best practices when designing flow calibration stands

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Best Practices When DesigningFlow Calibration Stands

Brandon HansenDryCal National Sales ManagerMesa Labs

Flow Calibration is a Broad Term1) Flow Proving

Physical and/or theoretical validation of installed instrument

2) Flow VerificationIn-situ test to achieve reasonable belief in accuracy

3) “Wet” Flow CalibrationCompare instrument against standard in a controlled manner

Today, we’re reviewing best practices in flow stand design for #3

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Metrology Terms & DefinitionsIUT, UUT, or DUT

Instrument, Unit, or Device Under TestFlow instrument to be calibrated for our purposes

Measurement StandardReference with a stated quantity value and uncertaintyDevice’s measurement that will be compared to DUT

UncertaintyDispersion of measured values in sample populationVarying interpretations of uncertainty vs. accuracyUncertainty is always found using prescribed methods

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More Metrology TermsCombined Standard Uncertainty

Standard uncertainty found using all input quantitiesCalculated using root sum square method

Coverage FactorMultiplier used with CSU to find expanded uncertaintyOften denoted by “k” – typical value of 2 or 3

Expanded UncertaintyInterval that includes a large fraction of all measurementsAKA: the coverage probability or level of confidenceWhen k=2, EU=95.5% When k=3, EU=99.7%

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Discipline Early Pays OffUseful information to gather about the instruments to be calibrated includes:

Flow Ranges Including range and units

Quantities Each model or type of instrument

Application ConditionsPressure, temperature, & media that instrument measuresInclude all signal conditioning methods with uncertainties

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Know Your Meters!

Each technology operates differentlyand has unique uncertainty influences

Identify sources of error for each instrument typeTemperature change, pressure change, flow profile, etc.Process media vs. calibration media correction factors

Performance at different and changing flowsInstrument response time will impact calibration process

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Develop Performance GoalsDesired Calibration Uncertainty

Likely varies by instrument and application

Determine Calibration Speed Needs/GoalImpacts calibration process and standard to be usedMay include consideration of calibration automation

Set “Real World” Limits of Calibration CapabilityFlow rates, temperature, pressure ranges deliveredCalibration media that will be availableEnvironmental control capabilities

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Choose the StandardPrimary Calibration Standard

Standard calibrated using a “fundamental” measureIncludes time, distance, weight, temperature, and others

Secondary Calibration StandardDevice calibrated with reference to a primary standardSequential calibrations away from primary add to uncertainty

Working Calibration StandardDevice with calibration traceability to primary & secondaryTypically used in a production environment

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Ensure Standard Traceability

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Choose Measurement StandardShould provide the measure needed directly

Volumetric flow, mass flow, etc.Inferential math adds to uncertainty

Avoid IUT/UUT technology where possibleDecreases risk of hidden bias in results

Know the standard’s measurement technologySuitability for instruments and calibration goals

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Continuous vs. Batched FlowContinuous Settling vs. Batched Population Size

Continuous measurements require stabilization timeStabilization needs add to potential for operator errorSample size of batched flow standards influence resultsBoth concerns are a balancing act: uncertainty vs. speed

Flow Stand Mechanical DesignImpact varies by flow technologies involvedAllows a measure of control over operator errorBest to minimize connecting volume

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Differential PressureSecondary StandardTyp uncertainty: 0.2 – 1% rdgContinuous Volumetric Flow

Challenges:Prone to Thermal DriftUnstable at Low Gas Flow & PressureFlow Profile SensitivityTemp & Pressure Control NeedsPositives:High Flow & Pressure Cal’s

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CoriolisSecondary StandardTyp uncertainty: 0.1 – 0.7% rdgContinuous Mass Flow

Challenges:Vibration SensitivityLimited Gas Flow Range

Positives:No Flow Profile SensitivityWorks Well for Liquids

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ThermalPrimary or Secondary StandardTyp Uncertainty: 0.5% rdg – 2% FSContinuous Volumetric or Mass Flow

Challenges:High Thermal Sensor DriftMFC’s are Pressure SensitiveTypically Gas Flows OnlySome Flow Profile Sensitivity

Positives:Readily AvailableEasy to Implement & Use

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GravimetricPrimary StandardTyp Uncertainty: 0.02 – 0.2% rdgBatched Volumetric Flow

Challenges:Complex to MaintainLong Calibration Cycle TimeInflexible Install Location

Positives:Highly Accurate

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Bell ProverPrimary StandardTyp Uncertainty: 0.17 – 0.4% rdgBatched Volumetric Flow

Challenges:Temp Shift SensitivityComplex to MaintainInflexible Install LocationLong Calibration Cycle Time

Positives:Flow Range Flexibility

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Piston ProverPrimary StandardTyp Uncertainty: 0.15 – 0.5% rdgBatched Volumetric Flow

Challenges:Connecting Volume SensitivityPossible Hazmat ConcernsBest for Low Flow Ranges

Positives:No Thermal DriftShort Calibration Cycle TimeFlexible Install Location

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What TUR is Best?TUR

Test Uncertainty RatioRatio of DUT to measurement process uncertaintyMUST use same expanded uncertainty values

Consumer RiskProbability of non-conforming part passing calibration

Producer RiskProbability of conforming part failing calibration

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Flow Measurement Distribution

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Flow sample populations follow a normal, Gaussian distribution

This distribution applies to both the DUT & the calibration standard

Visual of Consumer Risk

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Probability that flow rate reported by standard is within the test’s specification limit when the “true” value of the flow rate is outside of the specification limit.

Visual of Producer Risk

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Probability that flow rate reported by standard is outside the test’s specification limit when the “true” value of the flow rate is inside of the specification limit.

TUR Helps Manage Risks

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A narrow standard deviation in the calibration standard vs. the deviation of the UUT results in reduced probabilities for both consumer and producer risk.

Visual of Guardbanding

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Adding a test limit inside of the spec limit reduces the probability that the calibration standard reports a “pass” result when true flow should be a “fail”

Uncertainty is More Than the StandardType A Uncertainty

Flow dependent, analyzed via statistical methodsTypically the flow calibration resultsMay be a variable uncertainty throughout flow range

Type B UncertaintyFlow independent, analyzed via alternative methodsEnvironmental & flow media uncertaintyComponent uncertainty & operator error

Combined Total UncertaintyCalculated using weighted root sum square methodCTU is used as TUR value for loss & guardbanding

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Review Performance RequirementsCan all instruments be calibrated to meet desired results?

Revise guardbanding strategyConsider spares inventory or alternate labAlter “real world” lab limitsReview cases of performance “overkill”

Does expected calibration time suit performance needs?Consider calibration automation optionsReview uncertainty expectations for increased speed

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Further Resources

Guide to the Expression of Uncertainty in Measurement-Also known as the GUM-JCGM member organizations (ISO, OIML, IEC, and others)

International Vocabulary of Metrology-JCGM member organizations

Measurement Decision Risk – The Importance of Definitions-Scott M. Mimbs, NASA

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Can I Help With Questions?

DryCal Manufacturing LocationMesa Labs10 Park PlaceButler, NJ 07405Main Phone: (973) 492-8400Main Email: CSButler@MesaLabs.com

Brandon HansenDryCal National Sales Manager12100 West 6th AvenueLakewood, CO 80228My Phone: (303) 987-8000 ext. 10522My Email: BHansen@MesaLabs.comLinkedIn.com/in/BrandonDHansen

www.DryCal.MesaLabs.com