Online Ozone Analyzer for Water – OzoSense

For ozone analyzer options such as communications, relays and controls, please click here.

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.

When you add ozone to seawater it reacts with the bromide salt in seawater and you get bromine. Pi offers a total bromine sensor for this application. Click here.

This depends on the application. The online residual ozone sensors have a very low drift so most people calibrate it either once a week or once a month.

Every 3-6 months.

Yes, the OzoSense is resistant to the effects of tensides.

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.

No.

If stored in a cool dry place, two years.

There will be a small response to residual chlorine (<2%) and also to chlorine dioxide (<7%) for the membraned sensor. There could be other interferences. Please contact Pi for details.

PVC, stainless steel, perspex (sensor plus overflow cell).

>3°C – <50°C.

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.

Nothing! The sensor has a thermistor that measures the temperature and does an automatic compensation.

Use a handheld meter. These are available from a variety of suppliers and nearly all of them utilize colorimetric DPD to determine the ozone concentration in the sample.

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.

During calibration the analyzer 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 analyzer prevents calibration to avoid the calibration routine introducing errors.

Ozone is an allotrope of Oxygen having the chemical formula O₃. It is three Oxygens joined together. It decays to O₂.

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

Ozone disinfects and leaves no taste or odor. In addition, it completely breaks down urea so doesn’t form organochlorine or combined chlorine products.

There is no difference between an Ozone monitor and an Ozone analyzer. They are just two expressions used interchangeably by different people. An Ozone controller is an Ozone monitor or analyzer that has on board control functionality. The Pi OzoSense has such on board functionality, so for a Pi OzoSense there is effectively no difference between an Ozone controller, analyzer 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.

You probably know that most chlorine, ozone and chlorine dioxide analyzers 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 color depending on the concentration of the oxidant present. A handheld colorimeter measures light passing through the colored 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).
• Minimizing DPD measurement error (sampling, alignment and cleaning).
• Things to look out for (low concentrations, pink color, 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 color, 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 color, 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 coloring agent, which is how it gives the oxidant the color 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 color 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.

Minimizing 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!

Stained Glass

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.

Tap water

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 deionized water to wash out your vials, but if this isn’t available (deionized 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.

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 analyzer.

Recommended Routine for calibrating ozone analyzer

Perform a DPD test, and compare the reading to your analyzer.

• Is the reading within 10% of your analyzer? If yes, leave the analyzer 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.

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?

Sensor Fouling

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.

Autoclean

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.

Autoverify

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.

Autoflush

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.