Toxic gas leak detection

Traditionally detectors for toxic gas leaks n chemical plants use catalytic beads or semiconductor or photo ionization detector (PID) sensor technology. When it comes to certain gases, however, we need more. Consider the toxic properties of phosgene, of which the threshold limit value is as low as 0.1 parts per million (ppm) in the U.S., and even 20 parts per billion (ppb) in Europe. For this application, it's obvious we need accurate sensor technology specific to the actual gas, while avoiding common cross-sensitivities.

The atmosphere, even in a nonindustrial environment, contains up to 5 ppm methane from natural gas sources. In urban areas, you might also find 10-20 ppm carbon monoxide (CO) and a certain amount of nitrous gases from traffic and combustion processes. Other substances from a variety of different sources can have a bearing on the environment. Compared to the low threshold value of phosgene, sensors having cross-interferences to one of the gases mentioned are inadequate for either monitoring or leak-detection purposes.

Another important issue is the fact that most phosgene manufacturers and users have a strict zero-emission policy. You can't comply with that if you don't have the tools to determine a potential phosgene concentration from the background of other gases.

The oldest method is the catalytic bead. It is stable, fast, and robust, and it has a well-defined zero. The most commonly used sensor for leak detectors is the semiconductor. It's cheap, rugged, and responds quickly. What makes it inadequate for phosgene detection is the comparatively high detectable limit, which is, depending on its design, between 1 and 10 ppm. It will also be sensitive to any background substance, so you can only locate major leaks with this technology.

The PID method is the high-end technology for leak detection. It's very sensitive, fast responding, stable, with a well-defined zero, unlike the semiconductor. Because it won't respond to any kind of hydrocarbon ionized by high-energy ultraviolet radiation, it's not vciy useful in detecting phosgene.

With colorimetric paper-a sensitive method-the paper will turn red when exposed to phosgene, but only in total exposure, not by concentration. You can get the same color change if you expose it to a high concentration for a short time, close to a leak, or if you move for a long time in an area with a lower concentration, far from the leak. You must store the spare paper you carry in an airtight container; otherwise it'll turn red after a while. Colorimetric paper is a useful and economic method if you've already located a potential leak and just want to check if your guess was right. If you really have to locate a leak, you'll need a continuous detection method.

Using adequate technology, electrochemical sensors can be specific to certain substances. Typically, electrochemical sensors require more time to respond and recover than physical methods. When locating gas leaks, you'll want rapid response to increasing concentrations when approaching a leak and fast recovery when going away from it.

How does it work?

An electrochemical sensor is nothing more than a gas-operated battery. Engineering and manufacturing know-how of electrodes and electrolyte will render an excellent sensor for various gases. The phosgene sensor uses a silver electrode as a working electrode. The electrolyte is a special gel. This means you don't need an additional membrane you'd otherwise require when using liquid electrolyte. Membranes may increase response time and make a sensor temperature-sensitive. The primary working electrode reaction is as follows:

2 Ag [arrow right] 2 Ag^sup +^ + 2 e

This reaction will take place as soon as the sensor sees manufacture. It will stop as soon as it reaches an equilibrium (Ag versus Ag+).

AgCl is a nonsoluble salt, thus taking silver ions out of the working electrode equilibrium. This will make the working electrode produce more silver ions and electrons. All you need now is a sensitive current amplifier, between the working and counter electrodes.

A combination of a flow chamber with optimum flow characteristics and a powerful pump reduces the response time to a few seconds. Another benefit to this configuration is that the pump forces more molecules per second to hit the working electrode, resulting in a higher signal.The new leak detector is capable of detecting as little as 2 ppb! If you force too much flow through the calibration adapter, you may under-span the sensor.

Behind the byline

Bernd Rist is general manager at Compur Monitors in Munich, Germany.

Copyright Instrument Society of America Oct 2004
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