Covers all the various measuring instruments and auxiliary instruments required to pick up a measured variable, relay and modify a measured signal and output a measured value to represent a measured variable (Figure M 19). Between these can be found transfer elements such as measuring amplifiers, transducers and measuring converters. Since these devices are connected in a "chain", the measuring device is also called a "measuring chain".
The measuring device must be chosen very carefully in accordance with the respective requirements, since errors occurring between the comparison position and the measuring position in the forwards branch of a control circuit are suppressed in accordance with the loop gain. Errors occurring between the measuring position and the comparison position in the feedback section will, however, have a real impact on the controlled variable, which can never therefore be more accurate than its measuring device!
Figure M 19: Tasks associated with a measuring device