How to Choose the Right Spoke Type Load Cells for Your Tank Weighing System

Step 1: Determine the Total Weight Capacity Needed

This is the foundational calculation. You must account for the maximum possible weight the system will ever need to measure.

• Tare Weight (Empty Tank Weight): The weight of the empty tank, its structure, and any permanently attached piping, insulation, or platforms.
• Net Weight (Product Capacity): The maximum weight of the material the tank is designed to hold. Use the material's highest possible bulk density for conservative sizing.
• Dynamic Factors: Consider potential weight from snow, wind loading, internal pressure (for pressurized tanks), and process forces (like agitation).

Formula: Total Max Weight = Tare Weight + Net Weight (Max Product) + Safety Factor for Dynamic Loads

Step 2: Calculate the Load per Cell & Select Cell Capacity

Tanks are typically supported by 3 or 4 load cells. The load must be distributed evenly for accurate measurement.

1. Decide Number of Supports (N): 3-point support offers inherent stability; 4-point is more common but requires a summing junction box with balancing resistors to handle minor tank floor deflection and uneven loading.
2. Calculate Load per Cell: Max Load per Cell = Total Max Weight / N
3. Apply Safety Factor (SF): To ensure longevity and protect against overloads, never select a load cell with a rated capacity equal to the calculated max load. Industry standard is to apply a safety factor of 1.25 to 1.5.
   Selected Cell Capacity ≥ Max Load per Cell × Safety Factor (1.25-1.5)
4. Standard Capacity Selection: Choose the nearest standard rated capacity above your calculated value (e.g., 5t, 10t, 20t, 30t). Spoke type load cells excel in the 1-50 tonne range.

Step 3: Assess the Installation Environment

The environment dictates the required build material and protection rating.

• Material:
  • Alloy Steel: Standard choice for indoor, dry, non-corrosive environments. Most cost-effective.
  • Stainless Steel (e.g., 304 or 316L): Essential for outdoor installations, food/chemical/pharmaceutical applications, washdown areas, or any environment with moisture, vapor, or corrosive agents.
• Ingress Protection (IP) Rating:
  • IP65/IP66: Dust-tight and protected against strong water jets. Suitable for most industrial settings.
  • IP67/IP68: Submersible for limited periods/permanent immersion. Critical for tanks in flood-prone areas or with aggressive washdown procedures (e.g., food & beverage).
• Temperature Range: Ensure the load cell's compensated range covers your site's ambient and process temperature variations.

Step 4: Choose the Correct Mounting Configuration

Spoke type load cells are versatile but require proper mounting hardware.

• Compression Mounting (Most Common for Tanks): The tank sits on top of the load cell.
  • Requires a load button on top of the cell and a base plate underneath.
  • Ensure the tank support structure (legs, stools) is stiff and provides a flat, level mounting surface.
• Tension Mounting: For suspended or hanging tanks.
  • Requires clevis pins or threaded rods.
• Check Alignment: The load must be applied vertically and centrally to the cell. Use self-aligning load buttons or spherical bearings to compensate for minor structural shifts or thermal expansion.

Step 5: Consider System Accuracy & Signal Requirements

Define your precision needs to select the right performance class.

• Accuracy Class: Spoke type load cells typically conform to OIML R60 or NTEP standards.
  • C3 Class: Standard industrial accuracy (0.02% FS). Suitable for most inventory and batching control.
  • C4/C5 Class: High precision for critical batching, custody transfer, or pharmaceutical applications.
• Output Signal:
  • mV/V Analog Output: Standard output; requires a nearby weight indicator or transmitter.
  • Digital Output (RS485, CANbus, IO-Link): Offers superior noise immunity, longer transmission distances, and easier integration with PLCs and SCADA systems. Ideal for new installations.
• Cable Length & Entry: Specify sufficient cable length to reach the junction box. Choose top or side cable entry based on conduit routing and splash risk.

Step 6: Select Accessories for a Complete System

Load cells are only one part of a reliable weighing system.

• Junction/Summing Box: Mandatory for 4-cell systems. Balances signals from each cell and sums them into one output. Choose a sealed, high-quality model.
• Weight Indicator or Transmitter: Converts the summed mV/V signal into a readable weight value and provides outputs (4-20mA, Ethernet) for your control system.
• Overload Stops/Check Rods: Safety devices that prevent catastrophic damage to the load cells if the tank is overloaded or experiences extreme lateral forces (e.g., from earthquakes).
• Cable Protection: Use flexible conduits and proper strain relief to protect cables from damage, pests, and weather.

Common Mistakes to Avoid

• Undersizing: Selecting a cell capacity too close to the actual operating load, leading to early fatigue failure.
• Ignoring Side Forces: Not accounting for forces from connected pipes, thermal expansion, or agitators, which can cause binding and inaccuracies. Use flexible couplings on all piping connections.
• Poor Foundation: Installing on a flexible or uneven foundation that shifts, causing load redistribution and measurement errors.
• Mixing Old & New Cells: Never replace just one failed cell in an existing system. Always replace the complete set and recalibrate.
• Skipping Professional Calibration: On-site calibration by a certified technician is essential after installation and during periodic maintenance.