Active and Passive Transducer

ACTIVE TRANSDUCER

What is meant by Active Transducer?

         Active transducer is defined as transducer that does not requires an external energy source. In this type of transducer, the non-electrical quantity which is to be measured is converted to electrical output without any external source. As it does not require any external source this type of transducer is also known as a self-generating transducer.

Examples of an Active Transducer: Thermocouple, Piezoelectric Pick up & Photovoltaic.

Block Diagram of Active Transducer

          The block diagram of an active transducer is shown above. Pressure, temperature, vibration, heat radiation and flow are the various input variable that can be directly converted into electrical output.

Why active transducer is called as self-generating Transducer?

         The transducer is a device that converts one form of non-electrical signal into a corresponding electrical signal. The active transducer generates electrical output directly in response to stimulation

Active transducer Classification

Active transducer is classified based on the internal physical property of the transducer. They are

 listed below.

• Thermoelectric based active transducer
• Piezoelectric based active transducer
• Photovoltaic based active transducer
• Electromagnetic based active transducer
• Chemical based active transducer

List of Active transducers with its principle.

Active Transducer	Parameter to measure	Principle
Thermocouple	Temperature	It works in the principle of the Seeback effect. An emf is generated when the junctions of two dissimilar metals have a change in temperature.
Thermopile	Temperature	Works on the principle of the Seeback effect. Thermal energy is converted into emf.
Piezo electric pickup	Sound, Vibration, Acceleration & Pressure	An emf is generated when an external force is applied to crystal materials such as quartz.
Photovoltaic transducer	Solar cell, Light Meter	An emf is generated in a semiconductor junction device when light radiation falls on the device.

PASSIVE TRANSDUCER

What is meant by Passive Transducer?

         The transducer which requires external energy to produce its output is called a passive transducer. This transducer requires external energy to process the output from the sensing element to produce respective electric output. The electrical parameters in a passive transducer are resistance, inductance, and capacitance which require external energy for conversion.

Examples of Passive Transducer: Potentiometric Transducer, RTD, Thermistor, Capacitor microphone, LVDT etc.,

Block Diagram of Passive TransducerThe block diagram of a passive transducer shown above has external energy. This passive transducer consists of a sensing element and a transduction element. The pressure, temperature, stress and displacement are the various input that can be measured.

         Sensing element like RTD, strain gauge and LVDT is used to measure temperature, applied force and displacement respectively. This sensing element is used for sensing the change in input quantity which is to be measured and convert the physical change into the electrical property such as resistance, inductance & capacitance change. This is fed to the transduction element.

 The transduction element consists of a signal conditioning circuit to measure change in electrical parameters & convert it into electrical output.

What is the key difference between active and passive transducer?

         In an active transducer, the energy required for its operation is taken from the quantity that has to be measured but in passive transducer energy needed for operation is derived from an external source.

Passive Transducer Classification

Passive transducer is classified based on the transduction medium. They are listed below.

Variable resistance based passive transducer

• Variable reactance [capacitance & inductance] based passive transducer
• Optoelectronics based passive transducer

List of Passive transducer with its principle

Passive transducer	Electrical Parameter	Parameter to Measure	Principle of operation
Potentiometric transducer	Resistance	Pressure, Displacement	When an external force varies the slider position of the potentiometer, then the resistance varies in the bridge circuit.
Resistance thermometer (RTD)	Resistance	Temperature, Heat radiant	Resistance of pure metal wire with a large positive coefficient of resistance varies with temperature.
Thermistor	Resistance	Temperature	Resistance of certain metal oxides with a negative temperature coefficient of resistance varies with temperature.
Strain gauge	Resistance	Applied Force, Pressure, Torque, Displacement	When stress is applied there is a change in the resistance of the wire by elongation or compression.
Hot wire meter	Resistance	Gas flow, Gas pressure	Resistance of a heating element is varied by convection cooling of a stream of gas
Photoconductive cell	Resistance.	Photosensitive Relay	Resistance of the cell as a circuit element varies with incident light.
Resistance hygrometer	Resistance	Relative humidity	Resistance of conductive strip changes with moisture content.
Capacitor microphone	Capacitance	Sound	Sound pressure varies between a fixed plate and a movable diaphragm.
Reluctance pick up	Inductance	Pressure, Vibration, Displacement & Position	When changing the position of the iron core of the coil, the reluctance of magnetic circuits is varied.
LVDT	Inductance	Pressure, Force, Displacement, Position	When there is a change in displacement there is a change in inductance.

ACTIVE TRANSDUCER VS PASSIVE TRANSDUCER

Difference between Active & Passive Transducer

Parameter	Active Transducer	Passive Transducer
Principle	The quantity being measured provides operational energy.	Energy for operation is taken from an external source.
Other name	Self-generating transducer	Externally driven transducer
Type of output	Electrical current or Voltage	Changes in quantity associated with passive elements is observed.
Conversion technique	Simple	Complex
External Energy	Not required	Required
Amplification	Needed	Not Required
Resolution	Low	High
Example	Thermocouple, Piezoelectric crystal, etc.	Potentiometer, Thermistor, Differential Transformer, Photovoltaic cell, etc.

List some advantages & disadvantages of Active transducer & Passive transducer

         The below table shows advantages and disadvantages of some active and passive transducer based on transduction mechanism.

ACTIVE TRANSDUCER

Transduction Mechanism	Advantages	Disadvantages
Thermocouple	1.Available in a compact dimension  2. Rapid temperature response 3. No need for a bridge circuit	1. Nonlinear  2. Hard to re-calibrate 3. Reference junction at cold temperature
Piezoelectric	1. Excellent Spatial Resolution 2. Precise sensitive 3. Not dependent on temperature.	1. Lack of spatial resolution  2. Charge related leaks present 3. Rely on dynamic sensing

PASSIVE TRANSDUCER

Transduction Mechanism	Advantages	Disadvantages
Capacitive type transducer	1. High sensitivity 2. High Spatial resolution 3. Large dynamic range 4. Temperature independent	1. Difficult measuring circuit 2. Stray capacitances 3. Cross-talk between parts of elements 4. Vulnerable to noise
Inductive type transducer	1. Output is linear  2. Sensitivity is high 3. Power output is high 4. Dynamic range]s  high	1. Low frequency response 2. Unreliable  3. Higher Energy consumption
RTD	1. Stability is high 2. Most precise output is obtained when compared to 3. thermistor 4. Excellent repeatability	1. High cost 2. Complex circuit design 3. Current source required

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Working Principle of Transit Time Flowmeter

• The working of the transit time flow meter is based on the time difference between flow sound propagation intervals occurring at upstream flow and downstream.
• Transit Time Flowmeter works by calculating the transit time of ultrasonic sound pulses transmitted from one transducer to another transducer through the fluid.
• The received rate of the ultrasonic pulses is then converted into the flow rate.
• Depending on the type of mounting either Vertical or Horizontal, the signal may intersect the pipeline only once, twice, or four times.
• The flow meter evaluates the time duration between transmitted and received signals.
• These Ultrasonic signals are propagated from the upstream transducer and then the downstream transducer functioning as transmitters and receivers.
• The transit time flow meters measure the flow rate of fluid nevertheless the direction of the flow.
• The transit time of the signal in the flow direction is always greater than the transit time against thefluid flow.
• This transit time Flowmeter can accurately compute the flow rate by comparing the difference with precision timing circuits.
• Because this ultrasonic signal is forced to cross the pipe to determine the average of the flow profile.
• So compensation between laminar flow and turbulent flow is automatic.

Mounting of Transit type Flowmeter

• The choice of V, Z, or W mounting method depends on the application and diameter of the pipeline.
• V-Mounting is the most common method in all process industries..
• Z-Mounting is used for larger pipe diameters or for fragile signal applications.
• W-Mount is most suitable for or smaller pipes.
• Generally, “V-mode” installation is recommended installation method because this method gives the highest trade-off between propagation time values, and signal strength when compared to remaining “W-mode” and “Z-mode”.
• These transducers can be installed on both vertical and horizontal pipelines.
• The fluid flow inside the pipe must be full.

Describe various types of Transmit time flow meters

Based on their installation these transit time flow meters are classified as

1. V – Type Transmit time flow meters

• In a V-shaped transit time ultrasonic flow meter, the receiver, and transmitter are fixed on the same side of the pipeline so that this ultrasonic wave from one meter travels down to the pipe and bounces back to another meter in the form of a V-shape.
• In this case, both flow meters are installed one near the other with some spacing between them.

2. W – Type Transmit time flow meters

• The Two Transmit time flow meters are fixed on a pipeline so that this ultrasonic wave from one meter to another meter propagates in the form of a W-shape.
• Here, an ultrasonic wave bounces at the bottom side of the pipe, back to the top side of the pipe, and then travels and again bounces at the bottom side of the pipe before reaching the receiver.
• In this case, both flow meters are installed one near the other with some spacing between them.

3. Z – Type Transmit time flow meters

• The two Transmit time flow meters are fixed on a pipeline so that this ultrasonic wave from one meter to another meter propagates in the form of a Z-shape.
• In this case, one flow meter is installed at the top of the pipeline and another one at the bottom of the line.

What are the factors to be considered while selecting a transit time flow meter?

1. Safety: If the flow rate is not controlled, Temperature and Pressure may reach dangerous levels.

2. Product Integrity: Flow plays a role in ensuring the right amount of blended materials is constantly present.

3. Efficiency: A process’s efficiency can be assessed by comparing the amount of each input that went into a product to the amount generated.

4. Process Variable Control: The flow rate is measured and controlled during energy transfer applications.

5. Understanding the Process: Before specifying a flow meter it is important to understand the entire process. Involve plant operators during designing and discuss the issues of maintenance, and calibration.

6. Process Media:

• When selecting a flow metre, the temperature and pressure conditions of an application must also be addressed.
• Temperature and pressure characteristics, like flow rate, determine the meter’s material capabilities to withstand the effect of thermal energy and the pressures imposed by the flowing fluid.

7. Process Parameter:

• When selecting a flow metre, the temperature and pressure conditions of an application must also be addressed.
• Temperature and pressure characteristics, like flow rate, determine the meter’s material capabilities to withstand the effect of thermal energy and the pressures imposed by the flowing fluid.

8. Output Indication:

• Whether flow measurement data is needed locally or remotely and how the meter will translate the flow rate into a usable data form need to be considered in the selection process.
• These are established by the purpose of the data, such as billing, regulatory reporting or monitoring, or process control.
• The remote indication can be transmitted via analog, digital, or shared through protocols such as HART, FOUNDATION Fieldbus, or Modbus.

When or where to use transit time flow meters?

• Transit time flow metres are the industry standard for measuring cryogenic liquids at temperatures as low as -300°C, and they are also used in molten metal flow metering.
• Transit time flow meters are usually used in domestic and various industrial sectors to measure the volume or mass of a liquid or gas, depending on the application.
• Regarding flow meter installation for gas or liquid, the flow meter should always be placed in a position where it is filled with fluid, even if there is no flow. There should be a way out for the second undesirable phase as well.

Advantages of Transit Time Flow Meters

• Installation is easy.
• Easy to calibrate.
• High resolution.
• Applicable to higher flow velocities.
• No moving parts to wear out.
• No pressure drop.
• Functions better for clean and ultrapure fluids.
• No leakage potential.
• Insensitive to temperature, viscosity, density, or pressure variations of flowing fluid.

Disadvantages of Transit Time Flow Meters

• Its performance may endure pipe wall interference.
• These Flow meters do not operate on dirty, bubbly fluids.
• Accuracy and Repeatability problems may arise if there is a space gap between the pipe wall and flowing fluid.

Applications of Transit Time Flow Meter

• The clean water flow rate in water treatment plants.
• Hot or cold water in power plants.
• Pure and ultra-pure liquids in all pharmaceutical, food, and beverage industries.
• Liquid to medium crude oils in the petroleum refining industry.
• Water distribution systems are used in agriculture and irrigation.