thermal flowmeter in-situ calibration validation - 5 methods explored
TRANSCRIPT
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Thermal Flowmeter In-Situ Calibration Validation
5 Methods Explored
SierraInstruments.com
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In-situ calibration is a highly marketed benefit
of thermal mass flow meter technology.
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Beware: Not all methods of in-situ calibration validation are
created equal.
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Learn the methods and pitfalls of the five most common forms of in-situ
calibration validation.
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1.
Validation Using
Resistance
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Method: Resistance
Only measures the resistance of the platinum
wire that is wrapped around the platinum mandrel.
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Validation Using Resistance
— Velocity Sensor
— Platinum Windings
— Resistance of Windings (20 Ω typical)
— Multimeter
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Pitfall: Resistance
Doesn’t measure drift or factors related to heat transfer
from the wire through the organic epoxy fillers and
sheath out into the flowing gas.
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2.
Validation Using
Zero Flow
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Method: Zero Flow
Many manufacturers provide data for checking zero at
another set of more reproducible conditions—zero flow at atmospheric
pressure and temperature.
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Validation Using Zero Flow Calibration
— Velocity Heater Coil
— Power to Heater Coil (500mW typical)
— Multimeter
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Pitfall: Zero Flow
Requires the meter to be removed from the process
and allowed to come to equilibrium at ambient
conditions.
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3.
Field Adjustments
Using K-Factors
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Method: K-Factors
K-factors only provide a linear offset, not a complex polynomial function, typically at least to the
fifth order needed for accurate in-situ calibration validation.
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Sensor Output Versus Increasing Flow Rate
Charlotte to add Fig. 5 here from page 8 of the In-Situ white paper.
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Pitfall: K-Factors
The application of a global k-factor inherent response curve of a thermal sensor to flow is non-
linear and is best represented by a complex polynomial function,
typically at least to the fifth order.
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4.
Validation Using
Full-Flow
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Method: Full-Flow
Complex and expensive technique that validates
beyond a zero flow condition by generating a series of
known flow rates.
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Validation Using Sonic Nozzles
— Internal Flow Tube
— Calibrated Nozzle Known Flow Rate
— Velocity Sensor
Temperature Sensor
Pressure Regulator
Test Value —
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Pitfall: Full-Flow
Depends on the nozzle not becoming plugged or dirty, and it requires precision pressure
gages—which themselves need periodic recalibration.
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5.
Validation Using Actual
Flow Audit
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Method: Actual Flow Audit
Uses a high-accuracy flow standard to prove the accuracy of the flow device under test.
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Audit-Meter with Hot-Tap
Low Pressure Hot-Tap
Flow Audit Meter
— Device Under Test
Flow
—
—
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Advantage: Actual Flow Audit
Sierra’s QuadraTherm 640i can be used across multiple pipe
sizes and gases, and features a patented no-drift, dry sensor that
results in stable, reliable measurements.
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SierraInstruments.com
For more information, download Sierra’s In-Situ Calibration
Validation White Paper.