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:

 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:

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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:

  

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.

  

4 Wire Configuration RTD:
This wire configuration provides two connections to each end of the RTD sensor. This construction is used for measurements of the highest precision.

2 Wire Configuration RTD with Compensating Loop:
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:

• How to Specify an RTD Sensor
• Fundamentals of RTDs
• Application Limitations, Types and Failure Modes of RTD
• How to Convert RTD Resistance to Temperature