Load Cell Troubleshooting Guide
Field Testing & Diagnostic Procedures for Load Cell Technicians
Load cells are used in all types of electronic scales from laboratory scales to truck scales. Each application presents unique challenges of engineering design to ensure that the load cell continues to perform to its original specifications under adverse conditions.
Field Testing Capability: There are four specific areas you can inspect in the field to determine if a load cell is functioning properly.
1) Visual Inspection
Material Construction Overview
Load cells are typically fabricated from three materials, each with specific advantages:
Aluminum
- Lightweight construction
- Good for moderate environments
- Cost-effective for many applications
Tool Steel
- High strength properties
- Good for demanding applications
- Excellent durability
Stainless Steel
- Corrosion resistant
- Ideal for harsh environments
- Food-grade applications
Physical Damage Assessment
Single-Ended Beam Inspection
- Visual Check: Look for obvious bending or deformation
- Flat Surface Test: Lay the cell on a flat surface on its back
- Level Check: Place on level surface to detect subtle bending
Diagnosis: If the cell rocks on a level surface, it has likely been overloaded or shock loaded.
S-Type Cell Inspection
- Check arm alignment – arms should be parallel
- Look for twisting or bending of the S-shape
- Inspect threaded connections for damage
- Verify mounting point integrity
Critical Inspection Points
Crack Detection
Critical Areas to Inspect:
- Strain gage area (most critical)
- Load application points
- Weld seams on sealed cells
Note: Cracking usually results from overload, not manufacturing defects.
Cable Entry Point
Inspection Checklist:
- Check for cuts or abrasions in cable jacket
- Verify strain relief boot condition
- Look for signs of moisture entry
- Inspect for damage from installation or handling
Moisture Damage Recovery
Recovery Procedure: Pack damaged cell in warm area at 125-150°F for several hours. However, you must eliminate the root cause of moisture intrusion.
2) Zero Balance Testing
Understanding Zero Balance
When manufactured, a load cell is adjusted to have minimal output under no-load conditions.
Test Procedures
- Place cell on flat surface without hardware attached
- Connect to appropriate readout device
- Check output with no load applied
- Compare reading to specification listed on calibration certificate
Typical Values
| Condition | Typical Output | Factory Specification |
|---|---|---|
| Good Cell | 0.003 mV/V | Up to 0.3 mV with 10V excitation |
| Acceptable Range | Within ±1% of specification | Varies by model |
| Failure Threshold | 10% or more deviation | Requires re-manufacturing |
Failure Criteria: If zero balance exceeds 10% of specification, the cell should be re-manufactured as performance within original specifications is questionable.
3) Bridge Resistance Testing
Standard Resistance Values
Load cells typically have nominal bridge resistance of:
- 350 ohms (most common)
- 480 ohms (some models)
- 700 ohms (heavy-duty models)
Measurement Procedure
Key Measurement Points:
- Input Resistance: Between red and black wires
- Output Resistance: Between green and white wires
- Individual Gage Resistance: Each strain gage separately
- Reference Comparison: Against calibration certificate values
Tolerance Evaluation
| Test | Acceptable Tolerance | Damage Indication |
|---|---|---|
| Overall Bridge | Within 1% of certificate | Outside 1% indicates damage |
| Individual Gages | Within 1-2 ohms of each other | 500+ ohms indicates open circuit |
| Input Variation | May be higher due to compensation | Span/temperature resistors affect readings |
Important: Input resistance may be higher than nominal due to compensation resistors for span and temperature correction.
4) Resistance to Ground Testing
Final Field Check
This test can identify issues not detected by previous checks, including poor connections and water contamination.
Connection Check
- Rap the cell gently with small wrench
- Observe readout for jumping or erratic behavior
- Erratic behavior usually indicates poor connection
Insulation Resistance Test
- Shield to Body: Check resistance between shield drain wire and cell body
- Shield to Conductors: Check resistance between drain wire and all conductors tied together
- Acceptance Criteria: Both readings should exceed 5000 megohms
- Failure Indication: Less than 5000 megohms suggests water contamination
CRITICAL SAFETY WARNING: Use only low-voltage test equipment (45-50 volts maximum). Higher voltages can destroy delicate strain gage grids.
Understanding Strain Gage Sensitivity
Think of strain gages as microscopic wire grids. While they can sense thousands of pounds, they are extremely vulnerable to electrical damage. High voltage testing can literally burn the grid pattern off the gage.
Factory Return Procedure
Documentation Process
Documentation Process
- Detailed problem description
- All field test measurements
- Service history and previous repairs
- Operating environment and usage details
Return Procedure
- RGA Number: Call factory for Return Goods Authorization
- Documentation: Include written description with returned cell
- Service Level: Specify urgency (standard or expedited)
- Communication: Inform service department if replacement already installed
Factory Evaluation Process
Inspection
Visual and mechanical examination of returned cell
Testing
Electrical and performance verification procedures
Diagnosis
Root cause analysis of failure mode
Reporting
Findings communicated to customer with recommendations
Summary Checklist
Complete Field Testing Sequence:
Visual inspection for physical damage
Cable and connection point examination
Zero balance measurement and evaluation
Bridge resistance testing (overall and individual gages)
Insulation resistance testing (ground fault check)
Documentation of all findings
Comparison with specifications and history
Professional Tip: Proper field testing can save time and money by accurately diagnosing problems before returning equipment to the factory.