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CO2 Sensor Response Time

GSS offer CO2 sensors with different gas sampling methods.

GSS manufacture Carbon Dioxide sensors with two different types of gas sampling methods, diffusion and flow through. Different sampling methods are needed to optimise the gas measurement method to meet the varying application requirements.

TERMINOLOGY

The response time of a sensor is usually quoted as the time to reach a proportion of the target value. For example, T90 30s indicates the sensor will reach at least 90% of the final value within 30s of the measured property being changed.

DIFFUSION

Diffusion of the gas into the measurement chamber is the most common method for sensors used in Indoor Air Quality applications, either as part of a control system for the HVAC or just monitoring conditions within a building. For low-power sensing, diffusion is the obvious choice as it requires no artificial air circulation (e.g., from a pump). The ambient range of CO2 sensors from GSS have small openings on the outer surface of the optic. These are typically protected from view under a PTFE membrane dust filter. Natural gas diffusion allows the outside sample to fill the volume of the optic, typically with a T90 response time of 30s. The sensor response time is largely defined by the pore size of the PTFE membrane and therefore cannot be changed without modifying the membrane.

FLOW PORT

A flow port is typically used where either high speed of response is required, or the sensor is used to sample a process environment, where the sample is side streamed through the sensor and returned to the process. In these cases, the response time is influenced by the flow rate and the measurement rate of the sensor. This application note will primarily address these variables.

MEASUREMENT SPEED

GSS sensors use a technique called Non-Dispersive Infra-Red (NDIR) where the gas concentration is determined by the amount of light absorption by the target gas in measurement chamber.
There are three factors that affect sensor response time; the gas flow into the measurement chamber, the amount of digital filtering done by the sensor, and the measurement rate.
For GSS diffusion sensors with a measurement rate of 2Hz, the limiting factor is the diffusion rate through the membrane. For sensors with a gas flow port, the response time can be reduced by increasing the measurement frequency.