ION Science PID Gas Sensor Response Factors

PID Gas Sensor Response
ION Science MiniPID 2 VOC gas sensors respond to a broad range of organic and a few inorganic gaseous and volatile chemicals (‘volatiles’). For the PID sensor to respond to a volatile, the photon energy of the lamp must be greater than its ionisation energy (IE). Ion Science PIDs are available with lamps emitting light of maximum energy of 10.0 eV, 10.6 eV, and 11.7 eV. This Technical Article lists the response factors (‘RF’s’) for over 900 volatiles with PID incorporating these lamps. The RF relates the sensitivity of PID to a volatile to the sensitivity to the standard calibration gas isobutylene.  The higher the RF, the lower the sensitivity.

Isobutylene as Reference Gas
Ideally, the PID response to a chemical volatile would be calibrated by using a low concentration of the chemical in air. However, this is often not practical. Isobutylene is then used to calibrate PID, and a Response Factor (RF) used to convert the isobutylene calibrated measurement to a measurement of the target volatile:

Concentration of target chemical = isobutylene calibrated measurement x RF

For example, the RF 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 10 ppm response to anisole, from an isobutylene-calibrated unit would indicate:

Concentration of anisole = 10 ppm x 0.59 = 5.9 ppm

Compound Mixtures
The MiniPID 2 sensor will respond to all detectable volatiles present and it is not possible to measure the concentration of each individual component. When the formulation of a given mixture is known, the overall response factor for the mixture can be calculated as follows:

RF =  1/[X₁/RF₁  +  X₂/RF₂  +  X₃/RF₃  ….]

where X₁, X₂, and X₃ are the mole fractions of components 1, 2, and 3, etc.  For example, if a mixture consisted of 20% anisole (RF = 0.59), 30% acetone (RF = 1.17) and 50% ethanol (RF = 11), the overall RF would be:

RF = 1/[(0.2/0.59)  +  (0.3/1.17)  +  (0.5/11)]  = 1.56

This factor can then be used to reconstruct the concentration of each component. For example, to measure worker exposure near a vat generating the above volatile mixture, the PID is calibrated with isobutylene and the response factor is set to 1.56. If the overall reading is 100 ppm, then the concentration of anisole is 20 ppm, acetone is 30 ppm, and ethanol is 50 ppm.

Want to learn more? Click the button below to download our full TA-02 Gas Response Sheet.