Module 6 Advanced
Process Control Loop and Calibration, Advanced Topics
Learning objectives
• Explore advanced calibration concepts.
• Guardbanding
• Understand HART smart device technology.
• HART lab
• Applicability of Calibration Management Software (CMS)
• Key concepts in review
• Lab review:
• mA loop calibrators, mA input indicator
• Fluke 744 Calibrator overview
• Temperature transmitters, temperature switches
• Pressure transmitters, temperature switches
Test uncertainty ratios revisited
• The Test Uncertainty Ratio (TUR) is the ratio between the accuracy of a calibration device to device under test (DUT).
• Ideal ratio is 10X or better.
• 10:1 ratio is ideal but impractical.
• Typical accepted ratio is 4:1.
• Based on outdated MIL-STD-45662A
• Ratios of as small as 1.5:1 with guardbanding
• Test the DUT to 80 % of specification
• Provides same confidence level as 4:1
Calibrator adequacy: When is a test tool accurate enough?
• Field instrumentation is becoming more and more accurate and stable
• Achieving a 4:1 TUR is becoming difficult.
• The calibrator must be more accurate than the UUT.
• Manufacturers specifications often drive calibration practices.
• Full accuracy not always needed in the process.
• Exceptions to manufacturers specifications need to be documented.
• ISO documents
• Alternate test methods are becoming necessary.
Guardbanding: Applying metrology to field calibration
• Guardbanding is a technique that can be used to set test limits that help:
• Deal with low TURs (1.5 to 4).
• Comply with new standards.
• Simply put, guardbanding is testing to a tighter tolerance than the manufacturer’s specification.
• Advantage:
• More accurate devices can be tested with higher confidence.
• Disadvantage:
• Good devices can be rejected.
How accurate does your measurement need to be?
All UUT’s have specification limits...
Anything within limits passes!
We assume that, because the uncertainty of the standard is small...
If the reading is near the nominal, we can say with a high level of confidence that the UUT is within specification
What if the reading is at the limit?
There is a 50 % probability of passing an out-of-tolerance device...
Or failing an in-tolerance device...
There’s a 50 %-50 % chance to call it right.
We cannot say with any level of confidence if the UUT is in or out spec.
Suggested method
• No guardbanding if TURs >= 4:1
• For TURs between 1.5:1 and 4:1, set the test limit at 80 % of the specification.
• There is some some consumer risk here, but it is low.
HART Communication “101”
• What is HART?
• Highway Addressable Remote Transducer
• An industry standard developed to define the communications protocol between intelligent field devices and a control system.
• The most popular digital communication standard in the field today.
• The HART Foundation
• An independent not-for-profit organization that supports the application of HART technology worldwide
• Fluke is a member of this organization.
Why HART?
• Enhances operation of field devices by allowing digital communication
• Important benefits:
• Allows simultaneous 4-20 mA analog signal and digital over same wiring.
• Existing cabling can be used.
• Current control strategies remain secure.
• Additional information can be transmitted.
• Tag numbers, measured values, range and span data, product information and diagnostics.
• Reduces operation costs.
• Important information is available for installation, calibration and maintenance.
• Improved management and utilization of “smart” instrument networks.
Simultaneous Analog and Digital
• Asynchronous serial communication
• Very much like RS-232
• HART uses a Frequency Shift
Keying (FSK) signal superimposed
upon the 4-20 mA analog signal.
• FSK used is the same as
American Bell 202 standard.
• 1200 baud
• 1 start bit, 8 data bits, 1 parity
bit (odd), 1 stop bit, signal
4-20 mA Analog
• Average value is zero; no DC signal is
added to low level (4-20 mA) analog.
• Low pass (noise) filtering easily removes communication signal.
A Smart System
HART Operation
• Point to Point or Multidrop
• Message length allows two transactions per second.
• Burst mode allows three.
• Master-Slave operation
• Field device only replies when commanded.
• Multiple Masters allowed
• Primary - Control System; Secondary - Communicator
• Specified pause time after transaction allows other Master devices to send command.
Point to Point Wiring
Multi-Drop
• Multi-drop mode must use digital communication.
• Loop current value is meaningless.
• Typically, it’s the analog sum of the power draws of all the devices on the loop.
• Each device needs a defined address.
• Maximum of 15 devices on a single loop
• A non-zero address sets output current to 4 mA or less, conserving power.
• All messages carry source and destination addresses.
Command Sets
• Universal commands
• Provides functions that are implemented in all field devices
• Read manufacturer and device type; read primary variable (PV); read current output and percent of span
• Common practice commands
• Provides functions that are common to many - but not all - field devices
• Read multiple variables; calibrate (set zero, set span); perform self-test; read or write dynamic variable assignments
• Device-specific commands
• Provides functions that are unique to a particular field device
• Start, stop or clear totalizer; read or write density calibration factor; choose primary variable (mass flow or density)
744 HART “Communication Tree”
744/HART Terminology
• PV - Primary measured variable
• PVAO - Digital representation of analog output
• PV LRV - PV lower range value
• PV URV - PV upper range value
• SV - Secondary measured variable
• TV - Tertiary measured variable
• QV - Quaternary measured variable
• Trim - Digital adjustment
Analog vs. Smart Transmitters
Smart Transmitter Adjustments
• When and what do you adjust?
• If only analog signal is used:
• Treat just like other analog transmitters; adjust the zero and span as needed.
• If operated in the digital mode:
• Sensor trim is needed – only the PV is used.
Do smart transmitters need maintenance ?
• Yes!!!
• True, smart transmitters are generally more stable and are much higher performance.
• They have to be calibrated (or at least verified) with real, traceable standard - this is no different than any other analog measuring device!
• At regular intervals
• Requires a communicator and a calibrator
<or>
• A calibrator with an integrated HART communicator.
Commissioning Utilities
• In initial commissioning, the transmitter needs to be:
• Tagged.
• PV units set.
• Ranging set.
• Damping adjusted.
• Transfer function changed .
• Pressure only
• Sq, root, linear
• Use Fluke 744 or HART Communicator
Configuring the temperature sensor on a smart temperature transmitter
• Smart temperature transmitters can be configured for a variety of sensors.
• Thermocouples (many types)
• RTD (many types); 2-, 3- or 4-wire
Multi-variable Transmitters
• Many smart transmitters have multiple temperature and pressure sensor inputs.
• Temperature:
• Sensor burnout protection (back up sensor)
• Temperature differential measurements
• Temperature averaging measurements
• Pressure:
• Static pressure
• Differential pressure
HART Utilities
• “Loop test” sets the transmitter output to a specified 4-20 mA signal by issuing a PVOA command.
• Tests output stage of transmitter, input of PLC or other device
• Input trim, trim the sensor block
• Output trim, adjust the output D/A, mA output circuit
HART Summary
• HART is the most widely used digital communication protocol in the process industry.
• Supported by an industry backed non-profit organization
• HART allows traditional 4-20 mA signals to co-exist with more modern digital communication.
• Offers an opportunity for Fluke to continue its leadership position in the high-end of the Process Calibration market.
HART hands-on lab
• Calibration utilities
• Service utilities
• Commissioning utilities
Software for calibration data management
• Software solutions complete the documenting requirements for a documenting calibrator.
• Many different solutions from fully integrated CMMS to stand alone CMS.
• Vary in complexity (and price, $2K to $100K)
• CMMS = Computerized Maintenance Management Software
• CMS = Calibration Management Software
Calibration Management Software Solutions; Docking
• Fluke DPC/TRACK
• Honeywell DocuMint
• Fisher Rosemount AMS
• Yokogawa PRM
• AST Cornerstone
• Beamex QM6
• ProCal V
• Meridium
Example CMS Software: DPC/TRACK
What does a CMS package do?
• Creates custom procedures.
• Allows results to be uploaded and stored in database .
• Prints reports that support ISO 9000 documentation.
• Searches for particular tags and reports their history.
• Keeps track of all tags “touched” by a particular piece of instrumentation.
• Tracks due dates for specific tags.
Sample DPCTRACK (free)
• Sample software included with the 743B/744.
• Real Software, limited by:
• Includes 10 sample tags, 5 user tags; SampleTag1, SampleTag2...
• Only sample tags allowed in database.
• Sample tag information & history is editable.
• History data on sample tags is maintained.
• All DPC calibration data can be uploaded, reports printed, and exported to CSV data files.
• Perfect for use as an unloading utility.
Instrument View Search Screen
Search for tags by:
• Tag Id
• S/N
• Location
• Device type
• Location
• Other sort masks
Tag Instrument Data
Transmitter Data
Calibrator Setup Data
Develop Test Procedures
Custom Test Procedures
• Task prompts
• Pre- or post-test prompts
Use for procedure steps, location of tags, hook up instructions, lockout/tagout.
• User defined pick lists
• Imbed in procedures loaded into DPCs
• Document observations and related maintenance tasks.
• Documented with as found/as left results.
Schedule Workload
Upload and download information to DPCs
Review Test Results
Graph, Analyze Test Results
Evaluating Long Term Performance
Graphical trending analysis
Review
• Safety
• Loop calibration basics
• Temperature
• Pressure
• Labs
Review: Electrical Safety, 4-20 mA, 24V, No Harm
• Normal instrumentation signals are 4-20 mA and voltages 24 V and below.
• Older instrumentation systems can be 10-50 mA
• 48 V and 96 V supplies
• Many instruments require line power, 120 V ac
• Many instrumented systems switch 120, 240, even 480 V ac.
• Process heating circuits
• Pump controls
• Remember the electrical measurement safety video.
• Never assume low voltage, always take the same precautions you would take working with high power electrical circuits .
Review: Process loops
Process loop and control signals take many forms.
• Analog:
• 4-20 mA signal, most common
• 3-15 PSI, old control system technology, still in use
• 10-50 mA signal, old technology, isolated use
• Digital equivalents
• Foundation Fieldbus
• ProfiBus
• HART
• ControlNet – DeviceNet
• Ethernet IP
Review: Control Loop Basics
Review: Transmitter basics
• Primary function: Accurately “transmit” a signal proportionate to the measured PV.
• 4-20 mA (common),1-5 V (rare) analog outputs
• HART, Fieldbus, ProfiBus digital outputs
• Analog and smart versions
• HART smart versions can be mA, digital output or both
• Analog versions are fixed range.
• Basic signal conditioner technology
• Smart versions can be digitally re-ranged.
• Microprocessor controlled
Review: As Found / As Left Test
As found
• The verified condition of a device prior to adjustment
• If the test results are within the specified values no adjustment is required.
Adjustment if necessary
As left
• The verified condition of a device after adjustment
• If the results are within the specified values no further adjustment is required.
• If fails, readjust, re-run as left until the test passes.
• If the device fails as left after several re-tries, it may be defective and need replacement.
Review: What is calibration?
Calibration does not mean adjustment…
Calibration is:
• Comparison of a measured value to a traceable measurement standard.
• Adjust to agree with a standard, if necessary.
Reasons to calibrate:
• Product quality, quality standards
• Regulatory compliance
• Personal safety
Review: Calibration Documentation
A documented calibration requires:
• Tag, model and serial number of the device.
• Name of person performing the task.
• Calibration device and its’ certification information.
• When calibrated and when due for re-calibration
• Date and time performed.
• Test tolerance.
• As found data.
• Applied value, measured value, error calculation
• As left data if adjustment is performed.
Review: Typical Transmitter, Temperature
Review: Typical Transmitter, Pressure
Review: I to P Transmitters
I to P converts a 4-20 mA signal to a 3-15 PSI signal
• Often used with control valves
• Used as a bridge between 4-20 mA loop and 3-15 PSI pneumatic technology
• Typically operate from a 20 PSI or greater pressure supply
• “A current controlled pressure regulator”
Review: Transmitter terms
Commissioning
• Configuring a transmitter and the installation and verification of its performance.
• Setting the span, tag number, dampening and transfer function
Turn-down ratio
• The ratio between the factory calibrated span and the commissioned span.
Tag
• An identification number assigned to a location in a loop that is attached to the transmitter installed there.
Transfer function
• The relationship between applied signal and the output
• Linear, square root or other
Review: Transmitter performance
Linearity error
• The deviation of performance between 0 and 100 %
Offset error
• Error at zero that effects the entire measurement range
Span error
• Gain error, error at full span
Review: Current loop devices
• Transmitters
• Temperature, pressure, flow, analytical
• I to P, 4-20 mA input, 3-15 PSI output