why there is no pressure Gage on carbon DI oxide fire extinguisher?

Answer 1

Since CO2 extinguishers are filled with liquid CO2 a pressure gauge would provide little information on the state of charge, the pressure says almost constant (varying a bit with temperature) until the device is virtually empty. CO2 extinguishers are usually checked by weight.

Answer 2

Virtually all fire extinguishers used in multiple-occupancy buildings have pressure gages since periodic checks of the extinguishers are required and the pressure gage is the simplest, easiest, and most economical way of determining whether the extinguisher is full. Where inspection requirements are not dictated by some code, people may opt to forgo the expense of a gage.

Answer 3

There's a pressure gauge on the pump used to fill them. Most extinguishers have a gauge that shows empty or full, and that's all you really need.