How to use the GT58A 'air quality detector'

First of all, the GT58A works quite well.
But the 'how to use' information on sellers' webpages (like Amazon and Walmart) is partly wrong and it comes with a user manual in nothing but Chinese.
Here's how to actually use it:
1 - Give it fresh batteries, or better freshly charged NiMHs. Carry it to
a place in the shade where the air is clean and both temperature and relative humidity are similar to where you want to use the device.
Note that lots of things (for instance plants) emit VOC (volatile organic compounds) so pick your clean air location with care. Switch it on (button with 'ballpoint' action) and leave it in peace with all vent holes clear for half an hour. It has no stand, so you'll have to improvise something.
If you hang around for a few minutes you'll see a counter ticking down from 200 to zero; this shows that the device's (MOX) VOC sensor is being preheated, which happens each time it is switched on.
GT58A switched off
2 - Half an hour later the TVOC and HCHO readings may well be (very
nearly) zero. If not, then press and hold the 'set' button until you get all zeroes (about 3 seconds).
GT58A switched off
3 - take the device - don't switch it off - to where you want to check
air quality. Put it in an improvised stand, briefly press the 'pm' button to start a pm2.5 ug/m3 particle measurement cycle and wait at least ten minutes for the TVOC and HCHO to settle.
By that time, the PM2.5 indicator will be blinking the result of the finished cycle.
GT58A switched off
The LED above the three number displays will be green when air quality is good (TVOC below 0.6 mg/m3), red when it's not, accompanied by frantic beeping when it's so bad that you should either vacate the premises or use a breathing apparatus.
Note that the room where you're checking air quality should not be ventilated in any way.
GT58A switched off
4 - If you want to check somewhere else, take the device there without
switching off and leave it to settle again; start another particle measurement cycle if you need it, else leave it blinking to make your batteries or charge last longer. When all checking is done, switch off.
5 - To use it again after switching off, if possible start again in that place
where the air is clean, and leave it there for ten minutes. This cleans up its sensors and also allows you to check whether it rewards clean air with all zeroes on the TVOC and HCHO displays. If not, either your clean air spot leaves something to be desired or that half hour initial calibration needs repeating.
GT58A switched off
But how to interpret the results?
Right: Click on the photo to see a larger version
I've opened the device's case and found two sensors inside. One contains a laser LED and photo sensor to actually see tiny particles in the airstream blown past by a small fan. The other is a metal oxide (MOX) sensor which measures chemical reactions in a heated porous element. It responds to a long list of volatile organic components, making it pretty good at delivering a useful total VOX (TVOX) number. Formaldehyde is one of them, but I don't see any wherewithal under the hood which might differentiate reliably between that particular compound and all the others. Perhaps it can be done in some way I've never heard of; to be investigated.
And the effect of temperature and relative humidity?
At 20℃ ambient and 30%RH you might see for instance nearly 1.3 mg/m3 TVOC displayed, but the same concentration of volatiles would score only about 0.9 mg/m3 at 85%RH. So the four significant digits on the display may just be a little pretentious.
I consider this device useful when I suspect that some kind of VOC is becoming dangerous. For instance in case of sudden headaches or funny smells and such. I also note that MOX sensors are particularly sensitive to methane (natural gas) so a gas leak should be easy to check for and roughly locate.
As to the pm2.5 particles/m3 sensor - it seems to work, is all I can say.
GT58A switched off
Bottom line:
The device is neatly built, sturdy, simple but functional, and easy to use in so far as you know what you're doing. Unbeatable value for money. But there should at the very least be documentation in a European language somewhere, if only in 'Chinglish'.
GT58A switched off
The user guide for a very similar gadget can be found on this page.
Here is a direct link to that guide, which is of course in only slightly understandable 'Chinglish'.
What the user should know about Metal Oxide VOC Sensors
And about formaldehyde (HCHO) sensing in particular.
GT58A switched off