Configure a Transmitter for pressure control system

Prerequisite

Standard Instrumentation Signal range

Transmitter is a device that translates the signal produced by a sensor into a standardised instrumentation signal either 3-15 PSI air pressure Or 4-20 mA DC electric current.

The standardized signal to sent to an indicator , a controller or both.

The standard 4 and 20 mA current transmitters are the classical analog transmitters.

Evolution of transmitters:

  • 4 wire
  • 3 wire
  • 2 wire
  • Intelligent/SMART
  • Wireless

4 Wire Transmitter Configuration:

3 Wire Transmitter Configuration:

In 3 wire/4 wire transmitters separate power supply is used to power the sensor and its signal conditioning circuits. These Power supply wires are separate from signal wires carrying 4-20 mA.

  • These are also called "self powered"

Advantage: No loading effect

Disadvantage: Expensive due to requirement of more cable length

2 Wire Transmitter Configuration:

only two wires which carry power supply as well as output current signal

  • These are also called as "loop powered"

  • 4-20 mA current loop interface is the standard for almost all the process control instruments.

  • Features

  • Only two wires connect sensor (transmitter) and receiver

  • DC supply voltage is nominally 24 V DC

  • When process parameter is at lowest value

  • Current in the loop is 4 mA

  • When process parameter is at highest value

  • Current in the loop is 20 mA

  • 4 mA are used for powering the sensor electronics

  • Advantages

  • Signal transmission over long distances without signal degradation.

  • Broken wire detection ( as the current would drop to zero)

  • Short circuit detection ( as the current would drop to zero )

  • Dead sensor detection( as the current would drop to zero )

  • Low voltage and low current are safe.

  • A range of improved transmitters are available in two broad categories

  • smart analog transmitters

  • intelligent digital transmitters

Smart Analog Transmitters

  • Main features
  • Essentially 4-20 mA current output
  • Transmitter has micro processor/ Microcontrollers based circuits with on chip memory

Other Features

  • Built in signal conditioning
  • Linearization
  • Offset adjustment
  • Gain
  • Arithmetic capability
  • signal can be converted into engineering units for direct use
  • Digital display for
  • Sensor output
  • Parameter setting
  • Diagnostics

Intelligent Transmitters

Main features

  • Outputs
  • 4- 20 mA current output
  • Digital data transmission
  • Support to popular protocols like
  • HART ( most popular )
  • Foundation field bus
  • Profibus
  • Wireless ( most recent development)
  • Transmitter has Microprocessor/ Microcontrollers based circuits with on chip memory.

    Other features

  • Remote trouble shooting
  • Two way communication digital
  • Built in signal conditioning
  • Linearization
  • Offset adjustment
  • Gain
  • Arithmetic capability
  • signal can be converted into engineering units for direct use
  • Digital display for
  • Sensor output
  • Parameter setting
  • Diagnostics