In this section you will find answers to the questions that we are asked often.
Before requesting direct assistance from us, please kindly check that your question has not already been listed and answered in this section. Thank you.
In this section you will find answers to the questions that we are asked often.
Before requesting direct assistance from us, please kindly check that your question has not already been listed and answered in this section. Thank you.
A load cell is an electronic component (or transducer) to which a weight/force is applied.
It converts the weight/force applied to the load cell into an electrical signal (in units of millivolt or mV) that is proportional to the mechanical deformation (stresses and strains) caused by that weight/force.
A high quality load cell is able to deform under that weight/force in an extremely repeatable manner, just like a good mechanical spring.
Typical applications:
Typical materials used to manufacture load cells are special alloys with high strength, high fatigue life, high repeatability, high linearity and low hysteresis. For example steel alloys, stainless steels, aluminium alloys, beryllium-copper and others.
A load cell consists of a metal body (usually stainless steel or aluminium) that behaves in a linear elastic fashion, an electrical circuit and a protective housing (cover).
Strain gauges are applied to the elastic metal body (also called an element). The strain gauges each consist of a grid of thin metal wire (constantan) applied to a support of insulating material and carefully glued into specific areas inside the load cell that are designed to concentrate the mechanical stress and strain in that area.
As the weight/force is applied to the metal body, the strain gauges follow the deformations of the surface of the metal body to which they are bonded, increasing and decreasing in length as the metal body does; these dimensional changes create a variation in the electrical resistance (in Ohms or Ω) of the strain gauge.
Inside the load cell is a “Wheatstone Bridge” electrical circuit, which magnifies the small changes in resistance of the strain gauges and generates an electrical signal. This is normally expressed in mV (millivolts, or thousandths of a Volt) per Volt of excitation supplied to the load cell. During load cell manufacture, small adjustments in resistance are made for calibration purposes and to compensate for thermal effects in the material.
Load cells must be protected from dust, moisture and electromagnetic interference.
Thames Side can offer you a comprehensive range of load cells, consisting of all the following types:
Some load cells have a cable with 4 wires and a screen; others have a cable with 6 wires and a screen. Those with 6 wires, in addition to the +input, -input, +signal and -signal terminations, have 2 wires called +Sense and -Sense. These are sometimes called +Reference (or +Ref) and -Reference (or -Ref).
The main difference in function of these 2 types is that load cells with a 6 wire cable can compensate for variations in the actual excitation voltage they receive from the amplifier/indicator. The resistance of an electrical cable (conductor) varies according to its length and any temperature changes, resulting in variations in the excitation voltage at the load cells. With long cables, there will be a drop in voltage from the original value supplied by the amplifier/indicator and the advantage of a 6 wire load cell is that this drop in voltage can be quickly and effectively compensated without it affecting the weight measurement.
4 wire load cells
4 wire load cells are already calibrated and thermally compensated together with the permanent length of cable supplied during their manufacture. We recommend that you do not shorten the cable of a 4 wire load cell if it is too long; it is better to coil up the cable. This is because the factory calibration and compensation of a 4 wire load cell will be compromised if you shorten the cable. There are no sense wires to compensate for the new cable length.
When connecting 4 wire load cells together into a junction box before the weight indicator/transmitter, we recommend using a dedicated 6 wire load cell cable (for example the Thames Side polyurethane cable CA-PU-5.7MM-6C) to connect the junction box to the indicator/transmitter. This will compensate for any voltage drops over the length of cable between them. In any event, the cable should be well shielded and have sufficient cross section (at least 0.2 mm2) to limit the voltage drop along its length.
6 wire load cells
The above precaution about cutting the cables does not apply to load cells with a 6 wire cable. The two sense wires are capable of measuring the actual excitation voltage seen at the Wheatstone bridge inside the load cell, therefore the mV signal from the load cell can be adjusted according to the actual excitation it experiences. If an installation engineer wants to shorten the cables, they can do so without compromising the load cell performance.
The Sense (reference) wires are connected to the sense terminals of the weight indicator, so that this can measure and adjust the amplifier on the actual voltage that arrives to the load cells. The 6-wire load cells are therefore preferred over those with 4 wires, also on these load cells there are no limitations in the event the installer wants to shorten the cables.
A non-automatic weighing instrument (NAWI) is a weighing instrument (e.g. a weight indicator/controller) that requires the intervention of an operator during weighing processes.
With reference to the European directive 2009/23/EC dated 23/04/2009, the use of approved weighing instruments is mandatory for legal use with third parties in the following applications and/or weighing tasks:
Instruments for legal use with third parties must be subjected to an initial verification prior to commissioning and will be subject to periodic verification after that, with a frequency determined by the individual national regulations of the relevant Member State of the European Community.
An automatic weighing instrument is a weighing instrument (e.g. a weight indicator/controller) that determines the mass of a product without the intervention of an operator and follows a predetermined program of automatic processes that are characteristic of such an instrument.
Automatic weighing instruments (AWI) used for commercial purposes must comply with the requirements of Directive 2004-22-EC (Measuring Instruments Directive, M.I.D.), the essential requirements of Annex I and the specific requirements of Annex MI-006.
The main types of automatic weighing instrument subject to the Measuring Instruments Directive are:
Automatic catchweigher
An automatic weighing instrument that determines the mass of discrete loads (for example, pre-packaged goods) or single loads of loose material.
Automatic weight batcher
An automatic catchweigher that subdivides articles of different mass into two or more subsets, depending on the value of the difference between the mass of the object and a nominal set-point.
Weight labeller
Automatic catchweigher that affixes labels to individual articles showing the weight value.
Weight/price labeller
An automatic weighing instrument that affixes labels to individual articles showing the weight value and price information.
Automatic gravimetric filling machine
An automatic weighing instrument that fills containers with a predetermined, and virtually constant mass, of product.
Discontinuous totalizer (weighing instrument/totalizing hopper)
An automatic weighing instrument that determines the mass of a product, by dividing it into discrete loads. The mass of each discrete load is determined in sequence and summed. Each discrete load is then physically brought together.
Continuous totalizer
An automatic weighing instrument that determines the mass of a loose product on a conveyor belt, without systematic subdivision of the product and without interrupting the movement of the conveyor belt.
Weighbridge for railway vehicles
An automatic weighing instrument with a load receptor included within the rails used for conveying railway vehicles.
A company operating with a Quality System certified according to ISO 9001 must verify all weighing instruments at their disposal on a periodic basis, the frequency of which is to be determined at the discretion of the company in combination with their Quality Inspector and depends on the use for which the instrument is intended.
If the weighing instrument is crucial to the major activities of the company and is used with high frequency, the required verification period may even be monthly or half-yearly, but if the weighing instrument is used rarely, e.g. for minor activities within the company, inspections should be less frequent, for example every 2-3 years.
The verification can be carried out using certified weights traceable to national standards, following a properly documented procedure.
An initial verification will be required when the weighing system is used in applications for legal use with third parties, in accordance with EN 45501 – OIML R76:2006 – Directive 2009/23/EC.
According to the law, subsequent periodic verifications must be carried out with a frequency dictated by the individual national regulations of the relevant Member State of the European Community, by the Weights and Measures Office of the relevant Member State, or by a nearby accredited Metrology Laboratory.
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:
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:
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.).
legs).
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.
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”.
about 50°C.
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.
maximum load operative in the installation. Do not load them over their nominal capacity.
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.
problems with rodents and any other risks.
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.
Both the load cells as the wires must never be submerged for long periods of time.
the main direction of measuring.
recommended one and prevent the load cells from overloads and electrical discharges.
for the cell or higher than the necessary for the user of the application.
practices of the sector.
not the only ones to take into account. The person in charge of the installation should
analyze the needs of each concrete case.