AirGradient’s SGP41 post, a manufacturer test against an Empa PID, found that the official output is a relative index that often returns to 100 after 24 hours. An electrochemical HCHO listing can be humidity- and alcohol-sensitive. A ten-minute outdoor zero is not a traceable calibration. Neither number is a medical result, and neither proves a newly decorated room is safe.
What the SGP41 write-up actually measured
Krause and Brooke, writing on the AirGradient blog and thanking Empa for a PID reference, compared two SGP41 sensors. Raw signals tracked each other (adjusted R² 0.85). After a 1/raw linear fit against the PID, R² was 0.27: the trend could be followed, the amplitude could not. The official output is a relative index. After 24 hours it often returns to 100, so a lasting high concentration can disappear from the display.
Those figures are the manufacturer’s, on that sample, against that PID. They are not AirMeasure UK measurements. These pages do not publish a ppb “toxicity” number from a consumer TVOC channel.
Electrochemical HCHO is still a cell
A related shop page for vibtest.co.uk wrote “electrochemical”, 0.000–1.999 mg/m³ and 0.001 mg/m³ resolution. A review mentioned a ten-minute outdoor fresh-air zero. Outdoor air is not a formaldehyde-free standard, and a ten-minute rest is not a laboratory calibration. Selectivity, humidity response and a chip identity are not shown on that listing. Those gaps are why the page is not treated as a traceable HCHO method, not a measured error on any named unit.
SAMHE records TVOC in classrooms with low-cost sensors. The project page is a research network, not a statement that those TVOC values are reference VOC.
What these pages do not say about renovation
These pages do not say a room is safe, that formaldehyde has “passed”, or that a consumer HCHO reading is a traceable concentration. A colourimetric tube, DNPH or a laboratory method would be a different document. This publication has not used those methods.
Sources and scope
Sources checked on 23 September 2026. Numbered references point to the original publishers. Their sample measurements are not AirMeasure UK measurements. Manufacturer, tutorial and official material is identified separately.
- AirGradient, How accurate is the Sensirion SGP41 TVOC sensor? ↗Anika Krause and Ethan Brooke, 10 December 2024. Manufacturer test thanking Empa for a PID reference. Two sensors’ raw signals had adjusted R² 0.85 with each other. After a 1/raw linear fit against the PID, R² was 0.27. Official output is a relative index that often returns to 100 after 24 hours, hiding a lasting high concentration.
- AirGradient, Photo-Acoustic vs NDIR CO₂ sensors ↗Achim Haug, 22 September 2024. Manufacturer science post. Indoor two-week tracks of SenseAir S8 and SCD40/41 were close. Outdoors, two S8 units correlated at R² 0.90 and two SCD units at 0.36; versus a reference, S8 about 0.64–0.69 and SCD about 0.16–0.24. eCO₂ from SGP30 is written as highly unreliable; a deodorant can raise it. These are AirGradient’s tests, not an independent laboratory and not AirMeasure UK numbers.
- Yoctopuce, Testing Sensirion’s STC31 CO2 sensor ↗22 December 2023. Author’s humidity-disturbance trial. An uncompensated reading collapsed toward negative values when a finger approached. After SHTC3 humidity injection the disturbance remained but stayed inside the stated ±0.5%. The author writes that STC31 does not replace SCD30-class accuracy. Manufacturer tutorial, not an independent laboratory.
- SAMHE, About the project ↗UK school research project. Classrooms record minute-level carbon dioxide, PM2.5, TVOC and temperature. The project is a research network, not a monitor-buying guide. Low-cost TVOC in that network is not a reference VOC measurement.
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