Dashboard: air quality

The Dashboard shows what CAT's sensors measure right now and turns it into a single letter grade.

Opening the Dashboard#

On the home screen, highlight the first tile (clouds and sun) and press A, or tap it. The Dashboard has two pages. Switch between them with ◀ and ▶ (they wrap around). Press B to go home. It always opens on the summary.

LIVE SUMMARY#

A large letter grade for the air overall, from A (best) through A-, B+, B, B-, C+, C, C-, D+, D to F. Its colour follows the grade: green, teal, orange, then red.

Clean indoor air: CO2 540 ppm, PM2.5 2 µg/m³. Grade A.

A stuffy room: CO2 780 ppm, PM2.5 6.5. Grade B. Three warning boxes.

Poor air: CO2 1450 ppm, PM2.5 38, high VOC and NOx. Grade F.

To the right of the grade, one box appears for each pollutant group that scores B+ or lower: CO2, fine particles, and gases (VOC or NOx). The number of boxes tells you how many groups need attention; the statistics page tells you which.

LIVE STATISTICS#

One row per sensor: the name, the current value, a colour bar and that sensor's own grade. The marker on the bar sits further right the better the reading, so a marker at the green end is good. Values refresh every 5 seconds.

Fair air: CO2 B-, PM2.5 C, VOCs C+, NOx B.

Row	Measures	Unit	Sensor
CO2	Carbon dioxide. Rises when people share a closed room; a good proxy for stale, rebreathed air.	ppm (parts per million)	Senseair Sunrise, pressure-compensated
PM 2.5	Fine particles under 2.5 µm: smoke, cooking, traffic.	µg/m³	Sensirion SPS30
VOCs	Volatile organic compounds (solvents, cleaning products, cooking). An index where about 100 is "normal for this place".	index 0–500	Sensirion SGP41
NOX	Nitrogen oxides (gas stoves, traffic). An index where about 1 is "normal".	index 0–500	Sensirion SGP41

After switching onThe particle sensor needs about 10 seconds to spin up and the gas sensor about 45 seconds to condition. Until then the rows read 0.00.

How the grade is calculated#

Each reading gets a score from 0 (worst) to 1 (best) by sliding between the breakpoints below. The overall grade is the average of six scores: the four sensors above plus temperature and humidity.

Sensor	Perfect up to	Breakpoints (getting worse)	Worst at
CO2 (ppm)	420	800, 1000, 1400	4500
PM2.5 (µg/m³)	0	5, 12, 35, 55	150
VOC index	0	100, 150, 250, 400	410
NOx index	0	1, 90, 150, 300	310

Score	≥ 0.93	0.90	0.87	0.83	0.80	0.77	0.73	0.70	0.67	0.65	< 0.65
Grade	A	A-	B+	B	B-	C+	C	C-	D+	D	F

A single sensor drops to F at roughly these readings: CO2 above about 950 ppm, PM2.5 above about 10 µg/m³, a VOC index above about 140, or a NOx index above about 75.

Temperature and humidityCAT has a temperature and humidity sensor, but its readings are not accurate enough yet, so they are hidden. Both count in the grade as a fixed, comfortable 24 °C and 50% humidity. That nudges the overall grade up a little compared with the four sensors alone.

The e-paper air badge#

The separate black-and-white e-paper screen updates every 30 seconds. It shows:

a unicorn and a cloud whose moods follow the air (happy above 75%, sad below 50%, a smoke cloud below 25%);
CO2 in ppm and PM2.5 in µg/m³ in large digits;
the NOx and VOC indices;
% rebreathed: roughly how much of the air you breathe has already been breathed, estimated from CO2;
CritAQI: a percentage driven by the worst pollutant, with a penalty for each other pollutant that is also bad;
the time of the reading, and --- for any sensor that failed.

CritAQI is calculated differently from the Dashboard grade (worst pollutant rather than average), so the two can disagree. Faint ghosting between updates is normal; a full refresh clears it about every 25 minutes.