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Diffusion vs. Sampling Sensors: What's the Difference?

Reliable carbon dioxide and oxygen measurement and control is critical in many applications. When comparing gas measurement devices, vendors use the terms "diffusion sensor" or "sampling sensor" to differentiate between the two major uses of gas sensors. Having a general understanding of these terms can help you better evaluate which sensor is right for your application.

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.

Diffusion Sensors

All gas sensors are diffusion sensors. In fact, your nose is the perfect example. Like a mechanical gas sensor, your nose has a space inside it where it measures molecules of different gases diffused in air.

Why is diffusion important? Diffusion sensors rely on the tendency of different gases to mix together evenly over time.

For example, consider a pot of home-made soup. The aroma of the soup starts in the pot, then diffuses throughout the house. Put a sensor in the house (your nose), and you can smell the soup anywhere.

Diffusion, while useful, has 2 major drawbacks. First, it takes time for gas to diffuse, and second, it isn’t consistent for precise measurement. For example, it ignores the fact that some gas molecules are heavier than others. To use our soup analogy again, the aroma is strongest in the pot, and is milder in the next room. It is stronger at the floor than at the ceiling. Now open a window, and the aroma is diluted with outdoor air. If you are want to measure aroma, where do you put the sensor (your nose) to take the most accurate measurement? In the pot, of course.

Sampling Sensors

To solve the measurement problem, sensor manufacturers created sampling sensors. They start with a diffusion sensor, put it inside a sealed container, and then attach tubing for inlet and outlet ports. A small vacuum pump pulls the gas over or through the sensor at a consistent velocity.

Imagine putting a towel over your head and the pot of soup, then taking a deep breath. You’ve just turned your nose into a sampling sensor.

That’s why diffusion sensors are best for measuring room CO2 levels, while sampling sensors are more commonly used in sealed environments.

For example a sampling sensor may be mounted to a bio-reactor column or a cell incubator with the tubing inlet and outlet ports vented into the enclosed space to form a closed-loop gas measurement system, or a hand-held unit could be attached to the ports via tubing for spot-check measurements. The added expense of using sampling sensors is offset by their ability to more quickly and accurately measure the gas in a closed environment.


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