Technology and performance - AQT530

Electrochemical cell technology for gas sensing

The most widely used technology for ambient pollutant gas sensing in compact air sensors is the amperometric electrochemical technology. Electrochemical gas sensors use the reduction/oxidation reactions at a gas sensitized electrode surface of a galvanic cell to convert gas concentration to an electrical current running through the cell. This signal is converted into pollutant concentration. From a measurement quality standpoint, the advantage of electrochemical gas sensors is their sensitivity. Due to the large absolute number of measurand gas molecules present in ambient air, even in parts per billion (ppb) concentrations, the current from electrochemical reactions due to pollutants can be measured with state-of-the-art electronics. The challenges in using electrochemical gas sensors in field instrumentation is in their susceptibility to environmental conditions such as temperature and humidity, sensitivity to other gases (cross-sensitivity), as well as their limited lifetime.

Electric current is not only dependent on gas concentration, but there is a dependency to temperature and humidity. In AQT530 these dependencies have been taken into consideration by performing compensations in the signal processing algorithms, as well as by calibrating the products. In normal conditions these compensation methods perform quite well. However, the mechanisms are quite complex, especially in extreme conditions, such as in very hot and dry conditions, there are challenges and the technology has its limitations.

Electrochemical gas sensors are consumables and must be replaced after their service time has ended. The typical maintenance interval is 2 years, but this depends strongly on the usage conditions. AQT530 includes a health index parameter, which indicates when it is time for maintenance. When the health index is below 30 %, maintenance is recommended. Maintenance can be carried out as a Vaisala service or alternatively a spare part with new calibrated electrochemical gas sensors can be purchased.

After installing AQT530 to a measurement location, it is essential for the unit to stabilize at least 24 hours before using the measurement results. This stabilization time varies depending on the usage conditions, but typically 3 - 5 days after installation and switching on the device is enough to ensure optimal measurement accuracy. During the first 24 hours, which are the most important, the measurements are marked as invalid. The invalidation takes place also after power breaks and when temperature inside AQT530 is too high to get reliable measurement results.

Optical particle counters for particle measurement

The Vaisala technology used in air quality sensors is based on an optical laser particulate counter (LPC) measuring single particulates. The single particulate measurement principle enables more reliable and accurate measurement than the commonly used photometer technology, which assumes particle size distribution to be known.

In AQT530 the scattered intensity of the light from single particulates is measured. These single pulses define the size of the individual particles and the number of the pulses corresponds to a number of particles. Knowing the number and size of the particulates algorithms are applied to compute particle mass concentrations (µg/m³) in different size fractions (PM₁, PM_2.5, PM₁₀). As with all optical methods, certain density for particulates is assumed and calculations to mass concentrations are based on this assumption.

A limitation of optical particle sensing is that the amount of light scattering from particles does not primarily depend on the quantity of interest (that is, particle mass), but there are many factors affecting the way the scattered and detected light signal corresponds to the particle mass. Factors such as color, shape, and size have an effect on the scattering capability.

Due to the physics of the scattering process, the amount of scattered light gets extremely small as the particle size falls below the wavelength of the light used for the measurement. This limits the minimum particle size that can be measured with compact optical particle sensors and with AQT530 this is currently 0.6 um. For measuring particle sources such as fine particles from car exhaust or smoke from forest fires, where main particle component is smaller than 0.6 um, AQT530 is not optimally suited.

The other limitation is water uptake of particles. The water uptake of particles increases the optical diameter of a measured particle, enhancing light scattering during the measurements and therefore might result in high readings from the sensor although the dry mass of the particles remains unchanged. In AQT530 the readings that may be compromised by humidity are marked as invalid.

Field performance and co-location study

The performance values given in the AQT530 datasheet are derived from field tests carried out at major climate zones. The values given are non-corrected using only factory calibration.

To have the best performance for AQT530, Vaisala recommends carrying out a co-location study. By applying a correction based on this study, the sensor output is tuned to match the typical conditions in the area and the absolute difference between the reference grade sensor readings versus the AQT530 sensor can be decreased. This may be particularly relevant for the particulate measurements, where it can be seen as a way to adapt the sensor response to the local particle composition.

Linear correction

AQT530 enables adjusting the linear correction parameters, gain (span) and offset (zero), to Modbus registers, meaning that the sensor data can be corrected based on the co-location study.

For details and instructions, see Applying Correction Equation to AQT530 Data Application Note.