PE2-4.5 Applications: Sound, Temperature & Humidity Measurement

What this is and why it exists

Temperature is the most measured physical quantity in industry. There are four common ways to measure it and none of them is best.

That is the useful shape of this topic. Choosing correctly means starting from the range and accuracy you need, not from the device you happen to know.

The vocabulary

• Microphone — a transducer converting sound pressure into an electrical signal.
• Thermistor — a resistance that changes steeply and non-linearly with temperature.
• Resistance thermometer — a metal element whose resistance rises nearly linearly with temperature.
• Semiconductor thermometer — a temperature sensor built into a chip.
• Thermocouple — two dissimilar metals joined, producing a small voltage that depends on temperature.
• Cold junction — the second junction, whose temperature must be known.
• Cold junction compensation — measuring or fixing that second junction's temperature.
• Hygrometer — an instrument measuring humidity.

The mental model

Microphones open the topic as the acoustic case of the transducer principles from earlier. Sound pressure moves a diaphragm, and the movement is read. How it is read defines the type. One design reads it as a capacitance change and needs a supply. Another moves a coil in a magnetic field, generating its own signal, which makes it the active case. A third uses a piezoelectric element, and it inherits that family's properties. Same problem, three principles you have already met.

Then temperature, and the four devices are best held as a set of trades.

A thermistor is a semiconductor whose resistance changes steeply with temperature. That steepness is its sensitivity, and it is large. The same steepness makes it very non-linear, so the reading needs correction, and its useful range is modest. It is cheap and it is the right answer when you want fine resolution over a narrow range.

A resistance thermometer uses a metal element, usually platinum, whose resistance rises nearly in proportion to temperature. It is far more linear and far more stable, and it is much less sensitive. It costs more. It is the right answer when accuracy matters over a wide range.

A semiconductor thermometer is a circuit rather than a component, giving an output that is close to linear over a modest range. It is cheap and often already inside the microcontroller you are using. That makes it the right answer whenever the range it covers is enough.

A thermocouple works on a different principle again. Join two dissimilar metals and a small voltage appears that depends on the temperature of the junction. It spans enormous ranges, well beyond what the others survive, and its output is small and non-linear.

It also carries a problem that catches everyone building their first one. There is not one junction but two. Wherever the thermocouple wires meet the instrument's terminals, a second junction forms, and the voltage you measure depends on the difference between the two temperatures. So the reading is meaningless unless the second junction's temperature is known.

The answer is cold junction compensation. Measure the temperature at the terminals with something else, usually a semiconductor sensor, and correct for it. That is why a thermocouple instrument contains a second, different kind of temperature sensor inside it.

Choosing between the four is the item that makes the rest worth reading. Start with the required range, because that alone rules out most of the options. Then accuracy and linearity. Then cost and whether one is already available in the hardware you have.

Humidity closes the subject and deserves a note of caution. It is harder to measure than temperature, and the calibration drifts with time and exposure. A sensor left somewhere dirty or condensing will read confidently and wrongly. Knowing that before trusting one is the practical value of the item.

What you should now be able to explain or do

Relate the microphone types to the transducer principles they use. Compare thermistor, resistance thermometer, semiconductor sensor and thermocouple on range, linearity, sensitivity and cost. Choose one for a stated requirement, starting from range. Explain why a thermocouple has a second junction and what compensation does about it. Say why a humidity reading should be treated with caution.

Check yourself