Diluting device for aerosol measurements

An aerosol diluting apparatus comprising:

• a first diluting unit to provide a first modified sample flow by combining an aerosol sample flow with a first diluting gas flow,
• a control unit, and
• a control valve,
• wherein the apparatus further comprises an ejector unit to draw the first modified sample flow from the first diluting unit to the ejector unit and to provide a second modified sample flow by mixing a second dilution gas flow with the first modified sample flow, wherein the first diluting unit comprises a gas permeable tunnel element or a nozzle to form a protective annular layer from the first diluting gas flow such that the protective annular layer surrounds a central portion of the first modified sample flow, and
• wherein the control unit and the control valve are arranged to adjust flow rate of the first dilution gas flow based on a pressure signal indicative of an inlet pressure of the aerosol sample flow and/or to adjust the flow rate of the second dilution gas flow based on the pressure signal indicative of the inlet pressure, such that the dilution factor of the diluting apparatus is kept substantially constant in a situation where the inlet pressure varies, the dilution factor being the ratio of the mass flow rate of the second modified sample flow to the mass flow rate of the aerosol sample flow, wherein the control unit and the control valve are arranged to maintain the flow rate of the second dilution gas flow higher than a predetermined value such that the minimum pressure in the ejector unit during operation is lower than or equal to 500 mbar, so as to ensure a choked flow condition for the first modified aerosol sample flow in the ejector unit.