Abstract: A retrofit unit for retrofitting a fire extinguisher, the retrofit unit including a body portion, having a connector for connection to the fire extinguisher, an electronic pressure sensor, connected to the body portion, and adapted to sense pressure within the fire extinguisher, a processor adapted to control operation of the pressure sensor and to receive at least one pressure signal sensed by the electronic pressure sensor, a power source adapted to provide power to the processor and to the electronic pressure sensor. The retrofit unit may be fitted onto a fire extinguisher for measuring pressure therein, instead of, or in addition to, a mechanical pressure gauge.