MODULE V Electrical engineering - Basics Page 45

8. Overview, structure and function of measuring instruments, sensors and arrangement of feelers

8.1 Overview, structure and function of measuring instruments

The following types of measuring instruments can be distinguished:

• Analog measuring instruments
  These measuring instruments indicate the measured value on a scale, mostly with an indicating needle or a pointer (e.g. pressure manometer). If the to be measured quantity (e.g. amperage, voltage or temperature) changes, the position of the indicating needle also changes or, as in case of the mercury thermometer, the length of the mercury column.
  Analog measuring instruments can indicate every value.
  
  
• Digital measuring instrument
  Digital measuring instruments always indicate the measured values with numbers, e.g. 0, 1, 2, ... , 8, 9, sometimes also in combination with special characters like e.g. +, -, %, etc.... These measuring instruments have the advantage that the measured values can comfortably and easily be read.
  The indication of measured quantities and values however, also depends on the digital display. If e.g. the layout of the display is such that only integers can be displayed during e.g. a measurement of the voltage, intermediate values (e.g. 213,5V) can not be indicated.

• Single-purpose precision measuring instruments
  These devices are designed for the measurement of a single particular quantity and only have a very small measuring range. With them one can furthermore normally only measure a fraction of the basic unit.
  Example: The basic unit for measuring voltage is Volt [V]. If measurements in the mini volt range [mV] have to be done, a volt scale can not be used. Instead a special mini volt scale is necessary which in turn can only show a particular measuring range, because it otherwise would become too large.
  
  
• Multi-purpose precision measuring instruments (multi-function precision measuring instruments)
  These measuring devices can indicate various measured values and quantities in different device-specific measuring ranges. Corresponding adjustment and connection possibilities make this possible.
  
  
• Single-purpose measuring instruments
  For measurements that have to be frequently repeated and where always the same values and quantities have to be measured or controlled, usually single-purpose measuring instruments are used. These inexpensive and easily to handle measuring devices are normally sufficient for many measurements .
  
  
• Multi-purpose measuring instruments (multi-function measuring instruments - multimeters)
  Multi-purpose measuring devices are used if different electric values and quantities have to be measured which each have different measuring ranges. Because of the multi-functionality of these devices however also the measuring error probability is increasing. Measuring errors can e.g. be the result of wrong device adjustments. So, operating this kind of devices requires increased attention. Furthermore, multi-purpose measuring devices normally are relatively more expensive and susceptible to errors than other measuring devices.