Alternative operational modesDirect-indirect modes:Direct: the desired measurand can be interfaced directly to a sensor and derived thus.Indirect: the desired measurand is not accessible directly, then either another measurand that bears a known relation to the desired one or some form of energy or material that interacts with the desired measurand to generate a new measurand that is accessible, e.g. cardiac output (volume of blood pumped per minute by the heart).