Electrometers are instruments that measure electric charge and/or electric potential difference. Many devices can measure voltage and charges with very-low leakage currents to 1 femtoampere (fA) or less. There are three main types of electrometers: vibrating reed, valve, and solid-state. Vibrating-reed electrometers contain a variable capacitor and a fixed-input electrode. As the capacitance varies, an electric charge is forced into and out of the capacitor. Valve electrometers contain an unbiased vacuum tube with a very high gain and input resistance. The input current flow into the high-impedance grid, amplifying the generated voltage. Solid-state electrometers contain a solid-state amplifier with field-effect transistors (FETs).
Selecting electrometers requires an analysis of product specifications. Basic parameters include height, width, and rate. Rate and charge are specified at both low range and high range. Choices for charge collections are triggered, timed and continuous. Electrometers with triggered charge collections can automatically start, stop, reset and save data based on user-defined thresholds. Timed devices have a user-set duration, usually in 1-second increments over a range of 0 - 600 seconds. Products with continuous charge collections have an unlimited duration with manual stop. Range switching with electrometers is user-selectable and available in two modes: high and low. Specifications for electrometers also include repeatability, stability, response time, stabilization time, zero drift, zero shift, bias voltage and power supply.
Electrometers are tested in accordance to standards from national and international organizations. For example, the European Economic Community (EEC) requires CE marks for all electrometers that are sold or put into service for the first time within any member state. The International Electrotechnical Commission (IEC), an organization that also prepares and publishes standards, has defined IEC 60731 as its relevant specification for electrometers. The IEC is the European equivalent to Underwriters Laboratories (UL), an independent testing agency that provides the UL Mark.
Electrometers are used in medical applications such as brachytherapy, external beam radiation therapy, and diagnostic radiology. Devices that are used in brachytherapy are designed to provide rapid measurements of low-activity isotopes and higher iridium sources. Electrometers for external beam radiation are suitable with ion chambers, data acquisition in water phantoms, and quality assurance (QA) tests. Products for diagnostic radiology are used in conventional applications, as well as with mammographic and computed tomography (CT) scanning.