Ozone sensor (Membraned)
The OzoSense Online Dissolved Ozone Monitor from Pi for the measurement of the ozone concentration in water, is designed to be simple to install and operate, and offers unrivalled stability, accuracy and precision, whilst offering excellent value for money.
The OzoSense range of online dissolved ozone analysers and dissolved ozone monitors utilise the very latest and best ozone sensors available in the world today.
sensor (Non Membraned)
There are two sensors in the OzoSense range. The first is a membraned device which is insensitive to changing pH, uses no reagents, is extremely stable, and has reduced maintenance and reduced whole life costs. The second is a non membraned three electrode sensor, that can operate at higher temperatures and pressures.
The online dissolved ozone monitor sensors connect to a range of electronic instrument controllers from Pi resulting in world class online dissolved ozone monitoring and controlling solutions. For more information on Pi’s dissolved ozone controllers, click here.
Principle of Operation
The OzoSense M is a membraned amperometric ozone sensors are two electrode sensors which operate at an elevated applied potential which in turn eliminates zero drift. Its unique design means that no reagents or buffers are required at all.
The OzoSense NM is a non membraned three electrode sensor that requires no buffer or reagents. Choosing which sensor is best for you is always tricky but a good place to start is with the OzoSense M membraned device and then go to the NM (non membraned) sensor if you need to run the sensor at elevated temperatures (up to 70°C) or elevated pressures.
Click here for a guide on how to choose your ozone sensor.
- Stable and reliable – excellent ozone process control – online dissolved ozone controllers
- Suitable for all potable, process and salt waters
- Up to 6 months between maintenance
- Up to 3 months between calibration
- Does not respond to residual chlorine (OzoSense M)
- Resistant to detergents in the water
- Up to 70°C (OzoSense NM)
- Onboard PID controls – Online dissolved ozone controller
- Onboard data logging and graphing capability
- Onboard comms such as TCP/IP, Modbus, Profibus and more.
The OzoSense sensors and flow cells are available with different dissolved ozone controllers giving you the same great performance with different communication, display, and control options.
The OzoSense Residual Ozone Monitor (Membraned) suffers no interference from other oxidants such as Chlorine. Coupled with its excellent zero and calibration stability it is extremely well suited to the monitoring of the residual or dissolved ozone concentration in water in clean or harsh environments. The OzoSense Residual Ozone Monitor (Non-Membraned) can be used in high pressure, high temperature environments making it particularly suitable if the sample must be returned to the process.
The OzoSense (M) online dissolved ozone monitor is bufferless and reagent free, meaning it has a low total cost of ownership and with maintenance intervals for up to 6 months, it is fast becoming the instrument of choice for the engineer who wants the best online residual ozone monitor at the best price.
The OzoSense online dissolved ozone monitors are currently installed on many applications including:
- Bottled Water Plants – Ozone monitors
- Ozone Dosing Control – Ozone controllers
- Remote Sites
- Cooling Towers
- Food Preparation
- Secondary Ozonation
- Waste Water Ozonation
Anywhere you have a requirement to measure online residual ozone in water is a suitable application for the OzoSense ozone monitor. The OzoSense range is particularly suited to working in sites where reliability and ease of use are most important. The sensors are resistant to the presence of tensides making them suitable for use in many washing applications.
Dissolved Ozone Analyser Options
The whole range of OzoSense online dissolved ozone monitors and online dissolved ozone controllers can be fitted with additional sensors such as more ozone sensors, turbidity, pH and many others. Such flexibility can save considerable sums of money when putting together and monitoring or controlling solutions for ozone.
Can I measure ozone in seawater?
How often do I calibrate my dissolved ozone monitor?
How often do I change the online ozone sensor electrolyte (membraned)?
How often do I change the online ozone sensor reference electrode (non membraned)?
Can I measure ozone in the presence of tensides (detergents)?
Can I operate the ozone sensor at an overpressure so I can return the sample to the process?
The membraned sensor can operate at 1 bar overpressure but the pressure must be constant or the reading will vary. This is usually not enough to put the sample back into a process so most customers use an open flow cell to ensure a constant pressure.
Will changing pH affect the reading?
What is the shelf life of the membranes and electrolyte?
What are the interferences?
What are the materials of construction?
PVC, stainless steel, gold, perspex and epoxy (non-membraned sensor).
What is the Temperature range of the sensor?
>3°C – <70°C – Non-membraned sensor.
Why is there no zero adjustment on the membraned ozone sensor?
What will happen if the temperature varies?
What should I use to calibrate the sensor?
What do I have to think about when I am taking a sample to do a DPD test?
I have tried to calibrate and the analyser says that the sample wasn’t stable?
What is Ozone?
Why is Ozone used to disinfect water?
Dissolved ozone is a powerful oxidant and can therefore be used to disinfect water. Specifically it can:
- React with and remove manganese
- Destroy some pathogens resistant to chlorine such as Giardia and Cryptosporidia.
Why Is Ozone favoured for some applications?
What is the difference between an Ozone monitor, an Ozone analyser and an Ozone controller?
There is no difference between an Ozone monitor and an Ozone analyser. They are just two expressions used interchangeably by different people.An Ozone controller is an Ozone monitor or analyser that has on board control functionality. The Pi OzoSense M and OzoSense NM all have such on board functionality, so for a Pi OzoSense there is effectively no difference between an Ozone controller, analyser and monitor.
If you have any other questions relating to the online measurement and control of dissolved ozone, please don’t hesitate to contact us.
Focus Ons are a series of short articles distributed by email providing technical information regarding instrumentation, process measurement in potable, waste, process and pool waters. If you would like to join the mailing list, please contact us.
Focus On… Calibrating Sensors Using a DPD Kit
You probably know that most chlorine, ozone and chlorine dioxide analysers are calibrated using hand held DPD kits but…
… did you know that DPD can’t tell you when you have no residual?
… did you know that errors on DPD performance can be up to ± 100%?
… did you know that a significant number of service calls received by Pi relate to poor calibration?
DPD (N.N-diethyl-p-phenylenediamine) is a chemical that when mixed with water containing an oxidant, changes colour depending on the concentration of the oxidant present. A handheld colorimeter measures light passing through the coloured solution. The absorption of that light by the liquid gives a concentration value. It is usually used to check concentration of, for example, free chlorine, total chlorine, ozone and chlorine dioxide etc. in water.
When the DPD kit gives a value, it is often used to calibrate online instruments……and that is where Pi comes in!
As a manufacturer of online instruments we have to understand DPD in order to help our customers when they have problems calibrating their online monitors.
This Focus On will look at:
- The limitations of DPD (turbidity, zero oxidant, bleaching, pH and interferents).
- Minimising DPD measurement error (sampling, alignment and cleaning).
- Things to look out for (low concentrations, pink colour, stained glass).
- Little known chemistry (measuring bromine, chlorite versus chlorine dioxide).
- Rinse and repeat: is it really worth repeating my measurement?
What are the limitations of DPD?
DPD cannot measure zero oxidant well.
DPD works using the absorption of light, and turbidity in the sample will give a positive reading. This means if there is no oxidant in the sample, any turbidity introduced to the sample after ‘zero’ such as undissolved tablet or powder will cause the DPD test kit to give a small reading, this is why…
DPD cannot measure below approximately 0.05 ppm.
If you suspect there is zero oxidant in your sample, hold the vial up to a white surface. If you cannot see any trace of pink colour, it is likely any reading you are getting is from the unreacted DPD tablet.
DPD cannot measure free chlorine above 6 ppm (and won’t always give a ‘high concentration’ reading error).
Many people are unaware that past a certain level of oxidant, DPD will not form its characteristic pink colour, and instead will ‘bleach’ to form a clear solution. This can lead people to think there is little or no oxidant in their water, when in fact there is so much that it is bleaching their DPD. Be on the lookout for a flash of pink when the tablet or powder is added if you suspect your sample is being bleached. NB. special kits and reagents are available for measuring oxidant above 6 ppm.
DPD cannot measure in extremes of alkalinity or pH.
DPD tablets, powdered pillows, and drops contain buffers that will change the pH of your solution in order to facilitate DPD reacting with your oxidant. There is only so much buffering capability in the powder or tablet, and if your sample has an extreme of pH or alkalinity this could affect the concentration reading from the DPD handset.
DPD cannot distinguish between oxidants such as: chlorine, chlorine dioxide, chlorite, ozone, organochlorides, bromine and more, meaning interferents are a big problem.
DPD is a fantastic chemical, in that it is very versatile as a colouring agent, which is how it gives the oxidant the colour that we measure. This versatility does come at a price, DPD is not very specific as an analysis tool, and so if other chemicals are present in the sample, they can interfere with the reading, giving an inaccurate result. Common interferents include chlorine dioxide (for chlorine measurement, and vice versa), sodium chlorite, ozone, organochloramines, peroxides, and many more.
DPD cannot distinguish between colour and turbidity.
Any undissolved solids, including unreacted DPD tablet, will affect the reading. Sample turbidity should be accounted for in the zero measurement. If the zero measurement has a high turbidity, this will affect the sensitivity of the colorimeter, due to the large correction it must perform to account for absorption by undissolved solids. Allowing any solids in the sample several seconds to settle after mixing is the best way to counteract this.
Minimising DPD measurement error
Here is an easy to read, printable checklist to ensure accurate DPD readings every time.
Things to look out for
When was the last time your DPD was calibrated?
Like all measurement devices, handheld DPD colorimeters can drift over time, and need to be calibrated. Check your device manual for how often it should be calibrated, if you can’t remember the last time it was calibrated, chances are it needs doing again!
The pink solution formed after DPD tests can leave a residue behind on the glass, which will affect the DPD reading. This residue can be easily cleaned off using what is in your DPD kit.
If you use normal tap water to wash out vials, droplets left behind can affect your reading due to the residual chlorine in drinking water. It is best (but not always practical) to use deionised water to wash out your vials, but if this isn’t available (deionised water can be purchased as car battery top up water from any car parts supplier) then you can use cooled boiled tap water, as boiling gets rid of any chlorine. If not then simply make sure the vials are perfectly dry before use.
Little Known Chemistry
DPD has a wide range of interferents. This means recurrent problems can sometimes be caused by the chemical makeup of the sample. For example, chlorite (ClO2–) and chlorine dioxide both affect DPD, but only chlorine dioxide is measured by most chlorine dioxide amperometric sensors.
DPD can be used to track bromine, but DPD No.1 tablets measure FREE chlorine or TOTAL bromine. As combined bromine is just as effective a disinfectant as free bromine, this generally doesn’t pose too much of a problem, however some amperometric sensors measure free bromine, and cannot be calibrated using DPD No.1 tablets. For more information on measuring bromine, or chlorine in seawater, please see Pi’s technical note on Seawater Chlorination.
Rinse and repeat
How important is it to repeat my DPD measurement? Isn’t it a waste of time?
A sensor is only as good as its last calibration, and the sensor will be as accurate as you calibrate it to be. If you need your sensor for tight process control, such as a pool or dosing controller, then it is essential to repeat the DPD test at least twice, if not more. The reason it’s important to repeat the test is mainly due to human error, but variation in DPD tablets has been known, or it could be a slight concentration spike that you happened to pick up in your sample. With each repetition these circumstances become less and less likely, giving you more confidence in the value you use to calibrate your analyser.
Pi recommends the following routine for calibration:
Perform a DPD test, and compare the reading to your analyser.
- Is the reading within 10% of your analyser? If yes, leave the analyser alone.
- If the reading is not within 10%, repeat the DPD test.
- Is the second test within 10% of the first test? If yes, calibrate your instrument to this reading. If not, keep repeating the DPD tests until 2 consecutive tests are within 10%, then calibrate the machine to this reading.
Focus On… Probe Fouling
Many different sites ranging across the whole water industry have a daily struggle to keep instrumentation functioning correctly due to fouling. However did you know that…
… self cleaning and self flushing systems are now available from Process Instruments for most types of sensors?
… these fouling removal systems can extend the life of sensors and drastically reduce maintenance regimes?
… Pi’s self cleaning/flushing systems are affordable, simple and trouble free by design?
What is the problem?
Whatever the process being monitored is, there is often something in the sample water capable of fouling a sensor, and therefore causing erroneous results. The obvious solution to this problem is to clean the sensor, but how regular should inspection and cleaning programs be for each piece of instrumentation? Too regular and the inspection and cleaning regime is time consuming and unnecessarily costly. Not often enough and the instrumentation will give false results and probably fail prematurely.
What is the solution?
Process Instruments’ Autoclean and Autoflush Systems
Simple, reliable and easy to maintain, Process Instruments’ Autoclean/Autoflush systems are an alternative to mechanical cleaning mechanisms which can clog and break. By regularly spraying the sensor/probe with clean water or air, the sensor remains clean and free from fouling for extended periods of time. The sensor cleaning cycle is activated by Pi’s controller for a user selectable length of time and frequency so that no matter how dirty the application, the probe remains clean. With no moving parts in the sensor body or in the cleaning attachment there is nothing to replace or check other than a simple valve positioned in an easy to reach location.
Pi’s Autoclean and Autoflush systems can give trouble free and fouling free functioning of sensors for weeks, if not months, at a time.
A solution for each application
This option can be added to our pH, ORP, Turbidity, Suspended Solids and Dissolved Oxygen (DO) sensors. Consisting of an end cap to direct the flow of clean water (or air for a DO sensor) across the face of the sensor blasting any dirt away. The cleaning is controlled by a single valve positioned in an easily accessible location.
If using air to clean a DO sensor the system can also automatically verify that the sensor is still responding correctly, removing any need to remove the sensor from the sample for months at a time.
For sensors that require flow cell mounting like Chlorine, Ozone and Chlorine Dioxide, an Autoflush system has inbuilt valves which automatically start/stop the sample flow and control the flow of clean water past the probe. The user can set the flushing interval and duration to keep the flow cell and sensor clear from fouling. For particularly dirty or stubborn contaminants, warm water can be used as the flush water to aid cleaning.
With the above options, whatever the application or parameter being measured, Process Instruments will be able to provide a monitoring system that will not only be accurate, precise and long lasting but that will also remain free from fouling and save the operator both time and money.