Reliability of the sensor: Part 1

I was very worried that the sensors wouldn’t be very good. After all, they’re cheap, poorly documented, and come from a virtually unknown manufacturer.

Happily, there are statistical tests to to tell how good the sensors are—and the TL;DR? They’re not bad at all!

I used a statistical test called the Intra-Class Comparison to measure whether the four different Pis I built agree with each other. And they do! When one Pi reports bad air quality, the other Pis tend to do so, too.

Of course, that’s the short version. Herewith, the longish version:

In a perfect world, the air quality monitors would all report exactly the same number, like in Figure 1. They would give us results that are perfectly correlated.

Figure 1: Perfectly correlated AQMs

Of course, it’s not a perfect world, and these are cheap sensors. They vary. The issues, then, are how much they vary, and whether that is an acceptable amount.

Figure 2 shows another imaginary example. The AQMs make wild swings, and, worse, the swings are uncorrelated. When one AQM reports an increase, another reports a decrease. When one goes up a lot, another goes up a little. It’s a mess.

Figure 2: Uncorrelated results

These are uncorrelated results, and if we received them, we would know our sensors are random number generators.

There are middle grounds between perfect correlation and no correlation at all.

In Figure 3, the results are loosely correlated. Generally, when one sensor reports a change, the others do, too. However, in each case, the sensors measure different sizes of change.

Figure 3: Loosely correlated results

And in Figure 4, we have an excellent result, and one that somewhat approximates the results I found with the four Pis I used: each sensor reports a consistent change. If Pi1 reports, say, a PPM of x, then Pi2 reports a PPM of x+2.

Figure 4: Excellent results

 

Of course, it would be best if there was no variation between the sensors, and if they reported exactly the same results. But, if they’re going to vary, this is just the kind of variance we want, because it’s easily corrected for. And, more or less, that’s what we got.

The actual correlation for a 24-hour period

Our Pis varied, but they varied by a reasonably consistent amount. Pi1 was always a little higher than Pi2, which was usually a little higher than Pi3 and Pi0.

This is great, because it means we will be able to make meaningful comparisons between different parts of the neighbourhood. We can take the results from one AQM, adjust them by the constant, and compare it to the other adjusted AQMs. Thus, we will be able to see whether local pollution conditions are better (or worse) than other locations in the area.


I can hear you in the back. Correlation does not equal causation. Not quite right, but I catch your drift.

Correlated results aren’t necessarily good results. For instance, our air quality monitors could, unbeknownst to us, be measuring humidity, not pollution. As long as they are all measuring humidity consistently, we would never know, because they are all correlated.

Quite so. Still, I can’t think of any way to check every possible, non-particulate, cause. It could be that they are all measuring humidity, temperature, sunshine, the radio waves, sunspots, or the Blue Jays’ score. At some point, we have to just have faith that the sensors are doing what they say they are doing and look like they are doing.

Your first results

When your Pi boots, it should start recording air quality data. It won’t flash or bing or do anything science-y sounding. Your only chance to notice it will be once an hour, when the fan starts spinning to suck air into the monitor. (If you really want to check, you can log into the Pi over VNC and see if it’s working by searching through running processes.)

As it measures the air quality, the Pi is recording data to a spreadsheet in the AQM folder called “alldata.csv”. It is also trying to send data to my webserver, because I haven’t got around to fixing that yet—not to worry, though; no data is being sent because your Pi is not able to log into my server (and I’m not able to log into your Pi).

The Pi saves a lot of data to alldata.csv. It saves 40 measurements an hour (20 for each of PM2.5 and PM10). There’s no good reason for this, and I should make it save only an average, but it has proven useful¹, and there’s no discernible harm (after 15000 measurements, alldata.csv is still less than a megabyte in size).

The number of measurements does make drawing inferences a little difficult. The trick is to use the moving average function on your spreadsheet software of choice. Chart 1 shows the AQM data for a week in May, 2018, in my  backyard. I’ve drawn two moving averages, one for each of PM2.5 and PM10.


¹ The Pi takes 20 measurements, once an hour. Weirdly, the first measurements are always lower than the others. I’m glad I kept all the data (over Mohammad’s objection) because we were able to find this flaw. It doesn’t make a lot of difference to the results because we are making relative comparisons and the error is consistent. But it’s there.

Figure 2: Measurements over one minute. Note that the first measurements are lower than the final measurements. This is a consistent error.

Assembling the air quality monitor

We wanted our air quality monitors (AQMs) to be weatherproof, so I used cheap, dollar-store tupperware enclosures with snap-tight lids. Each was $1.50 CAD.

The SDS011 must be connected to a short hose if it’s going to be enclosed. I used some hose I had around the house from making beer.

The tupperware needs three holes: one for the intake, one for the power cable, and one for the air outlet—which I, stupidly, forgot at first.

I used a hot glue gun to melt holes in the sides of the cases, pushed the tubes and cable through, and then sealed them up with hot glue. (I did try using caulk; hot glue worked better.) The power cord was rather larger, so I also sealed it with Gorilla Tape, just to be extra sure.

Hot glue to the rescue!
Version 2, with caulk. I made a third version with the case of 2 and the glue of 1. You get the idea.

The local software

The Raspberry Pi needs software to power the SDS011 and collect the data it produces. Ours improves on other software available online, we think, because:

  • It measures consistently, without wild swings due to mathematical errors
  • It produces a local and an online copy
  • It extends the useful life of the sensor by powering it down between measurements

To use our software in its basic form, there are three steps, explained below:

  1. Download this package to your main computer and unzip it
  2. Copy it to the desktop of your Pi with RealVNC.
  3. Create a startup script that will start the software every time the Pi boots

Step 1: Downloading the package from adamnorman.com

This should be easy. I’ll leave you to it. Download this.

Step 2: Copying the files to the desktop

RealVNC allows you to copy files between your server (the Pi) and your viewer (your computer). As of June, 2018, the process for doing so (on a Mac) is as follows:

Open "File Transfer" from the hamburger.
Open “File Transfer” from the hamburger menu.

Open RealVNC on the Pi by clicking on the black and blue VNC icon in the top right of the Pi’s menu bar. Open “File Transfer” from the hamburger menu.

Select the desktop for the location your files will be saved to under the “Fetch Files to” option box, and close that dialog.

Select the Desktop as the location in “Fetch Files to”

Placing your cursor on the menu bar of the RealVNC client window. Click on the two-way arrows.

Click on the two-way arrows.
Click on the two-way arrows.

Click on “Send Files” to send the files you unzipped on your computer to the desktop of the Pi.

Click on “Send Files”.

Your files should now appear as a folder on the Pi’s desktop. Important: the program will only work if it’s installed on the desktop.

Step 3: Creating a boot process

You will now force the Pi to start the measurement software every time it boots up. Unfortunately, this step requires using the Terminal, which is a pain. Not to worry, though; you only need to type, and you won’t need to understand what you’re typing.

Open the Terminal by clicking on the raspberry in the top left, then “Accessories”, then “Terminal”.

Open the Terminal.

Now type: “sudo nano /etc/rc.local”. This will open a very ugly, very tiny version of Microsoft Word right in the Terminal window.  You’ll use this word processor to edit one of the files the computer reads when it starts (the file is called rc.local).

Move the cursor (with your keyboard arrow keys) to the line that says “fi” in green. Press Enter or Return on your keyboard to make a new line.

Type (or copy) the following words into the document, on the blank line below the word “fi”: “sudo python3 /home/pi/Desktop/AQM/main.py &”

Type the line “sudo python3 /home/pi/Desktop/AQM/main.py &”

Press Ctrl-X, and save your work.

The computer will read rc.local when it boots, and will start the program main.py when the it boots.

We still need to build the sensor, but the programming part is done!

Getting the Pi running

To set up the Pi, hook it up to an HDMI-compatible TV, a keyboard, and a mouse. They keyboard and mouse must be attached with a USB hub and an OTG USB cable–an adapter that converts the full-size USB cable to a micro-USB male end.

Configuring the operating system is  straightforward, with a series of dialogs to help users configure the settings.

Only four small customizations are required:

  1. Setting the time to local time,
  2. Installing VNC
  3. Changing the system password
  4. Getting the wifi running

Setting the local time is required to get the Pi to report pollution data accurately, and VNC is used to control the Pi from a remote computer. It is software that ‘projects’ the Pi’s desktop onto your desktop, and allows you to control it as if it were in front of you, with a keyboard and mouse.¹

Both VNC and the time are set within the System Preference dialog, which is under the Raspberry icon at the top left.

You can set the Pis preferences in the Raspberry menu in the top left.

Click on the Raspberry, then Preferences, then Raspberry Pi Configuration

Chance the password while you’re at it.

It would be a good idea to change your password.

Enable VNC, which will let you connect to your Pi over the internet using a keyboard and mouse.

Enabling VNC will let you use the software RealVNC to connect remotely to your Pi.

Set the timezone in the same panel.

Set your timezone.

 

Next, you will need to connect to your wifi, which is very straightforward, though confusingly named. When the Pi asks for your “shared key”, enter your wifi password, if any.

After you’ve enabled VNC on the Pi, it’s easy to connect to it with RealVNC. You will need ‘client’ and ‘server’ software, on your home computer and the Pi respectively, but it’s no harder to use than GMail.

Finally, once RealVNC is up and running, you’ll may want to allow your Pi to be remotely administered over the internet (and not just your local network). If so, enable cloud connections under the RealVNC options menu.

 

 


¹ There are other ways to do this, using the terminal and SSH. They are agonizing.

 

The Raspberry Pi Zero W

The air sensor connects via USB to a computer. For indoor use, hooking the sensor up to any computer would be good enough for spot readings, but our plan was to put the computers outside. We decided to use a tiny and very cheap computer called the Raspberry Pi Zero W.

Image from [1]
Raspberry Pis are bare-bones computers that cost between $10 and $50, but do not include screens, storage, or any peripherals—not even an electrical cable to power them with.[1] They do, however, run a full operating system, which makes them quite easy to use compared to the alternatives we considered for this device (notably Arduino).

We settled on the second-cheapest Raspberry Pi because we have very simple computing requirements. Though the Pi Zero W is very slow compared to any other modern computer, it is certainly capable of doing the computations we need. We did not use the cheapest Raspberry Pi, the Pi Zero (without a W), because we wanted our computers to be able to report wirelessly.

We were forced to purchase the Pi Zero Ws in kits, which included a case, a power supply, and a MicroSD card, because the computers alone are rationed out at one per customer. Unfortunately, this drove the price of each Pi up from an advertised price of $13  to $65 (CAD, including tax and shipping).

The Pi Zero W (which I’m going to call “the Pi” from now on) runs Raspbian, a free operating system, which can be installed using a utility called NOOBS. Raspbian comes with the free programming language Python preinstalled. We used Python to collect and manipulate the measurements from the SDS011 air-quality sensor.

Setting up the Pi is fairly straightforward if you have the cables and peripherals—but to do it over a graphical interface, you’ll need quite a few of those, including:

  • A MicroSD card writer
  • A USB hub
    • And a USB keyboard and mouse
  • A mini-HDMI to HDMI adapter
  • An HDMI cable
  • A USB OTG cable
  • A somewhat beefy USB power adapter

In addition, you’ll need a MicroSD card. These peripherals add considerably to the cost of the $13 Raspberry Pi if you don’t have them in a drawer somewhere.

There is excellent help available on the internet, particularly Reddit, to get you to the point where you can boot the operating system.


1. New product! Raspberry Pi Zero W joins the family – Raspberry Pi. Available at: https://www.raspberrypi.org/blog/raspberry-pi-zero-w-joins-family/. (Accessed: 30th May 2018)

The SDS011

After looking over many options for an air-quality sensor, I settled on the SDS011. It costs about $25, and can be ordered on many Chinese retail sites, such as AliExpress.

Footnote 1.

The SDS011 is well reviewed, and “developed by inovafit, a spin-off from the university of Jinan”[2][sic].  It reports the concentration of ultra-fine (2.5 micron) and fine (10 micron) airborne pollution in μg /m3, which are standard measures called PM2.5 and PM10, respectively.

There are several other sensors, but the SDS011 had the benefit of being well reviewed and capable of being connected over USB. I found that soldering joints and using breadboards were very difficult and unreliable.

The sensor has problems, though. The documentation is sparse, and it does not come with a program to make it function and record the data. These have to be written (or downloaded). The specifications are also written in poor English.

Finally, I found it hard to believe that a $25 sensor would do a good job—that it would be accurate, reliable, and consistent with other sensors. I was glad to be mistaken about these concerns.


 

1. sds011-large.png (500×419). Available at: http://aqicn.org/aqicn/view/images/sensors/sds011-large.png. (Accessed: 30th May 2018)

2. The World Air Quality Index. The SDS011 Air Quality Sensor experiment. aqicn.org Available at: http://aqicn.org/sensor/sds011/. (Accessed: 30th May 2018)