HP INCFollow

Abstract

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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