PPubMed9 Mar 2017Cited 23×

The 3-Omega Method for the Measurement of Fouling Thickness, the Liquid Flow Rate, and Surface Contact

Casper Clausen, Tim Pedersen, Anders Bentien, Vittorio M. N. Passaro

Abstract

The 3-omega method is conventionally used for the measurement of thermal conductivity in solid samples. The present work includes the experimental characterization and proof-of-concept measurements of sensor concepts, based on the 3-omega method. It is shown that this method can be used to measure fouling layers with a thickness of 10 to 400 µm, to conduct the measurement of flow rates with a high precision, and finally, as a simple on-off contact sensor with a fast response time.

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The 3‑omega method can simultaneously measure fouling layer thickness (10‑400 µm), liquid flow rate with high precision, and act as a fast on‑off contact sensor.

What’s new

i.Demonstrated 3‑omega sensing of fouling layers from 10 µm to 400 µm with a reverse‑exponential response (R² > 0.936)
ii.Showed high‑precision liquid flow measurement, detecting linear speeds 0.208–1.04 mm/s (R² > 0.999) and the laminar‑to‑turbulent transition around 1 m/s (Re≈2000)
iii.Implemented a rapid on‑off contact sensor distinguishing air from water with adjustable response time (1 s–10 s) and signal‑ratio control
iv.Provided temperature‑invariant signal processing by raising U₃ω to the power 1.28

How they did it

A commercial thin‑film platinum sensor was driven with a 30 mA AC current while the 3‑omega voltage component was extracted via Fourier transform. Experiments varied frequency, flow conditions, and added controlled fouling layers, and the signal magnitude was correlated to thickness, flow speed, or contact state. Temperature effects were corrected by empirically scaling U₃ω to the power 1.28.

What they found

Fouling thickness 10‑400 µm produced U₃ω amplitudes 2.7‑4.6 mV (correlation coefficient > 0.936)
Low flow rates 0.208‑1.04 mm/s gave U₃ω 3.196‑3.187 mV with slope 0.0105 V/(m/s) and R² > 0.999
Flow transition detected at ≈1 m/s (Re≈2000) with a change in U₃ω slope
Air vs. water contact gave U₃ω ratios 1.67 (1 Hz, 10 s response) and 1.26 (10 Hz, 1 s response)

Caveats

✕Measurement uncertainty of fouling layer thickness up to 25 % due to non‑uniform coating
✕Maximum measurable thickness limited by thermal penetration depth (~200‑400 µm)
✕Temperature‑dependence requires empirical correction (n = 1.28)
✕Protective coating material unknown and treated as constant offset

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