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It is always interesting to know what is going on in your indoor environment. With that in mind, the gremlins at SwitchDoc Labs decided it was time to test out the indoor air quality sensor and see just how sensitive it is. After running the system for a while at the Labs, our CTO said he would take it home for the weekend and try it out in the home environment. His results were interesting and showed that the unit was a lot more sensitive than we initially expected.
SwitchDoc Labs recently released the first expander kit for the OurWeather product, the Air Quality Sensor kit. The Air Quality Sensor kit has everything you need to build an indoor air quality measurement system. Pure Python driver software for Raspberry Pi Available at https://github.com/switchdoclabs/SDL_Pi_AirQualitySensor.
What is the TP-401A?
The TP-401A sensor is very sensitive to a number of air-contaminating substances at low concentrations. Some of these are: Second hand smoke, carbon monoxide (CO), alcohol, volatiles of cosmetics (especially hair spray as we will see later), acetone, thinner, insecticides, benzene, formaldehyde, etc.
The sensor is composed of SnO2 materials doped with a catalyst making it a metal oxide semiconductor type of gas sensor. The resistance of the sensor will decrease when there are contaminating gases and will increase when the gases are removed from the environment. To operate the device needs to be heated to about 200-400 degrees C and this is what causes the current for this device to be about 50mA. A typical response of the sensor to 20 ppm (parts per million) of CO will to take the voltage up about 0.25V ~ 0.60V when the CO is detected.
The Software and Setup
We used the recent released SDL_Pi_DataLogger software to read the data values from the Air Quality Sensor and used the Grove INA3221 Sensor to read current (which was really boring – always at 49mA) . We also used the Pi2Grover – Grove connectors for the Raspberry Pi to hook things up.
Annotations from the Weekend at SwitchDoc Labs CTO House, all times in UTC (+8 Hours to Pacific Time):
A – Dryer Door opened, ironing begun. Could smell fabric softener.
B – Door opened to outside first for Panther The Cat, then door left open for a while. Finally dinner was cooked in oven.
F – Night time – ripples are probably auto fan and air conditioning effects.
C – Door opened for cat. Cat slow to go out. Very slow.
D – First hairspray test. Short sets of hairspray in the bathroom – noted air conditioner was on.
E – Dinner was cooked. Spinach Salad (with fresh Bacon bits).
F – Night again.
G – Hairspray in bathroom, this time the real thing as the family got ready for work.
We were amazed at how sensitive this inexpensive sensor was. We could detect all sorts of events in the entire house. One thing to point out is that virtually all of the time the sensor was under 3200 (rated fresh air) and the average was 2727 across the entire period.
We waited for the morning particulate count to drop and we ran one more test. We decided to do the ultimate hairspray test and use the hairspray near the sensor to see how fast it would react and how bad it would conclude the air was.
Test findings? The Air Quality System really does not like hairspray. The hairspray was sprayed about 18 inches above the sensor and not directly into the sensor. It peaked about 11,000 (High Pollution) and quickly trailed down in the next 15 minutes.
The TP-401A sensor is a pretty sensitive device. While it is not very discriminating (with the possible exception of Hairspray), it is sensitive to a wide range of different air contaminants. The system was very easy to build using Grove devices and was a piece of cake to setup to log the data using the DataLogger software.