Resistance Output Versus Rated Output

Understanding load cell specifications is an essential threshold in determining the best sensor for your force measurement requirements. It is necessary to evaluate the accuracy and quality by having clarity in the differences between the resistance output and the rated output.

The first step is to explore how a load cell works and the interplay between these terms. In this article, we aim to provide a better definition to guide you through the optimal application and performance.

Interface load cell performance relies on the use of strain gages. Imagine a precisely machined metal element, often made of high-grade aluminum or stainless steel, that, when a force (load) is applied to it, undergoes a minute deformation – it either stretches (tension) or compresses (compression). Bonded to this form factor are tiny electrical conductors called strain gages. These gages are designed to change their electrical resistance proportionally to the deformation they experience.

Simplified Version of How a Load Cell Works

1. Application of load occurs when a force is applied to the load cell, causing the internal flexure, which serves as the sensing element, to deform.
2. Strain gage deformation is when the gages bonded to the flexure deform along with it.
3. Resistance change is the deformation that causes a change in the electrical resistance of the strain gages. Deflection causes an offset or unbalanced state, which creates a change in resistance. The result is a ratio metric output directly proportional to the force applied.
4. The Wheatstone Bridge is a four-strain gage configuration that forms a Wheatstone bridge circuit. This configuration is highly sensitive to small changes in resistance, providing a stable and accurate output.
5. The voltage output measures the resistance change of the strain gages, causing the Wheatstone bridge to become unbalanced and produce a small differential voltage output. This voltage is directly proportional to the applied load.

TIP: For a more detailed review of how a load cell works, read How Do Load Cells Work?.

Output Specifications and Terms

To fully grasp the significance of resistance output and rated output, let’s dive into some fundamental load cell specifications.

• Excitation Voltage (EV) is the input voltage, which is the stable DC voltage supplied to the Wheatstone bridge circuit of the load cell. Interface load cells typically operate with a recommended excitation voltage, often in the range of 5 V DC to 10 V DC. A stable and accurate excitation voltage is critical for reliable measurements.
• Output Signal (Output Voltage) is the differential voltage produced by the Wheatstone bridge circuit in response to an applied load. It’s a very small millivolt (mV) signal.
• Rated Output (Full Scale Output – FSO) This is one of the most critical specifications for a load cell. Rated Output is the nominal output signal (in mV/V) that the load cell will produce when the maximum rated load is applied, with the specified excitation voltage expressed in millivolts per volt of excitation (mV/V). For example, if a load cell has a Rated Output of 2 mV/V and an excitation voltage of 10 V, then at its full rated load, it will produce an output of 2 mV/V × 10 V = 20 mV. The Rated Output is a measure of the load cell’s sensitivity. A higher mV/V rating generally indicates a more sensitive load cell, producing a larger output signal for a given load. For more details, refer to the Detailing Sensor Rated Output Millivolts per Volt.
• Resistance Output (Bridge Resistance) refers to the electrical resistance of the strain gage bridge circuit. Load cells typically have two primary resistance specifications. Input Resistance (Bridge Input Resistance) is the resistance measured across the excitation (input) terminals of the load cell. Output Resistance (Bridge Output Resistance) is the resistance measured across the signal (output) terminals of the load cell. These resistances are usually specified in ohms (Ω). Typical values for Interface load cells might be 350 Ω or 700 Ω.
• Zero Balance (Zero Offset) is the output signal from the load cell when no load is applied. Ideally, it should be zero mV, but due to manufacturing tolerances, there’s always a slight offset. This is typically expressed as a percentage of Rated Output (e.g., ±1% of RO). Read Why Is Load Cell Zero Balance Important to Accuracy?.

EXTRA! TECHNICAL TIP: It is crucial to understand that Resistance Output is a static electrical property of the strain gage bridge itself and does NOT directly represent the force being measured. It’s a characteristic of the load cell’s internal wiring and strain gage configuration. While it’s essential for ensuring compatibility with data acquisition systems and amplifier instrumentation, it doesn’t fluctuate with the applied load in the same way the voltage output does.

The Relationship of Resistance Output and Rated Output

Rated Output is the dynamic electrical signal that directly represents the applied mechanical force. It’s the information you use to calculate the load. It’s a measure of the load cell’s sensitivity to force. Crucially, when you see performance specifications expressed as a percentage of “RO” or “Rated Output,” this percentage always refers to the Rated Output (Full Scale Output) of the load cell. This indicates the performance relative to the cell’s maximum measuring capacity.

For example, when you see specifications like:

• Non-linearity: ±0.02% RO
• Hysteresis: ±0.02% RO
• Repeatability: ±0.01% RO (meaning the maximum difference between output readings for repeated loadings under identical loading and environmental conditions is expressed as a percentage of the Rated Output)
• Zero Balance: ±1.0% RO
• Temperature effect on zero: ±0.005% RO/∘F
• Safe Overload: 150% RO

Note that all these percentages quantify the load cell’s performance relative to its full measurement range.

Resistance Output is a static electrical characteristic of the load cell’s internal circuitry. It describes the electrical impedance of the bridge. It’s vital for electrical compatibility, but it remains constant regardless of the applied load and is not a measure of the force itself.

Why is this distinction important for metrology? It is fundamental to accurate load measurement.

When you are performing force measurements, you are primarily interested in the Rated Output. This value, in conjunction with your excitation voltage, enables you to accurately convert the measured millivolt signal into engineering units of force in lbf or Newtons.

System compatibility resistance output is crucial for ensuring proper electrical interfacing with your data acquisition system or amplifier. The input impedance of your measurement device should be sufficiently high relative to the load cell’s output resistance to minimize loading effects and ensure accurate signal transfer. Mismatched resistances can lead to signal degradation and inaccuracies.

If you suspect an issue with your load cell, checking its input and output resistance can be a valuable diagnostic step. Significant deviations from the specified resistance values could indicate internal damage or wiring issues. However, changing resistance output with applied load would indicate a severe, likely catastrophic, failure of the load cell.

Output References

Understanding the difference between resistance output and rated output is fundamental for anyone working with load cells. While resistance output is a critical electrical specification for system compatibility and diagnostics, it is the rated output that truly quantifies the load cell’s sensitivity to force, allowing for precise and reliable measurement.

Watch our discussion on load cell specifications:

At Interface, we are committed to delivering highly accurate and reliable load cells and measurement tools. Quality remains our top priority, as ensured by our meticulous engineering and detailed specifications. For further definitions, please refer to our online technical glossary.

By grasping these core concepts, our customers can confidently select, install, and utilize our products to achieve the highest levels of metrological precision in their applications. If you have any further questions or require assistance with your specific application, our expert technical support team is always ready to assist.

Be sure to subscribe to our new InterfaceIQ Podcast for more discussions about force measurement and bookmark ForceEDU for future reference. It’s the ultimate resource for load cell users.