The disadvantages of pocket dosimeters begin with their cost — generally around $150 per unit. Readings should be obtained at least daily, as the electric charge tends to escape the chamber over time, resulting in an inaccurate reading. Pocket dosimeters also can discharge if dropped or subjected to other mechanical shock during the normal working day, again resulting in an inaccurate reading. The ionization chamber must be charged to a pre-determined voltage before use so that the indicator reads zero. Unlike film badges, pocket dosimeters provide no permanent, legal record of exposure. Therefore, readings must be recorded diligently by radiation safety personnel. 2