The manipulation of odor availability of training aids used in detection canine training

Abstract

Detection canines can identify numerous substances for which they have been trained. Historically, and a point of ongoing contention, detection canine threshold (i.e., sensitivity or limit of detection) training has primarily focused on changing the weight of the training aid substance used. There has been minimal focus on other principles, such as surface area, confinement, and temperature, which can be manipulated to affect odor availability. That said, trainers have been manipulating odor availability for years without necessarily understanding the governing scientific principles. The aim of this review is to highlight the principles that control odor availability of a substance and how an end user can apply these principles for operational detection canine training needs.

Keywords: canine; dispersion; dog training; odor movement; olfactory science; scent detection; training aids.

Figure 1

The journey of an odorant from source to sensor. In this example, the source material (also known as the training aid, true material, or “bulk” material) is emitting its odor, or odorants, via outgassing. Once the odorant is in its gaseous form, the molecules then move from the source to the sensor by dispersion (i.e., spread). Dispersion allows the odor to travel from the source to the sensor (i.e., detector) to be made available for analysis. Typical sensors are detection canines, laboratory instruments (gas chromatography-mass spectrometry), and handheld detectors.

Figure 2

Relationship between surface area and time on the diffusion of training aid odor in a closed system. Whereby X is the surface area (SA) of the training aid (TA), the number that proceeds the X is the multiplier that either increases or decreases the SA. TA. ^‡Time = 0 (T₀) represents when the TA is first placed in the hide location. *Time = 30 min (T₃₀) represents the typical “set time” for the training aid employed by the canine detection community for certifications. ^†Time = equilibrium reached (T_equilibrium) represents the amount of time (T) it takes for the headspace around the TA (e.g., the box in this example) to become saturated. In this example, this amount of time will be shortest for the 2× surface area TA and longest for the ½× surface area.

Figure 3

Notional headspace composition images of a pure training aid and a mixture. Note, there is significantly less TNT found in the headspace of the TNT training aid due to competition; DNTs and DNBs are more prevalent. Published headspace compounds for cocaine (34) and TNT (11, 35).

Figure 4

Permeation of odor as described by Dravniks’ equation. This figure illustrates the elements in Dravniks’ equation that govern the principles of odor permeation.

Figure 5

Odor movement within a closed system and throughout an open system. (a) In this closed system, illustrated by a TADD®, permeation (green box) is the mechanism by which training aid gases are transported across the microporous membrane. Within the TADD® (blue box), condensation and sublimation occur continuously as the training aid aims to reach equilibrium. There are also surface interactions (adsorption) between the gas and glass jar. Diffusion is the main mechanism by which gas moves within this closed system. (b) Once the system is opened to the environment, several processes occur. (1) Permeation determines how much odor gets into the air and enter small-scale flows which determine the local intensity of the odor signal. (2) Intermediate-scale flows then determine how well-mixed the odor signals become as they are transported away from source. (3) Finally, large-scale flows determine where in a large space odor ends up. The odor signal that is available for canine detection (red box) are described by the strength and consistency of the localized odor plume. The plume anatomy (orange box) is composed of odor filaments that can reach far beyond the source material due to air flow. There are two physical processes in plume dispersion: advection (bulk movement) and diffusion. They are perpetual processes and can be described as a ratio of which one is larger or more dominant (the ratio is the Péclet number). Diffusion is dominant in the closed system TADD® because it outweighs whatever miniscule amount of airflow is occurring. Once the TADD® is opened and odor permeates out of the membrane, air flow is significantly larger than the scale of diffusion that advection dominates.

Figure 6

Linear relationship between permeable surface area and flux (dissipation rate). Reproduced with permission from Beltz, 2013 (16).

Figure 7

Methods of altering odor availability from source material. Odor availability can be manipulated by using some of these methods. Training aid (TA) odor can be increased or decreased by modifying the surface area, set time, degree of concealment/confinement, barriers to odor permeation, and temperature of the training environment.