Chemical detectors on drones A Gate to Airborne Chemical Analysis
Chemical detectors on drones A Gate to Airborne Chemical Analysis
The UAV (Unmanned Aerial Vehicles) as a platform to carry chemical sensors, detectors, analyzers
Using UAVs as a platform for carrying the chemical detector has significant benefits; moving into harsh and dangerous environments without jeopardizing human operators is the most important one. Other benefits include remote sensing, fast screening of large areas and a capability to overlay 3D data on maps and GIS.
With chemical detectors/analyzers on drones you can monitor or analyze:
during flight
while hovering or
while landed
The alternative of using UAVs as a carry platform is to use handheld instruments or Unmanned Ground Vehicles (UGV). The handheld devices are established devices but do not allow fast screening of areas. They are too slow to deploy at certain suspect coordinates and in harsh environments operators need to wear special protection suits. Although UGVs have the advantages of simplicity, low cost, capability of transporting bulky/heavy payloads and high autonomy, they suffer from serious restrictions, especially when moving over a rough terrain.
According to US FAA, small Unmanned Aerial Systems (sUAS) are those with weight less than 25 kg. In T4i engineering we target chemical detectors miniaturization based on payloads requirements for micro-UAS. sUAS advantages include: wide range of applications, low cost, versatility and precision. Current limitations of small and micro-UAS include autonomy, payload capacity, and safety. In order to address these problems, demanding requirements and standards are set for the chemical detectors. For example, to resolve problems in spatial resolution due to UAS speed, chemical detectors with response time equal or less than 1s are sometimes needed.
Beyond chemical analysis: Critical challenges for chemical detectors on board UAVs
Weather conditions, the terrain and the types of air flow are critical for reliable and accurate measurements.
When using UAVs as a carrying platform for the chemical detector, the weather conditions during sampling need to be carefully considered and addressed.
The physical quantities that characterize the state of the atmosphere are temperature, pressure, density, humidity (water vapor), wind speed and wind direction. The gradients and the interactions of each of these variables together with their rate of change in time constitute the weather parameters. It is important to consider these parameters when designing and running measurements. Measuring weather parameters with small, portable weather stations is significant both for the flight of the UAVs but also for the chemical detector/analyzer performance and reliability.
The atmospheric pressure can affect the chemical detection technique (atmospheric or vacuum), the wind speed can affect the sampling method and humidity can also be critical for potential condensation or other phenomena. The ambient temperature can have serious impacts on the properties of substances to be detected.
The physical layout, the site description, the site history can strongly affect airborne chemical detectors/analyzers. The term ‘field terrain’, is commonly used to describe the morphology of the land surface. The characteristics of terrain include altitude, slope and orientation and can affect air motion. Sampling depends significantly on the terrain at the sampling area, since weather phenomena are strongly related to the land surface. Sampling in urban areas populated with high buildings is different from that of rural areas or valleys surrounded by mountains. “Street-canyon” phenomena will occur in cities with high buildings that can affect sampling conditions and strategies.
The types of air flow are governed by fluid dynamics. In the case of chemical detectors on-board UAVs, fluid dynamics play an important role in air samples acquisition. The type of flow can also affect the limit of detection. Various properties of air (e.g., pressure, temperature) may significantly change from one point to another and due to the speed of the UAVs these changes can occur rapidly.
How to radically address the critical challenges of “flying chemical detectors”: Sampling strategy and miniaturization
Using the right type of sampling technology can effectively address critical challenges such as the need for iso-kinetic sampling, effects of fluid dynamics and field terrain, condensation and other phenomena such as sudden changes of pressure and temperature.
T4i engineering chemical detectors are equipped with a unique, sampling system (proprietary knowledge) called the SMS that serves both as sampling unit and separation system (based on Gas Chromatography). It is a dynamic, valveless, miniaturized sampling unit in front of a fast, miniaturized, LTM GC. It can hyphenate as a front-end to various atmospheric and vacuum detectors.
The SMS (Sample Modulation and Separation) is based on well-proven principles of fast, pneumatic, periodic sampling and is designed to address the critical challenges of “flying chemical detectors”. The basic principles of SMS are presented in the following diagram when SMS is hyphenated to an IMS:
An SMS unit consists of two concentric, tiny tubes, and one COTS capillary column located inside the inner tube. By utilizing miniature valves and pumps, the unit can pneumatically prevent the capillary column from sampling. During ‘sampling mode’, this is reversed and sample flows through the column that acts as transfer line if uncoated or as a fast GC if coated. As a transfer line, it retains all specifications of the hyphenated IMS and as a GC (commercially available columns as long as 15 m), it provides fast GC or analytical-scale gas chromatographic separation.
The SMS unit operation is supported by the pneumatics board where the sample modulation and GC control takes place by combining heating elements, miniature pumps, valves and flow sensors. An on-board microcontroller with specially designed firmware allows the user to define sampling times as short as 100 ms and pre-program the alternation between sampling and non-sampling modes in a fully customizable manner. In environments where the compounds of interest are found in ultra-low concentrations, the SMS unit can be complemented with a pre-concentrator. This pre-concentrator uses special materials and temperature modulation algorithms to capture the compounds of interest, over a pre-programmed period of time, and release them in an instant.
The SMS has been designed using advanced and integrated CAD tools and is characterized by low footprint, low power consumption (less than 20W) and low weight (less than 150 g). Special software (T4i Symbiotic engine) provides, depending on the type of detector, remote operation, data transmission, automation, detection, identification and monitoring. Multivariate data analysis (on-line) and AI algorithms allow autonomous application of sampling profiles depending on the type and concentration of Toxic Industrial Compounds and Volatile Organic Compounds.
SMS is sampling at atmospheric pressure. The advantages include:
sampled vapor molecules only touch GC-grade surfaces and coatings. Thus, with proper temperatures and flow design, any substance that can be analyzed by capillary GC is therefore suitable for analysis
the rapid modulation of the SMS inlet allows direct coupling with high-resolution capillary GC systems
very low hysteresis effects permit near-real time, on-line monitoring of rapid dynamic changes in compound concentrations
Compared to membranes as inlets, the SMS technique, has certain advantages against the limitations of membranes: many large and or polar molecules that can be analyzed by capillary GC are unlikely to pass through membrane inlets, membranes are far too slow to act as injectors for capillary GC, and require a system of injection valves behind the membrane, thereby introducing the potential for sample losses and hysteresis (“memory”) effects (e.g., peak tailing and ghost peaks)
T4i proprietary knowledge provides SMS with:
Sharp ‘injection-like’ sampling periods (as short as 100 ms) resulting in sharp peaks
Iso-kinetic sampling during flight
Self-cleaning of the SMS from contamination that may cause memory effects or ghost peaks
Valve-less sample introduction. The pneumatics system acts as a “virtual valve” and no electromechanical valve comes in contact with the actual sample
Elimination of surface chemistry phenomena during the presence of the sample in the inner tube
No pressurized gas cylinder for use as a carrier gas is required. Air is being cleaned and utilized as a carrier gas using well-engineered air scrubbers. An advantage for use on field.
Hyphenation and the Limit of Detection (LOD) strategy
Various strategies are used for enhancing LOD when using SMS. Hyphenation with ppb level Photo-Ionization Detector (PID), Ion Mobility Spectrometry (IMS), as well as, Mass Spectrometry (MS) provides different advantages.
Fluid dynamics for airborne chemical detectors: The long road ahead.
T4i has carried out CFD studies to define the impact of multi-rotor UAV downwash flow on the chemical detector on-board UAV. An indicative simplified CFD model is shown in the following figure.
Research on the impacts of fluid dynamics on the reliability of chemical sensors/detectors/analyzers is ongoing and T4i engineering is embracing strategies and specific technology roadmap to address these impacts for enhancing reliability, efficiency and performance. In these studies, strong support is provided by T4i FemtoMachine® PRO, a commercially available miniaturized vapor generator and calibrator that can be used in the field.
