Guide to Taking a Representative Sample

2.1 Overview

Pneumatic truck probe sampling systems are used to sample truck conveyances with the use of a sampling probe (compartmentalized or core) that is inserted into a bulk lot of grain. When the probe reaches the maximum depth of a conveyance, a pneumatic recovery system transports the sample from the probe and delivers the sample, through a series of delivery lines, to a sample collection unit.

Compartment type and core type probes with controlled air pressures are the recommended probe types for use by the industry. The installation, components, adjustments, operation and maintenance of the pneumatic truck probe sampling system are all important to ensure that the system is operating efficiently, and as designed.

2.2 Installation

The pneumatic truck probe foundation, drive mechanism, base structure, booms, sampling component, sample delivery system and sample collector must be installed as prescribed by the manufacturer. This will help ensure that the sample taken from the sampled lot and delivered to the inspection office for analysis is representative.

The foundation and base structures mounting plate must be installed at the proper height from ground level (not too high or too low). If the probe is installed outside of manufacturer specifications, it may not be able to reach the entire depth of a conveyance, depending on the probe's length and reach of the boom.

Pneumatic truck probes and supporting sample delivery components come in various makes and models. Purchase and install equipment that best suits your environmental working conditions and expected work volumes.

• Protect probes from adverse weather conditions (during sampling and when at rest) to help alleviate issues with moisture contamination and equipment performance.
• Adverse weather such as rain, snow and cold will negatively affect compartment type probes’ performance more so than core type probes due to their design differences (see description of each type in section 2.3: Components).
• Manufacturers design and build various models to accommodate a range of workloads. Consider the expected work volumes, such as annual tonnage and/or number of trucks to be sampled, when selecting this type of sampling equipment.

Sample delivery lines from the truck probe to the inspection area must follow the most direct route with as few bends as possible. Follow manufacturer installation guidelines and the information in the following section to help minimize impacts to the primary sample, such as cracked corn and foreign material (CCFM) in corn. Sample delivery systems are typically installed above ground, but underground installations are also an option.

In both types of installations:

• Delivery lines must not have a turn radius greater than 90 degrees and must be laid vertically or horizontally as much as possible.
• The delivery line conduit must be cut squarely and the inside edges honed to remove any roughness or burrs.
• Conduits for negative or positive air pressures must be connected with airtight couplers where necessary.
• An electrical path must be maintained over the entire delivery system with suitable grounding points to discharge any static build up.

Aluminum or Stainless Steel

Morris Couplers

Vacuum pumps, air supply pumps and sample collector installations are very important to the effectiveness of sampling, transporting and collecting of a representative sample. Manufacturers supply these components based on configuration and installation requirements.

2.3 Components

All components, including drive mechanism (hydraulic or electric), pneumatic truck probe structure, sampling device, sample delivery system and sample collection unit, must be constructed of materials meeting the industrial durability required to operate in the environment where the sampling system is located. Sample delivery lines should be either metal conduit construction, aluminum straight pipe, stainless steel elbows or PVC hose with a smooth inner lining that is appropriate for grain handling.

Hydraulics and electrical power are the main sources used for the drive mechanisms to manoeuver pneumatic truck probes when sampling a conveyance. These power sources must be maintained at a constant and uniform pressure or voltage to ensure the smooth and unaffected operation of the truck probe and any associated equipment during sampling.

Side View

Closed Ports

Open Ports

Open-ended constant vacuum suction probes

Open-ended constant vacuum suction probes use a constant vacuum draw at the intake opening when sampling. They can over-represent lightweight material when sampling a conveyance and are not recommended for taking a grain sample.

The length of the probe selected in combination with the boom’s telescopic extension should enable the operator to reach the bottom of each conveyance to be sampled. Probes are available in various lengths from the manufacturers, ranging from 5’ to 9’.

The operating radius of truck probes have various ranges but are limited. The reach of the probe must enable the operator to follow the recommended probing patterns for each conveyance. If the truck probe cannot reach all areas to be sampled, reposition the conveyance being sampled.

Sampling with a probe that does not reach the maximum depth of the conveyance and/or not following the recommended probing pattern increases sampling extraction error and the risk of collecting a sample that does not accurately represent the lot.

Truck probes with telescopic booms can generally extend the probe out to a maximum reach of ~ 17’ from the supporting structure.

The vacuum and forced airflow system(s) are an integral part of the sampling and sample delivery systems. These components must have the capacity required to effectively draw a sample with the selected type of pneumatic truck probe and allow the sample to travel through the entire delivery system to a sample collector with minimal impact on the sample. Manufacturers will recommend specific makes and models of vacuum and forced airflow systems based on the installation configuration of each system. For a description of sample delivery lines, see section 2.2: Installation.

There are various makes and models of sample collection units. The vacuum system installation will determine the required type. The sample collection boxes require airtight discharge gates and should be installed in the inspection office.

Sample delivery systems utilizing a vacuum pump with a cyclone will not require a sample collection box with a separation screen inside. The cyclone allows the vacuum air pressure to exhaust while allowing the sampled grain to decelerate and transition to the sample collection unit.

Sample delivery systems installed with only a vacuum pump will require a sample collection box with a separation screen inside. The separation screen allows the vacuum air pressure to pass through the screen and exhaust out while holding the sampled material on the intake side of the screen.

2.4 Adjustments

Compartment type probes

Compartment type probes are designed with 3 chambers. The first chamber is of neutral air pressure when sampling. The second chamber has an air intake/vent located at the top of the chamber/probe, allowing air to be drawn into the chamber as needed. The second and third chamber have a vacuum air pressure to draw the sample from the second chamber, through the third chamber and to the inspection office. Manufacturers will recommend specific makes and models of vacuum systems based on the installation configuration for each system, as there is no forced airflow requirement or adjustment for these types of probes.

Core type probes

Core type probes are designed with 2 chambers; the outer chamber has forced airflow travelling through it to the bottom portion of the core tip opening, and the inner chamber has vacuum air pressure drawn up through from the core tip opening. In order to achieve a neutral/vacuum pressure at the sample entry point of the core tip opening of the probe, the forced airflow going through the outer chamber must be controlled. To control this, the core probe is manufactured with one or more air adjustment relief holes. Some manufacturers also include air adjustment relief holes on the air supply motor assembly.

The amount of forced airflow through the outer chamber of the core type probe is controlled by either exposing or covering the air adjustment relief hole(s) with the flexible tube that is used to plumb the air to the probe from the air supply motor.

Properly adjusting and balancing the forced airflow with the vacuum air pressure creates a neutral/vacuum air pressure at the core tip opening of the probe.

Follow these steps to determine if the adjustment(s) and air pressure balance are correct:

1. Position the pneumatic truck probe system so they are operating with the bottom of the probe positioned at an accessible height.
2. Place a piece of lightweight paper so it completely covers the core tip opening of the probe.
   • If the paper is blown away from the opening, there is too much forced airflow pressure. Expose the air adjustment relief holes more to create a neutral/vacuum air pressure at the tip opening.
   • If the paper is being sucked into the opening, there is not enough forced airflow pressure. Cover the air adjustment relief holes more to create a neutral/vacuum air pressure at the tip opening.
   • If the paper holds to the core tip opening by itself without being sucked in or blown away, the air pressures are considered balanced.

Sampling problems caused by improperly adjusted forced air or excessive vacuum pressure

• Inadequate/low forced airflow causes the air pressure at the core tip opening to have excess vacuum pressure, which could lead to over-representation of lightweight material by selecting more dust, small seeds or broken grain from the surrounding grain in the conveyance.
• Excessive vacuum pressure within the sampling apparatus and sample delivery system could impact the composition of the primary sample, such as increasing the amount of CCFM in corn.

• Excess/high forced airflow causes the air pressure at the core tip opening to have excess forced airflow pressure, which could lead to under-representation of lightweight material by not allowing the selection of that material from the surrounding grain in the conveyance.

Manufacturers recommend that the air pressure at the core tip opening be checked at least once a month. Air supply and vacuum units will weaken over time; evaluate them as prescribed by the manufacturer.

2.5 Operation

• Ensure staff are trained in the proper use of the equipment in accordance with the manufacturer’s operating manual.
• Follow recommended probing patterns of conveyances and extract a sample from the entire depth of each conveyance to ensure a good representation of the conveyance has been sampled.
• Conveyances with shallow loads may need to have additional probes taken to obtain enough sample for inspection purposes.
• Draw additional probes in a representative manner (e.g. one area of a truck lot shall not be probed twice unless the entire truck is probed twice).
• Exercise extreme care when inserting the probe to prevent damage to the sides or bottom of the conveyance.
• During sampling operations, monitor the sample collection and delivery system for possible malfunctions.
• After all samples are drawn from the conveyance, check to ensure the sample delivery system and sample collection box are empty before sampling another conveyance.

Operating instructions for a compartment type probe

1. The operator manoeuvers the probe vertically downward into the load. This type of probe operates on the principle that only grain that flows via gravity into the first chamber compartments are taken as the sample.
2. Once at full depth the operator will then mechanically rotate the inner tube on the first chamber to the open position.
3. Material will then flow via gravity into the individual sample compartments within the first chamber; sufficient time is required for the compartments to fill and will depend on grain type and dockage content.
4. The operator will mechanically rotate the inner tube to the closed position.
5. The sampled material will then flow via gravity from the back of the first chamber into the second chamber.
6. During the probe’s retraction or while the probe is still in the load, the sampled material is pneumatically drawn from the bottom of the second chamber up through the third chamber and delivered to a sample collector in the inspection office.

Material flows into first chamber

Sampled material transitions from first to second chamber

Sampled material transitions from second to third chamber

Operating instructions for a core type probe

1. The operator manoeuvers the probe vertically downward into the load. This type of probe operates on the principle that only grain that is forced into the inner chamber is taken as the sample.
2. When the product has reached the inner sample chamber at the core tip opening, it mixes with the air that is being forced down through the outer chamber of the probe.
3. The sampled material is then pulled away through the inner chamber via vacuum draw to a sample collector in the inspection office.

Illustration of core type probe and opening

Sampled material drawn up through inner chamber

2.6 Maintenance

• Regularly monitor the condition of all components and maintain all equipment according to manufacturer specifications.
• Replace damaged equipment with adequate replacement parts to ensure the system functions properly and takes representative samples.
• Replace parts (such as rubber hoses) affected by prolonged exposure to weather such as rain, snow, heat and cold as required.
• Check the rubber gasket on the sample collector unit’s grain discharge gate on a regular basis, and replace the gasket if there are any air leaks.