PIDs for Benzene Monitoring – Unlike other health and safety hazards, the only way to carry out a risk assessment for benzene exposure is to monitor quantitatively. The photoionisation detector (PID) has proven to be an ideal solution but there are several considerations that must be borne in mind when choosing an instrument.
In our Guide you will find out about:
- PID theory of operation
- Which lamp do I need?
- Effect of humidity
- Fixed, portable or personal monitoring?
- Environmental protection
- Calibration and maintenance
Unlike other health and safety hazards, the only way to carry out a risk assessment for benzene exposure is to monitor quantitatively. Like other volatile organic compounds (VOCs), benzene evaporates easily and most people can just detect its distinctive ‘aromatic’ smell at a concentration between 2.5 and 5 parts per million (ppm) in air but regulatory occupational exposure limits (OEL) are typically 1 ppm. However the ‘direction of travel’ for the OEL is towards 0.1 ppm and knowing that benzene is a hazardous, carcinogenic chemical, it is imperative that the measurement solution is sensitive and accurate.
It must also be capable of operating in harsh process plant environments in the likely presence of dirt, dust, high humidity and interference from other VOC/aromatic compounds. The photoionisation detector (PID) has proven to be an ideal solution but there are several considerations that must be borne in mind when choosing an instrument.
A UV lamp generates high- energy photons, which pass through the lamp window and a mesh electrode into the sensor chamber. Sample gas is pumped over the sensor and about 1% of it diffuses through a porous membrane filter into the other side of the sensor chamber. The inset on the ‘lower right’ of figure 1 shows what happens on a molecular level. When a photon with enough energy strikes a molecule M, an electron (e-) is ejected. M+ ion travels to the cathode and the electron travels to the anode, resulting in a current proportional to the gas concentration. The electrical current is amplified and displayed as a ppm (or part per billion (ppb)) concentration. Not all molecules can be ionized, thus, the major components of clean air, i.e., nitrogen, oxygen, carbon dioxide, argon, etc., do not cause a response, but most VOCs do give a response.