Thermal Flowmeter In-Situ Calibration Validation - 5 Methods Explored
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Transcript of Thermal Flowmeter In-Situ Calibration Validation - 5 Methods Explored
Thermal Flowmeter In-Situ Calibration Validation
5 Methods Explored
SierraInstruments.com
In-situ calibration is a highly marketed benefit
of thermal mass flow meter technology.
Beware: Not all methods of in-situ calibration validation are
created equal.
Learn the methods and pitfalls of the five most common forms of in-situ
calibration validation.
1.
Validation Using
Resistance
Method: Resistance
Only measures the resistance of the platinum
wire that is wrapped around the platinum mandrel.
Validation Using Resistance
— Velocity Sensor
— Platinum Windings
— Resistance of Windings (20 Ω typical)
— Multimeter
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.
2.
Validation Using
Zero Flow
Method: Zero Flow
Many manufacturers provide data for checking zero at
another set of more reproducible conditions—zero flow at atmospheric
pressure and temperature.
Validation Using Zero Flow Calibration
— Velocity Heater Coil
— Power to Heater Coil (500mW typical)
— Multimeter
Pitfall: Zero Flow
Requires the meter to be removed from the process
and allowed to come to equilibrium at ambient
conditions.
3.
Field Adjustments
Using K-Factors
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.
Sensor Output Versus Increasing Flow Rate
Charlotte to add Fig. 5 here from page 8 of the In-Situ white paper.
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.
4.
Validation Using
Full-Flow
Method: Full-Flow
Complex and expensive technique that validates
beyond a zero flow condition by generating a series of
known flow rates.
Validation Using Sonic Nozzles
— Internal Flow Tube
— Calibrated Nozzle Known Flow Rate
— Velocity Sensor
Temperature Sensor
Pressure Regulator
Test Value —
Pitfall: Full-Flow
Depends on the nozzle not becoming plugged or dirty, and it requires precision pressure
gages—which themselves need periodic recalibration.
5.
Validation Using Actual
Flow Audit
Method: Actual Flow Audit
Uses a high-accuracy flow standard to prove the accuracy of the flow device under test.
Audit-Meter with Hot-Tap
Low Pressure Hot-Tap
Flow Audit Meter
— Device Under Test
Flow
—
—
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.
SierraInstruments.com
For more information, download Sierra’s In-Situ Calibration
Validation White Paper.