Definition

An electronic instrument that measures relative humidity (RH), absolute humidity or dew point in air; the feedback input to a humidifier control loop. The most common types: capacitive RH sensors (industrial humidity sensor, room-type RH/T sensor) and chilled-mirror dew-point hygrometers (reference grade).

Detailed Explanation

Humidity sensors fall into three technology families: (1) CAPACITIVE RH — a polymer-dielectric capacitor whose capacitance shifts with RH; accuracy ±1.5–3% RH, response 10–30 s, drift over 5–7 years, price €200–800; the industry standard (industrial-class RH/T sensor, room-type RH/T sensor). (2) RESISTIVE (DEW-POINT) — hygroscopic salts like lithium chloride; low cost but quick drift. (3) CHILLED-MIRROR HYGROMETER — reference grade, ±0.1°C dp, €8,000–25,000; used only for calibration and very tight processes. 95%+ of field installations use a capacitive RH sensor.

Why It Matters

The humidity sensor is the measurement input of the PID loop — a wrong sensor or wrong placement defeats even a perfect device design. The three most common field mistakes: (1) Wrong position — too close to a steam manifold (inside the absorption distance); the sensor reads un-mixed steam and the system oscillates. (2) Calibration drift — without annual calibration, 5–10% RH error accumulates in 2–3 years. (3) Single-point measurement — controlling average RH in a large hall with one sensor; local variation can reach 5–15%. Correct sensor + correct placement + annual calibration = stable ±1–3% RH control.

Practical Example

A pharma plant in Istanbul reported up to 8% RH deviation in a cleanroom (setpoint 50%, swing 43–58%). NKT analysis found the sensor at 1.8 m from the steam manifold; the absorption-distance calculation returned 2.5 m. The sensor was moved to 3.2 m and a redundant second sensor was added (median control). Afterwards RH stabilised within a 50–52% band (±1%).

Engineering Note

Sensor-selection criteria: (1) Accuracy requirement — comfort ±5%, industrial ±2%, critical process ±1%; (2) Environment — temperature range, chemical exposure, IP rating; (3) Output signal — 4–20 mA as standard, 0–10 V optional, digital for Modbus/BACnet; (4) Calibration protocol — factory certificate + on-site calibration using saturated salt references (33%, 75%, 97% RH). Multi-sensor architecture: 2-3 sensors + median + calibration log for high precision. Annual calibration is mandatory; pharma and museums on a 6-month cycle.

NKT Application Link

For this application NKT – Nem Kontrol Teknolojileri supplies the Neptronic SKE4 resistive steam humidifier, the Neptronic SKH high-pressure atomisation system and the Neptronic SKD direct steam-injection humidifier. Unit-selection support is based on space conditions, humidification load and the feed-water analysis; technical service and spare parts are provided. Design and installation of water treatment, as well as engineering design, installation and commissioning, remain within the scope of the facility's designer, contractor and water-treatment specialist.