Category: Low-cost sensors

Yared Godine and I have installed the first four permanent PM2.5 measurement locations in Arba Minch. The locations are part of a planned low-cost measurement network. We have installed SPSA sensor systems at the Arba Minch University (AMU) main campus gate, at the AMU Kulfo campus gate, in the Limat neighborhood, and at the main commercial bank (CBE) compound in the center of Arba Minch. The sensor system measures PM2.5, relative humidity and temperature on a one-minute frequency. It stores the data on an SD card.

We plan to install sensors at six other locations over the coming weeks. The locations will be at the main hospital, at the bus station, in some neighborhoods, and near to a busy road.

Low-cost installation

A low-cost measurement network goes further than merely using low-cost sensors. We have conducted low-cost field installation as well. Yared Godine, staff of Meteorology and Hydrology, has many years experience with setting up weather stations. He is incredibly creative with waste materials, and has good connections with workshops at Arba Minch University. The following are some of the ideas that went into the field installation:

• Protective boxes were created from unused computer stand carts;
• Discarded metal sheets and pieces were used for rain protection and hanging the boxes;
• Individual copper wires of discarded internet cables were used to make power available.

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Unused computer stand carts are turned into protective boxes

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Parts of discarded sheet metal are turned into rain covers

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Yared’s good connections at the workshops make for quick help on all the metal work.

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Discarded power sockets are prepared for field installation.

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Interested audience are quick to help with installing the power supply. Copper wires of old internet cables are used.

Yared’s connections at the workshops made for quick metal work, and interested audience at the installation locations made for helpful support. The final ‘costs’ for installation were approximately 400ETB ($3.50) for a shared lunch and coffee.

Low-cost operation

With staff from different faculties, we aim to run the measurement network with student science. Students from electrical engineering, meteorology and hydrology, environmental sciences and environmental health will maintain instruments and collect, process and analyze measurement data, as part of their respective courses.

Last March, Professor Solomon Bililign donated thirty SPS30 Sensirion sensors, with the goal that apart from Arba Minch University at two other universities teams would start building PM sensor systems. A team at Debre Birhan University (DBU) constructed their first working sensor system. The team consists of Abebe Tsegaye, Abreham Lakew and Matiwos Zenebe, staff of Electrical and Computer Engineering. With their sensor system, PM2.5 data from the SPS30 Sensirion, together with time from a DS3231 real time clock, is stored on an SD card.

The DBU team received ten of the donated SPS30 sensors. They received build instructions and the sensor system software from me. The have added all other components and work from their own time and budget. It is a promising sign that the university team has done all this without any external project funding. While any future upscaling might make fund applications relevant, the DBU team is already showing and increasing their expertise. This makes any future fund applications more likely to succeed, but also shows that DBU can act independently.

The DBU team will install sensor systems at outdoor locations in Debre Birhan. They will install the first systems at campus and the university hospital. Other locations are planned close to industry and next to roads.

I have brought six SPSA sensor system to Addis Ababa. Tesfaye Mamo, staff of the department of Physics at Addis Ababa University, will collocate them over the coming weeks at some ground measurement locations of the MAIA mission in Addis Ababa.

Earlier, we tested the SPSA in Arba Minch, as well as at Ethiopian Meteorology Institute stations of Addis Ababa and Adama. Part of this data has been published. The new sensor systems will collect PM2.5 data next to Purple Air sensors (commercial low-cost sensors). Comparison of the data will show whether the SPSA sensor systems can produce data as reliable as (or even more reliable than) the Purple Air sensors. If this is the case, air quality monitoring capacity in Ethiopia could be increased with locally constructed sensor systems rather than commercial ones from outside. This would result both in cost reduction and local expertise increase.

Anticipating an increase in Arduino-based PM sensor systems, I have made an update in the low-cost sensor software code. I also shifted to another sensor identification system. So far, I used for my measurement instruments a four-letter abbreviation (e.g. SPSA for the Sensirion SPS Arduino sensor, IQAV for the IQAir Airvisual Pro), in combination with a two-digit identifier (e.g. SPSA01, SPSA02, …). This system accommodates a maximum of 100 unique identifiers per instrument type.

I have used up to 33 so far. Since my presentations in Addis and publication, some other persons and organizations have expressed their interest. We built ten sensor systems for a PhD student last June. A team at Debre Birhan University has recently built their own system according to our design. I plan to install some ten sensors in both Arba Minch and Addis Ababa the coming year. I am in discussion with universities across Ethiopia for building and/or using the sensor systems as well. Hence, I have decided to shift to the following naming convention: SPSA_XXXX (SPSA as four-letter abbreviation of SPS30 Sensirion with Arduino, XXXX being four digits).

The software

Apart from the naming convention, I have made the following changes versus the earlier software versions:

• Apart from a datafile, the sensor system also creates a metadata file. In the metadata file, for every restart of the sensor system it adds a line with the time, software version, device ID, serial number of the SPS30 Sensirion sensor, and the sensors of that specific SPSA.
• As data from the SPS30, not only PM values (PM1, PM2.5, PM4, PM10), but also particle numbers of the separate bins are saved.
• I have included a start-up sequence: first 50 seconds of low power mode, after which 30 seconds of SPS30 cleaning mode. I introduced the low power mode for the use of a Li-Ion 18650 rechargeable battery with battery shield as power bank. When the Li-Ion battery is fully depleted, and power comes back, the sensor system at the very start draws too much power. It therefore does not manage to start at all. Instead, at the moment the power comes back, the sensor system should start at low power. Within 50 seconds, there is some charge back in the battery, with which the system can start.

You can find the sketch (Arduino software code) for the set-up here. The sketch is for the following configuration:

 * SPS030......Mega
 * 1 VCC.......5V
 * 2 SDA.......SDA (nb: with 10k pull up resistor, or other I2C sensor)
 * 3 SCL.......SCL (nb: with 10k pull up resistor, or other I2C sensor)
 * 4 Select....GND (select I2c)
 * 5 GND.......GND
 *
 * BME280......Mega
 * VIN.........5V
 * GND.........GND
 * SDA.........SDA
 * SCL.........SCL

 * SD module...Mega
 * GND.........GND
 * VCC.........5V
 * MISO........50
 * MOSI........51
 * SCK.........52
 * CS..........53
 *
 * DS3231......Mega
 * GND.........GND
 * VCC.........5V
 * SDA.........SDA
 * SCL.........SCL
 * 
 * Green LED....Mega
 * +............8 [+: longer leg]
 * -..resistor..GND [-: smaller leg]

AirQo has agreed to host our PM2.5 measurement data on their platform. AirQo started at Makere University (Uganda). It is currently the only builder of professional low-cost PM sensor systems in the African continent. I met them at the international conference “Together for cleaner air in Ethiopia” in Addis Ababa, December 2023. Their sensor systems provide real-time data, and this data is hosted on their platform. Over the past years, I have collected data at some locations in Arba Minch, and the coming year I plan to install fixed locations. Possibly I will include real-time data transmission on those measurement systems. However, AirQo also agreed to host historical data, based on CSV files I submit to them.

As a sample, I submitted historical data collected between April – May 2021. You can download it by visiting www.analytics.airqo.net. This proof of concept shows the future opportunity of locally constructing sensor systems in Arba Minch, installing them across Ethiopia, and openly sharing the data.

I am glad to share the successful import of components for fifty PM2.5 sensor systems. For the low-cost sensor systems, most items are available in Addis Ababa – but for a much higher price due to import restrictions. Last month, fifty pieces of all components were ordered in The Netherlands, and I was able to bring all these items into Ethiopia. For total costs of 2500 euro, and additionally 90,000 ETB of import tax, we have enough materials to construct more than fifty new measurement instruments. Considering that all measurements up to now have been conducted with approximately twenty systems, this is a huge increase of the measurement capacity.

A list of all materials, with links to the sellers:

• 50 Arduino Mega 2560 microprocessor boards;
• 50 PCB prototyping boards;
• 50 microSD card SPI modules;
• 100 microSD 512MB cards;
• 56 SPS30 Sensirion sensors;
• 100 JST 1.5mm 5Pin female cables;
• 20 BME280 sensors;
• 70 Li-Ion 18650 battery shields;
• 10 battery shields for two 18650 batteries;
• 10 battery shields for four 18650 batteries;
• 50 Li-Ion 18650 batteries;
• 10 memory card readers;
• 30 1-meter microUSB cables;
• 20 2-meter microUSB cables;
• 50 USB adapters
• 50 DS3231 real-time clock modules;
• 50 USB A – USB B cables of 25 cm.