The OzoSense online Dissolved Ozone Analyser from Pi for the measurement of 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 Ozone Analysers, Ozone Controllers and Ozone Monitors utilise the very latest and best ozone sensors available in the world today. They are membrane devices which are insensitive to changing pH, use no reagents, are extremely stable, and have reduced maintenance and reduced whole life costs.
The ozone sensors connect to the range of electronic instrument controllers from Pi resulting in world class online dissolved ozone controllers and analysers. For more information on Pi’s controllers, click here.
The 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.
- Stable and reliable – excellent ozone process control – 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
- Resistant to detergents in the water
- Onboard PID controls
- 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 ozone controllers giving you the same great performance with different communication, display, and control options.
The OzoSense Residual Ozone Monitor 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.
Pi’s Ozone Monitor is bufferless and reagent free, meaning it has a low total cost of ownership and with maintenance intervals at 3 or even 6 months, the bufferless Ozone Monitor is fast becoming the instrument of choice for the engineer who wants the best ozone monitor at the best price.The OzoSense dissolved ozone controllers are currently installed on many applications including:
- Ozone Dosing Control – Ozone controllers
- Remote Sites
- Cooling Towers
- Food Preparation
- Secondary Ozonation
- Waste Water Ozonation
Anywhere you have a requirement to measure residual ozone is a suitable application for the OzoSense. The OzoSense dissolved ozone controller range is particularly suited to working in sites where reliability and ease of use are most important. The sensor is resistant to the presence of tensides making it suitable for use in many washing applications.
As described in a separate brochure, the OzoSense can come equipped to automatically clean itself at user defined intervals with all the benefits of no operator intervention for 6 months. The Autoflush is particularly useful in food preparation, pulp and paper, and many applications where there is likely to be a build up of solids in the sample.
The whole range of OzoSense Ozone Monitors and Controllers can be fitted with additional sensors such as more ozone sensors, chlorine, pH and many others. Such flexibility can save considerable sums of money when putting together and monitoring or controlling solutions for ozone.
Pi offers ozone sensors from a range 0.05-0.2, 0.05-0.5, 0.05-2, 0.05-5, 0.05-10, 0-20 mg/l (ppm)
Can I measure ozone in seawater?
When you add ozone to seawater you get bromine. Pi offers a total bromine sensor for this application.
How often do I calibrate my dissolved ozone sensor?
This depends on the application. The online residual ozone sensor has a very low drift so most people calibrate it either once a week or once a month.
How often do I change the online ozone sensor electrolyte?
Every 3-6 months.
Can I measure ozone in the presence of tensides (detergents)?
Yes, the OzoSense is resistant to the effects of tensides.
Can I operate the sensor at an overpressure so I can return the sample to the process?
Yes and No. The 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?
If stored in a cool dry place, two years.
What are the interferences?
There will be a small response to residual chlorine (<2%) and also to chlorine dioxide (<7%).
What are the materials of construction?
PVC, stainless steel and perspex.
What is the Temperature range of the sensor?
>3 °C – < 50 °C.
Why is there no zero adjustment?
The sensor operates at a positive voltage all of the time so any drift on the zero is negligible compared to the positive operating voltage so no zero is necessary.
What will happen if the temperature varies?
Nothing! The sensor has a thermistor that measures the temperature and does an automatic compensation.
What should I use to calibrate the sensor?
Use a handheld meter. These are available from a variety of suppliers and nearly all of them utilise colourimetric DPD to determine the ozone concentration in the sample.
What do I have to think about when I am taking a sample to do a DPD test?
Firstly take the sample from right at the instrument. Secondly don’t take the sample when the concentration is varying quickly, and thirdly use a good quality handheld and follow the instructions carefully.
I have tried to calibrate and the analyser says that the sample wasn’t stable?
During calibration the analyser looks at the stability (rate of change) of the signal from the probe and if it varies by more than 10% over the countdown then the analyser prevents calibration to avoid the calibration routine introducing errors.
What is Ozone?
Ozone is an allotrope of Oxygen having the chemical formula O3. It is three Oxygens joined together. It decays to O2.
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?
Ozone disinfects and leaves no taste or odour. In addition it completely breaks down urea so doesn’t form organochlorine or combined chlorine products.
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.
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.