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There are chiefly two options regarding where to install sensors: in a room close to the occupants or in the ventilation system to detect the CO level on outflowing air. How you choose to place your sensor depends on the nature of the space.

Let’s use a hotel as an example. In the restaurant or a big lobby, a sensor on one wall would most probably misread the other side of the room, so a sensor in the outgoing air might make more sense. In a small conference room or a regular hotel room, a sensor on the wall is sufficient.

There is also a difference between CO and temperature sensing and control. The temperature is quite even throughout a room. CO₂ levels, on the other hand, can vary from 3,000 ppm on one end of the room to 600 ppm on the other.

When installing sensor-controlled ventilation, there are a variety of strategies and algorithms to choose from. Depending on who you are (an OEM developer, a system developer, a distributor), we need to discuss what strategies you want to apply.

A sensor will deliver a signal strength depending on CO2 variables. The frequency of measurements depends on your needs and energy savings. 

Here are some different strategies:

PID

Most likely, our sensors end up in PID controlled systems (for more information on PID, see below), as they are suitable in multi-zone buildings that have high variables and unpredictable patterns of occupancy.

Helpful links ("PID for dummies"):

http://www.eurotherm.com/pid-control-made-easy
https://www.csimn.com/CSI_pages/PIDforDummies.html

On/Off

CO2 controls the damper with a set point and when ventilation reaches the correct level, the sensor closes the damper. This strategy works well in applications like schools, theatres, and conference rooms where the occupancy can vary greatly from one minute to the next.

Proportional

This strategy works with the difference between outside and inside air. For example, the setting point can be 100 ppm over outside level. When the desired CO level is reached, the control eases up on “the gas pedal”.