Where Johnson noise thermometry solves a different problem.
Johnson noise thermometry is not a more accurate version of a conventional thermometer. In many situations a good resistance thermometer or thermocouple will be simpler, faster and give lower short-term uncertainty.
The case for Johnson noise thermometry appears when the measurement problem itself is different. These four applications all contain a limitation that conventional thermometry cannot easily remove. Sometimes the sensor changes after calibration. Sometimes drift is discovered only after the process is complete. Sometimes the sensor cannot be retrieved at all. And in primary metrology the question is not sensor drift but how temperature is realised and traced.
Each application presents different requirements. Explore the limitations of conventional thermometry, what Johnson noise measurement can change, and the development work still needed for practical use.
- The sensor transmutes, and cannot be retrievedNuclearDecommissioning, waste storage and advanced reactors. A neutron flux rewrites a sensor’s calibration from the inside, and the places that matter most are the ones nobody can go back into.
- Drift is found at the next check, after the batchAerospace and high temperatureHeat treatment and high-value production qualified against periodic calibration. Includes the comparison that does not favour us: at 1,000 °C a thermocouple has less short-term scatter.
- The reading stays plausible while it goes wrongProcess and industrialContinuous processes where drift costs yield quietly over months, on plant floors electrically hostile enough to have stopped every previous noise thermometer.
- The question is what the number meansPrimary and scientific metrologyThermodynamic temperature after the redefinition of the kelvin, traceable to electrical standards instead of a chain of fixed points and comparisons.
What a noise measurement changes
A platinum resistance thermometer works because somebody established the relationship between its resistance and its temperature, once, by calibration. In three of these sectors what goes wrong is a version of that relationship quietly ceasing to hold while the instrument carries on reporting a plausible number. In primary metrology nothing is going wrong at all, and the question is how thermodynamic temperature is realised and traced in the first place.
We measure the sensor’s resistance from the same block of data as its noise, 6.5536 seconds of it in the published work, so the value used in the calculation is always the resistance the sensor has now. That leaves the sensor free to oxidise, foul, transmute or degrade without taking the temperature reading with it, and it puts the result on electrical standards instead of a chain of fixed points and comparisons. Everything on these pages follows from that.
If this is your measurement
The thermometer is a working second-generation prototype. Where it goes next will be shaped by real applications. If calibration drift is limiting a measurement you are responsible for, we would like to hear from you.