To achieve optimum performance of an electronic weighing system, accurate calibration of the system and correct selection of the load sensors, mount- ing hardware, and other installation assemblies are essential. During installation, the accuracy of the system may be affected by external factors such as restraints caused by the addition of process piping, surrounding equipment such as motors and valves, or any deformation of the supporting structure. Field calibration at the completion of installation, followed by verification of the calibration at peri- odic intervals thereafter, is recom- mended. There are many accept- able ways to calibrate a scale, all of which depend on the degree of accuracy required.
Using one calibration technique rather than another will depend on the accuracy you want to achieve, and the time you want to spend on the process. This article describes all calibration techniques in detail, so you can decide which method is best for you:
- Hardware calibration using certified weights.
- Hardware calibration using material substitution.
- Hardware calibration using test weights with material build.
- Alternate calibration techniques.
Hardware Calibration Using Certified Weights— the Most Accurate
This method uses known calibrated weights to apply a load onto the load-receiving elements. Because the traceability of the standard is known and maintained, it is the most accurate calibration tech- nique. Accuracies within 0.05% of the applied load can be achieved with this method. However, this procedure may be restrictive for some industrial scales, as large-capacity weights are not easily trans- ferred to the site or often unavailable in the field. In many other cases, the load-receiving element may not be able to accept placement of the weights, or is in a poorly or inaccessible location, such as a vessel hanging high above the factory floor.
To use the certified weights method of calibration, a set of Class F accuracy weights (within 0.01% of denom- ination) that meet the following regulations are required:
- A total weight of 80 to 100% of the system capacity.
- At least three weights between 10 and 100% of the system capacity to check the midrange.
- Several low-capacity weights, equivalent to one or two instrument divisions.
Most local Weights and Measures organizations have such weights available. Typical weight values are 10, 25, 50, 100, 500, and 1000 lb. If the weigh- bridge is a device other than a platform, some method of loading the weights on the structure
must be devised.