Guide to Taking a Representative Sample

Chapter 1: General information

1.1 Purpose

The Guide to Taking a Representative Sample outlines the single most important task related to determining the quality of a parcel of grain: obtaining a sample that represents a lot or consignment in all respects. These samples may be drawn by automatic or manual means. This guide describes methods that can be used to successfully collect a representative sample on farm, or when grain is delivered to a grain handling facility. It is important to understand the basic principles of unbiased sampling, and then to choose the correct sampling process for the lot you are sampling.

This Guide does not supersede the Sampling Systems Handbook and Approval Guide (Sampling Handbook).

The Sampling Handbook outlines the Canadian Grain Commission’s policies and procedures for automatic mechanical sampling systems used to obtain official samples for inward receipt and outward discharge of grain at licensed grain handling facilities. The Sampling Handbook also includes information on Canadian Grain Commission-approved sampling methods for the Accredited Container Sampling Program and the Certified Container Sampling Program. The Sampling Handbook’s section on manual sampling may be used at the discretion of the Canadian Grain Commission when a lot of grain cannot be sampled by automatic mechanical sampling means.

This guide is intended for use by Canadian Grain Commission staff, producers, and members of the grain industry. It contains the following chapters:

Chapter 1: General information

Describes the purpose of the document, sources of sampling errors, sampling methods and glossary of terms.

Chapter 2: Pneumatic truck probes

Describes the installation, components, adjustments, operation and maintenance of pneumatic truck probes when sampling truck conveyances.

Chapter 3: Manual stream sampling

Describes the equipment and methods used for manual stream sampling.

Chapter 4: Manual sampling of static grain in truck, railcar and container conveyances

Describes the equipment and methods used for manual sampling of static grain.

Chapter 5: Reduce your composite grain sample

Describes the equipment and procedures used to mix and divide grain samples into smaller representative portions.

1.2 Sources of sampling error

There are 5 principle sources of error in grain sampling. The first 2 cannot be controlled due to the very nature of the product (grain) being sampled. The remaining 3 sources can be managed by utilizing correct sampling processes that are fit for that purpose.

Cannot be controlled (see examples in Figures 1, 2 and 3):

  1. Grouping or segregation error: all components (grain or dockage material) may settle and form layers (stratification) according to their size and density (e.g. heavy components settle below light weight components).
  2. Long-range periodic and non-periodic heterogeneity fluctuation: components fluctuate during grain flow due to the process of moving the grain (e.g. bin unload). The composition of a sample taken at the beginning of the grain flow may be different from the composition of a sample taken later in the grain movement.

Can be managed (see example in Figure 4):

  1. Delimiting error: when any part of the grain does not have an equal chance at being sampled.
  2. Extraction error: when the sample is not taken by appropriate equipment and/or proper sampling procedures.
  3. Preparation error: when the sample is mishandled, including incorrect sample division, sample integrity not being preserved, etc.
An illustration showing how light, medium, and heavy weight material stratify into layers when in motion. Text version below.

Figure 1: Stratification of grain in motion

Figure 1 - Text version

During loading, all components (grain or dockage materials) may settle and form layers (stratification) according to their size and density (e.g. heavy components settle below light weight components).

For example, while an auger is used to fill a grain bin, heavy weight material will settle to the bottom and centre of the grain bin. Medium weight material will settle towards the walls of the bin. Light weight material will settle against the walls of the bin and at the top (surface) of the grain bulk.

Grain in motion will settle into layers due to the vibration of the conveyance (such as a truck or railcar). Heavy weight material will settle to the bottom of the conveyance. Medium weight material will settle on top of the heavy weight material. Light weight material will settle on top of the medium weight material, forming the surface of the grain bulk.

Components (grain or dockage material) fluctuate during unloading of a conveyance or grain bin. Text version below.

Figure 2: Stratification of grain when unloading

Figure 2 - Text version

Components fluctuate during grain flow due to the process of moving the grain. The composition of a sample taken at the beginning of the grain flow may be different from the composition of a sample taken later in the grain movement.

For example, while an auger is used to unload a grain bin, light weight material will be drawn away from the walls of the bin while heavy weight material will be pulled directly down.

During unloading of a railcar, lightweight material and medium weight material will be drawn down through heavy weight material. This causes light weight material to move to the centre of the grain stream.

Grain in a conveyance showing how light weight material settles on top of heavy weight material, flowing from the top of the grain bulk to the sides of the conveyance.

Figure 3: Example of stratification of material in a conveyance

Figure 3 - Text version

Product in a conveyance showing how light weight material separates from heavy weight material, flowing from the top of the mound to the sides of the conveyance.

Diagram of probe sampling patterns showing where samples should be drawn in different types of conveyances. Text version below.

Figure 4: Recommended probe sampling patterns to manage extraction error

Figure 4 - Text version

A minimum of 8 samples should be taken from a single truck.
A minimum of 10 samples should be taken from a partitioned truck or truck and trailer.

Sampling patterns

In a single truck, take a total of 8 samples from these locations:

  • each corner for a total of 4 samples
  • near the wall at the middle of each side of the truck for a total for a total of 2 samples
  • the middle of the truck, one-third of the length of the truck from the front of the truck
  • the middle of the truck, one-third of the length of the truck from the rear of the truck

In a partitioned truck, take a total of 10 samples from these locations:

  • each corner on both sides of the partition, for a total of 8 samples
  • the centre of each partitioned area

In a truck with a trailer, take a total of 10 samples from these locations:

  • each corner of the truck and the trailer, for a total of 8 samples
  • the centre of the truck
  • the centre of the trailer

1.3 Sampling methods

There are many opportunities to manually sample grain, such as when filling a bin, after loading a truck, while unloading a truck, or transferring grain. There are also many different methods and types of equipment available to sample with. This guide describes best practices to use to obtain a representative sample when using a selected method.

1.4 Glossary of terms

This section describes common terms and descriptions used in sampling grain in Canada.

Alteration

Modifications or changes made to the sampling system after the system was last examined and/or tested. This includes, but is not limited to, changes to the sampling device, drive mechanism or sample delivery system.

Composite sample

A sample formed by combining and mixing all of the primary samples taken from the lot.

Lockout control

Device or process used to disconnect the main power supply to all the sampling equipment and bring the entire sampling system to a zero-energy state.

Manual stream sampling

Obtaining a sample by inserting a recommended sampling device at alternating points (left, middle, right) and regularly timed intervals for each sampling action across a falling stream or moving belt stream from the start to the finish of the grain transfer.

Operating controls

Controls used by operators for the normal operation of a mechanical sampling system. These include, but are not limited to, on/off control panel switches, boom controls, joystick or light controls.

Primary sample

A sample taken from a lot of grain during one single sampling action.

Sample divider

A mechanical or gravitational divider used to reduce the size of the sample obtained from the sampling process.

Sampling action

The act of capturing grain from the lot; e.g., by one single entry and withdrawal of the pneumatic truck probe or double sleeve trier, or one scooping action of falling grain stream or grain flowing on a belt.

Sub-sample

A portion of the composite sample obtained by mixing and dividing the composite by a recommended sample dividing method.

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