Oxygen

QUICK GAS FACTS:

Target gas specific information is included below for both Open Path Gas Detection and In-Situ Monitoring Applications.

  • Open Path Gas Detection: Path Lengths greater than 5m

    Open Path Gas Detection

    Alarm Thresholds

    Open Path Toxic Gas Detection does not have a one-size-fits-all Alarm Threshold Guideline. However, other Learned Bodies have assembled their own technical reports and we applied their findings to fit with Open Path Gas Detection. This has been done to assist end-users set appropriate Alarm Thresholds for each of their unique applications.

    Referenced Learned Bodies Include:
    • Center for Chemical Process Safety – Continuous Monitoring for Hazardous Releases
    • UK HSE – Review of Alarm Setting for Toxic Gas and Oxygen Detectors (RR973)
    • ISA-TR84.00.07-2018 – Guidance on the Evaluation of Fire, Combustible Gas, and Toxic Gas System Effectiveness

     

    Tool to Select Alarm Thresholds:

    The Table below shows how the different Alarm Thresholds can vary with different Plume Sizes.

    The table below shows how the Maximum Alarm Thresholds fit within the Actionable Range.

    To help provide more clarity to the table above, the following terms have been further defined:

    Minimal Detectable Limit (MDL): This is the smallest concentration value that is reliably and repeatedly detected when exposed to a known gas concentration. In a comparable example, think of the Minimal Detectable Limit (MDL) in similar terms to a Signal-to-Noise Ratio where the MDL is the smallest signal that can be detected through the noise floor.

    Non-Detectable Range: Any ppm-m concentration below the Minimal Detectable Limit (MDL), regardless the value of the R2 Confidence Factor, should be strictly treated as noise.

    Zero Gas Noise: Typically, Zero Gas Noise will produce gas concentrations that are below our Minimal Detectable Limit (MDL) with low and high R2 Confidence Factors  and can be generated from spectroscopic, mechanical, or atmospheric sources. However, Zero Gas Noise can produce gas concentrations that also exceed our Minimal Detectable Limit (MDL) with “good” R2 Confidence Factors to two (2) times our published Minimal Detectable Limit (2x MDL).

    Non-Actionable Range: While measurements within the Non-Actionable Range (i.e. below 2x MDL) can be both above our Minimal Detectable Limit (MDL) and have “good” R2 Confidence Factor values, it is difficult to determine if the gas concentrations measurements are from Target Gas within the Active Measurement Path or just Zero Gas Noise. It recommended that Actionable Concentrations (i.e. Alarm Thresholds) not be set within the Non-Actionable Range and/or below 2x MDL.

    Actionable Range: This is where the results are repeatable, accurate, and linear. It is within the Actionable Range where end-users can confidently set Alarm Thresholds that are “As Low As Reasonably Practical” (ALARP) while taking into consideration the need to avoid false alarms.

    Lowest Actionable Concentration (LAC): As the name states, this is the Lowest Concentration that should be considered for use in setting an Alarm Threshold (i.e. 2x MDL). False Alarms between 2x MDL and 5x MDL while rare could be still be possible.

    Lowest Recommended Alarm Threshold (LRAT):To provide an additional margin of safety, Alarm Thresholds that are greater than five (5) times the Minimal Detectable Limit (5x MDL) will provide acceptable protection from less-common and/or circumstantial spurious/nuisance/false alarms. Concentrations above 5x MDL are to be considered valid.

    Sensitivity: We define Sensitivity as the smallest incremental change in concentration that is reliably and repeatability detected above the Minimal Detectable Limit (MDL). In a comparable example, think of our Sensitivity as Shot-to-Shot Noise. As a general rule-rule-of-thumb for all gases, the Sensitivity (i.e. Shot-to-Shot Noise) is a quarter of the Minimal Detectable Limit (e.g. 0.25 x MDL).

    Highest Actionable Concentration (HAC): This is the end of the Actionable Range and Alarm Thresholds should not exceed this Full Scale value.

    Non-Linear Range: Concentration measurements will continue to display beyond the Non-Linear Range. The measurements will be repeatable, but they will no longer be accurate. Typically, concentration measurements beyond the Non-Linear Range will display lower than actual.

    In Summary

    From the UK HSE, “Setting Alarm Thresholds should not be “fit and forget”. Initial Alarm Thresholds may be adjusted either up or down in the light of experience in the environment (i.e. process-related events and instrument characteristics). This requires analysis of the data over a suitable period. Minimizing the risk by lowering the alarm levels has to be balanced by minimizing spurious alarms, which reduce efficiency [of the overall Fire & Gas Safety System]. The Low Alarm Thresholds should be set such that this is not a spurious alarm, warranting an investigation, which should be carried out, and not ignored. The High Alarm Threshold should initiate a mitigating action.

  • In-Situ and Extractive Gas Analysis: Path Lengths less than 5m

    IN-SITU MONITORING

    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.

    It is important to note that the Path Integrated Concentration is completely independent from the physical path length (i.e. distance between the Open Path (OPX) Head and the Retro-Reflector).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.

    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 Concentrations: Parts Per Million

    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).

    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, Insertable (IPX) Probe and Extractive Measurement (EMX) Probe.

    Oxygen (O2):

    Path Integrated Concentration: Parts Per Million – Meter (ppm-m)

    The table below visualizes the Minimal Detectable Limit (MDL), Lowest Actionable Concentration (LAC), Lowest Recommended Alarm Threshold (LRAT), and Highest Actionable Concentration (HAC) with Path Integrated Concentrations (ppm-m) that are independent of path length.

    Path Average Concentrations: Parts Per Million

    The table below visualizes the how the Path Average Concentrations (PAC) for the Minimal Detectable Limit (MDL), Lowest Actionable Concentration (LAC), Lowest Recommended Alarm Threshold (LRAT), and Highest Actionable Concentration (HAC) are effected by path length. As a quick reminder, to obtain the Path Average Concentrations, simply divide the Path Integrated Concentrations (ppm-m) by the Path Length (m).

    To help provide more clarity to the table above, the following terms have been further defined:

    Minimal Detectable Limit (MDL): This is the smallest concentration value that is reliably and repeatedly detected when exposed to a known gas concentration. In a comparable example, think of the Minimal Detectable Limit (MDL) in similar terms to a Signal-to-Noise Ratio where the MDL is the smallest signal that can be detected through the noise floor.

    Non-Detectable Range: Any ppm-m concentration below the Minimal Detectable Limit (MDL), regardless the value of the R2 Confidence Factor, should be strictly treated as noise.

    Zero Gas Noise: Typically, Zero Gas Noise will produce gas concentrations that are below our Minimal Detectable Limit (MDL) with low and high R2 Confidence Factors  and can be generated from spectroscopic, mechanical, or atmospheric sources. However, Zero Gas Noise can produce gas concentrations that also exceed our Minimal Detectable Limit (MDL) with “good” R2 Confidence Factors to two (2) times our published Minimal Detectable Limit (2x MDL).

    Non-Actionable Range: While measurements within the Non-Actionable Range (i.e. below 2x MDL) can be both above our Minimal Detectable Limit (MDL) and have “good” R2 Confidence Factor values, it is difficult to determine if the gas concentrations measurements are from Target Gas within the Active Measurement Path or just Zero Gas Noise. It recommended that Actionable Concentrations (i.e. Alarm Thresholds) not be set within the Non-Actionable Range and/or below 2x MDL.

    Actionable Range: This is where the results are repeatable, accurate, and linear. It is within the Actionable Range where end-users can confidently set Alarm Thresholds that are “As Low As Reasonably Practical” (ALARP) while taking into consideration the need to avoid false alarms.

    Lowest Actionable Concentration (LAC): As the name states, this is the Lowest Concentration that should be considered for use in setting an Alarm Threshold (i.e. 2x MDL). False Alarms between 2x MDL and 5x MDL while rare could be still be possible.

    Lowest Recommended Alarm Threshold (LRAT):To provide an additional margin of safety, Alarm Thresholds that are greater than five (5) times the Minimal Detectable Limit (5x MDL) will provide acceptable protection from less-common and/or circumstantial spurious/nuisance/false alarms. Concentrations above 5x MDL are to be considered valid.

    Sensitivity: We define Sensitivity as the smallest incremental change in concentration that is reliably and repeatability detected above the Minimal Detectable Limit (MDL). In a comparable example, think of our Sensitivity as Shot-to-Shot Noise. As a general rule-rule-of-thumb for all gases, the Sensitivity (i.e. Shot-to-Shot Noise) is a quarter of the Minimal Detectable Limit (e.g. 0.25 x MDL).

    Highest Actionable Concentration (HAC): This is the end of the Actionable Range and Alarm Thresholds should not exceed this Full Scale value.

    Non-Linear Range: Concentration measurements will continue to display beyond the Non-Linear Range. The measurements will be repeatable, but they will no longer be accurate. Typically, concentration measurements beyond the Non-Linear Range will display lower than actual.

  • Effects of Pressure + Temperature

    Pressure + Temperature

    Effects to Detectable Range

    See the table below to see how changing Pressures and Temperatures can effect the detectable range.

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