Resistance output compared to nominal output

The understanding of the Load cell specifications is essential for determining the best sensor for your force measurement needs. It is necessary to evaluate accuracy and quality by gaining clarity on the differences between the resistance output and the nominal output.

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

The performance of load cells at interfaces depends on the use of Strain gauges Imagine a precisely machined metal element, often made of high-quality aluminum or stainless steel, that undergoes a tiny deformation when subjected to a force (load)—either stretching (tension) or compressing (compression). Tiny electrical conductors called strain gauges are bonded to this form factor. These strain gauges are designed to change their electrical resistance proportional to the deformation they experience.

Simplified version of how a load cell works

• A load occurs when a force is applied to the load cell that causes deformation of the internal flexure that acts as a sensor.
• The deformation of the strain gauges occurs when the strain gauges connected to the bending co-deform.
• The resistance change is the deformation that causes a change in the electrical resistance of the strain gauges. This deformation leads to an offset, or unbalanced condition, which causes a change in resistance. The result is a metric output that is directly proportional to the applied force.
• The Wheatstone bridge is a configuration of four strain gauges forming a Wheatstone bridge circuit. This configuration is highly sensitive to small changes in resistance and provides a stable and accurate output.
• The voltage output measures the change in resistance of the strain gauges, causing the Wheatstone bridge to become unbalanced and output a small differential voltage. This voltage is directly proportional to the applied load.

TIP: For a detailed overview of how a load cell works, see How do load cells work?

Output specifications and terms

To fully understand the meaning of resistance power and rated power, we should look at some basic load cell specifications.

• The excitation voltage (EV) is the input voltage, i.e. the stable DC voltage supplied to the Wheatstone bridge circuit of the load cell. Load cells at interfaces usually operate with a recommended excitation voltage, which is often in the range of 5 V DC to 10 V DC. A stable and accurate excitation voltage is critical for reliable measurements.
• The output signal (output voltage) is the differential voltage generated by the Wheatstone bridge circuit in response to an applied load. It is a very small millivolt (mV) signal.
• Full Scale Output (FSO) This is one of the most important specifications for a load cell. The full scale output is the nominal output signal (in mV/V) that the load cell produces 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 full rated load it will produce an output of 2 mV/V × 10 V = 20 mV. The full scale output value is a measure of the load cell's sensitivity. A higher mV/V value generally indicates a more sensitive load cell, which will produce a larger output signal for a given load. For more details, see Detailing of the sensor nominal output in millivolts per volt.
• The resistance output (bridge resistance) refers to the electrical resistance of the strain gauge bridge circuit. Load cells typically have two primary resistance specifications. The input resistance (bridge input resistance) is the resistance measured at the excitation terminals (input) of the load cell. The output resistance (bridge output resistance) is the resistance measured at the signal terminals (output) of the load cell. These resistances are usually specified in ohms (Ω). Typical values for interface load cells can be 350 Ω or 700 Ω.
• The zero offset is the load cell's output signal when no load is applied. Ideally, it should be zero mV, but due to manufacturing tolerances, there is always a slight offset. This is usually specified as a percentage of the rated output (e.g., ±1% of RO). Read Why is zeroing the load cell important for accuracy?

EXTRA! TECH TIP: It's important to understand that the resistance output is a static electrical property of the strain gauge bridge itself and does NOT directly represent the measured force. It is a characteristic of the load cell's internal wiring and strain gauge configuration. While essential for compatibility with data acquisition systems and amplifier instruments, it does not fluctuate with the applied load like the voltage output does.

The ratio of resistive power to rated power

Rated power is the dynamic electrical signal that directly represents the applied mechanical force. It's the information you use to calculate load. It's a measure of the load cell's sensitivity to force. Importantly, power ratings expressed as a percentage of "RO" or "rated power" always refer to the load cell's rated power (full scale output). This expresses power relative to the cell's maximum measuring capacity.

• Nonlinearity: ±0,02% RO
• Hysteresis: ±0,02% RO
• Repeatability: ±0,01% RO (i.e., the maximum difference between the output readings under repeated loads under identical load and environmental conditions is expressed as a percentage of the nominal output)
  Zero point adjustment: ±1,0% RO
• Temperature influence on the zero point: ±0,005% RO/∘F
• Safe overload: 150% RO

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

Resistance: The output resistance is a static electrical property of the load cell's internal circuitry. It describes the electrical impedance of the bridge. It is important for electrical compatibility, but 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 for accurate force measurement.

When performing force measurements, you're primarily interested in the rated power. This value, in conjunction with the excitation voltage, allows you to accurately convert the measured millivolt signal into engineering force units in lbf or newtons.

The output resistance of the system compatibility is critical to ensuring a proper electrical interface with your Data acquisition system or amplifierThe input impedance of your measuring device should be sufficiently high relative to the load cell's output resistance to minimize loading effects and ensure accurate signal transmission. Mismatched resistances can lead to signal degradation and inaccuracies.

If you suspect a problem 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, a change in output resistance with a load applied would indicate a serious, likely catastrophic, load cell failure.

Exit References

Understanding the difference between resistive power and rated power is essential for anyone working with load cells. While resistive power is a critical electrical specification for system compatibility and diagnostics, it is the rated power that truly quantifies the load cell's sensitivity to force, enabling precise and reliable measurement.

Check out our discussion on load cell specifications: