wheatstone bridge in strain gauge

In a two‑wire hookup, the lead resistance appears in series with the gauge arm, shifting the zero and reducing the effective gauge factor as temperature changes the wire resistance. With three wires, you bring two wires to one gauge end and one to the other. The bridge sees two equal lengths in adjacent arms, so the lead resistance cancels out—assuming the wire gauge and length are matched. It’s a quick upgrade that costs you one extra conductor, and it works best when all three wires are run together in the same cable bundle.

The choice mainly affects self‑heating and cable compatibility. A 120Ω gauge passes more current for a given excitation voltage, so it can locally heat the surface by a few degrees on low‑conductivity materials like plastic or wood—enough to skew readings if you don’t account for it. 350Ω gauges draw less current and tend to stay cooler. The other trade‑off: 120Ω circuits are slightly less sensitive to leakage resistance in wet environments, but 350Ω gauges are less affected by long cable resistance in large structures. Most of our long‑term monitoring installations default to 350Ω unless the client’s logger is optimized for 120Ω inputs.

The hardware method is to use a dummy gauge mounted on an unstrained piece of the same material, placed in an adjacent bridge arm. When temperature changes, both the active and dummy gauges shift similarly, so the bridge output cancels the thermal apparent strain. For full‑bridge sensors, you can arrange the four gauges so that two sense tensile strain and two sense compressive strain from the same temperature field—that gives both temperature compensation and roughly double the signal. Kingmach’s embedment gauges are built this way, so the output you get already has temperature effects largely canceled. If you’re using a quarter‑bridge with no dummy, you’ll need a reliable temperature curve for the gauge on that substrate, and even then, field results drift during rapid temperature shifts.

First, check the bridge completion wiring. A common mistake is misidentifying the sense and excitation leads on full‑bridge sensors, especially when using modular connector systems. Measure the resistance at the connector pins with a multimeter: between excitation lines you should see around the nominal bridge resistance (e.g., 120Ω or 350Ω), and between sense lines roughly the same. If one arm reads open, check for a broken gauge or solder joint. Also verify insulation resistance to ground—moisture ingress can create a parallel path that corrupts the millivolt output. If the readings jump with weather changes, look for galvanic corrosion at the terminals or insufficient cable shielding. We provide a step‑by‑step resistance check diagram with our sensors; it catches most field wiring errors before they become data headaches.

Some loggers only read full‑bridge sensors, expecting the completion to be done externally. For quarter‑bridge or half‑bridge setups, you need an external completion module with precision resistors. The module should match the gauge resistance and have low‑drift resistors. If you’re retrofitting an older logger, look for a completion box with a balance potentiometer and a shunt calibration switch so you can verify the output during installation. Kingmach can supply completion modules tailored to common gauge resistances and excitation voltages, or build the whole bridge into the sensor itself for a plug‑and‑play full‑bridge output.