Listed below are some of the frequently asked questions:
Units of Measure for Open-Path Measurements
Path Integrated Concentration: Parts Per Million – Meter (ppm-m)
The units of measure for Open Path Gas Detectors is Parts Per Million – Meter (ppm-m), which is a Path Integrated Concentration.
Think of Path Integrated Concentration as each of the target gas molecules within the Active Measurement Path being counted and added together to provide the total concentration.
Basically, this measurement methodology can be summed up as this being a molecule counter.
It is important to note that the Path Integrated Concentration is completely independent from the physical path length (distance between the Open Path (OPX) Head and the Retro-Reflector).
To help visualize what Path Integrated Concentration is, lets assume a background concentration of 10 ppm. The image below shows how the Path Integrated Concentration increases with path length even though the background concentration remains the same. Atmospheric gases like Methane (CH4) and Carbon Dioxide (CO2) will exhibit a similar result.
Path Average Concentration – Parts Per Million (ppm)
The Path Integrated Concentration (ppm-m) can be converted to a Path Average Concentration (ppm) by dividing your ppm-m concentration by the physical path length (m) of the Active Measurement Path. The path length can be programmed into the GasFinder to automatically convert the outputted concentration in a Path Average Concentration (ppm).
Use Applications for Path Integrated Concentrations and Path Average Concentrations:
Path Average Concentrations are typically used in short path length applications where the plume is either being fully measured or where the concentration assumed to be more or less homogeneous.
It is typical for Path Average Concentrations to be used with the following Measurement Heads: Remote Point (RPX) Probe, Stack/Duct (SDX) Probe, In-Line (ILX) Probe, and Extractive Measurement (EMX) Probe.
Path Average Concentrations can be used in Open Path Gas Detection Applications but the end-user needs to understand that this technology cannot spatially resolve the size or the concentration profile of the plume within the Active Measurement Path. The image below can show how a Path Average Concentration can be misleading depending on the size and density of the plume.
While a large and dispersed plume may not pose a threat – a small and highly concentrated plume may present a hazardous scenario. For Open Path Applications, we recommend using Path Integrated Concentrations for setting alarm thresholds.
Measurement Range Specifications
Measurement Range Specifications:
The GasFinder has three key performance specifications centered around it’s ability to detect and monitor a specific target gas: Minimal Detectable Limit (MDL), Sensitivity, and Full Scale.
The Minimal Detectable Limit (MDL) is smallest concentration value that is reliably and repeatably detected. Any values below this MDL is to be considered noise and not a reliable reading.
Sensitivity is the smallest incremental concentration change that is reliably and repeatably detected.
The Full Scale value is the end of the calibrated Linear Range. It is within this Linear Range where the GasFinder has an accuracy of +/-2% of Reading. While the GasFinder will continue to output repeatable readings beyond the linear range (i.e. Full Scale), they no longer have a specified accuracy associated with those values. It is typical for displayed concentrations that are beyond the Full Scale Range to be lower than actual.
The end-user should only have Actionable Concentrations such as Hi/Hi-Hi Alarms or Control Thresholds within the Linear Range of the GasFinder.
For more information about specifications for specific target gases click below:
Light Level: Performance Variable
The invisible Near Infrared Laser Light leaves the Measurement Head as a collimated beam. The image below shows the Open Path (OPX) Head on the left and the Retro-Reflector on the right.
The Retro-Reflector then returns the collected Laser Light back to the Measurement Head via a focused beam for analysis. The Measurement Heads used are also called Transceivers as they have both the sending and receiving optics built-in.
The amount of returned Laser Light can be affected by light scattering mediums such as fog, rain, sleet, snow, dust, etc. The functional test used to validated how much beam block the GasFinder can operate with is done with a Neutral Density Filter. This test verifies that the GasFinder can tolerate up to 97% Trans-Obscuration (or simply put, 97% Beam Block).
The laser light used is in the Near Infrared which is close to the Visible Spectrum. Light in the Visible Spectrum scatters more so than light within the Near Infrared.
Our anecdotal example of how to explain how much beam block the GasFinder can handle is simply as follows. If the Retro-Reflector can be seen from the Measurement Head then the Near Infrared Laser Light will be able to pass through as well. The image below is an example of looking through the Alignment Scope on the Open Path (OPX) Head at a Retro-Reflector.
If the Laser Light Levels were to decrease beyond tolerable levels then the GasFinder will automatically enter a Low Light or Beam Block status condition. This specific status/fault condition will be shown as 2.7 mA on an Analog Loop.
The status/fault condition for Low Light is non-latching and as soon as the sufficient Laser Light Levels return then the system goes back into normal operation.
To help avoid nuisance alarms caused by intermittent beam blocks (e.g. steam), the end-user can easily implement a time delay of 0-300 seconds on the HMI Touchscreen.