Load cells ideas and tips
Are you thinking of getting a scale for a small tank or a larger silo, or something in between, or perhaps you are interested in force measurement?
Read selected parts of this and you will probably save time in the end! This is a small document that you can use to get ideas when building a scale of components (load cells, mechanics, electronics, software, etc.).
We sell a number of completed scales such as floor, pallet and beam scales that you can choose if it suits. If it doesn't, you usually have to choose separate load cells and electronics. You achieve the same results as with a completed scale but it will be better suited to the application where it is to be used.
Here you will find a guide for general assembly advice for load cells and silo scales, click here.
Some examples of completed scales that can often work perfectly. There are many completed scales to be found on the website under products or industries.
Some examples of load cells and complete scales that can often be suitable for hanging loads. There is a really large selection of load cells and crane scales on the website under products and industries.
To build your own scale, you definitely need one or more load cells to pull or push on. The load cells can look in many different ways and can weigh from a few grams up to several hundred thousand kg. See some examples below.
There are many different accessories that can facilitate the assembly of load cells. See some examples below.
The electronics
The basic principle is always the same. One or more load cells mounted in a mechanical construction are needed and some form of electronics with or without a display is needed which often communicates with a computer, PLC, etc.
There are many types to choose from, ranging from simple transmitters that give an analog or digital signal to a remote computer or the like. Keep in mind that it is often appreciated by the staff on the floor that there is a display near the silo/tank.
The output signal can be, for example, analog 4-20 mA, digital RS232/485, Profibus, radio transmission, alarm contact/transistor.
A good compromise is a wall-mounted transmitter that also has a display and taring option, e.g. type LCD-A. There are now indicators that are fully sealed, IP69K, and even impact resistant. If you want one that can withstand most, choose the X320.
Some examples of different types of electronics you can use.
Usually you have 3 or more load cells that are placed in each corner of the tank, silo, feeder or whatever "silo/tank" you have to weigh in. Think like this:
1. A tank with 3 load cells often has a mechanically stable position but can sometimes become a little unstable. A tank with 4 load cells can often be more stable but can "jerk" a little. Compare with a three- or four-legged chair.
2. Most load cells that are intended for tank/silo weighing, pallet scales and many other applications have a "standardized" output signal, i.e. they give the same amount of signal for a given load, e.g. 1 ton. This means that several load cells of the same type can be connected in parallel. The scale will read the same regardless of whether it is biased.
3. For many scale plates only one single point type load cell is used. They usually give different signals for a given load and cannot be connected in parallel. The gain is adjusted in the indicator, so that the scale shows correctly. The weighing plate still shows the same weight regardless of where the load ends up on the plate.
4. The load cells are mounted so that any load you want to weigh loads the load cells and nothing else loads or takes the load. Input and output are isolated so that they do not affect the silo/tank. Of course, a tank/silo that is weighed must be mechanically isolated from nearby tanks/silos that are also weighed.
5. Remember that the silo/tank remains on the load cells and that it does not slide sideways or lift in e.g. storm. It is important that there is not too much friction in the construction between the part that weighs and the surroundings.
6. If there are strong vibrations, there is always the option of using rubber dampers for the load cells.
7. Temperature stresses in a load cell construction can occur if there is a long distance between the load cells and the silo/tank is exposed for temperature variations. There are simple means to reduce these lateral forces. You can e.g. give the tank legs the opportunity to slide a little laterally on balls/rolls/slides or to use rubber dampers.
8. Whether to choose load cells made of nickel-plated or stainless steel is determined by the environment. If it is only wet sometimes nickel-plated steel works, if it is aggressive and/or very humid, choose stainless steel. Some stainless load cells are rated so that they can withstand being submerged for a certain period of time, for example.
9. If it is an old silo/tank that you are going to install load cells under, you may lift the silo/tank and mount the load cell and possibly accessories. Be careful not to overload the load cells when setting down a tank. If necessary, blocks can be made and used instead of load cells. Do not weld in the silo/tank when the load cells are installed. Fit an earth braid past the load cell so that it is protected from earth current in case someone accidentally welds.
10. In the case of new workshop-made silos/tanks, you can prepare for load cell assembly with any accessories, but it is usually not possible to assemble the accessories in the workshop.
11. In applications where static electricity can be expected, e.g. in the case of silos that are filled with plastic granules, efforts must be made to reduce the impact as much as possible. For example. to ground filling hoses and sieves and to carefully isolate the scale galvanically during remote transmission of measurement data.
12. Almost all load cells today can be connected in parallel in a junction box. Remember that it is important to have a good connection and that cables are installed so that water cannot flow into the box. Preferably use screw connections that face downwards.
High temperature?
As standard, most load cells can handle approx. -25 to approx. +70 °C. The accuracy is then usually somewhat reduced outside the range of approx. -10 to approx. +40 °C. There are special load cells that can withstand significantly higher temperatures.
Application in water?
There are different types of stainless load cells that are rated IP68 and withstand 1 m water column for 100 hours.
Silo scales
It is common to choose tension load cells in nickel-plated or stainless steel of the "button type", e.g. VE420, VZ266AH or VZ266AS or C2S, These are very affordable nickel plated or stainless steel load cells in IP67/ IP68 and are available in capacities up to 200 tons. There are also models up to 500 tons.
This type of load cell can be mounted under most types of silos. One load cell per leg or one load cell on e.g. a three-legged silo if the accuracy requirement is not so high, for example in the case of CO2 weighing.
In the simplest form, you just lift the silo foot and install the load cell, see picture below. The screw-nuts must be loose to prevent possible up and side forces. Accuracy is usually excellent.
A more advanced form is complete assembly units that prevent lateral and lifting forces and facilitate service. Remember to always protect load cells when welding.
If the environment is vibrating, the load cells can be fitted with special dampers.
Installation example on an existing silo
For a silo such as this 60 ton pellet silo, above can be a "smart" model for mounting load cells without having to lift the silo several cm to accommodate the load cells. Here we have welded new plates on the inside with "ears" for extra reinforcement. In this way, the silo only needed to be lifted approx. 7mm, which meant that the "inelastic" construction, the feed screw was connected into the boiler room, so no rebuilding was necessary.