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Gas Sensor Response Time Increases with Length of Tubing

Sampling sensors and devices have different response rates depending on the length of the tubing used between the source gas and the sensor.

Factors Influencing Increased Response Time:

  1. Gas Flow Delay:

    • The longer the tubing, the more time it takes for the gas sample to travel from the point of entry to the sensor. This transit time adds a delay to the overall response time of the sensor.

  2. Adsorption and Desorption:

    • Tubing materials can adsorb and desorb gas molecules, which can delay the arrival of the gas at the sensor. This effect is more pronounced with longer tubing as there is a larger surface area for interaction.

  3. Diffusion:

    • As the gas travels through the tubing, diffusion can cause the gas molecules to spread out, potentially leading to a dilution effect and a slower response time in detecting changes in gas concentration.

  4. Dead Volume:

    • Longer tubing can introduce more dead volume (the volume of gas in the system that is not actively being sampled), which can cause a lag in the sensor’s response as the new sample must displace the existing gas in the tubing.

  5. Pressure Drops:

    • Longer tubing can create pressure drops, especially if the tubing has a small diameter or if the gas flow rate is low. These pressure variations can affect the speed at which gas reaches the sensor.

Mitigation Strategies:

  1. Minimize Tubing Length:

    • Where possible, reduce the length of tubing between the sampling point and the sensor to minimize delays.

  2. Choose Appropriate Tubing Material:

    • Select tubing materials with low adsorption properties to reduce the interaction between the gas and the tubing walls.

  3. Increase Flow Rate:

    • Increasing the flow rate can help reduce the transit time of the gas through the tubing, though this must be balanced against the risk of over-pressurizing the system or introducing turbulence.

  4. Use Larger Diameter Tubing:

    • Larger diameter tubing can reduce resistance and pressure drops, facilitating quicker gas transit.

  5. Precondition Tubing:

    • Preconditioning the tubing with the target gas before actual sampling can help saturate adsorption sites and reduce delays due to adsorption/desorption effects.

Understanding and addressing these factors can help optimize the response time of gas sensors, ensuring more accurate and timely measurements in various applications.

In a typical closed loop gas sampling system, the order of the components beginning at the source are:

  1. Particulate filters
  2. Moisture traps or hydrophobic filters
  3. The vacuum pump
  4. The sensor tube-cap adapter
  5. The sensor

Depending on your configuration, the distance from the gas source to the filters, combined with the tubing between the filters and the sensor, can vary from several inches to many feet. Since the 1/8 inch (3mm) tubing will already be filled with gas, it will take some period of time for the sample to reach from the source to the sensor.

In our lab, we performed the following tests using a CM-0003 100% CO2 Sampling Data Logger fitted with two different capacity micro-pumps to help us determine how long it would take from the time the pump was started until a 100% NDIR CO2 sensor inside detects 5% and 80% gas levels. We used 50 foot (15m) and 100 foot (30m) of 1/8 inch (3mm) tubing connected between the inlet port of the meter and a demand flow regulator on a 100% CO2 gas cylinder. All tubing was laid horizontal and purged with nitrogen gas. These were our results:

Using 50 feet of tubing and a 400 ml/min pump, it takes

  • 25 seconds to reach 5% CO2
  • 55 seconds to reach 80% CO2

Using 100 feet of tubing and a 400 ml/min pump, it takes

  • 50 seconds to reach 5% CO2
  • 80 seconds to reach 80% CO2

Using 50 feet of tubing and an 800 ml/min pump, it takes

  • 15 seconds to reach 5% CO2
  • 40 seconds to reach 80% CO2

Using 100 feet of tubing and an 800 ml/min pump, it takes

  • 25 seconds to reach 5% CO2
  • 80 seconds to reach 80% CO2

As you can see, doubling the length of tubing approximately doubles the length of time required to achieve full saturation. However, doubling the capacity of the vacuum pump does not cut the time in half. This is because the CO2 sensor response time is consistent, no matter how quickly the air over the sensor is changed.

What does this mean? As a practical matter, for short lengths of tubing and small changes of the gas level, a default pump “on” time of 10 seconds plus the sensors response time is adequate. If longer lengths of tubing are used, or if the sensor is required to test for a large variance in gas levels from normal, longer pump times should be used. This can easily be tested using a known calibration gas and your particular system design.

Finally, while upgrading to a higher capacity pump will improve response time, it cannot overcome the sensor’s response time which can vary greatly between NDIR and electrochemical sensors.

For more information on gas detection sensor solutions, click here or contact us to speak directly to a CO2Meter expert.

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