Measuring O3 Output

How to Measure the Output of your Ozone Generator

We are commonly asked how to measure output, or verify that an Ozone Generator produces the amount of ozone the manufacture claims. This tech sheet will cover how to measure the output of your high concentration Ozone Generator.

It is key to understand that the output of an Ozone Generator is almost always measured in grams per hour (g/hr), also known as ozone output. The ozone measurement instrument will read percent by weight (%), or grams per meter cubed (g/m3), also known as ozone concentration. To mathematically determine the g/hr (ozone output) from % by weight, or g/m3 (ozone concentration) you will need to determine the flow rate of the Ozone Generator in liters per minute (LPM). The calculations to use when you have determined the gas flow rate and concentration are below:

From percent by weight using oxygen as a feed gas:

Ozone Output (g/hr) =(( LPM x 0.001) x 60) x (14.3 x % by weight)

From percent by weight using dry air as a feed gas:

Ozone Output (g/hr) =(( LPM x 0.001) x 60) x (12.8 x % by weight)

From g/m3:

Ozone Output (g/hr) = ((LPM x 60) x 0.001) x g/m3

Another resource we offer is a handy online calculator. Just enter your flow and concentration values and we do the work!

Note: When using percent by weight to measure ozone concentration, the calculation to determine ozone output is different because the density of the gas is different. Due to the difference in the density of the gas, there are separate conversion formulas used to determine the output of an Ozone Generator (as shown above).

Ozone Measurement Device

A related question is which ozone measurement device should be used to facilitate these measurements. The short answer is, only a high concentration ozone analyzer. Note that the units of measure used are percent by weight and g/m3. This is due to the high concentrations of ozone produced by most Ozone Generators. We are also asked why parts per million (ppm) is not used as a unit of measure. Let me provide a quick example using a very small Ozone Generator.

An Ozone Generator producing 2 g/hr ozone from 10 LPM of air would produce 0.26% ozone concentration.

2 g/hr = ((10 LPM x 0.001) x 60) x (12.8 x 0.26% by weight)

0.26% ozone = 1573 ppm

At only 2 g/hr of ozone output the ppm value is already 1573 ppm. The parts per million (ppm) unit of measure is just not conducive to these calculations. Therefore it is rarely, if ever, used when calculating generator output.

The high concentrations that are created limit the number of ozone measurement devices that can be used, which is why the process is restricted to high concentration UV ozone analyzers.

Example System Setup to Measure Output of an Ozone Generator

Measuring Ozone Generator Output - Example System

Components Needed

  • Mass flow meter will measure overall gas flow accurately for a flow measurement.
  • 0 – 2 LPM Flow Meter will regulate and measure gas flow through the ozone analyzer.
  • Ozone Analyzer will measure the ozone concentration in % by weight, or g/m3.
  • Ozone Destruct Unit will convert ozone back to oxygen to safely be vented into the air.

Other Important Considerations

When choosing an Ozone Analyzer, ensure that the analyzer you are purchasing operates within the range that you will be operating within. Some Ozone Analyzers are limited to low ranges only, so ensure that you choose a device capable of reading a higher concentration of ozone than you will ever need to measure.

Also ensure that the analyzer you are purchasing will measure ozone concentration using the units of measure that you desire. If choosing an analyzer that measure ozone in percent (%) by weight, be sure to specify whether you are using oxygen or dry air gas. Some analyzers may be only factory configurable and cannot be configured by the user.

Gas flow is a very critical component of the calculations used to measure ozone output. A small deviation in actual flow measurement will result in a large error in the resulting ozone output figure. Since this is such an important value, we recommend using a high quality Mass Flow Meter to measure the total gas flow entering the Ozone Generator. A Mass Flow Meter will measure total gas flow. This measurement is also pressure compensated to eliminate that potential variable. Using the Mass Flow Meter prior to the Ozone Generator will remove the possibility of ozone corroding the delicate components inside the Mass Flow Meter.

Most high concentration Ozone Analyzers will only require a very small gas flow rate. This will require a small flow meter to regulate and measure that gas flow. An advantage of this small flow rate is that the Ozone Analyzer could be used as an on-line measurement device. If the Ozone Generator uses a gas flow rate of 20 LPM and the analyzer will only consume 1 LPM for accurate measurement, this setup only requires a 5% slipstream of the ozone gas.

Measuring O3 in an Occupied Room

Information provided by EcoSensors.

Ozone concentrations can vary greatly at various locations, and the concentrations are often highest in unexpected places. Key points to consider are:

  • Ozone is much heavier than air and tends to sink to lower levels.
  • Ozone has a low vapor pressure and so it does not try to fill the room uniformly. It tends to stay where it is.
  • Ozone tends to cling to rough surfaces such as fabrics and breaks down (converts back to oxygen) when passing through restricted and obstructed passageways.
  • Ozone reverts back to oxygen with a “half life” (time to go to half of its original concentration) typically of 10-30 minutes.
  • Ozone can be confused by instrumentation with other oxidizing gases such as chlorine compounds, acid fumes, and oxides of nitrogen (NOx). Strong “reducing” gases, such as vapors of alcohol and solvents, can reduce the apparent concentration of ozone.
  • Ozone has a sweet smell, but the odor threshold varies widely by the person and by ambient conditions. Therefore “smell” is not a reliable test for the presence or concentration of ozone.

The important measurement is:

What is the ozone concentration at the breathing level where room occupants will be?

For ozone introduced via HVAC systems with good room air circulation, the alternate point of measurement is near the entrance to the return air duct.

Ozone Sensor Technology Comparison
Not all ozone detection devices are created equal! This page compares the various technologies that are commonly used to detect ozone: Electrochemical sensors, semiconductor-based sensors (HMOS / GSS), UV absorption, and color-change badges.

Read on to discover the details of each technology, and find our side-by-side comparisons below.

Electrochemical Ozone Sensor

Diagram of an Electrochemical Sensor

Diagram of an Electrochemical Sensor

An electrochemical ozone sensor uses a porous membrane that allows ozone gas to diffuse 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 monitor interprets this signal and displays the ozone concentration in PPM (parts per million).

Advantages

  • Linear Response
  • Good repeatability and accuracy
  • Very quick response time – 1-2 Seconds
  • Long battery life
  • Can measure ozone accurately up to 20 ppm
  • Moderate resistance to interference

Disadvantages

  • Humidity can affect sensor readings
  • Sensitive to EMF/RFI
  • Limited sensor life (often max of 12-18 months), even if in storage
  • Decreased accuracy at low ozone levels (below 0.1 ppm)

Applications

  • Personnel (portable) safety monitoring (especially for shift-long use)
  • Ozone leak detection
  • Stationary ambient ozone monitoring
  • Ozone control scenarios (alarms, exhaust fans, ozone generators, etc)

Semiconductor-Based Ozone Sensors:
Heated Metal Oxide Sensor (HMOS)
Gas Sensitive Semiconductor (GSS)

A heated metal oxide semiconductor (HMOS) sensor works by heating a small substrate to high temperature (around 300-deg F / 149-deg C). At this temperature, the substrate is very sensitive to ozone and exhibits a change in resistance that is proportional to to the amount of ozone which contacts its surface. The circuitry of the monitor interprets this change in resistance and displays the corresponding ozone level on the display as either PPM or PPB.

Advantages

  • Very responsive to low levels of ozone ozone (below 0.1 ppm)
  • Least expensive monitoring technology
  • Excellent repeatability and accuracy
  • Long Sensor Life if stored properly

Disadvantages

  • Slow start-up (can require 8-24 hours warm-up time)
  • Slower response time to ozone (compared to electrochemical)
  • Very sensitive to interference
  • Shorter battery life due to heated sensor element
  • Not linear at ozone levels above 1 ppm
  • Max. Temperature threshold of 122F or less (depending on model)

Applications

  • Ambient ozone safety monitoring (especially below 0.1 ppm)
  • Portable ozone monitoring (especially below 0.1 ppm)
  • Ozone control scenarios (especially below 0.1 ppm)
  • Ozone leak detection

UV Absorption

UV Absorption ozone sensor

Diagram illustrating the basics of UV Absorption Technology.

An Ozone molecule has a UV absorption maximum at 254 nm. In a controlled environment, this means that the amount of 254 nm UV light absorbed in an airspace is proportional to the amount of ozone present.

UV ozone analyzers take advantage of this property of ozone for stable, accurate readings. A UV lamp emitting light at a wavelength of 254 nm produces a controlled amount of light within a sealed chamber. This UV light is measured via a photodiode that is also filtered to detect at the 254 nm wavelength.

The difference in UV light produced at one end of the chamber, vs the UV light measured at the other end of the chamber indicates the concentration of ozone in the air sample which is then displayed on-screen or output to a control system.

Advantages

  • Very accurate (within 1%)
  • Very linear at any ozone levels
  • Low ppb detection limits with accuracy
  • Minimal cross sensitivity to other gasses or interferences
  • Can read high concentrations (20% or higher)
  • Durable design with excellent longevity

Disadvantages

  • Higher cost than electrochemical or HMOS / GSS
  • Physically larger technology

Applications

  • Low Concentration
  • Cold storage facilities
  • Microarray applications
  • Semiconductor fabrication facilities
  • Ozone compatibility testing
  • Medium Concentration
  • Off-Gas Monitoring
  • High Concentration
  • Industrial ozone generator control
  • Ozone research
  • Ozone compatibility testing

Ozone Badges / Ozone Test Strips

Ozone badges are cards that use a color change indicator. The indicator used is a small paper strip or circle that is oxidized in the presence of ozone. The amount of oxidation (color change) that occurs during a set amount of time is proportional to the amount ozone present in the air.

Advantages

  • Easy to use
  • Low Cost
  • Specific to OSHA 8 hr ozone limit (PEL)

Disadvantages

  • One-time use (or limited use) cards
  • Very cross-sensitive to some other gases
  • Generally limited to ~0.1 ppm detection limits

Applications

  • Initial determination for presence of ozone
  • Temporary personnel monitoring needs
  • Widespread employee-level ozone monitoring
  • Residential use

How do they compare?

Now that you have an overview of the various technologies, you may want to know how they rate for various metrics. See the lists below!

Ranked by Resistance to Interference

(Descending order, from best to least)

  • UV Absorption (best)
  • Electrochemical
  • HMOS / GSS
  • Ozone Badges

Ranked by Price

(Descending Order, Highest to Lowest)

  • UV Absorption
  • Electrochemical
  • HMOS / GSS
  • Ozone Badges

Ranked by Operating Temp. Range

(Descending Order, widest operating band to narrowest)

  • Electrochemical
  • UV Absorption
  • HMOS / GSS
  • Ozone Badges

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