Frequently Asked Questions
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.
1. What is a load cell?
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.
- Packaging machinery
- Dosing and filling machinery
- Level/Inventory Control on Tanks and Silos
- Test Machinery
- Quality Control
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.
2. What is the principle of operation of a load cell?
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.
3. How are load cells protected from damaging substances and external influences?
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.
4. What types of load cell can be found in the marketplace?
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.
5. How can I check if a load cell is functioning correctly, or not?
Please refer to our Troubleshooting Guide for further information about this.
6. How can I find out more about the electrical circuit inside a load cell?
This information will be added to the website soon.
7. What is the difference between 4-wire and 6-wire load cells?
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.
8. What general measures should be taken when installing a weighing system?
- The cable connections between the load cells and the junction box, and from the junction box to the indicator/electronics, must not be located close to conduit, channelling or trunking containing other cables (especially any other signal or power cables).
- The installer of the load cells, junction box and indicator/electronics is responsible for ensuring the electrical safety of all items.
- It is good practice to always leave the indicator switched on, to prevent the formation of condensation inside by keeping the internal components slightly warmer than the outside ambient temperature.
9. What is an approved non-automatic weighing instrument for legal use with third parties (Non Automatic Weighing Instrument or NAWI)?
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:
- 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 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.
10. What is an approved automatic weighing instrument (Automatic Weighing Instrument or AWI) in accordance with Directive 2004/22/EC, also known as the M.I.D (Measuring Instruments Directive)?
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:
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.
Automatic catchweigher that affixes labels to individual articles showing the weight value.
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.
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.
11. What regulations must be followed during the verification of weighing instrumentation used by a company that operates with a Quality System certified according to ISO 9001?
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.
12. How can I periodically check that my weighing system is working correctly?
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. Your nearest Thames Side Partner can advise on the regulations that are applicable in your country.
If you are looking for detailed troubleshooting information, please visit our Troubleshooting page.
For general technical information, why not check if the answers to your questions can be found in our Technical Notes section?
Contact Thames Side now for further information on our products, application solutions, advice on legal metrology issues, troubleshooting questions or help/advice to configure a weighing system. We have the experience to provide expert guidance and we would be delighted to help You solve Your problem.