Suspension: solids dispersed in liquids
What does the term mean?
A suspension contains solid particles distributed through a liquid. They remain a separate solid phase rather than being dispersed at molecular level as a dissolved substance. Whether they remain suspended or settle depends on particle size, density difference and movement of the material.
Relevance to process measurement
A sensor detects conditions at its measuring point. Uneven solids distribution can make that point differ from the average in a tank. Mixing, flow and sampling therefore form part of a solids measurement task.
How can a suspension be measured representatively?
Consider shutdowns and startup phases as well as normal operation. Material may settle in a stationary line and redistribute temporarily on restart. Choose measuring and sampling locations that encounter comparable product states.
Investigate deposits, air entry and mixing changes when unexpected jumps occur. A new calibration curve cannot fix an unsuitable location. Laboratory samples also need handling that preserves a representative composition until analysis.
Take into account standstill, start-up and sampling
A suspension may distribute itself differently whilst the flow is in progress than it does during a standstill. If particles settle, a sample taken from the upper part of a vessel may not accurately reflect the composition of the entire contents. The same applies to a measuring point in a pipe where solids accumulate. Planning should therefore take the entire operational sequence into account. This includes start-up, steady-state operation and interruptions, insofar as these conditions are relevant to the measurement task.
Assign comparison samples to their respective states and document the degree of mixing present. Note whether solids may redistribute or settle again prior to laboratory analysis. A representative sample loses its value if only a portion of the sample with a different composition is subsequently analysed. The sampling point, sample collection and laboratory processing must therefore be consistent with one another.
When making an enquiry, it is helpful to provide details of the liquid, its solid constituents and the expected solids range. Please also include information on typical deposits, air ingress and product changes. This allows the desired measured variable to be distinguished from the effects of distribution. Calibration is subsequently verified using appropriate references; it is no substitute for a suitable installation and sampling setup.