Nota técnica

Humidity Fundamentals, Part 2: Practical Measurement

How the concepts from part 1 show up in practice: typical symptoms of a misread measurement and how to fix them by choosing the right variable.

Part 1 of this series covered the concepts —relative humidity, dew point, the effect of temperature and pressure—. This part shows how those effects appear in a real measurement and how to read their symptoms.

Most “strange” humidity readings aren't sensor faults: they're the expected physical behavior interpreted with the wrong parameter. Recognizing the pattern is what lets you decide what to measure and how to control the process.

This note covers

How humidity behaves when temperature and pressure change
The three most common measurement problems and their real cause
Two practical scenarios solved with dew point
What all of this means for choosing the instrument

How Humidity Behaves in Real Conditions

When temperature rises, saturation pressure increases rapidly: warm air can hold much more water vapor. That's why relative humidity falls even when no water has been removed —behavior that can mislead a control system reasoning only in terms of RH.

Saturation occurs when the partial pressure of water vapor equals the saturation pressure at a given temperature: that's the dew point. Below freezing it's called frost point. And per Dalton's law, compressing air raises the partial pressure of the vapor in proportion to the total pressure.

Three Common Measurement Problems

Sudden RH spikes

Usually caused by small temperature shifts rather than humidity changes —for example, opening a door to a cooler area—. The air's water didn't change; the reference temperature did.
Compressed air condensation

Appears when the partial pressure of the vapor rises during compression and the air is then cooled in downstream equipment, reaching the dew point.
Pressure-dependent measurements

Dew point measured at system pressure differs from that measured at atmospheric pressure: pressure must be taken into account when comparing values.

Two Practical Scenarios

Scenario	What happens	How to solve it
Heating air from 20°C to 30°C	Relative humidity falls even though the water content doesn't change.	Control by dew point instead of RH: it avoids unnecessary humidification, saves energy and prevents over-conditioning.
Compressing air from 1 bar to 7 bar	Dew point rises without the water content changing.	Measuring dew point at system pressure lets you predict where and when condensation will form.

What This Means for Choosing the Instrument

The practical conclusion is that the parameter to measure depends on the environment: low-humidity environments and processes like lithium battery manufacturing call for dew point sensors, while HVAC and building applications are solved with relative humidity and temperature transmitters.

In demanding industrial environments it pays to prioritize robust probes and transmitters with low drift, high accuracy and proven long-term stability, with long calibration intervals. How to land that choice product by product is developed in How to Choose the Right Humidity Instrument.

This technical content is based on Vaisala's educational material on practical humidity measurement. AKRIBIS is an authorized Vaisala partner for distribution and technical support in the region — explore the full Vaisala instrumentation line.

Instruments to Measure Humidity