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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

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

Working Principle of Transit Time Flowmeter2

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.

What are the major problems in a pressure transmitter and how to troubleshoot it?

Introduction to pressure transmitter

The major purpose of a pressure transmitter is to convert the measured pressure to an electrical signal. These transmitters are utilized in several industries such as chemical, oil and gas, paint, etc. A pressure transmitter must be able to produce precise readings; otherwise, it might lead to a number of malfunctions and expensive maintenance costs. These transmitters are designed to give a wide range of measurements in a variety of environments. In order to carry out the transmission process a transmitter requires a transmission medium, a power supply, and also a receiver (signal convertor or processing units)

How to troubleshoot the problems in a pressure transmitter?

There could be several faults in a pressure transmitter like pressure drop. Signal transmission over long distances will be sometimes difficult. Mechanical vibrations, nonlinearity, and low response time.

Some common real-world problems that can occur with pressure transmitters and how to correct them:

Zero drift:

If the pressure transmitter’s output changes over time even when the input pressure remains constant, it could be due to temperature changes, aging, or contamination. To correct this problem, you may need to perform a field calibration to determine the correct zero point, or you may need to clean the transmitter to remove any contamination.

Span drift:

If the pressure transmitter’s output changes over its full range of measurement, it could be due to the same factors as zero drift, as well as mechanical damage or wear. To correct this problem, you may need to perform a field calibration to determine the correct span, or you may need to replace the transmitter if it has been mechanically damaged.

Non-linearity:

If the pressure transmitter’s output is not proportional to the input pressure, it could be due to poor calibration, mechanical damage, or contamination. To correct this problem, you may need to perform a field calibration to determine the correct linearity, or you may need to clean or repair the transmitter if it has been damaged or contaminated.

Noise:

If the pressure transmitter’s output is unstable or fluctuates, it could be due to electrical interference, mechanical vibration, or a faulty sensor. To correct this problem, you may need to isolate the transmitter from any sources of electrical interference, or you may need to replace the sensor if it is faulty.

Hysteresis:

If the pressure transmitter’s output changes differently depending on whether the input pressure is increasing or decreasing, it could be due to mechanical wear or damage. To correct this problem, you may need to replace the transmitter if it has been mechanically damaged.

Some common real-world problems that can occur with pressure transmitters due to environmental conditions

Vibrations:

Vibration can cause several variations in the measurement and if this vibration is more than a limit then it can lead to the failure of the transmitter.

Temperature:

If the temperature is beyond the specified amount then there will be a lot of faults in the reading.

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HomePressure MeasurementWhat are the major problems in a pressure transmitter and how to troubleshoot it?

Pressure Measurement

What are the major problems in a pressure transmitter and how to troubleshoot it?

by sundareswaranDecember 29, 20220305

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Introduction to pressure transmitter

Introduction to transmitter1

Introduction to transmitter2

How to troubleshoot the problems in a pressure transmitter?

Some common real-world problems that can occur with pressure transmitters and how to correct them:

Zero drift:

Span drift:

Non-linearity:

Noise:

Hysteresis:

Some common real-world problems that can occur with pressure transmitters due to environmental conditions

Vibrations:

Vibration can cause several variations in the measurement and if this vibration is more than a limit then it can lead to the failure of the transmitter.

problems that can occur with pressure transmitters1

Temperature:

If the temperature is beyond the specified amount then there will be a lot of faults in the reading.

Vapor content:

If there is vapor content in the pressure transmitter then it will show a high reading and thus accurate measurement won’t be received. Electronics can’t work at all if they get wet. Pressure transmitters that could be exposed to water or other moisture must be completely sealed

Overpressure:

Overpressure will also make the transmitter read high. The pressure sensors will sense this over pressure and show abnormal readings.

Below image shows the damaged diaphragm due to over pressure

Faults in sensing lines:

If there are problems in the impulse lines, that will affect the readings. If the impulse line is cracked or punctured and there is a leak in the tube fittings, the reading will be much lower than the actual value.

Irregular power supply:

If the supply to the transmitter is lost then the readings will be really low or drop to zero.

Basic steps involved in troubleshooting problems with a pressure transmitter:

Check the installation:

Make sure the pressure transmitter is properly installed and that all connections are secure. Check for any signs of damage or corrosion.

Check the process conditions:

Make sure the process conditions (such as temperature, pressure, and humidity) are within the limits specified by the manufacturer.

Check the calibration:

Verify that the pressure transmitter has been calibrated correctly and that the calibration is up to date. You may need to perform a field calibration to determine if the transmitter is accurate.

Check the output:

Compare the pressure transmitter’s output to a known reference, such as a calibrated gauge or another pressure transmitter. This can help you determine if the problem is with the transmitter or the process.

Check the electrical connections:

Make sure all electrical connections are secure and free of corrosion.

Check the wiring:

Make sure the wiring is correct and that there are no shorts or breaks.

Check for contamination:

Clean the pressure transmitter and check for any contamination that may be affecting its accuracy.

Check for mechanical damage:

Inspect the pressure transmitter for any signs of mechanical damage or wear.

Troubleshooting specific problems in a pressure transmitter

In case there is any fault in the pressure transmitter make sure that the transmitter is installed properly. The pressure transmitter should not be utilized beyond its rated pressure and temperature range. Make sure all the electrical connections in a transmitter are correct and that the power supply is within normal limits. All these things should be checked in case there are any faults in the transmitter.

Certain communication devices can cause the transmitter to malfunction so make sure that there are no communication devices near the transmitter.

Problems in the pressure-sensing lines

The length of the pressure-sensing lines will affect the response time of the pressure transmitter.

In case a pressure transmitter is used in the fluid measurement process. The transmitter may be calibrated by considering that the reference leg is filled with a water column and its height will be known. During any faulty conditions in the process, there will be an increase in temperature and it can cause the fluid in the reference leg to turn into steam. This will cause an accuracy problem and the solution for this will be to utilize the isolation bellows in the sensing lines. For instance, if there is no water in the condensation pot in the steam line and the temperature is more than 300 degrees Fahrenheit, the transmitter will malfunction.

problems that can occur with pressure transmitters3

High output in transmitters

This could be because the impulse lines are blocked or leaking, so check for blocks and leaks. It will also be because of electronic failure, check the sensor connection. Also, check if the transmitter is grounded properly. Check if the supply power is proper and make sure that there is no electronic failure.

Low output or no output

Make sure the loop is wired correctly and check to see if the ground wire is properly connected. Check the polarity is proper and make sure the loop impedance is proper. Check for leaks and blocks in the piping. Check if there is any entrapped gas in the pipelines. Make sure there are no sediments in the transmitter flanges.

Variable outputs

Make sure that there are no open circuits and make sure that the sensor connection is proper. Make sure there is no electronic failure.

Utilizing the transmitter for proper applications

Make sure that the pressure transmitter which is utilized for the industrial process is the right one. Sometimes the industrial pressure transmitter is utilized in agricultural combines and due to this, the transmitter won’t operate properly. For example, use a tank level transmitter that is incompatible with the material that is contained within the tank.

Vibrations and shock

Most pressure transmitters are tested before it is on the market. Make sure that you are utilizing the pressure transmitter in a vibration-less environment.

Electrical Interference

Most of the Pressure transmitters are shielded to withstand electrical interference. For example, Electrical interference can be caused by nearby industrial equipment, power transformers, or a lightning strike. In case there is any degradation in the shield then there could be several problems. Make sure that the shielding is proper in the pressure transmitter and might need a transmitter that has signal conditioning built right in.

Damages due to chemicals

In case of the certain industrial processes, the pressure transmitter will be subjected to certain chemicals. Because of the potential for this to cause harm to the transmitter, it is important to ensure that the pressure transmitter that is utilised for the application is capable of withstanding the chemicals to which it is exposed.

Fault related with measuring error

The measurement error of the pressure transmitter is the difference between what the pressure sensor shows and what the actual value of the variable being measured is.

Fault related with measuring error in pressure transmitter

Following things can cause pressure transmitter measurements to be wrong.

Error in the measurement of the pressure transmitter caused by an incorrect Zero Reference.

Error in the measurement of the pressure transmitter caused by a shift in the full span.

Inappropriate range causes a pressure transmitter measurement inaccuracy.

Incompatibility with the process conditions caused a pressure transmitter measurement inaccuracy.

Error in the measurement of the pressure transmitter caused by mechanical wear or damage

What are the factors that must be considered while installing a pressure transmitter?

If the pressure transmitter is not installed properly then it won’t work properly.

During the installation of the transmitter make sure that the transmitter does not stick out from the connection parts

Make sure that there is no physical damage to the transmitter

Proper wiring should be done so that it will operate accurately