Carbon dioxide extinguishers do not have pressure gauges because carbon dioxide is a condensable gas. Thus, pressure does not tell you how much agent remains in the cylinder. Instead, the extinguisher should have a tare (empty) weight stamped on it. To determine the amount of carbon dioxide remaining in the extinguisher, subtract the tare weight from the current weight.
CO2 (carbon dioxide) extinguishers are for class B and C fires. They don’t work very well on class A fires because the material usually reignites. CO2 extinguishers have an advantage over dry chemicals in that they leave behind no harmful residue. That makes carbon dioxide (or Halotron I or FE-36; see below) a good choice for an electrical fire involving a computer or other delicate instrument. Note that CO2 is a bad choice for flammable metal fires such as Grignard reagents, alkyl lithium, and sodium metal because CO2 reacts with these materials. CO2 extinguishers are not approved for class D fires!