Design Con 2015

Gas Detectors to the Rescue

-December 01, 2013

When it comes to working in facilities, safety needs to come first. Hazardous gases in the workplace can be a serious danger and lead to fires or explosions. Despite their dangers, not all gases can be easily detected by smelling the air, which makes gas detectors very popular in industrial environments. Workers typically wear these detectors on their clothes so the presence of poisonous gas can be spotted. Area monitors are also permanently mounted near potential gas sources as a way to detect danger. There are several types of gas detectors, but the one we’ll explore is based on the electrochemical principle.

 

 

Figure 1. Gas detector

 

Portable applications require both small operating currents and battery power. This is ideal for gas-specific sensors that have gas concentration resolutions under one part per million. Gas sensors, particularly electromechanical sensors, are generally slow and take several minutes of response time to reach its output full-scale value. Equally, when the sensor is exposed to a step response in gas concentration, it may take as much as 30 seconds to settle to 90 percent of its new final output value.

 

Electrochemical sensors offer several advantages for instruments that detect or measure the concentration of toxic gases. These can be used to detect a number of different gases such as carbon monoxide, chlorine and hydrogen sulfide. All of these gases basically can be detected with this type of sensor.

 

Figure 2. Basic sensor circuit

 

When designing a sensor for gas detection, low-power consumption is the highest priority. The basic sensor is shown above in the circuit. The sensor has three electrodes: a reference electrode, a working electrode and a counter electrode. For the electrochemical action to work properly, the working electrode and the reference electrode must be at the same voltage. In order to do so, use a simple feedback loop, where you take the output and feed it back into the input. This forces the control electrode to go to the proper voltage, which makes the summing junction of the amplifier zero. This forces the reference and the working electrode to zero volts.

 

For a video demonstration on the principles related to high impedance sensing conditions in circuits, view my colleague Walt Kester’s on-demand presentation: “Instrumentation: Liquid and Gas Sensing” at ADI’s Virtual Design Conference. Here you will find on-demand online technical training presentations on how to solve complex design challenges until January 25, 2014.

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