Introduction
Electrochemical gas sensors and heated metal oxide sensors (HMOS) are the two leading technologies for the low-cost measurement of ozone. Both gas sensor types have strengths and weaknesses which we will outline here, then let’s discuss which sensor type is suited to some common applications.
Ozone (O3) is an important gas in industrial applications, medical or viral sterilization, ground-level pollution monitoring, and atmospheric studies. The best-known method for ozone measurement has been to use an analyzer based on the ultraviolet absorption principle. Despite significant cost reductions in recent years an ozone analyzer remains an expensive piece of equipment with prices starting at USD4,000. The expense of such equipment has driven many to look for lower-cost ways of measuring ozone.
Measuring O3 is also key to ensuring the safety of biological life within a concentrated area. The Clean Air Act requires EPA to set National Ambient Air Quality Standards for pollutants considered harmful to public health and the environment. Ozone is identified as harmful to both.
Electrochemical gas sensors
In an electrochemical ozone sensor, ozone gas diffuses across a porous membrane into a cell containing electrolyte and electrodes. When ozone comes into contact with the electrolyte, a change in electrochemical potential occurs between the electrodes causing electrons to flow.
In zero air, little or no electron flow occurs. As the presence of ozone increases, the electrical signal increases proportionally. The sensor interprets this signal and outputs the ozone concentration in ppm (parts per million) or ppb (parts per billion).
Strengths
Linear Response
Good repeatability and accuracy
Fast response time – 1-2 Seconds
Low power consumption
Able to measure up to 20 ppm
Limited cross-interference from VOCs
Weaknesses
Humidity can affect sensor readings
Changes in temperature can affect sensor readings
Sensitive to EMF/RFI
Limited sensor life (often max of 12-18 months)
Sensors age even when in storage
Reduced accuracy at low ozone levels (below 0.1 ppm)
Almost 100% cross sensitive to NO2
Applications
Detecting ozone leaks
Health and safety monitoring (above 0.1 ppm)
Controlling alarms, ozone generators etc (above 0.1 ppm)
Heated metal oxide sensors (HMOS)
In heated metal oxide gas sensors a metal substrate is heated which allows it to become very sensitive to ozone gas. An electrical current passes through the metal substrate. The resistance of the current changes according to the mount of gas present. The gas sensors output the resistance in ppm or ppb.
There are several kinds of HMOS gas sensors on the market. Different metals and temperature management can have a significant impact on the overall performance of the sensor.
The following strengths and weaknesses apply to a state of the art HMOS sensor.
Advantages
Very responsive to low levels of ozone (below 0.1 ppm)
Excellent repeatability and accuracy
Long sensor life if stored properly
Very low cross-interference from NO2
Disadvantages
Requires time to warm up (10 minutes after first use)
Slower response time (60 seconds)
Higher power consumption
Cross sensitive to VOCs
Reduced linearity above 1 ppm
Applications
Ambient (outdoor) ozone monitoring
Health and safety monitoring (especially below 0.1 ppm)
Portable ozone monitoring (especially below 0.1 ppm)
Ozone control scenarios (especially below 0.1 ppm）