Grab Sample

Definition - What does Grab Sample mean?

A grab sample is a sampling technique in which is a single sample or measurement is taken at a specific time or over as short a period as is feasible.

Grab samples provide an immediate sample, and are thus preferred for some tests. This is the most common type of sample and is the sampling technique used for most labs. An example of a grab sample would be collecting a beaker of raw water and testing it for pH.

A grab sample also known as a catch sample or individual sample.

Corrosionpedia explains Grab Sample

A grab sample has following limitations:

  • A grab sample takes a snapshot of the characteristics of the water at a specific point and time, so it may not be completely representative of the entire flow.
  • Grab samples are most appropriate for small plants with low flows and limited staff who cannot perform continual sampling.

Since pH, dissolved oxygen and total residual chlorine can change very rapidly in water once the sample is removed from the flow, grab samples are preferred for these tests. Dip discrete and pump samples are examples of types of grab samples.

Grab samples must be collected carefully so they are representative of the water as a whole. They should be taken at a time of day when the plant is operating near its average daily flow rate. If grab samples are used to determine plant efficiency by collecting a raw water sample and a treated water sample, then the collection of the effluent should be delayed long enough after collection of the influent sample to allow for the raw water to pass completely through the treatment process. Grab samples should not be mixed together.

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