Duct Humidity Measurement: Best Practices for Transmitters in High-Velocity AHU Systems
Proper transmitter placement, probe insertion depth, and protection against condensation are critical factors in achieving accurate relative humidity readings in high-volume air handlers.
In industrial air handling systems, maintaining accurate relative humidity (RH) is essential for product quality, energy efficiency, and regulatory compliance. However, high air velocities, turbulence, and rapid temperature fluctuations present significant measurement challenges.
1. Sensor Probe Location Matters
Never install duct transmitters immediately downstream of cooling coils or steam humidifiers. Allow adequate mixing distance—typically 3 to 5 duct diameters downstream—to ensure that the air sample is homogeneous and free of unevaporated moisture droplets.
2. Avoiding Thermal Bridging
Ensure the transmitter mounting flange is thermally decoupled from duct insulation. Extreme duct skin temperature differentials can conduct heat to the sensor head, causing localized temperature shifts that skew relative humidity calculations.
3. Regular Verification with Handheld Reference Probes
Utilize reference meters like the Vaisala HM70 or Indigo80 for periodic 1-point field audits. This allows maintenance engineers to catch sensor drift early without removing the duct transmitter from service.
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