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|When the Flow is laminar, the completely developed flow profile follows a logarithmic law, sharp and well developed in the middle of the flow profile.||When the flow is turbulent, the completely developed flow profile follows a power law, not as developed and relatively flat in the middle of the flow profile.|
The variation of the Reynolds Number from a change of flow rate or viscosity (variation of temperature, product, etc.) modifies the flow profile and then the "raw"
measurement of the flow meter. Thanks to their multiple measurement beams, the UFM "watches" the flow profile shape, determine a
Profile Factor (PF) and then is able
to correct the measurement versus this flow profile. This correction curve can be predefined and fitted during the calibration, or completely determined
during the calibration for maximum accuracy.
We can see that the correction only depends on the flow profile, so from the Reynolds Number.
Calibration in Reynolds
When the Reynolds Number is maximized, the flow profile is the flattest and the Profile Factor is minimized.
When the Reynolds Number increases, the flow profile develops, the Profile Factor increases and the measurement must have a calculated correction. This is the calibration in Reynolds, also known as calibration in Profile Factor (PF).
The calibration in Reynolds allows for calibration at a different flow rate and product than the future used one, as long as it is at the same Reynolds Number, so it is the same flow profile and with the same measurement correction. When the product range on the field is wide, the selection of the flow rates and calibration products is done in a way to span as close as possible the Reynolds Number range.
Example: UFM 8" (DN200), viscosity = 0.4cSt to 400cSt, flow rate = 220m3/h to 800m3h → Calibration with 4 products.
Calibration in Velocity of Fluid VOF
The calibration in VOF is pertinent when the viscosity is stable. This is the case when the UFM sees only 1 product (e.g. gasoline from refinery). The Reynolds Number changes only with the velocity of fluid VOF, so with the flow rate.
To go from upstream point A to downstream point B, the ultrasonic wave will take the time
To go from downstream point B to upstream point A, the ultrasonic wave will take the time TBA:
We can deduct the velocity of fluid VOF from the two previous equations:
The flow rate Q is proportional to the transit time difference ΔT = TBA - TAB and inversely proportional to the square of the average transit time T = (TBA * TAB)1/2:
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