Dasar dasar Instrumentasi

  Discrete process measurement

  

“Normal” status of a switch



  The “normal” status for a switch is the status its electrical contacts are in under a condition of minimum physical stimulus.

  Listing : “normal” definitions various discrete sensor types 

  Hand switch: no one pressing the switch 

  

Limit switch: target not contacting the

switch 

  Proximity switch: target far away 

  Pressure switch: low pressure (or even a vacuum) 

  Level switch: low level (empty) 

  Temperature switch: low temperature (cold) 

  Flow switch: low flow rate (fluid stopped)

  Hand switches

  Limit switches

  Proximity switches

   Exp. two pressure switches sensing the same fluid pressure

   Mercury tilt switches

  

Pressure switches

Level switches

  metal tuning fork structure capacitive switches

  Temperature switches

  Flow switches

  Analog electronic instrumentation

  

4 to 20 mA analog current signals

 example, a 4-20 mA

  

  Typically, a 4

  temperature transmitter

  milliamp current

  for measurement range 50

  value represents 0%

  to 250 degrees C

  of scale, a 20 milliamp current value represents 100% of scale.

  Graphical interpretation of signal ranges  Exp.

  

“manipulated variable”



  Most industrial control systems use at least two different 4-20 mA signals: one to represent the process variable (PV) and one to represent the command signal to the final control element (the“manipulated variable” or MV)

  

  Typically 4 milliamps commands a closed (shut) control valve or a stopped motor, while 20 milliamps commands a wide- open valve or a motor running at full speed.