Sash Position Sensor 

Model 873-b

Measuring the sash position coupled with the damper position pressure independence is a proven and accurate method of determining exhaust flow rates in a fume hood.


The Sash Position Sensor is a critical element in the control of airflow through a fume hood. The Sash Position Sensor is used to determine the open area of a fume hood sash, which is used by the fume hood controller, along with the face velocity through the sash, to modulate the valve / damper. The sash position and face velocity (airflow volume) are available for use in the Building Automation System (BAS), if desired.

The Waddell Sash Position Sensor is constructed of a linear sensor array (LSA), along with a protective cover and an activator, which interacts with the linear sensor array. The linear sensor array (LSA) is a non contact linear sensor which is solid state and does not use mechanical components to output a signal.


The Activator is located on the moveable sash and outputs a signal to the fixed array. The signal from the array is therefore proportional to the sash position.  The position of the sash is sent to the local fume hood controller.


The local fume hood controller then reads the face velocity from the measured exhaust volume and sash position and adjusts the valve or fan accordingly using a proportional integral program until the set face velocity is met. Additionally the sash position can be logged by the controller to track the hood usage or sash position during unoccupied times.


Applications and Benefits



  • Non-Contact System

  • Universal Mounting

  • Corrosion Resistance

  • Conformal Coated for Chemical and Corrosion Resistance



  • Compact and Discrete

  • Durable

  • Proven, Accurate Technology

  • Easy to Install and Low Maintenance

  • No Interference with the User or Sash Movement



873-b Product Bulletin