How to Choose Load Cells for Concrete Mixing Plant?

Selection of load cells in the weighing system usually considers the capacity, accuracy, installation space of load cells, possible influence of surrounding environment on load cells, loading types and service life of load cells. The weighing system of engineering mixing equipment is no exception, but the only difference is that different weighing systems focus on different aspects due to different requirements and working conditions of engineering mixing equipment.
Several problems need to be considered when selecting the load cell for the weighing system of engineering machinery mixing equipment. This article focused on analyzing the influence of the protective structure of the load cell on the operational reliability of the engineering machinery mixing equipment. It is pointing out that the protection class represented by the IP code cannot cover the full protection requirements of the load cell. Different types of mixing plant are recommended to use the load cells with different protection capabilities.
Engineering mixing equipment include concrete mixing plant, asphalt mixing plant, stabilized soil mixing plant and marine concrete mixing plant, the concrete mixing plant is most commonly used. Next analyses are made with the concrete mixing plant as an example.
I. Basic requirements of concrete mixing plant to the weighing system

1. Accurate Weighing
   Errors greatly affect the concrete strength, particularly the measuring accuracy of water cement ratio, because the strength keeps a linear relation with the water cement ratio. Relevant standards rule that the dynamic measuring accuracy of cement, water, admixture and additive is ±1%, the dynamic measuring accuracy of sand and stone is ±2%.
2. Quickly meet the working recycle requirement of mixing plant
3. Variety
   There should be many kinds of pre-selected weighing values, and they shall be convenient to exchange, so as to meet the requirements of various ratios and capacities.
4. Simple Structure
   The weighing device should be simple in structure, firm and reliable, stable in performance and easy to operate.
   

Obviously, the electronic weighing system can better meet the requirements than the mechanical scale, so load cells are more and more widely used in the concrete mixing plant. But objectively speaking, the electronic weighing system still has a huge improvement space for reliability and stable performance.
II. Operating Conditions of Load cell in Concrete Mixing Plant
Different from the electronic scale commonly used in the business measurement, the weighing load cells for concrete mixing plant work in extremely harsh environments, the stress environment is quite complex. Compared with the operating environment of common electronic products, it has greater randomness.

1. Ambient temperature and humidity
   The concrete mixing plant is usually installed outdoor, and load cells may be exposed to sunshine and rain with dramatic temperature changes. Moreover, many construction projects are in mountainous or remote areas where natural conditions are quite bad. Therefore, larger temperature ranges and higher humidity conditions must be considered. A certain amount of water vapor will also be produced during the water delivery and weighing, thus forming a relatively humid environment within a certain small range. In the temperature-controlled concrete mixing plant, requirements are different for high temperature condition and low temperature condition. When it runs in low temperature condition in summer, cold air below zero needs to be blown in and ice shall be added in the mixing. At this time, condensate water will be produced in the plant and the humidity would be relatively high.
2. Dust
   A lot of cement, fly ash and appropriate admixtures are required in the production of concrete, and these powders bring dust during transportation and weighing. Even the aggregate would also produce dust in the transportation. Some of the dust will adhere to the load cell surface. Under the combined action of dust and water, the load cell will be corroded seriously, and this is why the load cells of the powder scale break more frequently than other scales.
3. Impact and vibration
   Sand and gravel will cause impact in the feeding process, and load cells should be able to withstand 5g acceleration. During the mixing process, continuous vibration will occur, thus leading to fatigue failure of load cell.
4. Artificial environment
   Artificial environment is one of the factors that must be considered in product reliability design. The concrete mixing plant is usually installed on the construction site. If many temporary workers who lack necessary skills are employed on the construction site, they may make misoperation in maintaining or cleaning the equipment, such as sputtering of high pressure water to the load cells, as a result, overload may occur. Therefore, some special designs shall be made for long-term reliable operation of load cells under such conditions.
   

The above requirements and operating conditions can be used on the basis of choosing load cells for concrete mixing plants.
III. Subdivision of Load cell Selection for Different Types of Engineering Mixing Plant
There are many differences between the concrete mixing plant of large and medium-sized water conservancy projects, urban commercial concrete mixing plant, the concrete mixing plants used in small-scale water conservancy projects and general highway construction on their sizes, run-on times, installation environments, so are the performance of concrete thereof produced. Therefore, actual requirements for load cell reliability and protection class are also different.
The concrete mixing plant with long run-on time and harsh installation environment and used to produce high-performance concrete shall choose load cells with higher reliability and protection class. Other concrete mixing plants may have slightly lower requirements. For the marine concrete mixing plant, salt-spray resistant shall also be considered.