Olfactometer History

The first olfactometer was created by Hendrik Zwaardemaker in 1886 (Philipot et al., 2008). This device was very simplistic consisting of two glass tubes where the odor is sent, two hook-shaped corks to be placed in the nose, and a handle for convenient hold. Zwaardemaker was interested in finding out the elementary sensations of smell, how we become conscious of these stimulations, and why these sensations affect our thinking mechanism. In order to investigate these questions, he constructed this design which was first meant to be used for mixing odors. Using his first device, he conducted experiments to measure olfactive sensitivity where he set a standard value called ''olfactie''; the magnitude of an odorant substance that exceeds the threshold. This value could be used for comparing the individual results from patients. Zwaardemaker created several different versions of the olfactometer over time, with one and two tubes (Nicolas et al., 2021) After his developments in the field, many different types of olfactometers were made shortly after his invention, one version was from Charles Henry using an odorous liquid, then at the beginning of the 20th century, German and American brands started to sell olfactometers.

Figure 1: (A) Three different Zwaardemaker olfactometer illustrations, The Netherlands, 1886. (B) Zwaardemaker olfactometer, The Netherlands, 1886. (C) Charles Henry's olfactometer, 1891. (D) Olfactometers from (A) German (Zimmermann, 1897) and (B) American companies (Chicago Lab. Supply Co., 1901)

Latest Advancements in Olfactometers

After the 20th century, olfactometers have improved a lot since various models have been created for different purposes. Currently, there are two main types, namely; flow and dynamic dilution olfactometers. Flow olfactometers, created in 1985, have a constant flow of pure air to the nose of the experiment subject. It delivers a consistent stream of air to an individual's nose, creating precise and repeatable odor or pain stimuli. By using this method, researchers can record reflex reactions and alterations in the central nervous system.

Dynamic dilution olfactometer, is another kind that involves using a panel of trained human sniffers to rate the odor intensity of a sample in real time. This is done by sniffing the sample through an olfactometer that dilutes the odor concentration until it is at a level that can be accurately rated by the panel. Field olfactometer is another example that is used mostly in the industry as a portable alternative to laboratory olfactometers. Due to their portability, they are seen as the simpler versions of laboratory olfactometers where only one panelist is enough. In this olfactometer still, dilutions of odor mixtures are presented using airflow. They are mostly used for analyzing odor levels in different locations (Claros et al.).

Olfactometers were then started to be used for animal experiments as well. They were adapted to experiments involving insects using a specially constructed system with a Y-tube. In the system, insects are attracted to the forks of the Y-tube using light and when they reach the entrance of the forks they are encountered with odors where one fork works as an attractive or repellent side and the other fork as the control side.

Figure 2: Examples of different olfactometers (A) Dynamic dilution olfactometer (B) Static olfactometer (C) Field olfactometer (D) Y-tube flow olfactometer

Flow olfactometers were adapted to be used in mice for studying olfactory sense in behavioral research. There is a wide range of options available in terms of complexity, price, and portability. An example in the industry is the Aurora olfactometer depicted in Figure 3. This olfactometer features a compact design that offers automated delivery of odors. The design has space for odor vials, valves, and a control unit. The control unit contains several compartments that are essential to be used in the experiments, such as flow controllers and led indicators. Flow controllers are used for maintaining the flow at a certain level, while the led indicators show the ongoing incident. This unit also contains USB and LAN ports for connection to the computer. The other compartment contains a manifold that keeps the valves together. These valves are connected to the odor vials through input and output tubes, with the input tube being longer, thus creating a headspace between the two tubes. The output tubes are responsible for picking up the odor and carrying it to the mouse.

Figure 3: Aurora Olfactometer

Figure 4: Aurora olfactometer user interface

The creators of Aurora made a minimalistic visual interface where the user can select the odor, delivery duration, and flow. Furthermore, LEDs are displayed on the computer in addition to the box along with the control buttons that are implemented to manage the execution. The price for this olfactometer commercialised by Aurora scientific is 12 500€

Benchmark of other Olfactometers