This article provides detailed information about how the Correction Factor (CF) for a single VOC and VOC gas mixture is calculated. It also includes a listing of the most commonly targeted VOCs and their correction factors that you can use as a quick reference.
Every VOC has a specific Correction Factor (CF) based on its chemical composition. The CF indicates how much a specific VOC contributes to the overall signal detected by the PID sensor and helps to quantify this sensitivity—generally, a lower CF indicates higher sensitivity, and a higher CF indicates lower sensitivity.
To ensure accurate and actionable measurements by the sensor, you must understand which VOC or VOC mixture you expect to be present in the local environment, and then know how to set the thresholds, which will result in accurate alarms by the device.
WARNING: Consult with your Environmental Health and Safety group for your organization’s recommended PID Sensor thresholds and overall sensor configuration.
This article contains the following sections:For more information about the PID Sensor and VOC Correction Factors, please see the following:
CF Calculation for a Single VOC
PID sensor output is calibrated to a reference gas, most commonly isobutylene, which has an assigned CF of 1. This provides a relative comparison of the sensitivities to other VOCs.
The PID may under or over respond to a specific VOC depending on its ionization energy and how it interacts with the sensor’s 10.6 eV lamp.
Examples:
Anisole: The CF of anisole is 0.59 with a 10.6 eV lamp. That means 0.59 ppm anisole delivers the same PID response as 1 ppm isobutylene. A 100 ppm response to anisole, from an isobutylene-calibrated device indicates:
Isopropanol: The CF of isopropanol is 4 with a 10.6 eV lamp. That means 4 ppm isopropanol delivers the same PID response as 1 ppm isobutylene. A 100 ppm response to isopropanol, from an isobutylene-calibrated device indicates:
CF Calculation for VOC Gas Mixtures
You may encounter or anticipate more than one VOC in the local environment where the makeup of the gas mixture is less well defined. The amount of each chemical component in the air depends on how quickly a VOC can evaporate into the local air at a given temperature.
If you believe that the PID sensor will be exposed to a mix of component gases, you can apply an additional set of changes to the Blackline Live configuration for the sensor based on those assumed components. For more information, see Configuring the PID sensor.
The calculation for the overall CF for a blended gas sample is as follows:
Where X1, X2, and X3 are the mole fractions of components 1, 2, and 3, etc. For example, if a mixture consists of 20% anisole (CF = 0.59), 30% acetone (CF = 1.17), and 50% ethanol (CF = 11), the overall CF is:
You can use the calculated factor to estimate the concentration of each component. For example, to measure worker exposure to a process involving the above gas mixture, the PID sensor is calibrated with isobutylene and the CF is set to 1.56.
If the overall reading is 100 ppm, you can assume the concentration of anisole to be 20 ppm, acetone 30 ppm, and ethanol 50 ppm. Note that these mole fractions are estimated values. Changes in individual gas concentrations may vary throughout the day, depending on local environmental and process conditions.
TWA and STEL
For personnel safety monitoring, you may also need to convert TWA and STEL ppm levels of each VOC to an equivalent isobutylene concentration as ppm iBueq:
Blackline recommends that you use the lowest TWA ppm iBueq and STEL ppm iBueq as guidelines for measured exposure.
Commonly Used Correction Factors
The following table lists the most common CFs used by Blackline customers in their regular industrial applications.
Each chemical is listed according to both its chemical and common (alternate) name. To avoid confusion, use the CAS number to more precisely identify the chemical.
For a full list of CFs, please see the VOC Correction Factors Full Listing.
If you have any questions regarding Correction Factors or their use, please contact Blackline Technical Support.
