OPEN YOUR MIND TO OPEN PATH MONITORING - A FEATURE FOR ENVIRONMENT BUSINESS
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. Here Duncan Mounsor from Enviro Technology Services plc, the UK's leading supplier of AQM & CEMS equipment, discusses open path monitoring and dispels the negative, and mostly unsupported, myths surrounding this measurement technique.
Breaking with Tradition
So what's the difference in the techniques for AQM and CEMS and how does traditional ‘point method' analysing compare with ‘open path'? For those wanting to, the differences are easy to see. The traditional point method technique generally requires a different analyser for every gas that is to be measured. For example, if you were measuring for SO 2 , NO 2 and Benzene, you traditionally need one point analyser for each compound. These point analysers ‘suck' the air through a pump inside the analyser and the gases are measured from there. Calibration for point method analysers needs to take place approximately every two weeks, or 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 a receiver to measure compounds between a 100 – 500m distances, therefore giving customers a more representative air quality measurement. . The open path system works using a Xenon light path to measure stated gases. Open path systems, such as OPSIS, use DOAS (Differential Optical Absorption Spectroscopy) to measure these gases. A Xenon lamp transmits visible light from the transmitter to the receiver and gases such as NO 2 , SO 2 and O 3 absorb into the UV part of the light spectrum and are measured from there. Multiple gases (up to 10 or more) can be measured using the open path technique with just one analyser. The OPSIS System 300 is, in fact, the first ambient AQM system to receive MCERTS approval for the measurement of multiple gases including SO 2 , NO 2 , O 3 and Benzene (C 6 H 6 ).
Due to the nature of the open path measurement technique, maintenance and calibration of open path systems take s up less operator time than traditional point analysers. Systems such as OPSIS have few moving parts – unlike that of the traditional point analyser which, due to its hybrid electrical/pneumatic nature, includes various pumps, moving parts and general mechanics needed for operation. The OPSIS open path technique has fewer than 5 moving parts. Sampling takes place in the light path; this, again, cuts down on the amount of time an operator needs to dedicate to the operation of the open path system.
Dispelling the myths
Since its insurgence in the late 80's 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 - as with all new things - the open path system was expensive. However, due to more demand for the technology and less expensive parts becoming available, the price these days is at least 50% lower than at its inception. Open path systems, such as OPSIS, are comparable to their traditional counterparts due to their ability to sample multiple gases - which include Benzene, a traditionally expensive compound to measure.
The second main misconception is that open path measurement is not internationally approved, which is untrue. Systems such as OPSIS have approval from all over the world (spanning the past 10 years), including America (USEPA), Germany (T ü V) and, most recently, MCERTS – the Environment Agency's programme to increase the quality assurance levels of ambient and emissions monitoring systems. OPSIS System 300 is the first ambient air quality model of any technology to receive this certification.
The third of the major misconceptions is that open path systems are difficult to calibrate. The reality is that open path monitors are as easy to calibrate and service as traditional sampling methods and the frequency far less. This is due to the simplicity of the products, and the lack of moving parts, pumps etc means that, in most cases, systems such as OPSIS only need to be calibrated annually, as opposed to every two weeks, by a service provider such as Enviro Technology. Using an open path analyser means that it is likely that there will be a reduction in the resources needed for operation and maintenance – a good thing for any organisation.
Many people feel that open path technology doesn't work well for the measurement of hydrocarbons and, in the early days, there was an element of truth in that due to the complexity of this form of gas measurement. In recent years companies such as OPSIS have spent time, money and considerable R&D time in improving open path performance when measuring individual hydrocarbons such as Benzene. This hard work has culminated in the MCERTS certification of the OPSIS System 300 for Benzene measurement (along with NO 2 SO 2 and O 3 ).
The proof of the pudding is in the eating for any company or organisation looking to incorporate 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. Traditional point sampling wasn't the right avenue for them, so they decided to give the OPSIS system a try. “It was of great importance that we got a truly representative measurement when measuring AQ in various street canyons within Swansea ,” says Phil Govier of the Council's pollution control team. “Open path measurement has proved itself to us time and time again. We have one OPSIS system in use, another one on order – and a third in the planning stage. It is proving to be the best solution for our AQ monitoring/measurement requirements for street canyon situations and of paramount importance in the validation of the dispersion models under development.”
For further information regarding open path measurement, or for a demonstration on how OPSIS could work for you, please call Duncan Mounsor on 01453 733232 or visit www.et.co.uk