Why not? For precision glass thermometers in laboratories mercury is still the gold standard.I don't know how much a red liquid (don't use mercury) thermometer costs in Thailand.
There may be something wrong with your thermometer. Try another thermometer.No ventilation hoods, just in large pot, 60 liters with 50 000 btu gaz burner, I also tried a 2 liter pot in case of limited heat distribution, same result.
Thermometers are Taylor instruments:
1479 5* Commercial Wireless Remote Thermometer
Plus a standard glass candy thermometer, they all agree on the readings.
I am boiling tap water, maybe I'll try distilled or ionized to see if it makes a difference?
You shouldn't find mercury thermometers in a food grade manufacturing facility. The manufacturing facility I'm familiar with has specific instructions against having them on the property.Why not? For precision glass thermometers in laboratories mercury is still the gold standard.
Probably better to say it has ridiculously bad accuracy. It could indeed measure temperature with an acceptable precision but just not accurately.