Pressure instruments use many different sensing technologies. Mechanical deflection devices such as diaphragms, Bourdon tubes or bellows consist of an elastic or flexible element that is deflected mechanically by a change in pressure. Sealed pistons or cylinders are also available. Strain gauges are bonded to a larger structure that deforms as pressure changes. Piezoresistive devices sense shifts of electrical charges within a resistor. Piezoelectric pressure instruments measure dynamic and quasi-static pressures. Pressure instruments that use microelectromechanical systems (MEMS), variable capacitance, and vibrating elements are commonly available.