The system object of this article allows measuring the mass or volumetric airflow with very fast
response time, low noise and no added drift due to clogging. It is composed of:
‐ An airflow sensing device (such as, but not limited to, an annubar pipe or a venturi pipe) in
which at least one differential pressure sensor is used. In this device, the dynamic term of
the pressure in a turbulent flow has been characterized to obtain the relationship between
this pressure and the flow rate, so the measured pressure and the temperature define both
the mass and volumetric flow rate. This device has the advantage of adding a very low
pressure drop to the system and not changing with time due to clogging, but the
measurements it provides are noisy due to the turbulent nature of the flow and because of
this, fast changes in the flow (for example due to carriage movement or during diagnostics
to quantify leaks, characterize fans, etc.) are hard to measure accurately.
‐ A passive element (such as, but not limited to, filters commonly used in Metaljet Fusion
printers or Multijet Fusion printers to remove powder particles) that has an important
pressure drop, which is measured by a differential pressure sensor. This element has often a
linear dependence between flow and pressure drop (or parabolic if the flow is not
completely laminar) and could provide an accurate noise‐free instantaneous measure of the
flow, but the constant or constants that relate the pressure and the flow change with the
normal operation of the machine, as the filter gets dirty.
‐ An automatic closed loop process that calibrates the filter pressure drop constant with the
average measurements of the turbulent flow sensor. The instantaneous flow measurements
are then performed using the filter pressure drop instead.
This measurement mechanism applies to systems where:
‐ Accurate and fast measurements of airflows provide an advantage (such as 3D printing
machines in which airflows over the powder bed control the solvent removal rate or the
convective losses, or in cases where diagnostics are used periodically to characterize and
control the operating point).
‐ It is not desirable to introduce an additional pressure drop in the air ducts.
‐ There are existing passive elements in which a high pressure drop (that depends on the flow
but also varies with the use) can be measured.
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INC, HP, "COMBINED AIRFLOW MEASURING SYSTEM WITH NO DRIFT AND LOW NOISE LEVEL THROUGHT CONTINUOUS CALIBRATION OF AN INLINE ELEMENT USING A NON-VARIABLE TURBULENT PRESSURE DROP", Technical Disclosure Commons, (January 29, 2021)