1. Overview of gas concentration sensing

    As people pay more and more attention to environmental awareness; consequently, the gas concentration sensing market is gradually growing. The gas concentration sensing is to convert the specific gas contained in the gas into a signal that can be monitored or measured using appropriate components. Gas concentration sensor is widely used in the environment and industry. And it is extremely important and practical when it comes to disaster prevention, maintenance of worker safety and health, as well as pollution prevention. In daily life, most gases are colorless and odorless. Carbon dioxide (CO₂), oxygen (O₂), which are closely related to the human body, and harmful gases such as carbon monoxide (CO) can be detected by gas concentration sensors to protect health and personal safety. In the industrial field, pollutants and hazardous gases such as hydrogen (H₂), carbon monoxide (CO), sulfur oxides (SO_x), nitrogen oxides (NO_x) and ammonia (NH₃) are often used, generated or emitted in factory processes or certain areas. Gas concentration detection is required to provide critical assistance for work safety and accurate and stable operations.

    In general, gas concentration sensors were designed primarily based on semiconductor or electrochemical technologies. Different types of gas concentration sensors were used for different gases and detection accuracy requirements. The advantages of MOS-type gas sensor are detecting a wide variety of gases, long life, and low cost. However, it has poor selectivity of gas types and is mostly used in fields such as combustible gas leakage and air quality detection. Electrochemical-type gas sensors and detectors are more suitable in fields such as low-concentration toxic gases and oxygen monitoring that require higher accuracy. In addition, NDIR-type gas sensor, which has been increasingly developed in recent years, is suitable for monitoring various flammable, explosive and carbon dioxide gases, and has also occupied high-end markets in a wide range of fields.

    Ultrasonic gas concentration sensors, compared with other types of gas concentration sensors, have the advantages of high sensitivity, fast response, good stability, online and real-time detection, low power consumption, long life, and are not easy to pollute and wear. It does not as electrochemical or semiconductor gas concentration sensors that need to be replaced or recalibrated every 1-3 years, and can be used with different types of gases, such as nitrogen(N₂), oxygen(O₂), hydrogen(H₂), nitric oxide(NO), carbon dioxide(CO₂), argon(Ar), and helium(He). Ultrasonic gas concentration sensors are commonly used in ventilators, compressors and fuel monitors. An ultrasonic gas concentration sensor can measure the gas concentration without drilling holes in the pipeline to extract gas according to the change of the ultrasound speed in the pipeline. And it can also simultaneously and accurately measure gas concentration and gas flow. This is a unique advantage. Therefore, ultrasonic gas concentration sensors have been widely applied to  various equipment that need to detect the concentration and flow rate of oxygen, carbon dioxide and other gases in the fields of medical, industrial, chemical, mining and food. It is particularly suitable for situations where specific target gases are detected, such as medical oxygen generators, industrial oxygen generators, oxygen concentration detection equipment used in mining environment, oxygen concentration detection in food storage and production equipment, etc. In general, with the development of the industrial automation in the future, the application of gas concentration sensors will become more prevalent, and sensing technologies will become more and more advanced. The advantages of ultrasonic gas concentration sensing allows it to have its own and unique position in the market.

2. Operating principle and characteristics of ultrasonic gas concentration sensing

Operating principle:

Ultrasonic gas concentration detection is to measure the time of flight (ToF; Time of Flight) of the ultrasound emitted by the transmitter (TX) and reaching the receiver (RX) in the gas, so that the sound speed and the concentration of the mixed gas can be calculated. The basic structure is shown in the figure below.

Since different gas components have different sound speeds, we can detect the concentration of binary gases by measuring the difference of the sound speed or time of flight (ToF) of ultraso und in the gas. Furthermore, ultrasonic gas concentration sensing utilizes the relationship between the sound velocity of a gas medium and its gas equation (refer to Equation 1). It can measure the sound velocity of binary gas es to calculate the volume concentration of the two gases.

Taking hydrogen (H₂) concentration as an example, since the rising of H₂ concentration will increase the speed of sound (shorter ToF), the H₂ gas concentration can be obtained by ΔToF. The principle is shown in the figure below.

Since the binary gases are in the same pressure environment, Equation 2 can be derived based on Equation 1.

If the concentration (ratio) of the target gas (Object gas) is 𝓧, then the concentration of the reference gas (reference gas) is (1- 𝓧), expressed as Equation 3.

When the transmitting and receiving distance L of the ultrasonic sensor is fixed, the flight time t_R and t_M are measured to obtain the sound speed of the reference gas and the remixed gas (Equation 4).

Bringing equation 4 into the calculation formula of flight time difference Δt = (t_M-t_R), we obtain equation 5.

Bring Equation 2, Equation 3 to Equation 5, and then simplify it through Taylor expansion to obtain Equation 6.

where

C: speed of sound

𝛾: ratio of specific heat at constant pressure Cp and specific heat at constant volume Cv

P: gas pressure

𝜌: gas density

R: molar gas constant

T: temperature

𝑀𝑅: molecular weight of reference gas

MM: molecular weight of gas mixtures

𝑀𝑂: molecular weight of objective gas

𝓧: objective gas concentration

It can be oberserved from the results that there is an approximately proportional relationship between the target gas concentration 𝓧 and the flight time difference Δt. In theory, all binary gases can be measured. However, under the condition of limited measurement distance, in order to have a certain time difference discrimination ability, the molecular mass difference between the two gases should be relatively large.

Properties of ultrasonic gas detection transducers

• Durable and highly reliable
• Strong anti-interference ability
• Online and real-time detection
• No regular calibration needed
• Long lifetime
• Wide detection range (0%-100%)
• Simultaneously detect concentration and flow rate

3. Ultrasonic gas concentration detection application scenarios

    As a sensor that accurately measures gas concentration and flow, ultrasonic gas sensor can be widely installed in various equipment that need to detect the concentration and flow rate of gases such as oxygen and carbon dioxide in the fields of medical, industrial, chemical, mining, and food. For example: medical oxygen generators, industrial oxygen generators, oxygen concentration detection equipment used in mining environments, oxygen concentration detection in food storage and production equipment, etc.

    In addition, there is growing interest in the search for alternative energy sources. Among existing alternative energy sources, hydrogen (H2) is known as one of the most sustainable energy sources. Fuel cells powered by hydrogen fuel are designed and implemented by world-renowned manufacturers, and in Japan it is estimated that the ideal H2 economic plan will be fully implemented by 2050. Therefore, hydrogen might be used as a transportation fuel, industry, residential power generation and heating. The widespread use of gaseous hydrogen as an energy source means that hydrogen storage and processing will become increasingly important. Hydrogen is transported through pipelines. Since hydrogen is a colorless, odorless, and tasteless explosive gas, it is very important to measure the hydrogen gas concentration without drilling holes on the pipeline surface. The ultrasonic gas concentration sensor can measure the concentration of gas according to the changing of ultrasound speed in the pipeline without drilling holes to extract gas. It has certain prerogativein the potential market.

4. Development of ultrasonic gas flow and concentration detection system

Ultrasonic gas flow and concentration measurement requires a high-precision signal measurement system to measure the signal and calculate the flow rate. Generally, you can choose a public version test circuit suitable for detecting ultrasonic gas flow, such as TI EVM430-FR6043, as shown below:

The measurement accuracy of the circuit board EVM430-FR6043 can reach ±250 pico second, which can accurately measure important performance parameters of gas flow meters, such as frequency sweep, ADC peak, and static zero-flow drift (ZFD), etc. The picture below is the high-performance and high-reliability ultrasonic transducer A200M3 developed by Unictron. Its operating frequency is about 200 kHz. The test results using EVM430-FR6043:

(ΔToF = 0.73 ns)

Take the continuous detection of oxygen concentration as an example. When the gas in the pipeline is stationary, the gas is only air with an oxygen content of about 21%. This can be defined as the reference point of the system. In this state, the absolute ToF value is used to obtain t_R and c_R. When oxygen begins to pass through the pipeline, the absolute ToF also changes accordingly. At this time, the absolute ToF value is t_M, so that continuous oxygen concentration detection can be established.

5. Problems and diagnosis of ultrasonic gas concentration sensor

• From Equation 1, we can understand that temperature is also one of the factors that affects concentration calculation. Following the above example, every 1°C measurement error in the system will cause an error of approximately 2.75% in the oxygen concentration value. Therefore, temperature parameters must be considered in the system design and a more precise temperature sensor should be used.
• Ultrasonic gas concentration sensor designed based on binary gases ideally can only be used to measure a mixed medium of two gases. Once other gases are introduced, it will cause a significant error in ultrasonic gas concentration measurement.

6. Reference

• Shan Minglei, Li Xiang, Zhu Changping, Zhang Jiahua, “Gas Concentration Detection Using Ultrasonic Based on Wireless Sensor Networks”
• TEXAS INSTRUMENTS, “Application Note 氧濃度感應”
• TEXAS INSTRUMENTS, “Design Guide: TIDM-02003 Ultrasonic Sensing Subsystem Reference Design for Gas Flow Measurement”