Types, Principles, and Specifications of CO₂ Sensors for CO₂ Incubators

The core task of a CO₂ incubator is to provide a stable pH environment for cells. Since most cell culture media rely on a bicarbonate buffer system (CO₂/NaHCO₃), the precise control of CO₂ concentration inside the incubator directly determines the pH stability of the culture medium, thereby affecting cell growth. Currently, there are two main types of CO2 sensors used for CO₂ incubators: Thermal Conductivity (TC) sensors and Infrared (IR) sensors (especially NDIR – Non-Dispersive Infrared). Ultrasonic sensors have also been used in some incubators, but they are less common than the first two. This article focuses on the two primary types.

I. Sensor Types and Working Principles

1. Thermal Conductivity (TC) Sensor

Working Principle: The TC sensor measures CO₂ concentration indirectly based on the physical property that different gases have different thermal conductivities. Inside the sensor, two precision thermistors are arranged: one is exposed to the incubator environment, and the other is sealed inside a reference gas. The two thermistors form two arms of a Wheatstone bridge, and their resistance changes with the thermal conductivity of the surrounding gas.

• Core Principle: When the bridge is powered, both thermistors heat up. The surrounding gas carries away heat through thermal conduction, cooling the thermistors and changing their resistance. Since the thermal conductivity of CO₂ is significantly lower than that of the main components of air (nitrogen and oxygen), when the CO₂ concentration inside the incubator rises, the thermistor exposed to the environment cools less effectively, its temperature increases, and its resistance changes accordingly, creating a resistance difference from the sealed reference thermistor.

• Signal Conversion: The external circuit precisely measures the resistance difference between the two thermistors and calculates the CO₂ concentration based on the known relationship between CO₂ concentration and thermal conductivity.

Technical Characteristics: TC sensors have a simple structure and low cost. However, their measurement relies on the thermal conductivity of the gas, which is significantly affected by changes in temperature and humidity. Therefore, the measurement accuracy of this sensor is sensitive to fluctuations in temperature and humidity inside the incubator and is prone to drift.

2. Infrared (IR / NDIR) Sensor

Working Principle: The NDIR sensor is the standard configuration in modern high-end CO₂ incubators. It utilizes the characteristic of CO₂ molecules absorbing infrared light at a specific wavelength, using an optical method for direct concentration measurement. This method offers high sensitivity and selectivity and is not easily disturbed by other gases.

• Core Principle: An NDIR sensor internally contains a broadband infrared light source, an optical cavity, and a photodetector equipped with a narrow-band filter. The light source emits infrared light, which passes through the cavity filled with the sample gas and reaches the detector.

• Selective Absorption: The narrow-band filter in front of the detector only allows infrared light of a specific center wavelength (approximately 4.26 μm) to pass through. CO₂ molecules strongly absorb infrared light at this wavelength. When the CO₂ concentration in the cavity is higher, more infrared energy is absorbed, and the light intensity reaching the detector is weaker; conversely, when the CO₂ concentration is lower, the light intensity is stronger.

• Signal Conversion: The circuit converts the light intensity signal received by the detector into an electrical signal and calculates the exact CO₂ concentration in the measured gas according to the Beer-Lambert law.

Technical Characteristics: The NDIR sensor is an optical sensor; its light absorption characteristics are almost independent of temperature and humidity changes. Therefore, it offers high measurement accuracy and good stability, making it particularly suitable for cultivation scenarios where the door is frequently opened.

3. Ultrasonic Sensor

Working Principle: The ultrasonic sensor measures CO₂ concentration by utilizing the property that the speed of ultrasound propagation varies in different gas media. The speed of sound in air is approximately 334 m/s, while in pure CO₂ it is about 274.6 m/s. By measuring the speed change or phase shift of ultrasound after passing through the gas, the proportion of CO₂ in the gas mixture can be indirectly calculated.

Technical Characteristics: Ultrasonic sensors have no moving parts, are unaffected by high humidity, and have a long service life. However, due to cost and technical complexity, their application is relatively limited, especially in some markets.

Comparison Summary

II. Key Technical Specifications of Sensors

When selecting or evaluating sensor performance—whether TC or NDIR—the following key specifications are most important:

1. Measurement Range

The common CO₂ concentration control range in incubators is typically 0–20% (volume percentage), with set points generally at 5% or 10% to match the pH buffer system required by the culture medium. Most incubator sensors have a range of 0–20% CO₂. Some sensors used in infant incubators or low-concentration monitoring can have ranges as low as 0–5000 ppm (i.e., 0.5%).

2. Accuracy

Accuracy reflects how close the measured value is to the true value. For NDIR sensors, typical accuracy can reach ±0.1% CO₂ (at 5% concentration). TC sensors generally have lower accuracy, and deviations can be larger due to temperature and humidity effects. Some products specify an indication error ≤ ±0.1% or ±5% (whichever is met). Smaller accuracy values indicate better sensor capability.

3. Resolution

Resolution refers to the smallest change in concentration that the sensor can detect. For incubator CO₂ sensors, resolution is typically between 0.01% and 0.1% (depending on range). High resolution helps achieve smoother PID control and prevents the control system from oscillating frequently around the set point.

4. Response Time

Response time directly affects the speed at which the incubator recovers the set concentration after door opening. It is typically defined as T90 (the time required to reach 90% of the final stable reading after a concentration change). For incubator NDIR sensors, the typical response time is 30–60 seconds, with some products achieving less than 30 seconds. TC sensors have slower response speeds and longer warm-up times. Shorter response times enable faster reactions from the control system.

5. Warm-up Time

Sensors need some time after power-up to reach a stable measurement state. Typical warm-up time is 30 seconds to 4 minutes; achieving full specified accuracy usually takes longer (e.g., 5–15 minutes). Shorter warm-up times allow the device to be ready for normal operation more quickly after startup.

6. Temperature Compensation and Pressure Compensation

Because gas concentration measurement is affected by ambient temperature and atmospheric pressure (especially at different altitudes), sensors must have temperature compensation and pressure compensation capabilities. Sensors equipped with both compensations can maintain stable measurement accuracy in changing incubator environments (temperature fluctuations, pressure differences). When purchasing, prioritize products with automatic temperature and pressure compensation.

7. Long-term Stability and Drift

Long-term stability reflects how much the sensor's measurement value drifts during continuous use. It is typically expressed as drift per year. For incubator NDIR sensors, typical zero drift is ≤ ±0.1% CO₂/year; span drift is about ≤ ±1% F·S/year. Low drift means longer calibration intervals and lower maintenance costs.

8. Repeatability

Repeatability refers to the consistency of measurement results when repeatedly measuring the same gas concentration under identical conditions. The typical repeatability for incubator sensors is ≤ ±1% F·S. Higher repeatability indicates stronger reliability of the measurement data.

9. Sterilization Compatibility

CO₂ incubators require regular high-temperature sterilization to prevent microbial contamination. Therefore, whether the sensor can withstand the sterilization process is an extremely important specification when selecting an incubator. Common sterilization methods include:

• High-temperature dry heat sterilization: Some sensors support 140°C dry heat sterilization without needing removal.

• High-temperature moist heat sterilization: Some NDIR sensors can be used directly in moist heat sterilization environments.

• High-temperature tolerance: Some sensors designed for demanding applications can tolerate high-temperature sterilization up to 190°C.

Whether the sensor supports in-situ sterilization (no need for removal) significantly affects sterilization efficiency and ease of operation, and is an important consideration for evaluating whether a product is high-end.

10. Service Life

Sensor service life directly affects the total operating cost of the equipment. The design life of incubator CO₂ sensors can typically exceed 10 years. Due to having no moving parts and long-lasting light sources, infrared sensors generally have a longer service life than thermal conductivity sensors.

III. Selection Recommendations

1. Based on accuracy requirements: If high precision in CO₂ concentration control is required, or if the incubator door is frequently opened, an NDIR sensor should be the first choice, as it is almost unaffected by temperature and humidity changes, making it especially suitable for precise culture conditions and frequent door openings.

2. Based on budget: Although TC sensors have lower accuracy, they are inexpensive and suitable for applications with limited budgets and low accuracy requirements. NDIR sensors are more expensive but have become the de facto standard in high-end cell culture applications.

3. Pay attention to compensation and calibration features: Regardless of sensor type, ensure the product has temperature compensation, pressure compensation, and automatic calibration or easy manual calibration functions to maintain long-term measurement accuracy.

4. Confirm sterilization compatibility: Prioritize sensors that can withstand high-temperature sterilization without removal, as this significantly improves sterilization efficiency and ease of operation.

5. Verify specific performance specifications: When purchasing or accepting equipment, strictly check key technical parameters such as measurement range, accuracy, response time, long-term stability, and repeatability to ensure they meet the equipment requirements.

IV. Technology Development Trends

• Dual-wavelength design: Many NDIR sensors use a dual-wavelength detection scheme with a built-in reference wavelength channel. Automatic calibration algorithms further reduce zero drift, improving long-term stability and reliability of CO₂ measurement.

• MEMS miniaturization: Sensors are moving towards miniaturization and integration, allowing more flexible embedding into tight spaces inside incubators.

• Wireless and intelligent monitoring: Some models support digital communication protocols such as RS485 and Modbus, and can interface with wireless data loggers to achieve remote real-time monitoring and alarming of CO₂ concentration.

• Improved sterilization compatibility: New-generation NDIR sensors are continuously expanding their operating temperature and sterilization tolerance ranges. High-end models can now withstand high-temperature sterilization up to 190°C, reducing the operational risks and cross-contamination possibilities associated with sensor removal during sterilization.