In-Situ Calibration Validation of Thermal Flowmeters – Methods and Pitfalls

Fueled by the need to meet quality standards like ISO 9001, reliability engineers working in medium-to-large-sized campuses and facilities understand the time consuming truth of ensuring that their flow meters are carefully maintained and “in cal”.

One of the unique and hugely marketed advantages of Thermal Mass Flow Meter technology is “in-situ” or “in place” flow meter calibration. This definitely sounds like a captivating proposition to reduce time and money by avoiding the cost of returning the instrument to the factory for yearly recertification and recalibration. However, not all procedures of in-situ calibration validation are created equal.

Five of those methods have been explored in the article, “In-Situ Calibration: Methods & Pitfalls of Thermal Mass Flow Meter Sensor Field Validation,” as well as the question of how thermal flow meter sensor design determines the validity of in-situ calibration has also been addressed.

In-Situ Calibration Validation of Thermal Flowmeters – Methods and Pitfalls

When assessing flow instrumentation, it is imperative to assess the methods of in-situ calibration to prevent false positives and to get reliable results. Time is precious; the following sections talk about each calibration validation method in detail.

1. Validation Using Resistance

This easy method determines the resistance across the velocity sensor. The velocity sensor is usually a platinum resistance temperature detector. Hence, the measured resistance is directly proportional to the temperature of the sensor. This temperature of the sensor should be the same as the space surrounding the velocity sensor once everything reaches equilibrium. This technique only measures the resistance of the platinum wire that is wrapped around the platinum mandrel. Since the test does not take into account the factors related to heat transfer from the wire through the organic epoxy fillers and sheath out into the flowing gas, this method does nothing to calculate drift.

2. Validation Using Zero Flow

Zero flow is the only exact reproducible point between factory calibration and the site where the thermal mass flow meter is being used. As a result, many manufacturers furnish data for checking zero at another set of more reproducible conditions: zero flow at atmospheric temperature and pressure. Hence, it is imperative that the meter is completely removed from the process and is allowed to come to equilibrium at ambient conditions. To a certain extent, this stretches the definition of in-situ verification, as it is not “in place”.

3. Field Adjustments Using K-Factors

As a temporary step, many manufacturers enable the application of a global k-factor that works as a multiplier to the observed flow value. This is just a linear offset that is usually used to make the meter reading agree with another device. The complication with k-factors is that the inherent response curve of a thermal sensor to flow is non-linear and is best portrayed by a complex polynomial function, routinely at least to the fifth order.

In other cases, manufacturers may allow a number of points on the calibration curve to be adjusted. This is usually done for large pipes and ducts as part of a flow transit. This is sometimes incorrectly called an in-situ calibration.

4. Validation Using Full-Flow

One complicated and costly approach that validates beyond a zero flow condition checks the full-flow range by generating a series of known flow rates, from zero to full scale. The system uses a small sonic nozzle opening that guides a known flow past the velocity sensor. The diameter of the nozzle is fixed, and by applying a known differential pressure across the nozzle, the flow through the nozzle can be determined. This type of validation depends on the nozzle not becoming dirty or plugged. It also requires precision pressure gages — which themselves require periodic recalibration.

5. Validation Using Actual Flow Audit Method

The Flow-Audit Method is possibly the best in-situ calibration method. It uses a high-accuracy flow standard to prove the accuracy of the flow device being tested. The ideal meter for the flow audit method has the application flexibility to work on various gases and pipe sizes. Sierra’s QuadraTherm 640i insertion thermal mass flow meter is one such instrument that can be used across multiple gases and pipe sizes. It also features a patented no-drift, dry sensor that results in reliable, stable measurements.

This information has been sourced, reviewed and adapted from materials provided by Sierra Instruments.

For more information on this source, please visit Sierra Instruments.

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