Why not pressure gauge used on CO2 fire extinguisher ?

CO2 in a fire extinguisher has it's own vapor pressure. Which in a closed unit like a fire extinguisher is about 750 to 800 psi at 70 degrees F. So basically if there is ANY CO2 present, you have pressure. You really don't need the guage. You can check to see if the unit is full simply by checking it's weight

Because they must be weighed to determine the amount of contents.

There IS a pressure gauge. Notice that the gauge consists only of a pie chart-like display due to the fact that the amount of the pressure - hence, its amount of carbon dioxide - is more important to the end user and inspector rather than the numerical value of the pressure.

 

carbonic extinguisher CO2 anhydric can be used on most high electric voltage fears limiting heat temperature of conservtion 38c