How to Read a QTC Resistance-vs-Force Graph
A QTC resistance-versus-force graph shows how a quantum tunnelling composite sensor’s electrical resistance responds to applied pressure or force. Usually resistance drops as the load increases—but the relationship is rarely a straight line. Learning to read the axes, the test setup and repeated measurements matters more than memorizing a single curve.
What do the axes actually mean?
The horizontal axis often shows force in newtons (N) or pressure in kilopascals (kPa). These are not interchangeable. Pressure is force divided by the contact area: the same force acting on a smaller patch generates greater pressure. The vertical axis shows electrical resistance in ohms (Ω), kilohms or megohms. Check for a logarithmic scale: equal spacing may represent factors of ten, not equal resistance increments.
Why does the resistance fall when QTC is squeezed?
QTC contains conductive particles in an elastic host. Compression can bring the particles closer and change the probability of electrons passing between them. The resulting increase in conductivity can be pronounced. The full response also depends on the device’s contacts and mechanical structure. For context, see our guide to quantum tunnelling composites.
A hypothetical curve: read it step by step
The following values are illustrative numbers, not measurements of a commercial device. They demonstrate how to read a falling, nonlinear resistance curve.
| Applied force | Example resistance | Interpretation |
|---|---|---|
| 0 N | Greater than 10 MΩ | Near-insulating |
| 1 N | 600 kΩ | First strong response |
| 2 N | 150 kΩ | Resistance falls rapidly |
| 5 N | 28 kΩ | Smaller changes per unit force |
| 10 N | 15 kΩ | Response starts to flatten |
Plot the points on log-resistance paper. A nearly horizontal section can indicate saturation or a limited usable range. The most useful portion of a graph depends on the loads the sensor is intended to measure.
What are hysteresis and drift?
Hysteresis occurs when loading and unloading produce different resistance values at the same applied force. Drift means readings change with time even though the nominal load is held constant. Temperature, prior compression, contact geometry and the measurement circuit also matter. A published experimental characterization of QTC pills examined pressure, elapsed time and temperature—exactly the variables a single promotional sensor curve can conceal.
How to calibrate a QTC sensor for your own project
- Use a fixture with a repeatable contact shape and area.
- Apply several known loads that span the intended use range.
- Record stable resistance readings and the time after each load is applied.
- Repeat while unloading, then repeat the entire cycle on multiple occasions.
- Plot measurements with error bars or ranges; fit a model only within your tested region.
- Validate against fresh measurements instead of assuming the initial curve is universal.
Common mistake: a voltage-divider output is not itself resistance. If you use a fixed resistor to measure the QTC element, derive the conversion from the exact circuit before comparing data.
Does a steeper curve mean a better sensor?
Not by itself. High sensitivity can be useful for light touch, but a sensor may still be difficult to calibrate reliably or unsuitable for larger forces. Compare accuracy, repeatability, usable range and drift for the actual application. Our companion QTC versus FSR guide explains the broader trade-offs.
Technical reading: Evaluation of Flexible Force Sensors (2017); QTC Characterisation and Modelling (2011).
