Indoor Performance Testing Results

To demonstrate the performance of the Speck air quality monitor in a typical home environment, we installed a Grimm EDM 180 federal equivalence monitor (FEM) in a home kitchen along with three Speck units. Excluding a few hours where the monitor was switched off while guests were in the home, the test represents approximately 55 hours of co-located data collection.

The Grimm EDM 180 is an optical FEM that measures PM1, PM2.5, PM10, and particle counts within 31 bins representing particle sizes from 0.25 to 32 microns. Our EDM 180 is capable of sampling once every 6 seconds, while the Speck is configured in this experiment to measure every second. In order to compare coefficient of determination values, we generate 6-second averages of the Speck readings at each of the EDM 180 timestamps.

Figure 1
Figure 2
Figures 1 and 2 show the estimated PM2.5 values for each Speck as well as PM2.5 measured by the Grimm EDM 180 FEM. In this test, each of the three Specks slightly overestimates PM2.5 as reported by the EDM 180, but the general shape of the air quality events remains preserved. Speck 3 is unusual in that it settles at a much higher minimum value, which may indicate that this unit is in need of cleaning.

Figure 3

Figure 3 shows the correlation coefficient (r2) for each Speck with respect to each of 30 Grimm EDM 180 bins, plot according to size. The 32 micron bin, representing the largest measured particles, is excluded because no particles this large were recorded by the EDM 180 during this experiment.

From this plot, we see that the Speck's correlation coefficient is greater than 0.8 for particles from 0.7 to 4 microns. These values may vary somewhat by location because of different particle size distributions.

Figure 4

Figure 4 shows the r2 values between different size channels of the EDM 180. These values are predictably similar for similar sizes. This figure shows that similarly sized particles tend to appear and dissipate at the same time.

The Grimm EDM 180 also reports mass fractions for PM1, PM2.5, and PM10. These values represent total mass values for all particles below 1, 2.5, and 10 microns respectively. The r2 coefficients for the three Specks versus these three values are given in Table 1 below. The Speck exhibits a higher correlation with larger mass size fractions, despite having a highest correlation value with individual particle size bins between 0.7 and 4 microns in this experiment.

Speck vs Mass Fraction r2 Coefficients
Speck 1 Speck 2 Speck 3
PM1 0.20 0.19 0.17
PM2.5 0.46 0.44 0.42
PM10 0.83 0.83 0.81
Table 1
Table 2 shows r2 values for each Speck versus mass values from particles smaller than 1 micron, between 1 up to and including 2.5 microns, and 2.5 microns up to and including 10 microns respectively. Here we see that the correlation coefficients are similar for the latter two ranges.

Speck vs Ranged Mass r2 Coefficients
Speck 1 Speck 2 Speck 3
PM1 0.20 0.19 0.17
PM2.5 - PM1 0.82 0.82 0.83
PM10 - PM2.5 0.83 0.84 0.81
Table 2
Table 3 shows r2 values between each pair of PM1, PM2.5, and PM10 values from the Grimm EDM 180. As with individual particle bins, there is a non-zero correlation between each of these three mass fractions, indicating a relationship between the rise and fall of large and small particles. Independent control of smaller particle size ranges will require controlled chamber testing with aerosol size standards.
Mass vs Mass r2 Coefficients
PM1 PM2.5 PM10
PM1 1.00 0.90 0.40
PM2.5 0.90 1.00 0.66
PM10 0.40 0.66 1.00
Table 3

These results show that the Speck correlates strongly to fine particulates especially in the range of 0.7 to 4 microns, and exhibits decreasing sensitivity to larger particles. While the correlation is stronger between the Speck reading and PM10 with r2 = 0.83, there is also a nontrivial correlation of r2 = 0.44 between the Speck and PM2.5.

These test results are also available for download as a PDF document.