Tests
Below we detail some preliminary tests conducted on three MyPart prototypes.
##Outline
- Smoke tests overview
- Smoke test 1
- Smoke test 2
- PSL sanity check
- PSL voltage threshold test
- Real World Test
- Discussion
- Future Tests
One of the particle sources we tested our sensors with was wood smoke. The size distribution for burning wood tends to be in the sub-micron range, which is particularly important to sense accurately with respect to human health. In addition, these particles are easy to generate and decay naturally after being injected into our chamber, which allows us to collect a large amount of data across a broad range of concentrations.
We ran two tests with smoke in our testing chamber to find the correlation between MyPart sensors and commercial sensors. To produce smoke, we set fire to a wooden skewer, blow the flame out, then waft a small quantity of smoke into the chamber before sealing the lid. A set of fans inside the chamber mix the particulates. The sensors run an automated test sequence, all sampling once every minute, until the particles in the chamber decay to ambient concentrations.
| Device 1 | Device 2 | r^2 | slope |
|---|---|---|---|
| MetOne | MyPart 0 | 0.99074 | 0.016 |
| MetOne | MyPart 1 | 0.99657 | 0.0465 |
| MetOne | MyPart 2 | 0.99381 | 0.0401 |
| MyPart 0 | MyPart 1 | 0.99267 | 2.8933 |
| MyPart 0 | MyPart 2 | 0.99038 | 2.4938 |
| MyPart 1 | MyPart 2 | 0.99641 | 0.8616 |
We see very strong correlation between the three MyPart sensors and the MetOne. In addition, we see very strong inter-device correlations between the MyPart sensors.
Sensor 0 is seeing only 30-40% of the particles seen by the sensors 1 and 2. Of the three sensors fabricated, sensor 0 also had the highest noise floor. Though the source of the higher noise is currently unknown, it is likely the cause of the decreased sensitivity.
| Device 1 | Device 2 | r^2 | slope |
|---|---|---|---|
| MetOne | MyPart 0 | 0.98355 | 0.0094 |
| MetOne | MyPart 1 | 0.99355 | .0442 |
| MetOne | MyPart 2 | 0.98992 | .04 |
| MyPart 0 | MyPart 1 | 0.99403 | 4.6847 |
| MyPart 0 | MyPart 2 | 0.99484 | 4.2443 |
| MyPart 1 | MyPart 2 | 0.99684 | 0.9047 |
In this smoke test, we again see very good correlations between the MyPart sensors and the MetOne, as well as between other MyPart sensors.
In addition to being noisier, the noise floor for MyPart 0 also seemed to be inconsistent. The noise floor for MyPart 0 increased before this test, and the voltage cutoff for a sensed particle had to be increased accordingly. As a result, MyPart 0 saw less particles compared to MyPart 1 and 2, but still showed good correlation versus the MetOne.
##PSL Beads voltage threshold test
In this test, the raw voltage readings of the three MyPart sensors were recorded when the chamber was filled with 0.5 um PSL beads, and also for when the chamber was filled with 3.0um PSL beads. The data was analyzed in Matlab, and the number and amplitude of the peaks were found using the built-in findpeaks function. Below, we plot the voltage distribution for each sensor, with the distributions for both particle sizes on each plot.
Though there is an overlap between the distribution of the two particle sizes. (the 50th percentile for the large particles corresponds to roughly the 90th percentile for the large particles for all three sensors), there is a difference in the signal response for the two sizes of particles.
These plots show one source of ambiguity for optical particle counter particle sizing. Though an extremely high voltage peak likely corresponds to a large particle, a lower voltage peak could be a result of a small particle striking the laser at the center or from a larger particle striking the laser slightly off axis. Because we are trying to extract particle size information from intensity of light scattered, other sources of ambiguity include the composition, color, and shape of particle. However, the difference in the distribution suggests that some amount of sizing information can still be recovered.
Using the results here, we set the voltage threshold for a large particle to be at 3 volts to use for our first outdoor test. By setting such a high threshold, the particles sensed as large particles are likely to be actual large particles. However, the amount of large particles detected is sacrificed.
Since factors such as ambient light can affect the readings of optical particle counters, it is important to validate effectiveness in real world environments with varied lighting conditions. Testing in these environments is less controlled, but more representative of real use cases.
We brought the same set of sensors (MyPart 0-2, Dylos, MetOne) on a series of tests outside the lab. We carried them to various locations, some indoors in other buildings, some outdoors. At each location, we gathered two data samples using the same automated testing setup as the previous tests, and recorded the conditions.
Sample locations include:
- Near a group of restaurants
- A grassy field.
- Inside a library.
- Next to a busy road.
- An old classroom building.
