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