The use of ozone water treatment systems requires the measurement of ambient ozone levels as a safety measure. OSHA sets a 40 our exposure limit to ozone in spaces where people are present of 0.10 or less. Emissions from the ozone system or the down stream processes can exceed the OSHA levels. If this occurs, the ozone system should be shut down and the source of the ozone isolated. Normally, an ozone system will be interlocked with the ambient ozone monitor.
There are basically two different types of ambient ozone monitors: electrochemical and UV. The electrochemical sensors are less expensive and if there is not source of interference for these units, they are a good choice. They can see interference, especially from other oxidants that might be present, for example chlorine or one of its variants.
UV sensors are very accurate for low level ozone measurement, but much more expensive than electrochemical sensors. UV monitors are essentially UV spectrophotometers operating at 254 nm where ozone absorbs UV. They can also be affected by interferences, especially by aromatic hydrocarbons that also absorb UV at the 254 nm region. So, if these organic compounds are present UV monitors may not be a good choice for the application.
Because ambient ozone monitors are used for measuring ppb levels of ozone, it is a very challenging applications. Sometimes it is difficult to find the interference or to determine if there is an interfering compound. So, it is important to test the sensors against a known zero gas and calibrated ozone containing gas.
In general, electrochemical sensors are widely deployed for a variety of industrial and municipal drinking water applications. If problems develop a UV monitor might be a good option for checking the environment if calibration of the electrochemical sensors does not indicate the nature of the high ozone readings.