常见问题 (FAQ)
LAUMAS答复
In this section we have some answers to the questions we receive most often. Before requesting direct assistance please kindly check that your question has not already been reported and solved in this section.
1. What is a load cell?
A load cell is an electronic component (transducer) to which a weight force is applied.
The load cell is able to convert the weight force applied into an electrical signal in mV proportional to the mechanical deformation expressed in tenths of a millimetre caused by the same force.
2. What is the principle of operation of a load cell?
A load cell consists of a metal body (stainless steel or aluminium) to which strain gauges are applied. The strain gauges are constituted by a grid of thin metal wire (constantan) applied to a support of insulating material and glued into specific areas of the load cell. The strain gauges follow the deformations of the surface of the load cell to which they are bonded, lengthening and shortening along with it; these dimensional changes cause a variation in electrical resistance, which through the "Wheatstone Bridge" connection, is converted into a proportional electric signal normally expressed in mV (thousandths of a volt).
3. What types of load cell are available on the market?
Depending on the type of application for which they have been built, or the construction method used, we mention some common types of load cells that LAUMAS can offer you:
- single point/off-center/for platforms - placed at the centre of a plane surface, they are capable of correctly weighing the weight force applied at any point, they are normally employed in all single cell platforms and scales
- low profile compression - robust and easy to install, together with the corresponding weigh module suitable for weighing silos, tanks, hoppers, mixers and reactors even with large scale
- column compression - typically used in weighbridges or with related weigh modules for silos of additional capacity, even installed outside
- compression and tension - for material testing machines, special tests, etc.
- tension - for creating suspended weighing systems, batching plants or to be applied on tie rods to transform old mechanical scales into electronic scales
- shear beam - cells commonly used in platforms suitable for weighing small and medium-sized silos and transport systems with belts, rollers, etc.
- double shear beam - typically used in weighbridges or with related weigh modules for silos of additional capacity, even installed outside
- bending - cells commonly used for small-scale weighing systems with dosing machines and weight bagging and filling systems, bagging machines, filling machines, etc.
- pin - applied in lifting systems on ropes or pulleys and custom made according to the customer's specifications
- anchor - normally applied to monitor the tensioning force of tie rods present in civil works, dams, bridges, tunnels, etc.
- pre-amplified - cells with an internal amplifier that allows a pre-amplified analogue output 0-10V – 4-20mA
4. Are LAUMAS load cells compatible with any instrument of any brand and vice versa?
Yes. Our load cells are all compatible with any manufacturer's weight indicators and our weight indicators are all compatible with any manufacturer's strain gauge cells.
5. What are the rules to follow for correctly installing load cells?
Check that the load cell supporting surfaces are parallel and usually flat.
Use suitable mounting accessories to compensate if any of the support surfaces are slightly out of parallel.
The supporting surfaces must be sufficiently rigid and in theory non-deformable.
For load shear beam, bending beam and single point cells, the support value indicated on the datasheet must be respected.
Pay attention to the direction of load indicated on the datasheet or on the cell body: this must be oriented in the same direction as the force being applied.
For safety reasons, it is advisable to use the load cells at a maximum of 70-80% of their nominal capacity.
In the case of weighing of structures with 4 supports, keep in mind that the load will not be evenly distributed on them and that 85-90% of the applied load will be spread across just 3 supports.
6. What are the peculiarities of single point / platform load cells?
For example, using a cell guaranteed for an area of 400 by 400 millimetres, we are certain that, by applying a weight force at any point of a structure of such dimensions, the weight detected will be correct.
By installing a structure of larger dimensions than those indicated on the datasheet or by applying an object that protrudes out of the structure, the single point load cells may be damaged and in any case an incorrect weight value is obtained.
7. When installing a load cell or load cell system, how can friction that could compromise weighing accuracy be limited?
A weighing system is all the more precise when the weighed structure is free of friction. When there are pipes, before anchoring the pipe to the weighed structure, check that it is close to and in line with the nozzle to which it will be attached.
Friction can be limited by using flexible hoses and elastic joints or free inlets with rubber protection, such as of the bellows type.
Alternatively, position the first anchor bracket in the horizontal section as far away as possible from the structure being weighed, at least 40 times the diameter of the pipe.8. How to check if a load cell has been installed correctly?
To check the correct mechanical installation of the weighing system, reset the weight indicator and apply a force to the system, then remove it. The indicator should return perfectly to zero.
In the case of a weighing system with multiple load cells repeat this operation for each cell. At system loading the weight values will be similar on each cell, at system unloading they will indicate zero.9. How to install multiple load cells connected in parallel correctly?
To connect multiple load cells in parallel, use a watertight junction box, with a suitable terminal strip, or a multi-channel transmitter in the box.
The cables in and out of the box or multi-channel transmitter require the installation of cable clamps.10. What characteristics must the extension cables used for the connection between the junction box and the weight indicator/weight transmitter have?
The extension cables used for the connection must be shielded. We recommend using a 6-wire cable with reference/sense management, which can compensate for the voltage drop due to the distance between the devices. If possible, it should be inserted into the cable duct by itself and laid as far away as possible from the power cables.
When using 4-conductor connection cables, use a minimum cross-section of 1 square millimetre and preferably not more than 300 metres in length.11. What should I do if I need to weld with load cells that have already been installed?
12. In weighing systems with multiple load cells, how should constraints and accessories be created and positioned to counteract lateral forces and the danger of tipping over due to possible shocks, vibrations, wind forces, earthquakes, etc.?
We offer a variety of mounting accessories. Their purpose is to ensure the correct application of the load cell and maximum reliability and precision and compatibility with the mechanical, electrical and pneumatic connections present on the structure being weighed.
In multi-cell systems we recommend positioning the restraints so that they are able to withstand any lateral forces.
The system designer will have to assess whether the standard mounting accessories are sufficient for the purpose, or whether additional measures should be taken depending on: shocks and vibrations, wind thrust, seismic classification of the installation area, consistency of the supporting base.
Providing constraints that can counteract horizontal forces allows the load cells to work correctly, thus avoiding potentially damaging stresses.
The provision of anti-tip constraints is advised for weighing systems on silos, tanks or structures that are located outdoors and subject to potential wind thrust, earthquakes, accidental impacts with operating machines and other similar situations.
13. What is important to check if the weighing system is to be applied on leg structures or on silos and tanks?
14. How can a correct earthing system be implemented to prevent electrostatic charges from passing through and damaging the load cells?
Electrostatic charges have the potential to damage the load cells. For this reason, we recommend that you connect the top plate of each cell to the bottom plate of each cell using a copper conductor of suitable cross-section. Then, connect all the lower plates to each other and connect them to the same grounding network.
This allows electrostatic charges to be discharged to the ground without passing through and damaging the load cells.
A correctly grounded system removes the possibility of future damage to the cells and the instrument connected to them.
Extending the earthing system through the metal parts of the structure being weighedis absolutely forbidden.15. How can the proper functioning of a load cell be verified?
To verify the correct operation of one or more load cells, tests should be performed using a digital multimeter with mV range. The test must be performed by qualified personnel able to perform the measurements accurately. We can see in detail the tests to be performed:
Load cells resistance measure by means of digital multimeter:
- Disconnect the load cells from the instrument and check that there is no moisture in the cell junction box caused by condensation or water infiltration. If so, keep cleaning the system or replace it, if necessary.
- The value between the positive signal wire and the negative signal wire must be equal or similar to the one indicated in the load cell data sheet (output resistance).
- The value between the positive excitation wire and the negative excitation wire must be equal or similar to the one indicated in the load cell data sheet (input resistance).
- The insulation value between the shield and any other cell wire and between any other cell wire and the body of the load cell must be higher than 20 Mohm (mega ohms).
Load cells voltage measure by means of digital multimeter:
- Take out the load cell to be tested from underneath the container, or alternatively, lift the container support.
- Make sure that the excitation of two wires of the load cell connected to the instrument (or amplifier) is 5 Vdc +/- 3% (10Vdc +/-3% for instruments series TPS- TPZ).
- Measure the response signal between the positive and the negative signal wires by directly connecting them to the multimeter, and make sure that it is comprised between 0 and +/- 0.5 mV (thousandths of a Volt).
- Apply load to the cell and make sure that there is a signal increment.
16. What is the difference between 4 and 6-wire load cells? What advantages do the 6-wire load cells have with reference to the compensation of the voltage drop on the cables?
Depending on the model, the load cells may have a cable with 4 or 6 wires plus the screen. The 6-wire models, in addition to having the terminals of power supply + and power supply – and signal + and signal -, have 2 additional wires called Sense + and Sense – which may also be denominated reference.
The resistance of an electrical cable varies depending on temperature and length, given that we can say that as the temperature and the distance changes there is a voltage drop that the 6-wire system allows to be compensated without affecting the measurement.
The 4-wire load cells are thermally compensated and calibrated in relation to the length of the cable which is supplied as standard; to not compromise the calibration and compensation it is not recommended to shorten the cable of a 4-wire load cell; in the case of the weighing plant there is a weighing indicator or transmitter with a 6-wire input and a junction box-parallel, we recommend using a 6-wire cable to connect to the weighing indicator – transmitter to compensate the voltage drop on the stretch of cable between the junction box and the indicator itself.
If the indicator has a 4-wire input, it is recommended to use a shielded cable with a significant cross-section (minimum 1 sq.mm) in order to contain the voltage drop between the junction box and the weight indicator.
All these measures do not have to be considered with the 6-wire load cells.
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.
17. What are the standards that must be followed for the correct installation of electronic instrumentation (weight indicators, weight transmitters, weighing and batching instruments)?
The load cell cable must be self-contained and must not pass through cable ducts with other cables. We recommend that you connect it directly to the weight indicator or transmitter without interruptions and without terminal blocks.
We also recommend that you do not install the electronic instrumentation in a cabinet containing an inverter. If unavoidable, install special filters and separators between the inverters. The person in charge of the electrical control panel must prepare and install all the electrical protections necessary for the system.
We recommend that you always keep the equipment powered to prevent condensation forming.
18. What are the advantages for dealers-installers of the LAUMAS weight indicators with MULTI software (multiprogram)?
Instruments with "MULTI" program allow the installer to select independently the type of software required in order to be able to respond to different needs. Any dealer who decides to keep stocks of weight indicators with MULTI program will thus have "WILDCARD" instruments that will be able at any time to adapt to the various needs of supply that may arise.
List of selectable programs:
- BASE (basic): basic program, setpoint management without specific batching functions
- RIP (remote display): serial remote display program with setpoints without batching functions
- LOAD: monoproduct load batching program, 99 formulas
- UNLOAD: monoproduct unload batching program, 99 formulas
- 3 PROD.: load batching program up to 3 products, 99 formulas
- 6 PROD.: load batching program up to 6 products, 99 formulas (for this program the external relay module RELE6PROD is required)
- 14 PROD.: load batching program up to 14 products, 99 formulas (for this program both the external relay module RELE6PROD and the external relay module RELE14PROD are required)
19. What is meant by automatic weighing instruments (AWI Automatic Weighing Instruments) in accordance with Directive 2014/32/EU, better known as the M.I.D. directive (Measuring Instruments Directive) ?
An automatic weighing instrument is an instrument that determines the mass of a product without the intervention of an operator and follows a predetermined program of automatic processes characteristic of such an instrument.
Automatic weighing instruments (AWI) used for commercial purposes, must comply with the requirements of Directive 2014/32/EU (Measuring Instruments Directive, MID), the essential requirements of Annex I and the specific requirements contained in Annex MI-006.
Below we list the main types of automatic weighing instruments object of the M.I.D. directive
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
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 with the weight value.
Weight/price labeller
An automatic weighing instrument that affixes labels to individual articles with the weight value and price information.
Automatic gravimetric filling machine
Automatic weighing instrument that fills containers with a predetermined and virtually constant mass of product in grains.
Discontinuous totalizer (weighing instrument/totalizing hopper)
An automatic weighing instrument that determines the mass of a product in grains by dividing it into discrete loads. The mass of each discrete load is determined in sequence and summed. Each discrete load is then brought together.
Continuous totalizer
An automatic weighing instrument that determines the mass of a loose product on a conveyor belt, without resorting to a 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 inclusive of rails for conveying railway vehicles.
Laumas is able to provide weight indicators and weight transmitters in accordance with the M.I.D. directive ( Measuring instruments Directive)
20. I'm an end user, who should I contact if I'm going to buy LAUMAS products or if I need technical assistance?
LAUMAS has created a worldwide network of technical service centres and authorised dealers. Contact sales@laumas.it for information on your Local dealer or the technical service centre near you.
21. What are approved weighing systems or systems for legal use with third parties (NAWI - Non Automatic Weighing Instruments)?
A non automatic weighing instrument (NAWI) is an instrument that requires the intervention of an operator during weighing.
With reference to the European Directive 2014/31/EU the use of approved weighing instruments is mandatory for legal use with third parties in the following applications and/or tasks:
- the determination of mass for commercial transactions;
- the determination of mass for the calculation of a toll, tariff, tax, premium, fine, remuneration, indemnity or a similar type of payment;
- the determination of mass for the application of laws or regulations or for an expert judicial determination of mass in the practice of medicine in the context of weighing patients for the purposes of monitoring, diagnosis and treatment;
- the determination of mass for preparing medicines on prescription in a pharmacy and the determination of mass in analyses carried out in medical and pharmaceutical laboratories;
- pricing as a function of mass for direct sales to the public and the preparation of pre-packaged goods.
Instruments for legal use with third parties must be subjected to a first periodic verification prior to commissioning and will be subject to periodic verification time with frequency determined by the individual national regulations of the Member States of the European Community (In Italy every three years).
22. Where can I periodically check my weighing instrument?
LAUMAS is able to perform the first periodic verification/CE-M approval on its weighing instruments, both independently and through the use of its network of accredited assistance centres.
The first periodic verification is required when the weighing system is used in applications for legal use with third parties in accordance with OIML R76:2006 – Directive 2014/31/EU.
With regard to subsequent periodic verifications, according to the law, they will be carried out with a frequency determined by the individual national regulations of the Member States of the European Community (In Italy every three years) from the date of the first periodic verification through the weights and measures office of the chamber of commerce for the province of installation or an accredited metrology laboratory in the area.
Request information at sales@laumas.it on the accredited metrology laboratory nearest to you.23. What are the regulations to be followed for the verification of a weighing instrument that is used by a company that operates with a quality system certified according to UNI EN ISO 9001?
A company that operates with a certified Quality System, must check the weighing instruments at their disposal on a periodic basis, the frequency of which is to be determined at the discretion of the company in accordance with the quality inspector and depending on the use to which the instrument is intended.
If the weighing instrument is crucial to the productive activity of the company and is used with high frequency, the periodicity of verification may even be monthly or half-yearly, but if the weighing instrument is used sporadically, and for marginal activities within the company, inspections will be more dilated in time, for example every 24 – 36 months.
The verification can be done using certified sample weights traceable to national samples following a proper documented procedure. LAUMAS is equipped with a worldwide network of service centres able to perform calibration reports in accordance with UNI EN ISO 9001.
Contact sales@laumas.it for information on the technical service centre nearest to you.