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
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
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).
3 - take the device - don't switch it off - to where you want
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
By that time, the PM2.5 indicator will be blinking the result of
the finished cycle.
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.
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
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'.