Photo-ionization detectors (PID) are a valuable tool when measuring low level concentrations of organic vapours and hydrocarbons. Organic and inorganic compounds can be measured by a PID in levels anywhere from parts per billion (ppb) up to 10,000 parts per million (ppm).
Introduced originally as a handheld measuring device, photo-ionization sensors are now found in both single gas and multi-gas gas detection instruments.
The increase in the use of the PID’s for measuring VOC’s (volatile organic compounds) in different environments creates potential errors in the use of this valuable tool.
Like all sensor types there are a variety of issues, such as cross sensitivity or interference with photo-ionization (PI) sensors, which users must be aware of in order to use the instrument correctly.
Key Cross Interferences:
The two major cross interferences are methane and water vapour (humidity) on a PI sensor.
Methane:
In applications where methane is present such as landfills, wastewater treatment or natural gas measurement, users of PID’s must be aware of the effect of methane on the PI sensor.
Concentrations at or greater than 1% (10,000ppm) methane will reduce the response of the PI sensor dramatically. The effect is a lower than actual reading of the PID.
Methanol and butane have a similar effect.
Water Vapour (Humidity):
Similar to methane, water vapour can also reduce the response of the PI sensor.
Corrections will need to be made to the readings if the absolute reading is required. Alternatively, the use of filter tubes to dry the incoming sample can be used but this needs to be carefully evaluated as some filters may remove the vapour which needs to be analysed.
Oddly enough, condensation (condensing humidity) will cause the sensor to give a false positive output. Both issues need to be carefully addressed to obtain the most accurate readings.
All manufacturers will have humidity correction charts which can be used accordingly.
Calibrating and Bump Testing:
Most PID manufacturers recommend that instruments used for occupational health measurements be calibrated each day of use. The frequency of calibration can be extended based on experience in the field but typically the interval should not be longer than 30 days.
Various factors can cause changes in response including lamp degradation, coating of the lamp with dust and chemicals, temperature, pressure, gases measured and humidity.
Calibrating a PI sensor is typically done with isobutylene (other gases can be used).
As we have noted above, methane has a direct negative impact on the reading of the sensor. Therefore, calibrating the PI sensor must be accomplished with an individual gas mixture of isobutylene.
Although technically isobutylene can be mixed with methane, a mixture of 2.5% methane will reduce the response of the PI sensor to approximately 40% of the value or to 40ppm if the Isobutylene concentration was 100ppm. This means a multi-gas mixture with Isobutylene/CH4/CO/H2S/O2//N2 cannot be used to calibrate a multi-gas PID.
Always keep the calibration gas separated, with a 4gas mixture (H2S/CO/CH4/O2) and Isobutylene in different cylinders.
Could the 5gas mixture be used for Bump Testing?
Yes, provided the end user understands that the PI sensor will only read 40ppm not 100ppm. This test would only be valuable to determine if the instrument responds to the gas mixture. It cannot be used to make judgement on accuracy of the PID.
As most calibration gases are extremely dry, calibrating the PID to these gases can have a negative effect on accurate readings when measuring in a humid environment. Humidifying the gas mixture can be accomplished using moisture exchange tubes or with the use of humidity generators such as the Owlstone OVG-4.
PID are excellent tools but users should be aware of the issues when using a PID. Always consult the device manufacturer to obtain more information and to better understand your specific instrument.
