Measuring Gases: Diffusion vs. Sampling Method

Many of you have called us to ask about applications that start with the sentence, “How do I…” In every case, the application falls into one of two groups: measuring gas by diffusion or measuring gas by sampling. Knowing the difference can help you decide which method is best for you.

Diffusion – Measurement for Environment

From indoor air quality to plant growth to personal safety: all of these involve people and areas where gases are diffused in ambient air. In other words, the sensor measures the gas concentrations diffused around it. Since the sensor is physically in the ambient air, the readings taken around the sensor are assumed to be correct for all the nearby air.
Since trace gas levels in ambient air tend to rise and fall slowly, diffusion measurement results in a relatively slow response.

For example, CO2 sensors use diffusion measurement in agriculture to enhance the growth of plants or prolong freshness during transit. In either of these cases we are attempting to maintain the concentration for a specific purpose. Other examples of diffusion include monitoring CO2 levels inside buildings for indoor air quality or personal safety.

The diffusion method is used in any devices like our CO2 level controller, pSense CO2 Meter, and eSense CO2 Alarms. In addition, most of our CO2 sensors that use diffusion by default like the K-30 can be modified with a tube cap kit to become a sampling sensor.

Sampling - Process Measurement and Control

Anywhere a highly confined space is being monitored for an elevated gas level for a specific purpose, a sampling method is preferred.

Sampling normally involves a small pump in a closed-loop system that streams gas across the face of the sensor and returns it back to the enclosure.

Because of the volume of gas passed across the sensor, sampling tends to have a much faster response time. In fact, it is possible to measure a change in CO2 levels in the sub-second range. This makes sampling useful for leak detection, or in incubators where various gas levels must be tightly controlled.

Our sampling meters, K33-ICB and K33-BLG sampling sensors are all designed to sample CO2 levels for many different applications.

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