Gas detection techniques using pyroelectric sensors
01 October 2021
Mouser-KEMET_gas detection sensors application stock shot
Gas detection is an essential aspect of many industrial & commercial applications, covering many potential sources, including carbon dioxide, nitrogen dioxide & several anaesthetic gases. Most of us have detectors in our homes to protect us from smoke & carbon monoxide. Other applications include measuring carbon dioxide expelled, termed capnography, for medical diagnostic purposes, air quality measurement in smart buildings & commercial portable gas analysis detectors.
This article was originally featured in the October 2021 issue of EPDT magazine [read the digital issue]. And sign up to receive your own copy each month.
In this article, Mark Patrick, EMEA Technical Marketing Manager at electronic component distributor, Mouser and Derick Stevens, Global Product Manager for Sensors & Actuators at electronic component supplier, KEMET investigate gas detection using pyroelectric sensors and highlight why this approach is better than using a thermopile…
Thermopile-based sensing techniques suffer from several issues, such as a long settling period, long measurement times and susceptibility to environmental variations. With a focus on infrared measurement, we explain the principles behind gas detection and highlight the benefits of this approach. A complete system approach is showcased with I2C connected sensors to a microcontroller host.
Gas detection is all around us. In the home, smoke and carbon monoxide sensors protect us, modern offices keep air quality fresh by monitoring carbon dioxide levels, and factories employ detectors to monitor many different gases for process control and safety. Battery cell gas monitoring is an example of a more recent safety application in electric vehicles (EVs). Gas production from a cell provides an early indication of an overload or abnormal operation, in advance of any temperature changes. Hospitals and healthcare facilities use gas detection sensors in a variety of clinical and diagnostic equipment. An example apparatus is a capnograph, a portable instrument used to detect the amount of carbon dioxide exhaled by a patient. Capnography provides an overall assessment of a patient’s ventilation and metabolism, and is often used in emergency wards and by first responders. Portable, handheld capnographs are battery-powered and, especially for accident and emergency usage, need to yield a fast and accurate result.
Mouser-KEMET_smoke detection sensors application stock shot
Gas detection methods
Gas detection involves a sensor exposed to infrared (IR) light in the presence of a gas. Because different gases absorb more infrared light energy at specific wavelengths, it is possible to differentiate the type of gas detected by energy changes within the gas. Gas molecules absorb energy from the IR source and become more mobile and excited, generating heat. The higher the concentration of a specific gas, the higher the amount of infrared energy absorbed.
Thermopile sensors have traditionally been used in gas detection equipment such as capnographs. Thermopile sensors use a thermocouple to detect the temperature changes in the gas molecules and generate a voltage output proportional to a specific gas detected concentration.
However, thermopile sensors require a period of settling time, up to 2 minutes, from switching on the apparatus before the sensor is stable enough to conduct measurements. Another consideration is that measurement times can take over 200 ms. That measurement period might not appear an exceptionally long duration; however, for a battery-powered design, the time the sensor is used impacts the power consumption profile negatively, necessitating more frequent battery changes or recharge cycles. Thermopiles require additional analogue circuitry to operate, adding to development schedules and BOM (bill of materials) cost.
The pyroelectric method of gas detection uses the pyroelectric effect. This effect creates an output voltage by detecting changes in the amount of infrared radiation received.
Figure 1. The construction of a pyroelectric gas sensor, showing the principal components (source: KEMET)
Figure 1 illustrates a pyroelectric gas sensor operating in the near-infrared spectrum. A source of IR light energy is directed at sensors through an enclosed chamber, and spectral filters isolate the optical spectrum to match the gas being detected and provide a reference channel. The difference between the amount of IR energy transmitted from the source compared to the amount received at the sensor indicates the concentration of the gas. Two IR detectors are used, one for the reference signal to determine the amount of energy transmitted and the other for the specific gas detection.
Figure 2 highlights an example of a breath analyser application using a pyroelectric sensor. A pulsed IR heat source is used to transmit IR energy through a gas tube and transmissive windows to two pyroelectric sensors. One sensor provides a reference channel signal at 3.9 µm wavelength, and the second sensor is detecting the CO2 wavelength of 4.26 µm.
Designing a CO2 gas detection system using a pyroelectric sensor
An example CO2 gas sensor is the KEMET USEQGSEAC82180. Packaged in a compact surface-mount package, KEMET’s QGS series of thin-film digital interface, pyroelectric sensors exhibit extremely low power characteristics. They can perform gas measurements 15x quicker than a thermopile-based detector. Thanks to the piezoelectric (PZT) material’s low thermal mass, they can conduct measurements extremely quickly and initialise virtually instantaneously. Also, PZT-based sensors have a longer in-service life compared to thermocouple-based thermopiles. No additional analogue circuitry is required, and the digital I2C interface provides a convenient and easy method of connecting to a microcontroller host. A programmable ASIC (application-specific integrated circuit) device connected via the I2C interface is integrated inside the sensor, permitting complete control and configuration of the analogue filters, amplifier gain and analogue-to-digital conversion (ADC) parameters.
Figure 2. CO2 sensor example for breath analysis (source: KEMET)
Compared to a thermopile approach, KEMET’s highly sensitive, fast response time piezoelectric sensor approach to gas detection offers a reduced power consumption profile, increased battery life, increased product lifetime and reduced maintenance costs.
The QGS sensor family offers two modes of operation: a normal mode with a 1 kHz maximum sample rate; or a low power mode with a maximum of 166 Hz sample rate. In normal mode, the typical power consumption is 22 µA, and 3.5 µA in low power mode. A power-down mode disables the sensor and reduces the current to typically 1.1 µA.
Integrating all the key analogue signal chain components into a single package and using an industry-standard I2C interface dramatically simplifies constructing a low power CO2 sensor suitable for use in a battery-powered capnograph.
Sensor readings are stored in multiple registers accessible over the I2C interface. The whole measurement and signal conditioning process is managed by the sensor’s internal ASIC, without any resource loading on the host MCU.
Figure 3. Raw CO2 sensor register measurements across multiple gas concentration values (source: KEMET)
A complete capnograph system would include a pulsed IR source, a gas cell with a defined path length (typically 20 - 32 mm long), a reference sensor channel and a CO2 gas sensor. The IR source is typically pulsed up to 40 times per second.
A calibration process uses deterministic algorithms to find the specific ratios between reference and gas channels, which can later be used to predict gas concentrations accurately. The Beer-Lambert equation is used to linearise and determine accurate gas concentration levels based on the sensor values stored in the I2C registers.
Figure 3 illustrates the raw I2C register values read from a CO2 sensor across CO2 levels from 0 to 100%. Note the non-linear nature of the curve. KEMET provides an application note and a Microsoft Excel worksheet to aid sensor linearisation and gas % prediction.
Speeding gas detection prototyping
To assist in developing a gas detection system, KEMET supplies the USE-QGSK3000000 SMD CO2 sensing evaluation kit (see Figure 4).
Figure 4. The KEMET SMD CO2 gas sensing evaluation kit (source: KEMET)
Comprising a complete measurement system, it contains two KEMET sensors, the USEQFSEA391180 reference sensor and a USEQGSEAC82180 CO2 sensor.
The kit contains a host STMicroelectronics STM32F303K8T6 microcontroller, a 3D printed gas cell chamber, and a KEMET IR emitter and driver PCB. Microsoft Windows software is also provided to configure the sensor platform, capture and analyse data.
Pyroelectric gas sensors: your route to fast, low power integrated gas detection
Compared to traditional thermopile infrared sensors, pyroelectric sensors offer many advantages when used in portable, battery-powered gas detection equipment. Their low power, fast settling time and rapid sensing period allow product developers to advance their designs. This article highlighted the use of KEMET’s pyroelectric non-dispersive infrared CO2 sensors in a medical capnograph application. There are many potential use cases for measuring gas concentration levels from zero to 100%.
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