Sensing Hazardous Gases Can Save Lives
An increase in the number of hazardous gases poses a severe threat to humanity in general and to workers in many industries
These gases could come from natural or man-made sources like chemical industries, petroleum refining, stone, plastic, and food processing. Because of the risk that they leak into the environment, safety procedures are necessary to protect the environment and the workers. Different types of gas detectors are used to detect different gases such as commonly occurring pollutants like carbon monoxide, hydrogen sulfide, sulphonyl chloride, phosphine, and nitrosyl chloride.
CATALYTIC SENSORS
Pellistor/Catalytic Bead (CB) sensors, which have been around for nearly a century, can respond to flammable gases such as hydrogen, oxygen, hydrogen sulfide, methane, butane, propane, and carbon monoxide. They have two beads: an active bead coated with a catalyst, that reduces the temperature at which the gas surrounding it ignites. As a result of the combustion, this bead heats up, causing a temperature difference between it and a reference bead. The heat causes the resistance to change as a function of the type and concentration of the gas.
Since the bead is one leg of a Wheatstone bridge, this resistance change produces an output voltage signal.
Catalytic bead combustible sensors are quick and accurate when used for detecting a single gas. However, this technology is subject to sensor poisoning from exposure to silicones and lead compounds. Furthermore, in multiple gas detection sensors, these sensor detectors may give false readings for all other gases if they are calibrated to a single gas. The correct choice of combustible gas for calibration depends upon the industry application and the potential gas hazard(s) of that particular industry. For example, for a propane distributor, the hazard is propane; in sewers, the main hazard is methane.
In a complex environment with multiple combustible gases, a number of questions need to be considered when choosing a sensor, most importantly the Lower Explosive Limit (LEL) — the lowest concentration (by percentage volume) of a gas in air that is capable of producing a flash of fire in the presence of an ignition source.
- What are the combustible gases?
- Which gas is the most prevalent?
- What are their concentrations as a percent of LEL?
- What type of sensor do you need — single gas or multiple gas?
Molecular Property Spectrometer (MPS) sensors, a proprietary technology from NevadaNano (Sparks, NV), can detect the presence of flammable or combustible gases, including mixtures, and classify them as hydrogen, methane, light gas, medium gas, or heavy gas. They can quickly detect a wide spectrum of combustible gases, including hydrogen and heavy hydrocarbons.
MPS sensors, unlike catalytic bead sensors, cannot become poisoned because the measurement is based on physical properties rather than chemical reactions. They do not need calibration, and offer a fail-safe feature and built-in diagnostics to notify the user if a sensor becomes unusable or compromised.
The MPS Flammable Gas Sensor’s transducer is a micro-machined membrane with an embedded Joule heater and resistance thermometer. The micro-electromechanical system (MEMS) transducer is mounted on a printed circuit board and packaged inside a rugged enclosure open to ambient air. Presence of a flammable gas causes changes in the thermodynamic properties of the air/gas mixture that are measured by the transducer.
A SYSTEMS APPROACH TO PERSONAL SAFETY MONITORING
Universal Site Monitoring (USM) — Darwin NT, Australia — has designed and developed an integrated system of wearable personal safety monitoring devices, communications hubs, and reporting and management software. The Hero hand-held Personal Safety Monitor comes in a standard version, model 825, and an ATEX /IECEx certified version, model 715. It detects Carbon Monoxide (CO), Hydrogen Sulfide (H2S), explosive gases (LEL), and Oxygen (O2), using catalytic sensors. It includes microcontrollers, gas sensors, a temperature sensor, an accelerometer, a gyroscope, a speaker for audible alarms, a cellular module, GNSS, and Zigbee. The gas sensors use an external analog-to-digital converter (ADC) module, which converts the analog output from the sensors to a digital format.
The digital data is transferred to a microcontroller using an I2C serial communication protocol. The accelerometer and gyroscope, together with the GNSS, are used to determine the worker’s position, speed over ground, elevation, slips, trips, and falls. The Universal Data Interface (UDI) smart alert and navigation system is a web-based application developed for the control room, to monitor the live data coming from the Personal Safety Monitor.
The Personal Safety Monitor displays all the data on an LCD screen and sends it to the Universal Data Interface for real-time monitoring, analytics, configuring, and communication, using the available network, either cellular, GNSS, or Zigbee. The company has combined the hardware and the Universal Data interface (UDI) smart alert and navigation software into a connected worker safety solution with integrated 2G/3G/4G cellular connectivity.
Triggers and alarms can be set remotely from a control room for a high or low concentration of gas detection, and the level of alarms can also be configured according to the requirements of the site. The remote operator can define different stages of alarms, critical or warning alarms, and can set them in response to specific concentrations of the four detected gases. For example, the user can set the warning trigger/ alarm in response to 10 PPM of carbon monoxide or below or can change the alarm level from warning to critical in response to a 100 PPM of carbon monoxide or above.
This article was written by Umer Farooq, Technical Sales Engineer, Universal Site Monitoring (Darwin NT, Australia).
