Ozone: Frequently Asked Questions

Answers to Common Questions about Dissolved Ozone Measurement

Accurate dissolved ozone measurement is critical for maintaining process control, ensuring effective sanitation, and meeting strict compliance standards in pharmaceutical and industrial water systems. Whether you are installing a new METTLER TOLEDO pureO3™ sensor, troubleshooting measurement drift, or optimizing your maintenance schedule with Intelligent Sensor Management (ISM™) technology, finding quick solutions is key to minimizing downtime.
Explore our frequently asked questions below to get practical guidance on sensor calibration, interference factors, flow requirements, and regulatory compliance.

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What factors cause interference in dissolved ozone measurements?

The most common sources of interference are air bubbles at the membrane surface, depleted electrolyte, and an expired membrane, all of which cause unstable readings or drift in the measurement. When your readings become unreliable, these are the first things to check.

Flow rate also affects measurement. If flow past your sensor is too low, a depletion zone forms at the membrane surface where ozone is consumed faster than it is replenished, causing readings to appear lower than the actual process concentration. The recommended flow rate for the pureO3 flow housing is 200 to 500 mL/min.

Keep your sample line as short as possible and use stainless steel or an ozone-compatible polymer. Ozone decays rapidly, and a long sample line introduces measurement delay and concentration loss before the sample even reaches your sensor.

How long does the initial setup of an ozone sensor take before I can get an ozone reading?

The time to get an initial reading on an ozone sensor is variable based on the model and the technology used. Because the majority of process analytics ozone sensors are electrochemical, this time can typically range widely, from just a few hours to a full day. The two main types of electrochemical ozone sensors are polarographic and galvanic sensors.

For polarographic sensors, the sensor requires a polarization period. This period can last from two hours to almost eight hours, depending on the ozone sensor. After this polarization period, the sensor is ready to measure.

Galvanic sensors, although electrochemical, utilize a slightly different technology than polarographic sensors. Galvanic sensors require no polarization period, so they can begin to display ozone measurements quickly. However, in order to read low concentrations of ozone, a stabilization period of up to 24 hours is required before the measurements can become reliable. Otherwise, the sensor risks missing low concentrations of ozone.

My ozone sensor readings have become unstable or drifted significantly. What should I check?

For electrochemical ozone sensors, you should work through the most common causes based on the symptom you are seeing:

Unstable zero current

Check that the membrane surface is completely dry during zero calibration, then check for air bubbles trapped in the electrolyte. If the electrolyte is depleted, replace the full membrane and electrolyte assembly, reassemble, and then recalibrate.

Slope trending in one direction over several calibrations

This indicates that the electrolyte needs to be replaced.

Slope too low

Check for cathode contamination. Clean the cathode gently with warm, soapy water and rinse with deionized water.

Slope too high

This means the membrane could be expired and should be replaced.

What dissolved ozone concentration is required to sanitize a pharmaceutical WFI loop?

Dissolved ozone for pharmaceutical water sanitization is typically maintained between 0.02 and 0.5 ppm (20 to 500 ppb), though there is no single required figure. Your target depends on your loop volume, flow rate, temperature, and sanitization time, and should be set and documented as part of your validated operating procedure.

Temperature is a key factor too. Ozone is really only effective in systems stored or distributed cold or at room temperature, up to around 35°C. Because its half-life shortens sharply above 40°C, ozone sanitizations become ineffective above these temperatures.

Where you measure matters just as much as the target concentration. Three-point in-line measurement—after the storage tank, after the UV lamp, and at the loop return—confirms that ozone has reached the full loop during sanitization and that it is fully destroyed before water reaches points of use.

How often do dissolved ozone sensors require calibration?

Most ozone sensors use two types of calibration, each at a different frequency. Zero calibration is performed at startup and rarely after that, since the zero current changes very little in normal use.

Slope calibration is recommended every six months as a baseline, though your interval may be shorter or longer depending on your ozone concentration, process temperature, and accuracy requirements. Perform calibration during a sanitization cycle when ozone is at least 200 ppb and stable. This will help ensure accurate calibration, minimizing the effects of ozone’s short half-life. Never calibrate the slope at points where UV has removed the ozone.

Because the pureO3 includes ISM, the Adaptive Calibration Timer (ACT) replaces fixed intervals. ACT calculates when calibration is needed from actual time and ozone exposure, so you are not calibrating unnecessarily or waiting too long between checks.

How much maintenance is required for an ozone sensor, and how can I avoid significant downtime?

Ozone sensors have varying levels of maintenance. Electrochemical sensors often require changing the electrolyte and membrane semi-frequently. The anode and cathode often need to be replaced or polished at infrequent intervals, but each ozone sensor is different.

Routine maintenance on the pureO3 is minimal. A full electrolyte and membrane replacement is the most common task, and it takes only a few minutes thanks to the pre-assembled membrane body. The electrolyte is built for long-term use and lasts about six months at a continuous 200 ppb ozone exposure. Replace the membrane and electrolyte once a year as a baseline, or sooner if calibration slope values trend consistently in one direction.

To avoid unplanned downtime, use ISM diagnostics. The Time to Maintenance (TTM) indicator predicts when the membrane and electrolyte need replacing based on actual ozone exposure, before your measurement is affected, and the Dynamic Lifetime Indicator (DLI) gives advance warning for the inner body.

For sensor swaps, ISM's Plug and Measure feature lets you polarize and pre-calibrate a replacement at the bench. Calibration data is stored on the sensor and transfers to the transmitter automatically on connection, so no recalibration or repolarization is needed at the process.

How does Intelligent Sensor Management (ISM) reduce total cost of ownership compared to traditional sensors?

With a traditional sensor, maintenance and calibration are managed on fixed schedules regardless of what the sensor has actually been exposed to in your process. ISM bases sensor management on the sensor's actual condition instead, reducing unnecessary consumable use, operator time, and process interruption.

On the pureO3, four diagnostics work together to do this:

Adaptive Calibration Timer (ACT): Tells you when calibration is due based on actual ozone exposure, not a fixed interval.
Time to Maintenance (TTM): Predicts when the electrolyte and membrane need replacing, based on real process conditions rather than a calendar.
Dynamic Lifetime Indicator (DLI): Predicts inner body replacement before performance is affected.
Sanitization Counter (SAN): Tracks sanitization cycles separately, so the high ozone levels during sanitization do not distort the other diagnostics.

ISM also stores calibration data and sensor identification directly on the sensor. When you connect a pre-calibrated pureO3 to a compatible transmitter, the calibration transfers automatically, with no manual re-entry and no recalibration at the process. This reduces both the time and the documentation burden of each sensor change.

How does the pureO3 sensor support USP <1231> compliance in pharmaceutical water systems?

The pureO3 supports USP <1231> compliance by continuously measuring dissolved ozone at the critical points in a pharmaceutical water loop, giving you the documented evidence a validated ozone sanitization system needs.

Ozone is seen as an added substance in Pure Water and Water for Injection systems, so it is subject to USP <1231> and any equivalent section in other pharmacopeias. USP <1231> states that there should be “no added substance” in these waters, meaning ozone must be sufficiently removed before the points of use on the water system.

This can be proven by monitoring for ozone after UV destruction. Using a sensor that continuously monitors ozone, like the pureO3, is critical for compliance and protecting your product. Using batch measurement sensors can allow small amounts of ozone to pass by undetected, while continuous ozone sensors will catch momentary excursions.

Even though USP <1231> does not dictate any specific sanitization level, it is important to have accurate, fast-responding sensors that can help ensure proper sanitization to keep bioburden, TOC, and conductivity values within specification.

Intelligent Sensor Management (ISM) technology supports the associated documentation requirements by storing calibration history and sensor identification on the sensor, providing a traceable record that transfers automatically to the transmitter with each connection.

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Ozone: Frequently Asked Questions

Answers to Common Questions about Dissolved Ozone Measurement

Select Your Question

What factors cause interference in dissolved ozone measurements?

How long does the initial setup of an ozone sensor take before I can get an ozone reading?