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A dead weight tester is a very handy piece of instrument for calibrating a pressure gauge or other pressure transducers in an industrial plant. But how does this device work? Lets find out:
How a Dead Weight Tester Works
A dead weight tester is a very handy piece of instrument for calibrating a pressure gauge or other pressure transducers in an industrial plant. But how does this device work? Lets find out:
How to Use a Thermocouple: Practical Application Tips
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A thermocouple is said to be a ‘’simple’’ temperature measurement device. With a difference in temperature between its cold junction and hot junction, you have a voltage reading that gives you an indication of the temperature being measured. But is this really a simple device?
How to Calibrate DP Pressure Transmitters: 8 Effective Tips that Works
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Calibration of a DP pressure transmitter involves a process by which the output of the transmitter is adjusted to properly represent a known pressure input. Calibration is one of the most frequently performed maintenance operations on pressure transmitters. If well performed, the transmitter’s performance improves otherwise its performance could deteriorate with grave consequences. A pressure input is used to provide zero and span adjustments to the transmitter in the calibration process. Consult my previous post: How to Calibrate Your DP Transmitter for a detailed guide on how to calibrate a DP pressure transmitter.
Owing to the fact that a plant could go berserk, if one or two critical pressure transmitters are wrongly calibrated, it is important the calibration process and procedure be done properly. The following tips are general guides that you should have at the back of your mind when calibrating a DP pressure transmitter:
How to Calibrate a Pressure Switch with a Fluke Pressure Calibrator
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Owing to the fact that the pressure switch is a ubiquitous device, found regularly in every plant, the need to always calibrate the switch is always there. We have discussed how to calibrate and adjust a pressure switch in a previous post, but I do so again, this time using a Fluke pressure calibrator which is a very handy and popular device used by instrument technicians and experts.
How to Use a Fluke Pressure Calibrator for Calibration
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With the evolution of modern process plants, the analog pressure transmitter and pressure gauges have become very useful piece of instruments for transmitting pressure or for local indication by the pressure gauge. Often times it is required to calibrate these devices. What do you actually need to do this?
How to Use a Multimeter
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Having learnt the basics of analog and digital multimeters, you are now ready to make measurements. Resistance, voltage and current are the common measurements taken with the multimeter. Let us take a look at these common measurements using a digital multimeter.
If you are new to this page then please read the introductory post: Basics of a Multimeter: A Guide For Technical and Non-Technical People
How to Measure Resistance With a Multimeter:
To carry out resistance measurements, power to the component under test must be switched off. A resistor will scarcely short, but typically will open. If a resistor does open, the digital meter display will flash on and off or display OL (open line) because the resistor has an infinite resistance. To measure resistance with a digital multimeter, connect the multimter leads as shown below:
Follow the steps outlined below:
1) Turn off power to the circuit or component under test
2) Select resistance Ω function using the rotary selector switch
3) Plug the black test lead into the COM jack and the red test lead into the Ω jack (here you will see the letter V for voltage, Ω sign and diode sign)
4) Connect the test leads tips across the component or part of the circuit for which you intend to determine the resistance.
5) View the reading and be sure to note the unit of measurement: Ω or KΩ or MΩ depending on what you are measuring.
How to Measure Voltage With a Multimeter:
Before making voltage measurements, take all necessary precautions as any carelessness on your part could lead to injury or fatality depending on the value of the voltage you are measuring. Note for voltage measurements, there must be power in the circuit or component whose voltage is to be determined.
To start, connect the test leads as shown in the diagram below:
Follow the steps outlined below:
1) Connect power to the circuit or component under test
2) Select volts AC (V~), volts DC (V---), mvolts (V---) as desired
3) Plug the black test lead into the COM jack and the red test lead into the V jack
4) Touch the test leads tips to the circuit across across a load or power source as shown in the diagram above(parallel to the circuit to be tested)
5) View the reading and be sure to note the unit of measurement.
When taking DC voltage readings of the correct polarity (+ or -), touch the red test lead to the positive side of the circuit, and the black test lead to the negative side of the circuit ground. If you reverse the connections, a DMM with auto-polarity will merely display a minus sign indicating negative polarity. With an analog meter, you need to ensure the right polarity. Any mistake could lead to the damage of the meter.
How to Measure Current With a Multimeter
Most times during a troubleshooting process, hardly do we make current measurements. However if there is need for current measurements, connect the meter test leads in series as shown below:
Follow the steps outlined below to take the measurement:
1) Turn off power to the circuit under test.
2) Disconnect, cut or unsolder the circuit and connect the meter in the circuit as shown above.
3) Select Amps AC(A~) or Amps DC(A---) as desired
4) Plug the black test lead into the COM jack and the red test lead into the 10 Amps or 300mA jack depending on the value of the reading you are expecting
5) Connect the test leads tips to the circuit in series so that all current flows through the meter.
6) Turn the circuit power back on
7) View the reading and be sure to note the unit of measurement. Note if test leads are reversed, a negative sign will be displayed on the meter LCD.
I hope you have found these posts on multimeters useful.
Basics of a Multimeter: A Guide For Technical and Non-Technical People
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A multimeter is one of several test instruments often used by technicians and engineers in process plants and facilities for carrying out voltage measurements and troubleshooting to detect faults. This post introduces the basics of using a multimeter. The information provided here will be useful to technical people who are already aware of the importance of multimeters in measurements and troubleshooting as well as non-technical people who will find this guide very useful. Please read on.
The Digital Multimeter (DMM)
Digital multimeters (DMMs) have generally replaced the analog-type multimeter (VOM) as the test device of choice for engineers and technicians especially maintenance personnel because they are easier to read, are often more compact and have greater accuracy. The DMM performs all standard VOM measurement functions of AC and DC. Some offer frequency, capacitance and temperature measurements. Many have such features as peak-hold display that provides short-term memory for capturing the peak value of transient signals as well as audible and visual indications for continuity testing. Infact there are a great diversity of digital multi-meters in the market today. Whatever brand you possess or you wish to purchase, the key is to know how to use your multimeter for your work. Below is shown the picture of the popular fluke 77 model digital multimeter.
Digital multimeters suffer from the following disadvantages:
Multimeters come in various types, shapes and sizes. The most common type of multimeter is the volt-ohm-meters. They are grouped in two categories, ANALOG and DIGITAL.
The analog multimeter has a dial with a printed scale of a specific measurement range and a pointer to indicate measurements. It can measure resistance, voltage and current hence it is more commonly referred to as a VOM (Volt-ohm-meter). An analog multimeter is shown below:
The analog VOM is frequently used in conventional troubleshooting but is becoming more obsolete due to digital multi-meters. The VOM can tell the trouble-shooter if voltage is present or not, and can also tell how much is present. Sometimes this additional information can be helpful.
Digital multimeters (DMMs) have generally replaced the analog-type multimeter (VOM) as the test device of choice for engineers and technicians especially maintenance personnel because they are easier to read, are often more compact and have greater accuracy. The DMM performs all standard VOM measurement functions of AC and DC. Some offer frequency, capacitance and temperature measurements. Many have such features as peak-hold display that provides short-term memory for capturing the peak value of transient signals as well as audible and visual indications for continuity testing. Infact there are a great diversity of digital multi-meters in the market today. Whatever brand you possess or you wish to purchase, the key is to know how to use your multimeter for your work. Below is shown the picture of the popular fluke 77 model digital multimeter.
Digital multimeters suffer from the following disadvantages:
- Slow response to display the amount on the readout once it has been connected in a circuit. To compensate for this disadvantage, most DMMs have a bar graph display below the digital readout.A bar graph reading is updated 30 times per second while the digital display is updated only 4 times per second
- The phenomenon of GHOST VOLTAGE. This is the reading displayed by a DMM before the leads are connected to a powered circuit. A ghost voltage is produced by magnetic fields, fluorescent lights and such which may be in close proximity. These voltages enter the meter through the open test leads which act as antennae. These voltages are very low and will not damage a meter but can be confusing as to their source
Multimeters, digital or analog have rotary selector switches for selecting various ranges for current, voltages and resistance. These days, as the design of digital multimeters has improved, we now have digital meters with auto-ranging capability i.e. having the capacity to automatically switch to the desired range of measurement.
As can be seen from the pictures of both the analog and digital meters shown above, they all have terminal jacks where the leads for taking readings or measurement are inserted.
For the multimeter to be of any use it must first be connected to the circuit or device to be tested. Two leads, one RED and the other BLACK, must be inserted into the correct meter lead jacks. The black lead is connected to the meter jack marked COM or common. The red lead is connected to either of the appropriate jacks depending on what the user wants to measure: OHMS, VOLTS or AMPERES.
As shown in the picture for the analog meter above, when measuring resistance or voltage, one lead is inserted into the COM jack(black) and the other lead into the red jack marked VΩmA with the rotary switch used to select the appropriate range of voltage(AC or DC) or resistance value. To measure currents in the range of mA with a maximum of about 300mA for some meters (It is usually indicated on the manufacturers manual for the particular meter), the COM jack and red jack marked VΩmA is used. However, if a higher current is anticipated with a maximum of 12 Amps, the leads are inserted in the COM jack and the red jack marked 12A.
As shown in the picture for the Fluke 77 digital multimeter, the jack marked VΩ and the COM jack on the right of the picture are used for measuring voltages, resistance and for testing a diode. The two jacks on the left of the picture are utilized when measuring current, either in the 300mA or the 10 ampere range.
Most digital multimeters come with:
- A Low battery indicator shown on the display LCD when the battery is low
- An LCD display that shows what is being measured (volts, ohms, amps, etc.)
- Autopolarity function that indicates negative readings with a minus sign when the leads are connected incorrectly without any damage.
- Autoranging function that automatically selects proper measurement range
- One selector switch that makes it easy to select measurement functions
- Overload protection that prevents damage to the meter and the circuit, and protects the user
If you are looking out to buy a digital multimeter, endeavour to look out for all these features and much more to ensure you get value for your money.
Before I move on to the nitty gritty of how to use your multimeter for making various measurements, lets go through some tips and terminology that you must be familiar with before attempting to use a multimeter for any measurement:
TIP 1:
One of the most common uses of multimeters is for continuity test. A Continuity testing determines if an open, shorted or closed circuit exists in a circuit or device whether electrical or electronic.
TIP 2:
On a Volt-Ohm-Meter or VOM, infinity is signified by an open circuit. On an analog meter, infinity shows up as a static needle that won't move off the far left side on the display. On a digital multimeter, infinity reads “0.L.”
TIP 3:
On a VOM multimeter, “zero” means a closed circuit has been detected. The display needle moves to the far right side of an analog scale; “zero” reads “0.00” on a digital multimeter.
Check out my next post: How to Use a Multimeter For Making Measurements
How to Calibrate and Adjust a Pressure Switch
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Before we get down to the nitty-gritty of how to calibrate and adjust a pressure switch, let us get to understand some basic concepts with pressure switch calibration:
Setpoint:
Setpoint:
This is the pressure at which the pressure switch is required to operate. A pressure switch may be set to operate on either a rising pressure (high level alarm) or
How a Pressure Switch Works
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What is a Pressure Switch?
This is a device designed to monitor a process pressure and provide an output when a set pressure (setpoint) is reached. A pressure switch does this by applying the process pressure to a diaphragm or piston to generate a force which is compared to that of a pre-compressed range spring.
A pressure switch is used to detect the presence of fluid pressure. Most pressure switches use a diaphragm or bellow as the sensing element. The movement of this sensing element is used to
Factors to Consider When Selecting a Thermocouple for Temperature measurement Application.
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There are many different types of thermocouples. Each has its advantages and disadvantages over other types of thermocouples that you may find in the market. Some of the factors to guide your selection of thermocouple for any given applications are discussed below.
Common Process Switches and Their Symbols in P&IDs
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A discrete sensor is one that is only able to indicate whether the measured variable is above or below a specified setpoint. Discrete sensors usually take the form of switches designed and built to “trip” when the measured quantity either exceeds or falls below a specified value. They are very useful in the field of instrumentation for control and alarming purposes.Most process switches are discrete in nature in that they indicate whether a process variable is
Piping and Instrumentation Diagrams Tutorials III: Flow and Level Control
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In continuation of my series on piping and instrumentation diagram tutorials, we shall continue with the development of P&IDs when given some information about a process or control system. Let us take a look at the tutorial question below:
Tutorial Question:
It is desirable to have a small control system to control liquid flow and consequently level in an open tank. The description of the control system is as follows:
(a) A flow control valve will be used to regulate flow. This flow control will be based on flow measurement in an orifice meter
(b) We want to automatically adjust the setpoint of the flow controller with the aid of a level control loop. As level is being measured, the set point of the flow control valve is adjusted automatically. If the level goes up, the set point of the flow control valve should be lowered and vice versa
(c) The Orifice meter should have a secondary device to transmit a 4 – 20mA signal to the control room. The secondary device should be able to indicate flow rate locally at the Orifice meter.
(d) The secondary device on the Orifice meter is required to send this 4 - 20mA electronic signal to a controller in a central control room. The flow rate should be indicated on this controller
(e) The control room will send a 4 – 20mA signal from the controller to the control valve. At the control valve, we will use an I/P converter to provide pneumatic signal to control our valve. The flow control loop will have a loop number 100.
(f) The level of the tank will be measured using a transmitter, with local indication on the transmitter.
(g) We also want to send a 4 – 20mA level signal to a level controller in the control room. This controller will display the level of the tank in the control room.
(h) The level control instrumentation in the tank will make provision for activating high and low level alarms seen in the control room whenever the level goes too high or too low
(i) The tank should have a local sight glass or gauge for indicating level locally for plant operators
(j) The level controller will also send the level signal via wire to the flow controller in the control room, where the setpoint for the flow control valve will be adjusted. The level control loop will have the loop number 101
From the information provided above, develop the piping and instrumentation diagram (P&ID) for this control system.
Instrument Abbreviations Used in Instrumentation Diagrams (P&ID)
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Typically instrument abbreviations used in P&IDs consist of two letters: the first indicating the process variable and the second indicating the instrument/controller function. For example, the instrument abbreviation “PI” denotes a “Pressure Indicator”. Occasionally, a third letter is included in the instrument abbreviation to describe a simultaneous function or a special function. For example: the abbreviation “FRC” represents a “Flow Recorder and Controller” which describes both the recording and control functions and the abbreviation “PAL” denotes a “Pressure Alarm Low” which describes
Piping and Instrumentation Diagrams Tutorials II: Pressure Control
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In continuation of my series on piping and instrumentation diagrams tutorials, we shall look at how to develop and construct a simple piping and instrumentation diagrams (P&ID).
Before we start, I will advise you to go through Tutorials I . If you are completely new to P&ID, I will advise that you go through my various posts on piping and instrumentation diagrams to ensure that we are on the same page when we use the information provided to develop our P&ID.
How to Convert Thermocouple Milivolts to Temperature
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The voltage generated by thermocouples is very small. They are in the order of milivolts. In the application of thermocouples to measure temperature, it is often required to convert the milivolts signals of thermocouples to temperature values. To aid this conversion, several milivolts voltages for the different types of thermocouples are tabulated against known standard temperatures. With these thermocouple reference tables, it is then easy to determine any given temperature for known milivolts values.
Thermocouple reference tables are based on a reference junction of 0 degree C. If the reference junction is not at 0 degree C, then a correction factor must be applied.
The steps involved are:
- Select the correct reference table for the thermocouple type in use. e.g. J,S,T etc
- Locate the milivolt reading in the body of the table, and read from the margins the temperature value.
Reducing Noise In Thermocouple Installations
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As you may already know, a thermocouple is formed by joining two different metal alloys at a point called a junction. This junction is called the measuring or hot junction. The thermocouple leads are usually attached to a temperature indicator or controller. This connection point is called the reference or cold junction.
When the measuring junction is heated, a small DC voltage is generated in the thermocouple wires. The temperature controller measures the small voltage signal and converts it to a temperature reading. However, the voltage generated in the thermocouple is so small that
Piping and Instrumentation Diagrams:Tutorials I
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This post will begin a series of tutorials on P&ID to help many people seeking information on the subject to understand more about piping and instrumentation diagrams. Please read on and endeavour to go through all the posts on piping and instrumentation diagrams if you have the time. You will find the links to all my posts on P&IDs at the end of this post. Happy reading.
Ground Loops and Impedance Coupling: Causes and Reduction
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A ground loop is an undesirable current path in an electrical circuit. Ground loops occur whenever the ground conductor of an electrical system is connected to the ground plane at multiple points. Not only can ground loops induce noise in instrument signal cables, but in severe cases it can even overheat the instrument signal cable and thus present a fire hazard!
The phenomenon of ground loops is illustrated in the schematic diagram below:
There are several causes of ground loops in any instrumentation installation.
Some of them are itemized below:
Inductive Coupling in Analog Instrumentation and How to Reduce It
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When a wire carries an electrical current it produces a magnetic field; if this wire is in the vicinity of another wire also carrying electrical current or signal, the magnetic field they produce interact with one another resulting in noise voltage being induced in the wires. This is the principle through which inductive coupling takes place in instrumentation signal cable wiring
As we already know, Inductance is a property intrinsic to any conductor, whereby energy is stored in the magnetic field formed by current through the wire. Mutual inductance existing between parallel wires forms a “bridge” whereby an AC current through one wire is able to induce an AC voltage along the length of another wire. This become even more pronounced if we have power cables and instrument signal cables going through the same duct or conduit.
Ways to Reduce Capacitve Coupling in instrumentation signals
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If sets of wires lie too close to one another, electrical signals between the wires tend to couple or interfere with one another thereby introducing noise into the analog signal circuitry and corrupting the signals in the process. This can be especially detrimental when the coupling or interference occurs between AC power conductors and low-level instrument signal wiring such as thermocouples or pH sensor cables.
Capacitance is a property intrinsic to any pair of conductors separated by a dielectric (an insulating substance), whereby energy is stored in the electric field formed by voltage between the wires.
Sources of Noise in Analog Instrumentation Signals
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Noise
Many instrumentation systems involve the measurement of analog signals in which noise can be a prominent component. Analog instrumentation signals are commonly used for control purposes in most instrumentation facilities. These analog signals are very susceptible to various forms of noise which if not checked could corrupt the signals being transmitted for control purposes. The obvious result would be poorly controlled and dangerous systems with very low signal integrity that could potentially be hazardous.
How to Convert RTD Resistance to Temperature
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An RTD resistance can be converted into temperature using standard tables that gives values of temperatures for any given resistance value of the RTD.
The table below shows temperature versus resistance data in degree celsius with temperature coefficient of resistance of: 0.003916 ohm/ohm/°C.
How to Specify an RTD Sensor
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When a Resistance Temperature Detector (RTD) is required for a given application, many parameters need to be accurately documented for the particular RTD to be procured from the manufacturers. Since there are many different manufacturers of RTDs, there will be several different styles of RTDs in the market. Each manufacturer has their own way of specifying their product. In any case, when specifying an RTD you will always be required to select the following:
Comparison of The Common Temperature Sensors
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Semiconductor Temperature Sensors:
Semiconductors have a number of parameters that vary linearly with temperature and they form the core of today’s electronic temperature sensors. Normally the reference voltage of a zener diode or the junction voltage variations are used for temperature sensing. Transistors or diodes can also be used for temperature measurement. The outputs of these semiconductor devices are very linear and are good for
Resistance Temperature Detectors(RTDs): Application limitations, Comparison of types and Failure mode
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Application Limitations of RTDs:
RTDs can be quite bulky, which can inhibit their use in applications.Self heating can be a problem with RTDs. In order to measure the resistance of an RTD device, we must pass an electric current through it. Unfortunately, this results in the generation of heat at the resistance according to Joule’s Law:
RTDs can be quite bulky, which can inhibit their use in applications.Self heating can be a problem with RTDs. In order to measure the resistance of an RTD device, we must pass an electric current through it. Unfortunately, this results in the generation of heat at the resistance according to Joule’s Law:
P = I2 R
This dissipated power causes the RTD to increase in temperature beyond its surrounding environment, introducing a positive measurement error. The effect may be minimized by
RTD Construction and Lead Wire Configurations
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Platinum RTD elements are available in two types of constructions:
(a) Thin film and
(b) Wire wound.
Thin Film
Thin-film RTD elements are produced by depositing a thin layer of platinum onto a substrate. A pattern is then created that provides an electrical circuit that is trimmed to provide a specific resistance. Lead wires are then attached and the element coated to protect the platinum film and wire connections.
Thin film elements are available in the.
European standard (0.00385 Ω/Ω/°C), and in a special version, used primarily in the appliance industry, that has a temperature coefficient of 0.00375 Ω/Ω/°C. Thin film elements are not available in the American standard.
Wire Wound:
RTD elements also come in wire-wound constructions. There are two types of wire-wound elements:
(a)Those with coils of wire packaged inside a ceramic or glass tube(the most commonly used wire-wound construction), and
(b)Those wound around a glass or ceramic core and covered with additional glass or ceramic material (used in more specialized applications).
Wiring Arrangement of RTDs:
Thin film elements are available in the.
European standard (0.00385 Ω/Ω/°C), and in a special version, used primarily in the appliance industry, that has a temperature coefficient of 0.00375 Ω/Ω/°C. Thin film elements are not available in the American standard.
Wire Wound:
RTD elements also come in wire-wound constructions. There are two types of wire-wound elements:
(a)Those with coils of wire packaged inside a ceramic or glass tube(the most commonly used wire-wound construction), and
(b)Those wound around a glass or ceramic core and covered with additional glass or ceramic material (used in more specialized applications).
Wiring Arrangement of RTDs:
In order to measure temperature, the RTD element must be connected to some sort of monitoring or control equipment. Since the temperature measurement is based on the element resistance, any other resistance (lead wire resistance, connections, etc.) added to the circuit will result in measurement error. The four basic RTD element wiring methods according to the IEC/ASTM color codes are:
(a) 2 Wire configuration
(b) 3 Wire configuration
(c) 4 Wire configuration
(d) 2 Wire configuration with compensating loop.
2 Wire configuration RTD:
This wire configuration provides one connection to each end of the RTD sensor. This construction is suitable where the resistance of the run of lead wire may be considered as an additive constant in the circuit, and particularly where the changes in lead resistance due to ambient temperature changes can be ignored. This wire configuration is shown below:
(a) 2 Wire configuration
(b) 3 Wire configuration
(c) 4 Wire configuration
(d) 2 Wire configuration with compensating loop.
2 Wire configuration RTD:
This wire configuration provides one connection to each end of the RTD sensor. This construction is suitable where the resistance of the run of lead wire may be considered as an additive constant in the circuit, and particularly where the changes in lead resistance due to ambient temperature changes can be ignored. This wire configuration is shown below:
Note that the resistance of probe and extension is added to the RTD resistance and will increase the measured value. This could be a source of error in applications where high accuracy is required.
3 Wire Configuration RTD:
This is the standard wire configuration for most RTDs. It provides one connection to one end and two to the other end of the RTD sensor. Connected to an instrument designed to accept three-wire input, compensation is achieved for lead resistance and temperature change in lead resistance. This is the most commonly used configuration.
This is the standard wire configuration for most RTDs. It provides one connection to one end and two to the other end of the RTD sensor. Connected to an instrument designed to accept three-wire input, compensation is achieved for lead resistance and temperature change in lead resistance. This is the most commonly used configuration.
This wire configuration provides two connections to each end of the RTD sensor. This construction is used for measurements of the highest precision.
This is similar to 4 wire configuration RTD except that a separate pair of wires is provided as a loop to provide compensation for lead resistance and ambient temperature changes in lead resistance.
For more information on RTD Sensors, check out:
Common P&ID symbols used in Developing Instrumentation Diagrams
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The symbols used in piping and Instrumentation diagrams or drawings are many and varied. I have dealt with some of these symbols before but here I have given a comprehensive list of the common P&ID symbols of process equipment such as valves, flowmeters, piping line connections, and much more. Go through them and familiarize yourself with them. However they are by no means exhaustive. Getting to know these common P&ID symbols used in developing instrumentation diagrams will ensure that each time you see a P&ID, no matter how complicated you should be able to identify a symbol or two.
Piping and Instrumentation Diagrams : Piping Line Number Identification
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P&IDs play very important roles in plant maintenance and modification in that they demonstrate the physical sequence of equipment and system as well as how they all connect. During the Design stage they provide the basis for the development of system control schemes, allowing for further safety and operational investigations like HAZOP (Hazards and Operability Study).
Piping on a piping and instrumentation diagram(P&ID) is indicated by:
- Usage: For example, process, drain, nitrogen, blow down, etc.
- Line Number: The identification number of the line on the plant.
- Size: Usually in inches.
- Piping Class: The piping specification, both material and pressure rating
- The insulation class
Bi-Metallic Temperature Sensors
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Solids tend to expand when heated. The amount that a solid sample will expand with increased temperature depends on the size of the sample, the material it is made of, and the amount of temperature rise. The following formula relates linear expansion to temperature change:
Thermistors
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Thermistors are devices made of metal oxide semi-conductor material which either increase in resistance with increasing temperature (a positive temperature coefficient) or decrease in resistance with increasing temperature (a negative temperature coefficient). Their resistance changes a lot for a small change in temperature and so they can be made into a small sensor and they cost less than platinum wire RTDs. The major difference between thermistors and RTDs is
Troubleshooting Guide for DP Transmitters
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Just like every other equipment, DP transmitters can malfunction while in service. The ability to troubleshoot and locate the possible cause of malfunction of the transmitter is crucial for easy start up of the process the malfunctioning DP transmitter may have upset.
When troubleshooting DP transmitters, it is always valuable to consult the manufacturer’s manual for your DP transmitter. By so doing, you should be able to locate the malfunctioning parts of the transmitter and proffer a remedy immediately. However there are some problems that are common to most DP transmitters. If you suspect a malfunction, follow the guidelines below to verify that the transmitter hardware and process connections are in good working condition. Under each of the five common problems with DP transmitters, you will find specific guidelines/suggestions for solving the problem. It is wise to always deal with the most likely and easiest problems first.
Please follow the precautions below before and during troubleshooting of your malfunctioning DP transmitter:
1. Isolate the failed DP transmitter from its pressure source as soon as possible. Pressure that may be present could cause death or serious injury to technicians or personnel if the transmitter is disassembled or ruptures under pressure
2. Do not use higher than the specified voltage to check the transmitter loop. This may damage the transmitter electronics.
3. If there is need to open your DP transmitter while troubleshooting, please follow your manufacturer’s specific guidelines for dis-assembly of your transmitter. If you don’t serious injury or death to personnel may occur or your transmitter may be damaged.
The table below itemizes some common problems with DP transmitters and their possible remedies:
When troubleshooting DP transmitters, it is always valuable to consult the manufacturer’s manual for your DP transmitter. By so doing, you should be able to locate the malfunctioning parts of the transmitter and proffer a remedy immediately. However there are some problems that are common to most DP transmitters. If you suspect a malfunction, follow the guidelines below to verify that the transmitter hardware and process connections are in good working condition. Under each of the five common problems with DP transmitters, you will find specific guidelines/suggestions for solving the problem. It is wise to always deal with the most likely and easiest problems first.
Please follow the precautions below before and during troubleshooting of your malfunctioning DP transmitter:
1. Isolate the failed DP transmitter from its pressure source as soon as possible. Pressure that may be present could cause death or serious injury to technicians or personnel if the transmitter is disassembled or ruptures under pressure
2. Do not use higher than the specified voltage to check the transmitter loop. This may damage the transmitter electronics.
3. If there is need to open your DP transmitter while troubleshooting, please follow your manufacturer’s specific guidelines for dis-assembly of your transmitter. If you don’t serious injury or death to personnel may occur or your transmitter may be damaged.
The table below itemizes some common problems with DP transmitters and their possible remedies:
| Problem | Potential Cause | Corrective Action |
| Low output or No output | Primary Element | Check the insulation and condition of primary element |
| Loop Wiring |
|
|
| Impulse piping |
|
|
| Sensing Element |
|
|
| DP transmitter does not calibrate properly | Pressure source/correction |
|
| Meter | Determine if the meter is functioning properly | |
| Power Supply | Check the power supply output voltage at transmitter | |
| DP transmitter electronics |
|
|
| Sensing Element |
|
|
| High Output | Primary Element | Check for restrictions at primary element |
| Impulse piping |
|
|
| Power Supply |
|
|
| DP transmitter electronics |
|
|
| Sensing Element |
|
|
| Erratic Output | Loop Wiring |
|
| Process Pulsation |
|
|
| DP transmitter electronics |
|
|
| Impulse piping |
|
How to Calibrate Your DP Transmitter
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To calibrate an instrument involves checking that the output of the given instrument corresponds to given inputs at several points throughout the calibration range of the instrument. For the analog DP transmitter, its output must be calibrated to obtain a zero percent (4mA) to 100 percent (20 mA) output proportional to the DP transmitter’s zero percent to 100 percent range of input pressures.
If you got here looking for information on smart transmitter calibration please see : How to Calibrate Smart Transmitters
However there are general guidelines you need to follow before you calibrate any transmitter:
In other words calibration of the transmitter is required to make the transmitter’s percent input equal to the transmitter’s percent output. This is accomplished by adjusting screws located and clearly marked as ZERO and SPAN on the analog transmitter’s outer casing. The ZERO and SPAN screws may also be referred to as the ZERO and RANGE adjustment screws for some manufacturers of DP transmitters.
If you got here looking for information on smart transmitter calibration please see : How to Calibrate Smart Transmitters
Whatever the model/manufacturer of your DP transmitter, it can be easily calibrated according to the manufacturers specific instruction on how to calibrate it. For every calibration you need to do, consult your manufacturer’s specific instruction for calibrating the specific DP transmitter.
However there are general guidelines you need to follow before you calibrate any transmitter:
Common terms Used in DP Transmitter Calibration
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Lower Range Limit (LRL)
This is the lowest value of the measured variable that a transmitter can be configured to measure. This is different from Lower Range Value (LRV)
Lower Range Value (LRV)
Lowest value of the measured variable that the analog output of a transmitter is currently configured to measure.
Transmitter Re-ranging
Configuration function that changes a transmitter 4mA and 20mA settings
Upper Range Limit (URL)
This is the highest value of the measured variable that a transmitter can be configured to measure. This is different from Upper Range Value (URV)
Upper Range Value (URV)
Highest value of the measured variable that the analog output of a transmitter is currently configured to measure
This is the lowest value of the measured variable that a transmitter can be configured to measure. This is different from Lower Range Value (LRV)
Lower Range Value (LRV)
Lowest value of the measured variable that the analog output of a transmitter is currently configured to measure.
Transmitter Re-ranging
Configuration function that changes a transmitter 4mA and 20mA settings
Upper Range Limit (URL)
This is the highest value of the measured variable that a transmitter can be configured to measure. This is different from Upper Range Value (URV)
Upper Range Value (URV)
Highest value of the measured variable that the analog output of a transmitter is currently configured to measure
Span
Span is defined as the algebraic difference between the upper (URV)and lower range(LRV) values of the DP transmitter.
Span = URV – LRV
For example, if the DP transmitter is being used to measure a pressures in the range 0 – 300psig, then URV = 300, and LRV = 0
Therefore span = URV – LRV = 300 – 0 = 300
To have a better understanding of LRV and URV as used in instrumentation systems, please go through control signals
Calibration Range
The calibration range of a DP transmitter is defined as “the region between the limits within which a quantity is measured, received or transmitted, expressed by stating the lower and upper range values.” The limits are defined by
DP Transmitter valve manifolds
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An important accessory to the DP transmitter is the valve manifold. Most DP transmitters come with either a 3-valve manifold or a 5-valve manifold or a single block and bleed valve manifold depending on the application. The valve manifold is used:
- To isolate the DP transmitter from the process for maintenance and calibration.
- To ensure that the DP transmitter is not over-ranged
3-Valve Manifold:
This device incorporates three manual valves to isolate and equalize pressure from the process to the transmitter, for maintenance and calibration purposes. It consists of two block valves - high pressure and low pressure block valve - and an equalizing valve. The schematic below shows the configuration of a 3-valve manifold:
DPT in the schematic above is the DP transmitter. During normal operation
Applications of DP transmitters
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The DP transmitter is a very versatile pressure-measuring device. This one instrument may be used to measure pressure differences, positive (gauge) pressures, negative (vacuum) pressures, and even absolute pressures, just by connecting the “high” and “low” sensing ports differently.
In every DP transmitter application, there are means of connecting the transmitter’s pressure-sensing ports to the points in a process. Metal or plastic tubes (or pipes) are the means used for this purpose, and are commonly called impulse lines or sensing lines.
Let us now look at a few of the several applications using the versatile DP transmitter:
4 - 20mA Transmitter Wiring Types: 2 -Wire, 3 - Wire & 4 - Wire
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Today’s electronic process transmitters - pressure, temperature, flow and level are connected in different wire types or configurations. These connection methods are of great concern to the instrument engineer/technician. The 2 - Wire, 3 - Wire and 4 - Wire types are often used to describe the method of connection of electronic transmitters. However in today's rapidly evolving technological world, the 2 - Wire type transmitter is by far the most common. Evidently so because of the huge savings in wiring and other advantages it possess over the other transmitter wire configurations.
Two wire transmitters:
These are the simplest and most economical and should be used wherever load conditions will permit. They are often called loop powered instruments. In a 2 -wire system, the only source of power to the transmitter is from the signal loop. The 4 mA zero-end current is sufficient to drive the internal circuitry of the transmitter and the current from 4 to 20 mA represents the range of the measured process variable. The power supply and the instruments are usually mounted in the control room. The schematic diagram below shows the wire transmitter configuration:
An Introduction to DP Transmitters
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Differential pressure (DP) transmitters are one of the most common, versatile and most useful pressure measuring instruments in industrial instrumentation systems. A DP transmitter senses the difference in pressure between two ports and outputs a signal representing that pressure in relation to a calibrated range.
DP transmitters currently in use in most instrumentation systems are based on any of the following technologies:
a) Force-balance principle
b) Strain gauge
c) Differential capacitance
d) Vibrating wire or mechanical resonance
The force-balance principle is utilized in pneumatic pressure transmitters while most of today’s electronic pressure transmitters that have practically replaced the pneumatic pressure transmitters, use the technologies (b) – (d).
Linear Variable Differential Transformer (LVDT)
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A pressure sensor can be created using the motion of a high permeability core in a magnetic field created by the coils of a transformer. This principle is what is used in a Linear variable differential transformer. The movement of the core is transferred from the process medium to the core by the use of a diaphragm, bellows or bourdon tube.
The LVDT operates on the inductance ratio between the transformer coils.
Vibrating Wire Sensors
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It is a well known fact that the natural frequency of a tensioned string increases with tension. Mathematically, the relationship between the resonant frequency of a string and the tension applied on the string is given by:
Where,
F = Fundamental resonant frequency of string (Hertz)
L = String length (meters)
T = String tension (newtons)
μ = Unit mass of string (kilograms per meter).
This implies that a string can be used as a force sensor. In this type of sensor design, an electronic oscillator circuit, is used to keep a wire vibrating at its natural frequency when under tension. The principle is similar to that of a guitar string.
Differential Capacitance Pressure Sensors
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Like the strain gauge, differential capacitance sensors use a change in electrical characteristics to infer pressure. Here a change in capacitance is used to infer pressure measurement. The capacitor is a device that stores electrical charge. It consists of two metal plates separated by an electrical insulator. The metal plates are connected to an external electrical circuit through which electrical charge can be transferred from one metal plate to the other.
The capacitance of a capacitor is a measure of its ability to store charge. The capacitance of a capacitor is directly proportional to the area of the metal plates and inversely proportional to the distance between them. It also depends on a characteristic of the insulating material between them. This characteristic, called permittivity is a measure of how well the insulating material increases the ability of the capacitor to store charge. Mathematically this can be put as:
C = ε A/d,
here C = capacitance, A = area of plates, d = distance between plates of capacitor. ε = is the permittivity of the insulator between capacitor plates.
How a Strain Gauge Works
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Several different technologies exist for the conversion of fluid pressure into an electrical signal response. These technologies form the basis of today’s electronic pressure transmitters. One of such technology is the strain gauge discussed here.
A strain gauge is what may be described as a ‘’piezoresistive element’’. This means its resistance changes with changes in applied pressure. Basically, a strain gauge uses the change of electrical resistance of a material (wire, foil or film), under strain to measure pressure.
The electrical resistance of any conductor is proportional to the ratio of length over cross-sectional area (R ∝ L/A), which means that tensile deformation (stretching) will increase electrical resistance by simultaneously increasing length and decreasing cross-sectional area while compressive deformation will decrease electrical resistance by simultaneously decreasing length and increasing cross-sectional area.
The complete strain gauge pressure-measuring device includes:
Mechanical Pressure Sensors
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Pressure:
Pressure is defined as a force per unit area, and can be measured in units such as psi (pounds per square inch), inches of water, millimeters of mercury, pascal (Pa, or N/m²) or bar. Until the introduction of SI units, the 'bar' was quite common.
The bar is equivalent to 100,000 N/m², which were the SI units for measurement. To simplify the units, the N/m² was adopted with the name of Pascal, abbreviated to Pa.
Pressure is quite commonly measured in kilo pascals (kPa), which is 1000 Pascal and equivalent to 0.145psi.
Absolute, Gauge and Differential Pressure:
Pressure varies depending on altitude above sea level, weather pressure fronts and other conditions. The measure of pressure is, therefore, relative and pressure measurements are stated as either gauge or absolute.
Gauge pressure is the unit we encounter in everyday work (e.g., tire ratings are in gauge pressure). A gauge pressure device will indicate zero pressure when bled down to atmospheric pressure (i.e., gauge pressure is referenced to atmospheric pressure). Gauge pressure is denoted by a (g) at the end of the pressure unit , e.g., kPa (g)
Basic Functions of Instruments in a P&ID
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The primary functions of instruments and control components are monitoring, display, recording and control of process variables. Instrument and control symbols consist of an instrument bubble or circle with the instrument abbreviation lettered inside the bubble. The abbreviation completely describes the function of the instrument/control component.
Instruments/control elements can be grouped into different categories based on the process variable that the instrument or the control element is monitoring or controlling. The first letter in the instrument abbreviation indicates the process variable being monitored or controlled. The four common process variables are:
1) Flow (F)
2) Level (G)
3) Pressure (P)
4) Temperature (T)
Filled Bulb Temperature Sensors
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Filled-bulb systems use the principle of fluid expansion to measure temperature. If a fluid is enclosed in a sealed system and then heated, the molecules in that fluid will exert a greater pressure on the walls of the enclosing vessel. By measuring this pressure, and/or by allowing the fluid to expand under constant pressure, we may infer the temperature of the fluid.
There are basically four types of filled bulb temperature sensors in use in industrial applications. They are:
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