Sensors - ozone

Aeroqual ozone sensors may exhibit a response to gases other than ozone. The magnitude of the response is a function of both the interference gas and its concentration. Typical sensor responses to some common gases are shown in the tables below (these are indicative responses and the actual response of a specific sensor may vary). A negative response means the ozone sensor may under-read in the presence of that compound and this should be taken into account in ozone-control applications.

Volatile Organic Compounds (VOCs) are sometimes present in applications and can produce cross-sensitive readings. The term VOC applies to a wide range of hydrocarbons with different behaviours. At high concentrations of VOC, the sensor background compensation may become overwhelmed and the reading lower than actual. Users should also be aware that measurements of ozone in the presence of high concentrations of VOCs, particularly alkenes, may be lower than expected due to gas phase ozone reaction with the VOC.

 The Aeroqual GSS ozone sensor will be poisoned by exposure to specific compounds such as silanes, silicones, phosphate esters and organochlorides. Silicones and silanes are present in many products such as lubricants, polishes, mold-release agents and adhesives and can permanently damage the sensor due to the formation of an impermeable glassy layer on the GSS sensor. Exposure to these compounds must be avoided.

Cleaning processes in rooms fitted with Aeroqual ozone sensors should be carefully considered to ensure the process doesn’t impact the sensor accuracy. Cleaning systems that use mists or sprays may damage the sensor and the sensor should be powered down and removed or covered before cleaning starts.

Many applications require measurement and control of very low ozone concentrations below 200 parts per billion. At such low concentrations, you need to consider the following sampling issues to successfully measure and control ozone.

•  Ozone is highly reactive. Ozone will rapidly react with organic compounds and surfaces such as walls, flooring, plastic testing chambers and people.
• Ozone concentration gradients are common in rooms and are greatly influenced by air movement and mixing. Concentrations may be lower near walls and surfaces or in areas with low air flow.

The ozone sensor head has a clean stainless steel mesh to filter out dusts. If this becomes dirty over time, the sensor head starts to read incorrectly and will need to be replaced.

The Aeroqual GSS ozone sensor should be regularly tested and calibrated to ensure its correct operation. This is particularly important if used as part of a health and safety system.

In response to requests for an ozone sensor with a wide range and fast speed of response, Aeroqual introduced an electrochemical ozone sensor head for use with handheld and fixed monitors.

The EOZ uses a Kalman filter to achieve a fast speed of response (4s update) with good accuracy across a wide range (0-10 ppm). This makes it complementary to our gas sensitive semiconductor (GSS) sensors which offer unparalleled accuracy, sensitivity and stability at low ozone concentrations.

• The EOZ 0-10ppm ozone sensor specifications can be found here.
• The EOZH 0-30ppm ozone sensor specifications can be found here.

Note: Exposure to high levels of O₃ can temporarily affect the baseline of the EOZH.

The EOZ sensor is less affected by VOC cross-interferences than the GSS sensors, but it is sensitive to NO2 and Cl2. Therefore the EOZ is best suited to indoor and industrial applications while less suited to ambient outdoor applications.

The high accuracy makes it a good choice for health and safety monitoring. The speed of response and wide range make it the sensible option for leak detection.

Applications include ozone generator control and/or leak detection in mechanical rooms, leak detection in laundry, industrial health and safety monitoring and many more.

Note: Where greater accuracy is required below 0.1 ppm, you should use the GSS sensor head options – either OZL or OZU.

Unlike GSS sensors, electrochemical sensors do not have automatic baseline compensation which means they will have to be manually re-zeroed from time to time.

The required zero calibration frequency depends on the use of the instrument. If it is used to measure close to zero it will need to be zeroed more frequently than if used at higher concentrations.

There are two approaches to zero calibration – a high accuracy approach which does require zero air, and a more convenient re-zero in a low ozone environment (where known ozone level is <0.01 ppm).

Electrochemical sensors start to degrade from the moment they are taken out of their protective packaging. This means that operating life begins from the date of manufacture. Electrochemical sensors will degrade even when on the shelf.

The EOZ sensor has the following cross-sensitivities:

EOZH sensor heads contain an electrochemical sensor and are designed to measure high levels of O₃ (up to 30ppm).

Exposure to high levels of O₃ can temporarily affect the baseline of the electrochemical sensor, causing it to read lower (or negative) compared to the example concentration by up to 0.1-0.2ppm.

For example, on return to zero air following an exposure to 30ppm O₃ for several hours, the sensor may read -0.1ppm despite the concentration being 0.0ppm. This may cause an issue if the concentration in the measurement space is not actually 0.0ppm (i.e. if the actual concentration is 0.1ppm the sensor head may still read 0.0ppm due to this offset). This baseline offset can last up to 24 hours following exposure to high O₃ levels and a secondary O₃ sensor head should be used to validate low-level measurements, particularly when there are health and safety concerns.