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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 and sampling sensor to differentiate between the two major types of gas sensors. Having a general understanding of these terms can help you better evaluate which sensor is right for your application.

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 perfect 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.

CO2Meter.com offers almost every one of our diffusion sensors as sampling sensors, so no matter which type you need, we have a solution.

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