Photoionization Detector (PID) Sensor Technical Notes

This article provides an overview of the operation and performance of Blackline Safety’s Photoionization Detector (PID) sensor.

For more information about the PID Sensor, please see the following:

overview

The PID sensor detects Volatile Organic Compounds (VOCs) in the local environment using high energy ultraviolet (UV) light.

It can be installed in both G7 and EXO devices and supports both pump and diffusion sampling.

how the sensor works

The sensor uses a 10.6 electron volts (eV) UV lamp to ionize VOC molecules as they pass between two charged plates inside the sensor. This results in electric potential at the plates and generates a signal, which is used to calculate and show the total VOC concentration in the local area.

To be detected by the sensor, a VOC must:

Be volatile at the temperature in the surrounding environment.
Have an ionization energy of 10.6 eV or less.

Common VOCs that fall outside this range and cannot be detected by the PID sensor include methane, ethane, acetylene, propane, and some fluorocarbons.

The PID sensor does not detect vapors in mist or aerosol form.

Detection Range and SaFETY

The PID sensor has a detection range of 0.1 to 4000 ppm (total VOCs). The sensor is rated as intrinsically safe.

VOC Detection Selectivity

The PID sensor does not identify specific VOCs. It measures the total VOC concentration based on ionizable compounds present in the local environment.

To adjust readings for specific compounds or mixtures, a Correction Factor (CF) can be applied. The CF factor is based on the unique energy required to ionize each organic compound relative to isobutylene, the standard calibration gas.

By default, the sensor uses a response factor of 1.0 for isobutylene.

For the majority of VOCs, when the VOC is selected in Blackline Live, the low and high thresholds, TWA, and STEL (if enabled), are set by default based on the CF. The low and high thresholds are configurable. Consult with your Environmental Health and Safety group for your organization’s recommended PID Sensor thresholds and overall sensor configuration.

For more information on calculating and configuring CF values, see the VOC Correction Factor Technical Notes. For information on configuring the PID sensor, see Configuring the PID Sensor.

Sensor Response

Sensor readings reflect the total local VOC concentration and may fluctuate throughout the day due to variation in the local environment, air movement, proximity of the VOC to the sensor, or sensor movement if the user is moving between different locations.

Use caution when exposing the PID sensor to the following:

Concentrations of gas above a few thousand ppm (the concentration depends on the gas being measured). The sensor's accuracy can change depending on the type of VOC and result in a non-linear response. A non-linear response is when the sensor's signal output does not increase at an even rate with the concentration of the gas being measured.
Concentrations of gas that are at a low level directly following the sensor being exposed to concentrations of gas that are hundreds of times The sensor must first be flushed of the heavily concentrated gas before can react to a small concentration of gas in the environment. You can choose to flow synthetic air through the sensor cartridge to quickly diffuse the gas or you can wait until the gas diffuses naturally. The device is ready when the PID sensor reading is 0 ppm.

In either of these situations, perform a bump test to resolve the issue and confirm that the sensor is operating as expected.

Sensor Maintenance

Given the diversity of VOCs detected by the PID sensor, frequently calibrate and bump test the sensor. This ensures that the PID sensor is operating accurately and is responding to the current local environment. The recommended calibration gas is 100 ppm isobutylene balanced with synthetic air.

Best practices for proper maintenance of the PID sensor include the following:

Calibrate and manually zero the sensor in clean air to avoid offsetting the sensor’s zero reference. Calibrating the in VOC-contaminated air can produce inaccurate readings. If VOCs were present during calibration, recalibrate the sensor in clean air.
Calibrate the sensor to the target VOC(s) at the concentration you expect to detect.
Calibrate or bump test the sensor frequently, particularly when the sensor is put into an environment where it starts sensing less common VOC(s).
Bump test the sensor to resolve issues related to high or low VOC concentration levels.
Bump test the sensor when there is any doubt as to the sensor's operation.
Relocate the device to a known clean-air environment before manually zeroing the sensor. The device should not be in active use during this procedure.

Cleaning the Sensor

Certain groups of volatiles are identified as, or suspected of, causing temporary contamination of the PID window. To resolve this, clean the sensor window. For detailed instructions, see the PID Sensor Lamp Cleaning Procedure.

Environmental Considerations

High-humidity levels can reduce PID sensor response. Water vapor absorbs UV energy, limiting the energy available to ionize VOCs and lowering the concentration that displays on the device.

The reduction in sensor response is typically between 2 to 7%, depending on the relative humidity and temperature.

Alarm response

The PID sensor is highly sensitive and responds quickly to changes in VOC levels. Blackline’s application of the PID sensor is targeted at a quick, low concentration response to any local VOCs and is designed to provide a fast warning to let you know that the local environment contains one or more VOC chemicals. It is not intended for use as an analytical instrument. Treat PID alarms as potential exposure events and respond according to your safety guidelines.

Please consult your Environmental Health and Safety group for further information and guidance at your location.