A process for operating a polymerization reactor system, the process comprising: (I) contacting a transition metal-based catalyst system with an olefin monomer and an optional olefin comonomer in a loop slurry reactor within the polymerization reactor system under polymerization reaction conditions to produce an olefin polymer, the loop slurry reactor containing a liquid-solid mixture; (II) measuring a property of the liquid in the loop slurry reactor via a method comprising the steps of: (i) withdrawing a sample of the liquid-solid mixture from the loop slurry reactor; (ii) flowing the sample of the liquid-solid mixture through a vertical settling tube; (iii) periodically stopping the flow of the sample of the liquid-solid mixture in the tube for a time period sufficient for the solid to settle to a bottom portion of the tube and for the liquid to occupy an upper portion of the tube; (iv) removing a small fraction of the liquid in the upper portion of the tube and transferring the small fraction of the liquid to an analytical instrument for measuring the property of the liquid; (v) restoring flow through the tube; and (vi) returning an unused fraction of the sample of the liquid-solid mixture to the loop slurry reactor; and (III) when the property of the liquid in the loop slurry reactor has reached a predetermined level, adjusting a polymerization reaction condition. (I) contacting a transition metal-based catalyst system with an olefin monomer and an optional olefin comonomer in a loop slurry reactor within the polymerization reactor system under polymerization reaction conditions to produce an olefin polymer, the loop slurry reactor containing a liquid-solid mixture; (II) measuring a property of the liquid in the loop slurry reactor via a method comprising the steps of: (i) withdrawing a sample of the liquid-solid mixture from the loop slurry reactor; (ii) flowing the sample of the liquid-solid mixture through a vertical settling tube; (iii) periodically stopping the flow of the sample of the liquid-solid mixture in the tube for a time period sufficient for the solid to settle to a bottom portion of the tube and for the liquid to occupy an upper portion of the tube; (iv) removing a small fraction of the liquid in the upper portion of the tube and transferring the small fraction of the liquid to an analytical instrument for measuring the property of the liquid; (v) restoring flow through the tube; and (vi) returning an unused fraction of the sample of the liquid-solid mixture to the loop slurry reactor; and (i) withdrawing a sample of the liquid-solid mixture from the loop slurry reactor; (ii) flowing the sample of the liquid-solid mixture through a vertical settling tube; (iii) periodically stopping the flow of the sample of the liquid-solid mixture in the tube for a time period sufficient for the solid to settle to a bottom portion of the tube and for the liquid to occupy an upper portion of the tube; (iv) removing a small fraction of the liquid in the upper portion of the tube and transferring the small fraction of the liquid to an analytical instrument for measuring the property of the liquid; (v) restoring flow through the tube; and (vi) returning an unused fraction of the sample of the liquid-solid mixture to the loop slurry reactor; and (III) when the property of the liquid in the loop slurry reactor has reached a predetermined level, adjusting a polymerization reaction condition. <EOS>