What is Damping in Process Transmitters

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A process transmitter typically ”measures” a process variable – flow, level, temperature, pressure - and produces an output in response to changes in the input variable. Most transmitters incorporates a sensor which measures the input variable and gives out an output of which 4 – 20m A is common.

Of critical importance in the performance of a transmitter is a concept called damping. As the input variable changes, the transmitter output must update and change accordingly. Damping is the amount of time required, in addition to the update time, for the output of the transmitter to reach 63.2% of its final value after a step change has been applied to the input. A typical damping response curve of a process transmitter is shown below:

Transmitter damping is adjustable from 1 to 32 seconds. Damping reduces the effects of electrical noise and any other insignificant transient noise that may influence the transmitter output signal. It is often used to stabilize control loops and prevent false trips. In the absence of electrical or transient noise, damping may not be required in processes that are slow and have inherent lag time e.g temperature control loops. Damping should be minimized in fast changing process conditions.

Troubleshooting Guide for Pneumatic Field Transmitters

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Pneumatic instruments are still a critical component in most instrumentation systems of today even though recent technology is making them to become somewhat obsolete. For plants where pneumatic instruments especially pressure transmitters are still being used, there are common problems with these instruments that are encountered in their everyday use. These problems are highlighted below so that when troubleshooting any potential problem with such instruments, we know what the culprits are.

Pressure Transmitter  Problem	Possible Cause(s)
No Output	Bent flapper No air supply; plugged restrictor (this is  very common) Corroded pneumatic relay or components Dirty pneumatic relay seats Flapper is away from the nozzle due to freezing, improper adjustment, bent "C" flexure or transmitter has been dropped Leak in the feedback bellows Leak in the nozzle circuit Leak in the sensor pressure circuit Disconnected or broken links in a motion balance pressure transmitter
Partial Output	Plugged low pressure leg on a DP cell Worn pneumatic relay parts Partially plugged supply screen or filter Burr on the flapper assembly Hole in the flapper assembly Damaged feedback bellows Worn capsule diaphragms Warped or distorted "C" or "A" flexure on a DP cell Wrong range-sensing unit Pin hole leaks in the control relay diaphragm
Full Output	Plugged nozzle Ballooned capsule diaphragm Loose nozzle lock nut Blocked pneumatic relay vent Sensing capsule impacted with process solids Flapper assembly distorted or bent
Zero shift diaphragms	Dirty flapper assembly set point capsule problems - coating, fatigue, warped Temperature changes -  either ambient or process temperatures Process static pressure changes. Worn zero or span adjustments Flapper is depressed or hollowed on the surface Pin hole leak in the flapper Flashing and/or condensate on either leg of a DP cell installation
Output Oscillates	Liquid in the feedback bellows - water, oil etc "C" flexure look nut loose Close coupled pneumatic system Loss of capsule fill fluid Hole in the feedback bellows Loose bleed/vent valves Flashing due to pressure variations

The list of pneumatic field transmitters problem and their probable cause in the table above is by no means exhaustive. It is however a useful troubleshooting guide for pneumatic transmitters.

Read Also: Troubleshooting Guide for DP Transmitters

How a Pneumatic Pressure Transmitter Works

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The Foxboro 11GM pneumatic pressure transmitter is a force-balance instrument used to measure pressure and transmits it as a proportional 3 – 15psig signal.

Pneumatic Pressure Transmitter. (Photo Credit: Foxboro)

As shown in the diagram of the transmitter, the pressure being measured is applied to a bellows capsule. The force on the capsule is transmitted through a flexure to the lower end of the force bar. The metal diaphragm seal serves as both a fulcrum for the force bar and as a seal for the pressure chamber. The force is transmitted through the flexure connector to the range bar which pivots on the range wheel.

Operating principle of the force-balance system in pneumatic instruments

Any movement of the range bar causes a minute change in the clearance between the flapper and the nozzle. This produces a change in the output pressure from the pneumatic relay to the feedback bellows until the force on the feedback bellows balances the force on the bellows capsule.

The output pressure established by this force-balance mechanism is the transmitted signal and is proportional to the pressure applied to the bellows capsule. This signal can be transmitted to a pneumatic receiver or controller for recording/indication or control purposes.

Basics of the Flapper/Nozzle System and Pneumatic Relay in Pneumatic Instruments

How to Ground a Process Transmitter For Pressure, Flow, Level & Temperature Measurement

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Improper grounding of a pressure, flow, level or  temperature transmitter could compromise the successful  operation of the device. Most transmitters will operate with the current signal loop either floating (ungrounded) or grounded.

Ungrounded or Floating Process Transmitters
In a floating system, the extra noise affects many types of readout devices. If the signal appears noisy or erratic, grounding the current signal loop at a single point may solve the problem. The best place to ground the loop is at the negative terminal of the power supply. Do not ground the current signal loop at more than one point.

Grounded Process Transmitters
Because of noise, the majority of process transmitters – pressure, temperature, level, flow etc – are required to be grounded for good operation. A transmitter is said to be properly grounded when the signal wiring and transmitter case are grounded.

Transmitter Signal Wiring
The signal wiring of a process transmitter should be grounded at ONLY one point on the signal loop or may be left ungrounded as in floating systems. The negative terminal of the power supply is a recommended grounding point.

For good grounding practice, do not run signal wiring in conduit or open trays with power wiring or near heavy electrical equipment. See How to Reduce EMI in control Loops and Instrumentation Systems .

It is important that the instrument cable shield at the transmitter be:

•  Trimmed and insulated from touching the transmitter housing or casing.
• Connected to the next shield if signal cable is routed through a junction box
•  Connected to a good earth ground at the power supply end preferably at the negative terminal

The above grounding practice is very common in most plants however it pays to use the grounding options recommended by the facility for the specific transmitter type.

Transmitter Case
The transmitter case must be grounded in accordance with national and local electrical codes. The most effective transmitter case grounding method is a direct connection to earth ground with minimal impedance. The transmitter case may also be grounded through the process or conduit connections however this may not provide sufficient ground continuity.

Inside the field terminal side of most transmitters’ electronics housing, you will find the internal ground screw required to ground the transmitter case.  This screw is identified by the ground symbol below:

Transient Protection for Process Transmitters
Good transmitters are generally protected by integral galvanic isolation from potential damage from high voltage induced by welders, motor start¬ers, lightning strikes, switchgear and inadvertent exposure to power lines up to 500 to 700 VAC.

However, lightning strikes and other induced tran¬sient over voltage events can cause spikes and surges at much higher voltage levels. For this reason, additional protection for receiving devices may be a wise investment for installation areas prone to transient over voltages.

Many transmitters offer transient suppression options that can be integrally mounted onto the terminal strip within the housing. For other transmitters an external protection device may be used. External field mounted suppressors may not carry agency certification for explosion proof applications. A transmitter with an integral style suppressor should be the first choice for high risk applications.

Common Configuration and Calibration Tasks for Process Transmitters

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Process transmitters especially pressure transmitters are about the most popular piece of electronic device you will ever find in a process plant. Exactly what does it take to get your pressure transmitter up and running?

Listed below are common configuration and calibration tasks recommended to be done on a process transmitters before they become fully operational in the field. These tasks are divided into two basic tasks:

(a) Bench calibration tasks: This calibration task can easily be done on a bench or other suitable location before actual installation in the field or plant

(b)Field calibration tasks: This type of calibration task is done after the transmitter has been installed in the field or plant

Bench Calibration Tasks
The common bench calibration tasks recommended for a pressure transmitter before installation are:
(1) Setup of output configuration tasks:
     (a) Set the range points
     (b) Set the output points
     (c) Set the output type
     (d) Set damping value
(2) Perform a sensor trim if required - see how to perform a sensor trim

Basics of Smart Pressure Transmitter Calibration

Field Calibration Tasks
(1) Reconfigure parameters if necessary
(2) Zero trim the transmitter to compensate for mounting effects or static pressure 
     effects.
(3) Perform an analog output trim if required – see how to perform an analog
     output trim.
    See Smart Pressure Transmitter Calibration - Sensor Trim Basics

How a 2 Wire Transmitter 4 – 20mA Current Loop Works

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In two-wire 4-20mA control loops which are the most popular these days, the 2-wire transmitters convert various process signals representing flow, level, temperature, pressure, etc., to 4-20mA DC current for the purpose of transmitting the signal over some distance with little or no loss of signal.

Relationship between the Components in the 4 – 20mA Control Loop
There are three key components in the 4 – 20mA loop as shown below:

They are:

Smart Pressure Transmitter Calibration - Sensor Trim Basics

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In pressure transmitter calibration, sensor trim can be performed using either sensor or zero trim functions. Both trim functions alter the transmitter’s interpretation of the input signal. Also analog output trim is required to calibrate the output section of the transmitter.

Zero Trim

Zero trim is a single-point adjustment. It is useful for compensating for mounting position effects and is most effective when performed with the transmitter installed in its final mounting position. Zero trim should not be used in place of a sensor trim over the full sensor range. When performing a zero trim,

Basics of Smart Pressure Transmitter Calibration:

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Calibration is the process of optimizing transmitter accuracy over a specific range by adjusting the factory sensor characterization curve located in the microprocessor. Calibrating a smart transmitter is different from calibrating an analog transmitter. The one-step calibration process of an analog transmitter is done in several steps with a smart transmitter. These calibration steps involved are:

(a) Re-ranging - Re-ranging involves setting the lower and upper range points (4 and 20 mA) points at required pressures. Re-ranging does not change the factory sensor characterization curve.

(b) Analog Output Trim - This process adjusts the transmitter’s analog  characterization curve to match the plant standard of the control loop.

(c) Sensor Trim - This process adjusts the position of the factory characterization curve to optimize the transmitter performance over a specified pressure range or to adjust for mounting effects. Trimming has two steps, zero and sensor trims.

Factory Characterization Curve of Pressure Transmitter.

The characterization of a smart transmitter allows for permanent storage of reference information. In the factory setup, known pressures are applied and the transmitter stores information about these pressures and how the pressure sensor reacts to these pressure changes. This creates a transfer function of applied pressures versus output shown below:

Transmitters Used in Process Instrumentation

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For a process to be adequately controlled and manipulated, the variable of interest in the process (e.g. Temperature, Pressure or Flow) often called the Process Variable (PV) needs to be measured by a sensor which converts the measurement into a suitable signal format (4 – 20mA or digital) and then transmit it to a controller which makes the control decision and finally acts on a final control element in the control loop. What does this signal transmission is referred to as a transmitter. The schematic below illustrates the interactions between all the elements in the control loop:

Elements of a Process Control Loop

 
What is a Transmitter?

A Transmitter is a device that converts the signal produced by a sensor into a standardized instrumentation signal such as 3-15 PSI air pressure, 4-20 mA DC electric current, Fieldbus digital signal etc., which may then be conveyed to an indicating device, a controlling device, or both. The indicating or controlling device is often located in a centralized control room. The transmitter often combines a sensor and the transmitter in a single piece. The sensor measures the process variable and generate a proportional signal. The transmitter then amplifies and conditions the sensor signal for onward transmission to the receiving or controlling device.

How to Calibrate Smart Transmitters

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In our last discussion: Introduction to Smart Transmitters, we have seen that a smart transmitter is remarkably different from that of a conventional analog transmitter. Consequently the calibration methods for both devices are also very different. Remember that calibration refers to the adjustment of an instrument so its output accurately corresponds to its input throughout a specified range. Therefore a true calibration requires a reference standard, usually in the form of one or more pieces of calibration equipment to provide an input and measure the resulting output. If you got here looking for information on analog pressure transmitter calibration, you may consult: How to Calibrate Your DP Transmitter


The procedure for calibrating a smart digital transmitter is known as Digital trimming. A digital trim is a calibration exercise that allows the user to correct the transmitter’s digital signal to match plant standard or compensate for installation effects. Digital trim in a smart transmitter can be done in two ways:

Basics of Smart Transmitters

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Smart Transmitters are advancement over conventional analog transmitters. They contain microprocessors as an integral unit within the device. These devices have built-in diagnostic ability, greater accuracy (due to digital compensation of sensor nonlinearities), and the ability to communicate digitally with host devices for reporting of various process parameters.

The most common class of smart transmitters incorporates the HART protocol. HART, an acronym for Highway Addressable Remote Transducer, is an industry standard that defines the communications protocol between smart field devices and a control system that employs traditional 4-20 mA signal.

Parts of a Smart Transmitter:
To fully understand the main components of a smart transmitter, a simplified block diagram of the device is shown below:

Fig A Block Diagram of a Smart Transmitter

The above block diagram is further simplified to give the one below:

Fig B Simplified Block Diagram of a Smart Transmitter

As shown above in fig A, the smart transmitter consists of the following basic parts:

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: