Geotechnical Services, Inc

MiniRAE 3000 Correction Factors


Source: Geotechnical Services, Inc

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PID Correction Factors

Geotechnical Services, Inc. is proud to announce the introduction of the MiniRAE 3000 to the U.S. market. As a premier consulting organization and instrumentation sales company dedicated to providing laboratory and field analytical solutions, Geotechnical Services - a leader in elemental and materials analysis - to offer a complete range of XRF handheld and benchtop instruments for elemental analysis needs.


MiniRae 3000 Correction Factors


One of the handiest features of the MiniRae 3000 is the ability to use built-in correction factors.  The industry standard for calibrating PIDs (photoionization detectors) is isobutylene in concentrations of 100ppm (parts per million).  Correction factors have been developed for PIDs calibrated with isobutylene to allow the user to quantify specific gases of interest without having to calibrate with that particular gas.  This is an incredibly useful development because no one needs to be exposed to toxic gases just for calibration purposes. If you are operating a MiniRae 3000 with a 10.6eV lamp that is calibrated to 100ppm isobutylene and you are looking to quantify benzene, you would choose the correction factor for benzene which is .53.  This sets up your PID to read in benzene units even though it has been calibrated to isobutylene instead of benzene directly.  Therefore, if you are in an environment where benzene is the only gas present, you can actually quantify levels of benzene with the MiniRae 3000.

This feature is also used as a conservative approach even when other gases are present in the environment where you are trying to measure levels of contamination.  It is common to choose the most toxic gas expected in the environment detected by a PID, calibrate the instrument to 100ppm isobutylene then, apply the correction factor for that gas.  This way if concentrations reach a level of concern for that specific gas, you can respond accordingly.  Since PIDs are total VOC analyzers, the instrument is actually measuring all of the constituents in the air within the range of the lamp but it is reading in units specific to the toxic gas chosen as though everything it is reading is that toxic gas. 

As useful as this feature can be, correction factors are also the most commonly misused feature of PIDs in general.  The confusion most likely resides in the fact that there is another calibration gas that is almost as frequently used as isobutylene in the state of California.  This calibration gas is actually a directive of the SouthCoast AQMD (Air Quality Management District) Rule 1166 where the PID must be capable of being calibrated to hexane at concentrations of 50ppm.

However, the correction factors found in most PIDs have been developed for using isobutylene as the calibration gas.  If you set a correction factor in a PID and calibrate it to any other gas than 100ppm isobutylene, you will produce erroneous results.  The most common mistake in the field is to calibrate a PID to hexane while applying a hexane correction factor of 4.3.  Since the correction factor of 4.3 is derived from a 100ppm isobutylene calibration you are actually skewing your results by a factor of 4.3.  The only proper way to use the 4.3 correction factor for hexane is to calibrate your PID to 100ppm isobutylene and set the calibration memory to hexane with a 4.3 correction factor.  With this set up, you will then be measuring in hexane units.

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