12.To Calibrate or not to calibrate a fieldbus · PDF fileTo Calibrate or Not to Calibrate a fieldbus transmitter? Dale Perry Pressure Marketing Manager Rosemount

To Calibrate or Not to Calibrate a fieldbus transmitter?

To Calibrate or Not to To Calibrate or Not to Calibrate a fieldbus Calibrate a fieldbus transmitter?transmitter?Dale Perry Pressure Marketing Manager Rosemount

[File Name or Event]Emerson Confidential27-Jun-01, Slide 2

IntroductionIntroductionIntroductionl Digital Transmitters are here

– Wireless– Fieldbus– Same architecture as Smart-HART transmitter but

without the A/D component

l Traditional methods of calculating accuracy apply although most vendors don’t explicitly explain it

l Calibrating Digital Transmitters is easy


AgendaAgendaAgendal Digital Transmitter Theory of Operationl Accuracy Typesl Performing the calibrationl New at the Foundation


Basic Block Diagram of fieldbus TransmitterBasic Block Diagram of fieldbus TransmitterBasic Block Diagram of fieldbus Transmitter

Ff Electronics

Sensor TransducerSensorCorrection

& A/D

Sensor

AI

Function Block

Resource

Function Block


Sensor Correction is Called the Characterize/Verify (C/V) ProcessSensor Correction is Called the Sensor Correction is Called the Characterize/Verify (C/V) ProcessCharacterize/Verify (C/V) Process

SensorCorrection

SensorCharacteristic

Data

CHARACTERIZESet Temperature

Apply pressuresStore Data

Set TemperatureApply PressuresStore Data

…….

VERIFYSet Temperature

Apply pressuresCompare Characterize data

Set TemperatureApply pressuresCompare Characterize data

……….


A/D Converts a Measured Variable to a digital valueA/D Converts a Measured Variable to a digital A/D Converts a Measured Variable to a digital valuevaluel Converts an Analog signal

– infinite number of valuesl To a digital value

– Discrete Number of valuesl Number of digital values determine sensor resolution

Pressure

Infinite Values

UpperRange

Limit

LowerRangeLimit

A/D Pressure

Discrete number of

Values

12.435 PSI


A/D is a Generic TermA/D is a Generic TermA/D is a Generic Term

LH

C1 C2

C3 C4 C/D

Capacitance to Digital

Micro

V/D

Voltage to Digital

Micro


Output of Sensor Transducer Block is Channeled to an AI BlockOutput of Sensor Transducer Block is Output of Sensor Transducer Block is Channeled to an AI BlockChanneled to an AI Block

Ff Electronics

Sensor Transducer

SensorCorrection

& A/D

Sensor

AI

Resource


AgendaAgendaAgendal Digital Transmitter Theory of Operationl Accuracy

– Rangedown – Accuracy Types– Calculating Accuracy– Total Probable Error

l Performing the calibration


Knowing When to Calibrate Requires a lot of InformationKnowing When to Calibrate Requires a lot Knowing When to Calibrate Requires a lot of Informationof Informationl Accuracy

– Reference Accuracy• Linearity, Hysteresis, Repeatability• Accuracy type

– Accuracy at rangedownl Total Probable Error

– Accuracy, Temperature effect, Static Pressure effectl Long Term Stabilityl Accuracy Tolerancesl Test Equipment


Accuracy is Typically Stated in One of Three CategoriesAccuracy is Typically Stated in One of Accuracy is Typically Stated in One of Three CategoriesThree Categories

Accuracy wholly dependent on Full Scale Value

Error is significant at low end

Inexpensive transmitters

Accuracy dependent on Range

Error gets smaller at Rangedown

Standard Rosemount accuracy

Standard high end units

Accuracy dependent on Reading

Reading gets smaller, Error gets smaller.

Commonly used in calibration equipment

Very accurate at low end

% Full Scale% Full Scale

10:1 100:1

% Full Scale% Calibrated Span

10:1 100:1

% Full Scale% of Reading

10:1 100:1


Reference Accuracy Includes Linearity, Hysteresis, and RepeatabilityReference Accuracy Includes Linearity, Reference Accuracy Includes Linearity, Hysteresis, and RepeatabilityHysteresis, and Repeatability

Sensor Output

Pressure

Pressure Range

Linearity

Upper Range Limit

Lower Range Limit + Hysteresis+ Repeatability

Accuracy


Calculating the Rangedown is Simple Calculating the Rangedown is Simple Calculating the Rangedown is Simple l HART/Analog

– Sensor Rangedown is the Ratio between the sensor Upper Range Limit and the calibrated Span

l Digital Transmitters– Sensor Rangedown is the Ratio between the

sensor Upper Range Limit and the calibrated Range

Upper Range LimitCalibrated Span = RangeDown

Upper Range LimitCalibrated Range = RangeDown


Calculating the Rangedown is Simple Calculating the Rangedown is Simple Calculating the Rangedown is Simple l Sensor Rangedown is the Ratio between the sensor

Upper Range Limit and the calibrated Span

3051S Range 2

250 InH202.5 InH20

= 100

Example Rangedown

100:1

Upper Range LimitCalibrated Span = RangeDown

2.49 bar0.5 bar

= 4.98 4.98:1

Span= 0.5bar


Accuracy Statements are Typically Only up to 10:1 Calibrated RangeAccuracy Statements are Typically Only up Accuracy Statements are Typically Only up to 10:1 Calibrated Rangeto 10:1 Calibrated RangeTypical Accuracy Statement:± 0.055 % of span. For Spans less than 10:1 ± [ 0.015+0.005%( )]% of spanURL

Span

See full range but accuracy Depends on last calibration

+250 in H200.62 bar

-250 in H200.62 bar

0

1:110:1± 0.055 % of span

25 in H2062 mbar

0


Three areas need to be investigated to determine Total PerformanceThree areas need to be investigated to Three areas need to be investigated to determine Total Performancedetermine Total Performance

l Total (Installed) Performance– Accuracy of measurement at installed conditions

• Reference Accuracy• Temperature Effects• Line Pressure Effects

l Long Term Stability– Accuracy of measurement over time

l Dynamic Performance– How fast the measurement tracks process changes


Installed Performance can be Calculated by Using the Total Probable Error Equation.Installed Performance can be Calculated by Installed Performance can be Calculated by Using the Total Probable Error Equation.Using the Total Probable Error Equation.

Accuracy at Calibrated Range

2

Installation effectsOn Accuracy

2 2


Example: Total Probable Error Example: Total Probable Error Example: Total Probable Error

l Accuracy = ± 0.1% of spanl Total temperature effects = ± 0.5% of spanl LP effects = ± 0.3% of spanl Calibrated span is 5 psig

What is TPE in psig, given:

TPE = 0.12 + 0.52 + 0.32 = ± 0.59% of span

TPE = 0.0059 x 5 psig = ± 0.03 psig


Determine Calibration Frequency needed to Maintain Required AccuracyDetermine Calibration Frequency needed to Determine Calibration Frequency needed to Maintain Required AccuracyMaintain Required Accuracy

Time

Accuracy

Published Accuracy

Total Probable Error

Maximum Error

Long Term StabilityLo

ng Term

Stability


AgendaAgendaAgenda

l Digital Transmitter Theory of Operationl Accuracyl Performing the calibrationl New at the Foundation


Digital Transmitters Require One CalibrationDigital Transmitters Require One Digital Transmitters Require One CalibrationCalibration

Ff Electronics

Sensor Transducer

SensorCorrection

& A/D

Sensor

Resource

230 inH20

AI


Calibration ProcessCalibration ProcessCalibration Processl Select Accurate Test Equipment

– ANSI Suggest that Calibration Equipment be at least Four times More Accurate than the Device

l Calibrate at Temperaturel Perform Zero Transmitter (optional)l Perform Lower Sensor Trim

– Should be at or below typical lower end of operational range

l Perform Upper Sensor Trim– Should be at or above typical upper end of operational

range


Sensor Zero TrimSensor Zero TrimSensor Zero Triml Zeros out small linear offsets due to

mounting or static pressure effect.


Sensor Trim (Lower & Upper)Sensor Trim (Lower & Upper)Sensor Trim (Lower & Upper)

l Sensor Trim is a two-point sensor calibration where two end-point pressures are applied, and all output is linearized between them.

l Lower Sensor Trim is an offset– Shifts entire sensor characterization

curve up or down.l Upper Sensor Trim changes the

slope of the sensor characterization curve.– Pivots on the low trim point.

l Note: Always perform the low trim first!


Sensor Trim (Lower & Upper)Sensor Trim (Lower & Upper)Sensor Trim (Lower & Upper)

Actual

Your test equipment/ideal

Start 150

0

Pressure Input

1500

Reading

Pressure Input

Low Trim

150

0

1500

Reading

Pressure Input

High Trim

150

0

1500

Reading


Calibration DocumentationCalibration DocumentationCalibration Documentationl Last Calibration history stored in transmitterl Maintenance system

– As Found– As Left– Calibration History


AgendaAgendaAgenda

l Digital Transmitter Theory of Operationl Accuracyl Performing the calibrationl What’s New for Calibration


New for Calibration of Ff SensorsNew for Calibration of Ff SensorsNew for Calibration of Ff Sensorsl Transducer Block Standardizationl Human Centered Design User Interfaces


New at the FoundationNew at the FoundationNew at the Foundationl Sensor Transducer Block Standardization

– Easier to make Ff calibrators.

FF-903 – Pressure

FF-904 – Temperature

FF-906 - Positioner


Traditional Technology Driven User InterfaceTraditional Technology Driven User InterfaceTraditional Technology Driven User Interfacel Blockl Model Parameter


Human Centered Design User InterfacesHuman Centered Design User InterfacesHuman Centered Design User Interfacesl Task Based


Human Centered DesignAll Calibration Information AccessibleHuman Centered DesignHuman Centered DesignAll Calibration Information AccessibleAll Calibration Information Accessible


SummarySummarySummaryl Digital transmitters do not have an analog output that has

any correlation to the sensor inputl Digital transmitters use the Calibrated Range when

determining rangedownl Look carefully at the reference accuracy statementl Total Probable Error should be used when determining

calibration frequencyl The simulator used for calibration should be at least four

times better than the accuracy of the transmitterl Human Centered Design User Interfaces makes

calibration of Ff sensors easy.



Where To Get More InformationWhere To Get More InformationWhere To Get More Informationl American National Standard ANSI/CSL Z540.3-

2006– Calibration Laboratories and Measuring and Test

Equipment

l PlantWeb University– http://plantweb.emersonprocess.com/university

l Emerson Process Management Education Services– http://www.emersonprocess.com/education/

http://plantweb.emersonprocess.com/university

http://www.emersonprocess.com/education/

Documents

12.To Calibrate or not to calibrate a fieldbus · PDF fileTo Calibrate or Not to Calibrate a fieldbus transmitter? Dale Perry Pressure Marketing Manager Rosemount