Hygrometer
The relationship is one of category
There is no rivalry here, despite the way the question is usually phrased. Hygrometer is the general term for any instrument measuring humidity. Psychrometer names one particular design within that category.
So the comparison people actually want is between the psychrometric method and the capacitive one, since capacitive sensors are what "hygrometer" means in practice today.
How a psychrometer works
Two thermometers, side by side. One is bare — the dry bulb, reading the air temperature. The other has a cotton wick over its bulb, kept wet — the wet bulb.
Air is drawn across both. Water evaporates from the wick, and evaporation takes latent heat with it, so the wet bulb cools. How far it cools depends entirely on how readily the surrounding air accepts more water:
- Dry air — vigorous evaporation, large cooling, big gap between the two readings.
- Saturated air — no net evaporation, no cooling, both thermometers read the same.
The gap is the wet-bulb depression, and humidity follows from it through the psychrometric relation:
Divide that by the saturation vapour pressure at the dry-bulb temperature and you have the relative humidity.
Why it resists drift
This is the genuinely interesting property, and it is why the instrument survived so long.
A capacitive sensor depends on a material — a polymer film whose response is characterised at the factory and stored as a calibration curve. Over time the film changes, the curve stops matching it, and the reading drifts by around a percent a year.
A psychrometer depends on thermodynamics. There is no sensing material to age. If the two thermometers are accurate and the wick is clean and wet, the relationship holds — this year, and in thirty years.
That is why psychrometers remain in calibration procedures and reference work long after they stopped being convenient.
Sling versus aspirated
| Sling | Aspirated (Assmann) | |
|---|---|---|
| Airflow | Whirled by hand on a handle | A small fan draws air past both bulbs |
| Consistency | Depends on the operator | Controlled and repeatable |
| Radiation shielding | Minimal | Polished shields around both bulbs |
| Where used | Field checks, HVAC work, teaching | Laboratory and reference measurements |
Both need the air genuinely moving — around 3 m/s. Insufficient airflow is the commonest way to get a wrong reading, because the wet bulb never reaches its equilibrium temperature.
What goes wrong
- A dirty wick. Contamination changes evaporation. Replace it periodically and use distilled water; minerals from tap water build up on the wick.
- A dry wick. If it has dried out, the wet bulb is simply a second dry bulb and the reading is meaningless.
- Not enough airflow, or not for long enough.
- Mismatched thermometers. The whole method rests on a difference between two readings, so a systematic offset between the instruments goes straight into the answer.
- Below freezing the wick ices and the relationship changes. There are procedures for it, and it is not a casual measurement.
Where the wet bulb goes next
The wet-bulb temperature is interesting in its own right, beyond being an intermediate step here. It is the lowest temperature achievable by evaporative cooling, which makes it central to heat stress and to how well swamp coolers work.
That territory — heat stress, WBGT, survivability limits — belongs to a different instrument and a different question than this site covers. What matters here is the psychrometer as a humidity instrument: the reference method, the one that does not drift, and the reason a century of observations before electronics can still be trusted.