Boreal Laser Inc.

A Comparison of Point Sensors and Open Path Monitors For Toxic Gas (esp. HF) Monitoring

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Courtesy of Boreal Laser Inc.

Point Sensors

Ion Mobility Spectroscope (IMS) – a gas sample is ionized by a radioactive source (Ni63). Differences in ion drift under the influence of an applied electric field produce a spectrum that is dependent on the mass, charge, and shape of the component ions. The detector signal and strength is tuned to the drift time of the ion of interest (in this case HF) and signal strength is proportional to the concentration.

This is a low concentration detector, useful for personnel protection monitoring and early detection of a small leak. Typically, IMS devices are installed close to high risk sources of leaks. The signal is reversible if subsequent air samples do not contain the species being monitored and the recovery is rapid. There can be a problem with nuisance alarms if the detector is placed too close to locations that experience brief-exposure such as sampling points.

Silicon Chip - This is a high concentration detector and its primary application is to alarm a sudden and significant increase in HF concentration. The detector operates on the principle of an increase in reflection of a light beam as a silicon-coated chip is etched by HF. The detector does not provide concentration information but is used as a slope alarm. The etching of the chip is non-reversible and it must be replaced after exposure. This will be about every six months under normal conditions. There can be a problem with accelerated chip etching if it is deployed near the location of higher ambient HF concentration such as neutralization pits or where there can be brief periodic increases in HF concentration such as at sampling locations. The accelerated etching can be reduced by the use of protective socks during planned operations.

Electrochemical - This detector operates on the principle of a gas sample reacting with an electrolyte and generating an EMF with the current proportional to the concentration. The signal is reversible. The presence of other gases such as Cl2, HCl, NH3 will give false indications of HF. Electrochemical cells do not perform reliably in cold, dry conditions.

This is a low concentration detector and the primary application is in background monitoring for personnel protection, in early warning of a developing problem and to alarm a sudden increase in concentration, similar to the IMS detector.

Open Path

Open path sensors see the total amount of HF that absorbs a light beam transmitted over a measurement path. The output can be as ppm-meters or as an average ppm concentration along the path. Open path sensors are useful for monitoring the perimeter of a process unit, a specific path such as a pump row and air intakes or exhausts. Open path sensors can therefore be used for safety and environmental monitoring, and identifying process improvement opportunities. They can be affected by fog and steam plumes that scatter light and by obstructions that block the light beam. Most open path devices avoid false positives by being able to distinguish between a blocked beam and one that has strong absorption.

Infrared – infrared open path sensors use broadband IR sources to measure absorption in one of the HF absorption bands in the IR or near IR. Background interference from ubiquitous species such as water vapour and carbon dioxide is reduced by techniques such as Differential Optical Absorption (DOAS) and wavelength modulation. Despite using these techniques, strong IR absorbers such as hydrocarbon gases will still provide false responses.

Laser Diode – laser based open path sensors use single-line spectroscopy to practically remove any potential for interference from other atmospheric species. This spectroscopy is made possible by the very narrow and specific beam of light emitted by a room temperature diode laser. This light is rapidly scanned across an HF absorption line to generate a signal at a detector. The laser beam intensity is safe for human eye exposure. Laser based sensors are capable of much greater sensitivity, faster response and longer pathlengths than other open path systems. They can also be made smaller, have no moving parts and fewer consumables.

The following table compares some common features and specifications for the monitoring technologies listed above.

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