Over the past 15 years, open path monitoring has come on in leaps and bounds, yet many of the systems that are based on open path technology are still fighting against negative perceptions from potential buyers. So how does traditional ‘point method’ analysis compare with open path? The traditional point method technique generally requires a different analyser for each gas be measured. For example, if you were measuring for SO2, NO2 and benzene, you’d need one point analyser for each.

Calibration for point method analysers needs to take place approximately every two weeks – or even more frequently. These traditional analysers can only measure the ‘gas of interest’ (air in a close proximity) and therefore, in some cases, may not give a truly representative measurement. Open path measurement uses a transmitter and receiver to measure compounds at 100-500m distances, giving customers a more representative air quality measurement. The open path system works using a Xenon light path to measure stated gases.

Systems such as OPSIS use differential optical absorption spectroscopy to measure gases. A xenon lamp in the transmitter emits broadband energy to the receiver up to 1km away, which in turn is connected to the analyser (UV spectrometer) via a fibre optic cable. A wide range of organic and inorganic gases are measured including SO2, NO2, NO, O3, benzene, toluene and xylene.

Multiple gases (up to ten or more) can be measured using the open path technique with just one analyser. The OPSIS system is the first ambient AQM system to receive MCERTS approval for the measurement of multiple gases. Due to the nature of open path measurement, maintenance and calibration takes less operator time. Open path systems have few moving parts – unlike that of the traditional point analyser which includes various pumps, moving parts and general mechanics needed for operation. With open path, sampling takes place in the light path, cutting down on the amount of time an operator needs to dedicate to the operation.

Dispelling the myths

Since its emergence in the late 1980s, the open path technique has had a mixed response from organisations across the spectrum. Much of the speculation regarding costs, reliability and suitability has been based on untruths and scare-mongering. The first of these is to do with price. In the early days of the technology the open path system was expensive. However, due to more demand and less expensive parts becoming available, the price is at least 50% lower than at its inception.

The second main misconception is that open path measurement is not internationally approved. Open path systems have approval from all over the world, including America (USEPA), Germany (TüV) and, most recently, the Environment Agency’s MCERTS programme. The third misconception is that they are difficult to calibrate. The reality is that they are as easy to service as traditional sampling methods and in most cases only need to be calibrated annually, as opposed to every two weeks.

Many people feel that open path technology doesn’t work well for hydrocarbon measurement, and in the early days there was an element of truth in this. But manufacturers have spent time, money and considerable R&D time in improving performance when measuring hydrocarbons.

Repeatedly proven

The proof of the pudding is in the eating for any organisation looking to use open path monitoring techniques for air quality monitoring. Many of Enviro Technology’s clients, such as the City & County of Swansea, have incorporated OPSIS systems for ‘street canyon’ and ‘fence line’ monitoring. “It was of great importance that we got a truly representative measurement when measuring AQ in various street canyons,” says the council’s Phil Govier. “Open path measurement has proved itself to us time and time again.”

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