Engineering ToolBox - Resources, Tools and Basic Information for Engineering and Design of Technical Applications!

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An instrument  - or sensor - can be defined as a device that communicates a physical property to an observer. The observer can be a data acquisition system, a process control system and/or in the end a human observer.

Physical Property - Measured Medium

Typical properties to measure are temperature, speed, flow .. and many more.

Some properties - like temperature - are relatively easy to measure. Others - like blood sugar - requires more advanced sensor technology.   


The transducer senses the physical variable to be measured - and converts the physical value to a suitable signal. 

Example: a RTD - Resistance Temperature Detector - that indicates temperature with electrical resistance.

Example: a tachometer that indicates revolutions with electrical pulses from a magnetic sensor.

Signal Conditioning

The signal created by the transducer may need an adaption before transmission - like

  • integration
  • amplifying or attenuation
  • converting to digital or analog signal 


The transmitter responsibility is to connect and communicate the instrument value to the control or data acquisition system. The transmitter communication is based on standardized protocols and signal levels. Typical analog values are 4-20 mA for electrical communication or 3-15 psi for pneumatic systems. 

Data Acquisition or Control System

 A data acquisition typically accumulates value for historical use or to build or run empirical models.

Example - measured air pressure used to predict next days weather.

A control system use the values for process control.

Example - temperature measured in the outlet of an heat-exchanger used to control the heat-exchanger steam supply valve. 


The observer can be a human or an artificial intelligence system.

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Related Topics

Measurements and Instrumentation

Measurement and instrumentation strategies.

Related Documents


Measurements are required to monitor, analyze and control processes.

P&ID Diagram - Online Drawing Tool

Draw P&ID diagrams online in the browser with Google Docs.

PFD - Process Flow Diagram - Online Drawing Tool

Draw Process Flow Diagrams with this online Google Docs drawing application.

Process Control - Common Terms

Common terms used in the process control terminology.

Process Instrumentation - ISA Codes

ISA process instrumentation codes - and combinations.

Signal Transmission - Standards

Standard signals used for transmission from sensor to controller.

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