# Suitability of Low-Cost Sensors for Submicron Aerosol Particle Measurement
## 2 Experimental Setup
### Test Setup  
For the experimental investigations of the suitability of low-cost sensors for the measurement of submicron aerosol droplets and particles, the test rig shown in Figure 1 was used. The test aerosol is generated with a self-constructed Collison nebulizer using pressurized air (particle-free due to filtration with a HEPA filter) [45]. The mixing ratio between the stream after the nebulizer (A) and the particle-free air (B) determines the resulting particle concentration (C). After that, the aerosol is passed through a diffusion dryer with silica gel and an aerosol neutralizer (TSI Model 3012A) to obtain a bipolar equilibrium charge distribution [46, 47]. An optional second stream of the pressurized air (D) is passed through two packing columns with water to increase the relative humidity. The mixing ratio between this stream \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Model** & **SMPS 3934** & **AQ Guard** & **SDS011** &
\begin{tabular}{c} **HPMA** \\ **Compact** \\ \end{tabular} \\ \hline Company & TSI GmbH & Palas GmbH & NovaFitness & Honeywell \\ \hline Measuring device & Lab-scale devices & \multicolumn{3}{c}{Low-cost sensors} \\ \hline Measurement range & 0.014 to 1 & 0.175 to 20 & 0.3 to 10 & N/A \\ \hline Measurement method & Spectrometer & Spectrometer & Photometer & Photometer \\ \hline Mass conc. & - & max. \(2\cdot 10^{4}\) & max. \(10^{3}\) & max. \(10^{3}\) \\ \hline Number conc. & \(2\cdot 10^{6}\) & max. \(2\cdot 10^{4}\) & - & - \\ \hline Flow rate & 0.3 & 1.0 & 0.3 & 0.1 \\ \(1\cdot\text{min}^{-1}\) & 0.3 & 1.0 & 0.3 & 0.1 \\ \hline Costs & \textgreater{}50,000 & \textgreater{}5000 & \textless{}50 & \textless{}50 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Specifications of the used measuring devices. Figure 1: Test setup for the experimental investigations of aerosols. and the preconditioned aerosol determines the humidity and volume flow of the final aerosol (E). The conditioned aerosol is then led into the test section, where two samples can be measured simultaneously. Various measuring devices can be installed to detect the temperature, the particle size distribution and the particle mass concentration. In addition, a self-developed heating stage and humidity monitor can be placed in front of the sensors for further humidity control. To record the operating parameters, flowmeters from TSI (4000 Series) and sensors from Ahlborn (FHA646-E7C) were used for the aerosol temperature and humidity. The aerosol conditioning in terms of humidification is realized with two unsorted cylindrical packing columns made of PMMA (polymethyl methacrylate) tubes, each with a length of 300 mm and a diameter of 80 mm. The column is filled with approx. 1 L distilled water. A packing of irregular plastic particles with a mean size of 2 cm is added to the column, which increases the surface of the phase boundary and enhances the mass transfer. The airflow is humidified by passing through the deionized water. In terms of drying the aerosol, a heating stage is designed and constructed. It is based on a 200 mm long copper tube with an outer diameter of 10 mm and a wall thickness of 1 mm. The material is characterized by its very high thermal conductivity properties and its good availability, which makes it suitable for this heat transfer application. At each end of the tube, there are tapers to the 6 mm outer diameter to fit the heating section into the test rig. The heating resistance wire made of copper-nickel alloy is wound on the tube. The heating wire is connected to a laboratory power supply by means of two cables, whereby voltages up to 25 V can be set. The correlation between the heating voltage and the specific electrical energy is shown in Figure 2.  
For low specific energy inputs under 3.5 kWh\(\cdot\)m\({}^{-3}\), the temperature is nearly room temperature. In this case, the natural convection counteracts the heating process. For greater energy inputs, the temperature increases in an almost linear correlation.