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.