Two airflow sensing methods for fume hoods: displacement control vs. face velocity control, how to choose?

Variable airflow fume hoods require stable face velocity, with the prerequisite being that the controller can promptly determine two key pieces of information: the size of the sash opening and the current airflow at the access port. Sensing these two aspects corresponds to two types of sensors, with displacement sensors measuring the sash opening and face velocity sensors directly measuring the airflow at the access port. Both methods can achieve variable airflow, differing primarily in measurement paths, anti-interference capabilities, and commissioning/maintenance requirements. Prior to selection, it is essential to understand how each method operates.

How Displacement Control Works

Displacement sensors are installed on sash guides or hoisting mechanisms, outputting opening signals as the sash rises or falls. The controller calculates the required exhaust airflow based on the sash height and the preset face velocity, then drives the damper to the corresponding angle. It measures mechanical position without direct contact with corrosive gases, resulting in a longer sensor lifespan, minimal disturbance from internal airflow fluctuations, and relatively lower cost.

The premise of this method is that the relationship between sash opening and exhaust airflow is accurately calibrated during commissioning. If items inside the fume hood obstruct the access port or equipment modifications alter the airflow path, the airflow calculated based on the opening may deviate from the actual face velocity, requiring recalibration.

How Face Velocity Control Works

Face velocity sensors typically employ thermal or differential pressure measurement elements, installed at the access port or side wall sampling points to directly measure the current airflow entering the hood. The controller compares the measured value with the setpoint and adjusts the damper, forming a direct closed-loop system. It does not rely on sash opening estimation, and changes in internal items, filter buildup, or duct resistance variations are all reflected in the measurements and corrected accordingly.

The trade-off for direct measurement is that the sensing elements are continuously exposed to the exhaust airflow, facing corrosive gases and dust contamination. Regular cleaning and calibration are required, and the location of the sampling point can affect the representativeness of readings, demanding higher design and installation requirements.

How to Choose Between the Two Methods

Fume hoods used in stable conditions, with minimal internal items, primarily for conventional chemical operations, and seeking cost-effective and low-maintenance solutions can be satisfied with displacement control. Applications involving diverse materials, frequent changes in internal layouts, high safety margin requirements, multiple fume hoods in parallel, or significant duct network resistance variations benefit more from direct face velocity control. Alternatively, a dual-path approach combining displacement for rapid response and face velocity for validation can be employed, balancing responsiveness with long-term accuracy.