[leggomylibro]

Maybe add another sensor? Humidity and temperature seem like useful readings for these sorts of monitors, and a lot of those sensors include barometric pressure for free.

Although there is an old joke about their accuracy: "how do you use a cheap pressure sensor to determine altitude? Drop it out the window and count how long it takes to hit the ground."

There are also affordable gas concentration sensors which can be useful for this sort of thing. Some are even marketed as 'air quality sensors' because they exhibit sensitivity to several different types of gases, but they aren't always useful because the output is usually just the sum of the sensor's response to each individual gas. Still, if you want a project to probably depress you in 10 years, get a CO2 sensor like the MH-Z19B and make yourself a datalogger. (Keep in mind that it's a little power-hungry, up to 150mA @5V = 0.75W)

[joshefin]

The CO2 sensor would be a nice addition to my project. Do you have experience with the MH-Z19B, is it accurate enough for indoor use?

[leggomylibro]

I've only used it briefly, but I have also seen it used in DIY building management projects. The biggest issue I saw with it was power consumption: up to 150mA @5V is 0.75W, and I think it can take a bit of time to 'warm up' so you can't just turn it on for a few milliseconds every hour. But that should be fine if it's plugged into the wall.

It's also fairly expensive, on the order of $10 rather than $1 last I checked.