Calibration of Cement, Asphalt and Recycled Asphalt Planing (RAP) Weighing Equipment

With many years of experience in the calibration of cement, asphalt and RAP weighing equipment, Precia-Molen offer their customers the widest choice of calibration methods to suit individual site requirements.

Their preferred, well proven method of calibration for these sites is to use Force Calibration. This allows a traceable calibration to be performed quickly, non-invasively, and often can be completed without even taking the hopper or silo out of service, thus reducing disruption and downtime to a minimum.

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An approved process:
The process approved by QSRMC and covered by the Precia-Molen ISO9001 quality system is simple. Small hydraulic jacks located onto specially mounted reference load cells are used to apply a load to the hopper or silo. By comparing the weight read-out of the reference load cells with the system being calibrated, any errors can be determined.

Three or four reference load cells (pre-calibrated, certified and traceable to national standards) and their associated hydraulic jacks are positioned between a solid structure and the silo as close to the silos load cells as possible to give the optimum result. The jacks are then operated and the readings given by the reference load cells and the silo load cells are then compared at fixed points throughout the weighing range.

Force Calibration can be adapted to be either ‘push up’ or ‘push down’ to suit the silo and the site operations. Often this means that silos do not need to be emptied to carry out the calibration testing which is a major advantage as the product does not have to be stored or the plant stopped.

Using multiple reference load cells positioned as near as possible to the silo’s own load cells allows the calibration engineer to check the performance of individual load cells enabling the identification of any potential failure or malfunction. The Force Calibration equipment is both portable and flexible allowing a wide range of silo capacities to be calibrated by carefully selecting the reference load cell capacities. Precia-Molen has test rigs which can be used for silo capacities of 2,000kg, 10,000kg and up to 160,000kg.

Improved accuracy and stock control:
At the site of a major aggregate producer in Derbyshire, Precia-Molen calibrates the cement train loading system using this method. Utilising the Force Calibration system means the silos can be tested to their maximum capacity more accurately and in a fraction of the time taken by any other calibration technique.

The cement train loading system was installed by Precia-Molen some years ago and each silo has a maximum capacity of 80,000kg and a batch size of 60,000kg. Prior to using Force Calibration the silos had to be calibrated using the “Load Substitution” method using a combination of Calibrated Test Weights suspended by chains attached to the sides of the silo and material used as “Ballast Weight”. This method was both very time consuming and costly.

When asked to undertake a full calibration using Force Calibration Precia-Molen produced a special high capacity system utilising 4 x 40,000kg load cells and 4 x 30 tonne hydraulic jacks linked to an I200 battery powered weight indicator. Calibration tests were successful with repeatable errors of less than 20kg recorded throughout the weighing range.

Clearly the customer was very pleased with the improved speed and accuracy the Force Calibration System enabled them to achieve together with improved stock control, and subsequently entered into a service agreement which includes regular calibrations to ensure that performance is maintained.

Alternative methods and the challenges:
Although perfectly suitable for platform scale calibration, the use of calibrated test weights is often a challenge on cement and asphalt plants. With the silos or hoppers to be calibrated usually located many metres above ground the simple logistics of getting sufficient weights up to the silo can create health and safety issues in addition to the difficulty and time required to physically carry sufficient weights to the hopper location. Even then there is often insufficient space or provision to locate the test weights onto the silo in a safe manner. This method is very labour intensive and consequently time consuming. For larger capacity silos a combination of test weights and product (load substitution) can be used, which obviously extends the time required to complete the testing.

For chemical plants it has long been the practice to calibrate vessels using calibrated flow meters. This involves metering the amount of water which is loaded into a vessel and using a calculation based on specific gravity and water temperature to determine the weight of water loaded into the vessel. However due to its invasive nature this method is less widely used due to the high cost of vessel cleaning and contaminated water disposal. This method is not suitable for cement and asphalt plants.

A further method often employed is calibration by millivolt injection, however this method is applicable mainly where the weighing system is used for inventory control and where the large size of the system makes other techniques impracticable. The shortcoming of this technique is that it does not calibrate the complete measuring chain but only the weight display, resulting in a failed or drifting load cell remaining undetected. A variation of this method utilises small weights to partially calibrate the load cells over a very small part of their weighing range with extrapolation to the full span; but the technique is not likely to be acceptable as a part of an ISO 9000 accredited system.

Regardless of the size of the application Precia-Molen can provide a cost effective solution to all your calibration needs.