By far the most common US civil defense meter on the market today. This is a simple ion chamber radiological survey meter, specifically designed for high-radiation fields for which Geiger counters will give incorrect readings (see above). Survey meters do not read alpha or beta radiation. They work by radiation penetrating the case of the unit and the enclosed ionization chamber to produce a visible reading between 0.1 R/h and 500 R/h (× 0.1, × 1, × 10, and × 100 scales). The CDV-715 ion chamber controls a subminiature type 5886 tube, but no 22.5 volt batteries are necessary for the B circuit of this tube. A transistor oscillator coupled to a step-up transformer furnishes the necessary B current for the tube, with necessary rectifier diodes and filter capacitors.  The entire unit is thus powered by a single 1.5 volt D cell.