Selection of Weight Modules
Main keywords for this artile are Selection of Weight Modules. Weight Module Capacity. Weight Module Instrument Range. Weight Modules Supports.
Selection of Weight Modules
Three important factors should be considered when selecting weight modules for an electronic weighing system. These factors are as follows:
Vessel Mounting – Tension or Compression
- Weight modules are available for use in compression or in tension. Either method can yield high accuracy weighing systems and there is little to suggest one over the other. In most cases, plant layout is the governing factor. However, the following general guidelines can be applied to determine whether the vessel should be weighted in tension or compression:
- Maximum weighing system accuracy and stability can be achieved when the vessel is mounted in compression on a rigid concrete foundation. This type of installation avoids all the usual sources of deflection, variations in weight module alignment, and vibration that act to compromise accuracy and stability. Therefore, when high accuracy is required (0.05 percent), this approach should be considered first.
- Vessels weighing up to 3000 pounds are good choices for the simplest weighing system, a single canister type weight module or S-Cell in tension. Note that lateral restraints will be added if required to keep the vessel from tipping, rocking, and rotating.
Weight Modules Supports
- The number of supports is determined by the geometry of the supported structure – tank, silo, and hopper. The following guidelines can be used to determine the number of supports:
- Upright cylindrical vessels should have 3 supports. However, vessels requiring greater stability should have at least 4 supports; a round vessel with 4 supports is 20 percent more stable against tipping than the same vessel with 3 supports.
- Horizontal cylindrical vessels usually have 2 saddles positioned symmetrically apart from both ends. Three or 4 supports are placed under the saddles, depending upon the stability and accuracy required.
- Rectangular vessels such as hoppers and bins generally use 4 supports to accommodate the vessel geometry, symmetry, and understructure framework.
- There are a number of ways in which weight modules can be arranged for satisfactory performance in a weighing system. The following guidelines will be used to determine the number of weight modules for a weighing system:
- Single or double weight modules will be used for general weighing where accuracies in the order of ½ percent of full scale can be tolerated.
- Three or more weight modules per vessel will be used to achieve higher accuracies (1/4 percent of full scale or better).
Weight Module Capacity
- The total weight of the vessel, plus the anticipated maximum weight of the vessel contents, and the number of supports determine the weight module capacity. The following steps can be used to quickly determine the weight module capacity:
Step 1. Determine the dead load (tare weight) of the empty vessel plus all attached piping, agitators, vibrators, insulation, and the heating fluids.
Step 2. Determine the maximum weight of the vessel contents, or live load (net weight).
Step 3. Add deal load and live load to obtain the gross weight.
Step 4. Calculate the weight module capacity required for each support point;
Weight Module Capacity = C Gross Vessel Weight
Number of Supports where C – Compensation factor (1.25 – 1.30)
Step 5. Compensation factor is an allowance used to compensate for low tare estimates, dynamic forces produced by external effects such as wind and impact forces causes an unequal load distribution on the weight modules as installed.
Step 6. The ultimate choice of weight module is finally determined by using the manufacturer’s load cell selection chart to find which cells have a capacity equal to or slightly higher than what was calculated in Step 4.
Step 7. It should be noted that in some instances, where the installation is free from vibration or shock loads, weight modules can be used up to 150 percent of rated capacity, which is called safe overload of the weight module. Safe overload is normally given on the load cell product datasheet.
Step 8. It is important to note that if the calculations determine that 25 K pound weight module should be used, then is should be used. It is not advisable to use a 50 K pound weight module when a 25 K pound is indicated, although it would theoretically work. The reason for this is that if the weight modules are over sized for the application, the zero temperature stability will be affected.
Weight Module Instrument Range
Another design consideration of electronic weighing systems is to determine weight indicator range compatibility with a multiple weight module under structure. If it is a special build or if it s a conversion of an existing mechanical scale, the micro volts per increment should be calculated. Calculate the micro volts per increment, then check with the micro volt chart for the weight indicator to make sure that the selected weight modules are sized to generate a signal of sufficient strength to allow accurate measurement. It is suggested that the selected weight Instrument supplier be contacted to find out the proper procedure to check this compatibility.