nominal sensitivity it is a value of ±0,04 mV/V or ±0,4 mV,

supposing that we are feeding the cell at 10V. However, if the zero signal value observed in a used

cell is still inferior to 10% F.S., that is to say, inferior to ±2 mV feeding at 10V, and it still keeps

stable, the cell usually works and it is enough with carrying out a new calibration of the zero of the

instrument (scale).

However, in case of finding a zero displacement, it must be questioned if, when using the

application normally, there are shocks, mechanical overloads or perhaps it is already a fatigue. If

this is the case, change the cell and protect the installation or apply any other remedy to prevent

the problem from happening again.

Special attention must be paid to cells of very low nominal capacity, inferior to 10kg, where at the

moment of assembly, handling and/or fastening of anchoring bolts, it may overload and displace

them from zero just by the mere force of the hand remaining easily “bent”.

• Verify that the power source is in good conditions and it is stable.
• Connect another load cell to check.
• There may be a humidity problem in the connections, wires, summing box or inside the cell.
• Measure the insulation between the colored wires and the body of the cell.
• If there is humidity in the summing box, you may try to dry it using a hair drier.
• If there is humidity in the cell, you may try to dry it by keeping it 24 hours in a dry oven at

about 50°C.

• Check the choice of the cell in regards to environment.
• As much as possible, reduce the humidity potential of the installation. Protect it against high

pressure jets by means of screens and ensure a good drainage

All the load cells exit the factory individually verified and have individual compensations with

different values of their resistances within an input and output range of resistance values. A

significant increase of the input resistance indicates that there has been a change in the electric

circuit in respect to the moment of its manufacturing. In this case, the cell is damaged. The inner

damage is probable due to the breakage of one of its inner resistances, either in the component or

in its connections, soldering or connecting wires. It is not possible to determine the cause

accurately, whether it is due to an external agent or a manufacturing fault. In both cases, faults

may be produced or defects may accelerate for external reasons, such as shocks, mechanical or

electrical overload (electrostatic or lighting discharge). In some cases, the load cell keeps working

but with a different gain or sensitivity to the original.

There are load cells of the same shape appearance, but made of different materials, as steel or

aluminum. Both may even have similar accuracy, repeatability and linearity characteristics but not

the same mechanical resistance to overload, shocks or fatigue.

In order to reduce costs in manufacturing, different aluminum alloys may be used and they bear

good results in regards to accuracy but they have the disadvantage that they are much weaker

than the ones manufactured of steel alloy, in the sense that if certain stress levels are exceeded,

the cells are more easily deformed and undergo displacements of the output signal. Therefore,

they are also weaker in front of overloads and much more sensitive to shocks. Furthermore, they

wear out more easily with dynamic loads and last less.

Therefore, in case the choice is an aluminum load cell, precautions should be increased in

overload stoppers and choose load cell nominal capacities with a higher over‐dimension in respect

to the applied load than for a steel load cell.

The aluminum load cells are usually used in great consumption applications because they save

costs in great series, where the scales designing team may have studied, project, over‐dimension

and test this solution properly in order to avoid problems.

For industrial weighing processes, of little production, it is safer and more reliable to directly use

versions of cells made of high resistance steel alloys.

Also, to mention that there exist other materials with a very high resistance to fatigue, as the

Beryllium‐Copper, but very seldom used due to its high cost. Currently, they are only justified in

high fatigue applications.

As a company general policy, we do not supply information about the steel alloys used in the

manufacturing of our load cells. For years, we have invested a continuous effort in the selection of

the best types of steel for the manufacturing of our load cells, including the selection of the best

alloys, manufacturing processes and thermal treatments supplied by international suppliers. Steel

is an art! This is the reason why we beg your comprehension in regards to the fact that we

consider this information as an internal and confidential “know how” asset.

In order to define the Metrologic characteristics of a load cell, in general, the standard developed

by OIML (International Organization of Legal Metrology) is used, which in the case of a cell is the

OIML R60 recommendation “Metrological regulation for load cells”.

According to those recommendations, the vmin parameter is the “Minimum load cell verification

interval”, which is the data supplied by the manufacturer of the cell to indicate the recommended

minimum size in order to define the size of each division or resolution of the load cell.

The vmin value is in units of mass (weight). Usually, vmin is a value between 6.000 and 10.000

fractions of the nominal capacity of the load cell.

This data can be used by the manufacturer of the scale to validate if the chosen division for the

specific scale is compatible with the minimum division that the load cell can supply, according to

the following formula:

vmin ≤ e / √ N or e ≥ vmin * √ N

being, e the verification scale interval or division of the scale

N the amount of load cells in the scale

The precision that we may expect from a weighing system is the highest value obtained between

the two following calculations (a) and (b):

 

(a) Limit by minimum division of the load cell (related to repeatability):

emin(rep) = vmin * √ N (a)

Being,

emin(rep) =minimum error that can be obtained by the minimum division of the cell

vmin = the minimum load cell verification interval

N = number of load cells

 

(b) Limit by range of use of the cell (related to linearity):

emin(lin) = Max / nlc (b)

being,

emin(lin) =minimum error that can be obtained by range of use of cell

nlc = number of load cell verification intervals

Result: emin = the highest of emin(rep) o emin(lin)

 

Recommendation:

The precision is the error. The resolution or division of “display” is the fraction that is displayed. In

certified scales, the resolution or division of display should not be finer than the error of the

instrument itself. In certain industrial environments, an increased resolution is used, twice as fine

than the real error or the precision of the system.

Examples:

Ej. 1) Normal scale

Data of the weighing system:

Product Max= 600 kg

Dead Load DL = 120 kg

Total Load = 720 kg

Supports N = 3

Data of the Cells:

3 units Model 350 i 500 kg

Emax = 500 kg

nlc= 3000 divisions

vmin = Emax / Y = 500 / 10.000 = 0,05 kg

Calculations:

(a) Limit by minimum division of cell (related to repeatability):

emin(rep) = vmin * √ N = 0,05 * √ 3 = 0,0865 kg

(b) Limit by range of use of cell (related to the linearity):

emin(lin) = Max / nlc = 600 / 3000 = 0,2 kg

Result:

emin = 0,2 kg

For these scales we shall choose a display resolution of d = 0,2 kg

Ej. 2) Scale with quite a lot of dead load in respect to the weight of the product

Data of the Weighing System:

Product Max= 400 kg

Dead Load DL = 320 kg

Total Load = 720 kg

Supports N = 3

Data of the Cells:

Model 350 i 500 kg

Emax = 500 kg

nlc= 3000 divisions

vmin = Emax / Y = 500 / 10.000 = 0,05 kg

Calculations:

(a) Limit by minimum division of cell (related to the repeatability):

emin(rep) = vmin * √ N = 0,05 * √ 3 = 0,0865 kg

(b) Limit by range of use of cell (related to linearity):

emin(lin) = Max / nlc = 400 / 3000 = 0,133 kg

Result:

emin = 0,133 kg

For this scale we shall choose a resolution of display of d = 0,2 kg in an environment of certified

scales for commercial transactions or also the smaller d = 0,1 kg for an environment of industrial

control.

 

Ej. 3) Scales with a great amount of dead load and very little product weight

Data of the Weighing System:

Product Max= 220 kg

Dead Load DL = 500 kg

Total Load = 720 kg

Supports N = 3

Data of the Cells:

Model 350 and 500 kg

Emax = 500 kg

nlc= 3000 divisions

vmin = Emax / Y = 500 / 10.000 = 0,05 kg

Calculations:

(a) Limit by minimum division of cell (related to the repeatability):

emin(rep) = vmin * √ N = 0,05 * √ 3 = 0,086 kg

(b) Limit by range of use of cell (related to the linearity):

emin(lin) = Max / nlc = 220 / 3000 = 0,073 kg

Result:

emin = 0,086 kg

For these scales we shall choose a display resolution of d = 0,1 kg.

The signal that one or several cells of a weighing system deliver for a specific increase of load,

normally the display division, is:

Δu = (C * 1000 * Uexc * e) / (N * Emax )

Where,

Δu = Increase of signal in μV/div (micro‐volts/division)

C = Nominal Sensitivity of the cell in mV/V

Uexc = Excitation Voltage of the cells in V (Volts)

e = Size of the division in kg

N = Number of load cells

Emax = Nominal capacity of the cells

Typical values:

Δu = 0,8 a 5 μV/div (micro‐volts/division)

C = 1 a 3 mV/V (milli‐volts per volt)

Uexc = 3 a 12 V (Volts)

e = 0,001 kg a 100 kg

N = 1 a 10

Emax = 1 kg a 400.000 kg

Examples:

Ej. 1) Scale of Max range=15 kg, division e=0,005 kg (5g)

N = 1 cell of Emax = 20 kg, C= 2mV/V

Load Cell Excitation Uexc = 10 Volts

Δu = (2 * 1000 * 10 * 0,005) / (1 * 20) = 5 μV/div

Ej. 2) Same example as Ex.. 1) but with Load Cell Excitation at Uexc = 5 Volts

Δu = (2 * 1000 * 5 * 0,005) / (1 * 20) = 2,5 μV/div

Ej. 3) Scale of Max range= 600 kg, division e=0,200 kg

N = 4 cells of Emax = 500 kg, C= 2mV/V

Load Cell Excitation Uexc = 5 Volts

Δu = (2 * 1000 * 5 * 0,2) / (4 * 500) = 1 μV/div

Ej. 4) Scale of Max range= 1500 kg, division e=0,5 kg

N = 4 cells of Emax = 750 kg, C= 2mV/V

Load Cell Excitation Uexc = 6 Volts

Δu = (2 * 1000 * 6 * 0,5) / (4 * 750) = 2 μV/div

Ej. 5) Same scale as Ex..4) with slightly bigger cells:

N = 4 cells of Emax= 1000 kg, C= 2mV/V

Load Cell Excitation Uexc = 6 Volts

Δu = (2 * 1000 * 6 * 0,5) / (4 * 1000) = 1,5 μV/div

Ej. 6) Truck weighing type scale of a Max Range of =60.000 kg, division e=20 kg

N = 6 cells of Emax = 20.000 kg, C= 2mV/V

Load Cell Excitation Uexc = 10 Volts

Δu = (2 * 1000 * 10 * 20) / (6 * 20.000) = 3,3 μV/div

Ej. 7) Truck weighing type scale of a maximum Range =60.000 kg, division e=20 kg

N = 8 cells of Emax = 30.000 kg, C= 2mV/V

Load Cell Excitation Uexc = 6 Volts

Δu = (2 * 1000 * 6 * 20) / (8 * 30.000) = 1 μV/div

 

Recommendation:

As we have seen in the examples above, the signal values that the cells deliver per each “display”

division are very small; between 1 and 2 μV/div. Therefore, specific high sensitivity electronic

instruments should be used for the load cells, that have super‐stable power supply voltages, stable

differential amplifiers, high resolution analog‐digital converters of between 16 and 24 bits and

filters and suitable protections.

The shielding of the conduit of the cell wires and a grounding of the complete system will help to

protect these weak signals in environments with interferences, such as the industrial ones.

• Select the correct nominal capacity of the load cell, which should be higher than the

maximum load operative in the installation. Do not load them over their nominal capacity.

• Select the Model of load cell suitable for the application and the environment.
• Be aware that a cell is a delicate sensor, both electrically and mechanically. Its choosing

and installation should be carried out only by professionals of the sector. Take precautions

for the safety of the system. Do not allow that the safety of people or things depends on

the mechanical resistance of a cell or of the signals delivered by a cell. Properly overdimension

and use the safety external elements that you consider necessary.

• Use the cell accessories designed by the manufacturer for the specific cell.
• Assemble the cells and/or accessories on a clean, flat, solid and strong surface.
• Design suitable protection elements against mechanical overloads, wiring protection,

problems with rodents and any other risks.

• Avoid temperature gradients in the load cell. Temperature must be stable all through the

body of the cell. If there is a heat source nearby, insulate it by means of insulating plates in

order to reduce the transmission or radiation of heat towards any part of the cell.

• Do not hold a load cell by the wire or pull it. Properly protect the wires in the installation.
• Protect both the scales and the load cell from shocks.
• Returns must be properly packaged against shocks.
• Do not open the load cells or try to repair them.
• Do not carry out welding tasks near the load cells.
• Keep the area where the load cell is installed clean.
• Have a suitable drainage system in the installation in order to avoid floods for a long time.

Both the load cells as the wires must never be submerged for long periods of time.

• Do not strain the cells or submit them to force torsion moments, different of the ones in

the main direction of measuring.

• Use a stable and free of noise power supply. Do not feed it with a higher voltage than the

recommended one and prevent the load cells from overloads and electrical discharges.

• Do not set the electronic equipment to a higher resolution than the logical one available

for the cell or higher than the necessary for the user of the application.

• Do not exceed any specification limit of the load cell or its accessories, nor of the usual

practices of the sector.

• The above recommendations are just an orientation as general information and they are

not the only ones to take into account. The person in charge of the installation should

analyze the needs of each concrete case.