Humidity Sensor Placement

Detailed Explanation

Humidity sensor placement is the act of positioning the sensor where its RH/Tdb reading represents the real process condition. Three typical placements: (1) Duct sensor — mounted in the AHU supply or return, reads bulk flow conditions; (2) Room sensor — wall- or height-mounted, reads average room conditions; (3) Process-point sensor — close to a specific piece of process equipment (printing roll, product conveyor, museum vitrine).

Each placement implies a different control strategy: a duct sensor targets AHU global performance, a room sensor targets occupant comfort, a process sensor targets product quality. Wrong placement → wrong control → either wasted energy or lost quality.

Why It Matters

Bad sensor placement causes five typical problems: (1) Sensor too close to a steam manifold (inside the absorption distance) → wet droplets on the sensor, RH reads 95–100% continuously, the control loop assumes "enough moisture" while the room is still dry. (2) Sensor near a heat source (radiator, window, equipment) → reads high Tdb and low RH, the loop overhumidifies → wasted energy. (3) Near a door/window → outdoor leakage perturbs the sensor, control oscillates. (4) Single-point measurement in a large space → local condition is far from the average; at least 3–4 points are recommended. (5) Sensor in high-velocity flow (> 5 m/s) → turbulence error.

Correct placement: beyond the absorption distance, away from heat sources, in a region of laminar flow, at the centre of the room or at the process-critical point. For sensitive processes (pharma, museum, printing), two redundant sensors plus a median filter are recommended.

Practical Example

A spinning mill in Bursa complained of constant RH oscillation; the sensor sat in the AHU return duct at 0.8 m, but the return duct ran near a door. Site investigation showed the sensor swung 5–8% RH every time the door opened. Solution: the sensor was relocated 4 m further along into a straight duct section, and a second room-centre sensor was added (median control). RH oscillation dropped from ±6% to ±1%. Modulation time fell 22%, with ≈ 14% annual electricity savings.

Engineering Note

Sensor placement standards: ASHRAE 41.6 (laboratory humidity measurement), EN 16798-3 (HVAC sensor positioning), ISO 7726 (thermal environment). Industry practice: a duct sensor is placed at least 5× the duct diameter past a manifold, or at least 2 m; a room sensor is at 1.5–1.8 m height, 30 cm from any wall, and on an interior wall (not an exterior one); a process sensor is in the critical airstream and at least 1 m from any active heat source.

For sensitive applications (±1–2% RH band), two standard technologies: (a) capacitive RH sensors (industrial-class RH/T sensor, room-type RH/T sensor) — fast response (10–30 s), ±2% RH accuracy, 5–7 years drift resistance; (b) optical chilled-mirror dew-point meters (reference-grade, ±0.1°C dp) — used as calibration references. Calibration check every 6 months; full calibration every 12–24 months.

NKT Application Link

During commissioning NKT prepares a sensor-placement diagram for every facility: duct-sensor distance from the manifold, room-sensor location + height, and an additional sensor at the critical process point. Standard instruments include industrial-class RH/T sensor (duct/room), room-type RH/T sensor (high-precision duct), and the Neptronic on-board sensor (unit feedback). The NKT - Climate Track platform aggregates multi-sensor data with median/mean/max/min trend logs that simplify fault detection. A six-monthly on-site calibration check is a standard step in the periodic-maintenance contract.