120-ohm strain gauge

Why Were the First Strain Gages 120 Ohms? A Look Back and Forward

Here at Integrated Test & Measurement, we love diving into the nuts and bolts of how things work. While setting up a project using 120-ohm strain gages with the NI-9237 module, I found myself reflecting on the history and technical reasons why 120 ohms became the original standard for strain gages. As it turns out, this value has stood the test of time for good reasons, but newer applications are showing the advantages of moving to higher resistance gages. Let’s dig into both the history and the technical details.

The Origins of the 120-Ohm Strain Gage

The 120-ohm strain gage emerged as a standard because it hit a practical balance between material properties, manufacturing constraints, and compatibility with early measurement systems. Here’s why 120 ohms made sense:

• Material and Sensitivity: Early gages used constantan, a copper-nickel alloy. A 120-ohm resistance allowed for a practical wire size that was sensitive enough to detect strain while being durable and easy to manufacture.
• Wheatstone Bridge Circuitry: Most strain gages operate within a Wheatstone bridge circuit. At 120 ohms, the circuit provided a good signal-to-noise ratio without excessive power consumption.
• Heat Dissipation: Lower resistance values increase current draw and heat generation. At 120 ohms, the heat remained low enough not to affect the accuracy of strain readings.

With these advantages, the 120-ohm standard became widely adopted and remains in use today.

The Move to Higher Strain Resistances

While 120-ohm gages are still reliable workhorses, modern applications often use higher resistance gages, such as 350 or even 1000 ohms. Why make the switch?

• Reduced Heating: Higher resistance gages draw less current, which means less heat generation in the gage grid. This is crucial for maintaining accurate measurements, especially in high-precision applications.
• Improved Power Efficiency: In a Wheatstone bridge, less current means lower power demands. This allows more gages to be connected to a system like the NI-9237 without exceeding power limits.
• Higher Signal Quality: Higher resistance gages can operate with higher excitation voltages, resulting in stronger output signals and better data resolution.
• IOT Applications: High impedance strain gages should be considered in IoT applications where power is limited.

For instance, a 350-ohm gage powered by external excitation can enable a system to handle larger setups without overheating or maxing out power supplies.