Online, continuous Chlorine Dioxide Analysers, Chlorine Dioxide Monitors, Chlorine Dioxide Meter and Chlorine Dioxide Controllers make up the DioSense range which utilises the very latest and best chlorine dioxide sensor available in the world today. It is a membraned device which is insensitive to chlorine, uses no reagents, is extremely stable, and has reduced maintenance and reduced whole life costs.
The DioSense chlorine dioxide sensors which give a dissolved chlorine dioxide measurement in ppm or mg/l and flow cells are available with different chlorine dioxide controllers giving you the same great performance with different communication, display, and control options. With the DioSense range of residual chlorine dioxide analysers, you get everything that you need – and nothing that you don’t.
The DioSense Residual Chlorine Dioxide Monitor is used in many applications requiring the measurement of chlorine dioxide concentration simply for chlorine dioxide monitoring or residual chlorine dioxide dosing control.
Pi’s chlorine dioxide analyser is designed to have reduced maintenance, reduced calibration and reduced spares requirements. The DioSense ClO2 analysers are undeniably the most cost effective ClO2 analysers available.
According to the EU Directive 98/83/EC, the operators of waterworks must ensure that the levels of harmful chlorite (ClO2–) in drinking water remain below the specified limit of 0.2 mg/l. The monitoring of Chlorite at the same time as Chlorine Dioxide is also a European requirement.
Pi is able to provide both a ClO2 and ClO2– sensor which can be controlled from a single analyser. In most situations the ClO2 analyser should be able to control the dosing of ClO2 by adjusting flow rates, pump rates, or valve positions automatically to maintain the residual chlorine dioxide set-point. Automatic dosing can significantly reduce reagent costs, and increase the level of control over harmful chlorite levels.
Chlorine dioxide analysers are often used to monitor the residual ClO2 in difficult ‘dirty’ applications such as in hospitals or the food industry. In these cases, in order to maintain an appropriate Chlorine Dioxide residual it is necessary to automatically clean the sensor using the Autoflush.
If you think your application could need the Autoflush, please contact us and discuss it with our experienced technical sales people.
The membraned amperometric chlorine dioxide sensor is a two electrode sensor which operates at an elevated applied potential which in turn eliminates zero drift. Its unique design means that no reagents or buffers are required at all and calibration is a simple one point (no zero required) operation. In addition to the state of the art amperometric chlorine dioxide sensor the DioSense range of controllers has all the functionality that you need. Choose from the CRONOS® or CRIUS® controller to give you the highest quality chlorine dioxide monitor, with all the functionality you need, at the lowest price possible. This means that you pay for everything that you need, and nothing you don’t, without sacrificing the quality of measurement.
The DioSense is used in many applications from hospital water treatment to chicken washing often as part of a chlorine dioxide dosing control regime. Capabilities including remote access via the mobile phone network and full ClO2 dosing control.
- Amperometric sensors – continuous online chlorine dioxide ppm measurement
- No chemical reagents – lower cost of ownership
- Stable and reliable – excellent process control
- Suitable for all potable and process waters
- Up to 6 months between maintenance
- No interference from residual chlorine
- Tolerant of water containing detergents
The DioSense chlorine dioxide measurement sensors and flow cells are available with different controllers giving you the same great performance with different communication, display, and chlorine dioxide dosing control options.
- Chlorine Dioxide Dosing Control
- Remote Sites
- Cooling Towers
- Food Preparation
- Food & Drink Industry
Anywhere you have a requirement to measure residual ClO2 is a suitable application for the DioSense. The DioSense chlorine dioxide monitor range is particularly suited to working in sites where reliability and ease of use are most important.
Why not click here and ask for a demonstration of all that our chlorine dioxide analysers can do for you?
As described in a separate brochure, the DioSense can come equipped to automatically clean itself at user defined intervals. 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 DioSense range of chlorine dioxide ppm monitors are particularly resistant to tensides and are therefore particularly useful in food washing applications.
The whole range of DioSense Residual Chlorine Dioxide Monitors and Controllers can be fitted with additional sensors such as chlorite or pH. Please ask your local distributor for more details.
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.