Chlorine analysers from Pi are used in many applications requiring the measurement and control of online residual chlorine levels in water. The HaloSense range is suitable for total or free residual chlorine monitoring or control applications in potable water, seawater, process water, swimming pool water, wastewater, food washing, paper and pulp, etc.
The free and total residual chlorine sensors are three electrode amperometric sensors. Two of the electrodes (the gold working electrode and the silver/halide reference electrode) are behind a membrane that separates the sample from the electrodes and are submerged in an electrolyte. The membrane allows the movement of chlorine from the sample to inside the sensor where a low pH environment converts the vast majority of any OCl– present to HOCl. The HOCl is measured at the working electrode and a current proportional to the concentration of chlorine is produced which is reported back to the analyser.
Small water treatment plants, secondary disinfection plants etc. tend to suffer from similar problems wherever they are in the world. The first is lack of communications SCADA infrastructure, the cost of which to install can be prohibitive. The second is the lack of a central DCS control infrastructure, again the installation of which can be cost prohibitive. The third is the remote location. Often these water treatment plants are in remote and difficult to access locations.
Each Residual Chlorine Analyser from Pi has the capability to be an extremely capable Chlorine Controller. The controllers can have multiple control channels which can utilise chemical control (usually a relay (switch) turns dosing on when the chlorine is too low or off when it is too high) or PID control.The HaloSense sensors can come equipped to automatically clean themselves at user defined intervals, with all the benefits of no operator intervention for up to 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. For more information about AutoFlush click here.
For some free chlorine applications with high and variable pH, pH compensation can improve the accuracy of the analyser. For pH compensation to be valid it must be done with the highest quality pH sensors and with chlorine sensors that have a reduced susceptibility to varying pH, such as those used in the HaloSense range.
The graph shows the errors on a real HaloSense free chlorine sensor when a sample of 1 ppm free chlorine has the pH changed from pH 9 to more than pH 10, down to pH 7.5 and back again. The graph shows that the vast majority of applications won’t need pH compensation at all and for those that do that free chlorine sensor is the most appropriate sensor available to have that compensation applied.
When chlorine is added as a disinfectant to water it oxidises material in the water thereby killing any organisms. The ‘Residual Chlorine’ is the chlorine left over at the end of the process and is usually what we measure.
Pi offers Free and Total Chlorine sensors in the range 0.005-0.5ppm (total only), 0.005-2ppm, 0.05-5ppm, 0.05-10ppm, 0.05-20ppm and 0.5-200ppm (free only).
This depends on the application. The online chlorine sensor has a very low drift so most people calibrate it either once a week, once a month or even every six months.
Once a year (free and total), every 3-6 months (zero).
Once a year.
Yes, but only a very small amount and most users are happy to accept this.
Pi also design and install a range of pH analysers.
Both ozone and chlorine dioxide will interfere with the measurement. For more information, click here.
If stored in a cool dry place, two years.
PVC-U, stainless steel, hydrophilic membrane, PEEK (total and zero) and silicone.
0°C – 45°C (free and total), 0°C – 40°C (zero).
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 utilise colourimetric DPD to determine the chlorine 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 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.
No. Total Chlorine is the sum of Free Chlorine and Combined Chlorine, therefore Total Chlorine is always equal to or greater than both Combined Chlorine or Free Chlorine.
Free chlorine reacts with things in a pool and changes them. For example free chlorine reacts with viruses and bacteria. It changes and kills those organisms but is changed by them in the same time…in effect it is used up. Free chlorine can also react with ammonia in a pool to form combined chlorine. No free chlorine in a pool is always a result of either there isn’t chlorine going into the pool (a dosing problem) or it has all been used up (reacted).
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More than ten years ago, a large UK water utility company made the decision to remove all of their DPD based, chlorine measurement instruments and replace them with Pi’s HaloSense amperometric analysers. This puts Pi in a unique position to accurately calculate the savings made by this water company as a direct result of this change.
It is possible to purchase non-branded reagents for DPD based systems which does reduce the ongoing cost of these systems. However, real world data of the 330 amperometric systems installed also shows that maintenance times of the installed amperometric probes could be extended in most cases, well beyond the recommended frequency, which also reduces the cost of the amperometric systems. Taking both of these factors into account, the second graph shows the adjusted savings which, in this real world example, still adds up to £937,000.
Membraned sensors come with many advantages over non-membraned sensors such as higher resolutions, fewer interferents and a greatly reduced effect of flow rate changes. These advantages can make a huge difference to the bottom line, particularly if the cost of the chemical being dosed is quite high. For free chlorine sensors, using a membrane can make your measurement much less dependent on pH (if you are using sensors from Pi), meaning your measurement is a more accurate reflection of chlorine residual.
As such, membraned sensors are now largely the norm in residual chlorine measurement and are also prevalent for chlorine dioxide and ozone monitors, but did you know that…
…membraned sensors are sensitive to changes in pressure?
…flow cell outlets can airlock even when water is flowing through them?
…membraned sensors can still be used when the outlet does not go to drain?
…Pi has engineered and designed solutions to all of these potential issues?
Membraned sensors do have one property that needs to be carefully managed; they are sensitive to pressure. Pi was an early adopter of membrane technology, so we know that the installation of these sensors is just as important as the sensor itself. In fact, the same sensors in different flow cells can give very different results.
In order to prevent pressure variations affecting the probe, Pi typically uses open flow cells which eliminate variability in pressure before it reaches the probe.
Whether the sample to the cell is pumped, gravity fed, or comes from a pressurised line, it is important that the flow is controlled to within a range of 350-1000ml per minute, to ensure that sufficient flow is reaching the sensor and to prevent the flow cell overflowing.
If the flow to the cell is variable, Pi can provide a dole valve which controls the flow to approximately 500ml per minute, which prevents the cell from overflowing when pressure variations mean more flow than the cell can handle, while also ensuring adequate flow when the sample line flow/pressure reduces.
The outlet of the flow cell needs to be open to atmosphere, and completely unobstructed. Any system with a long outlet line (particularly flexible pipe) is prone to get airlocks, which will cause the cell to overflow. Outlets which are visually clear and even have water flowing through them, can be partially airlocked which causes backpressure to overflow the cell. This is very easy to diagnose as if you see the cell overflowing and remove the outlet pipe, you will see the cell go back to normal operations within approximately 10 seconds. If this is a persistent problem, consider putting in an air break using a commercially available tundish.
The water from a membraned sensor doesn’t have to go to drain. For processes where saving water is a high priority, a simple tank and pump system that will pump sample water back into your main process line will allow water losses to be reduced to almost zero. Pi’s CRIUS®4.0 controller can be used to control this return process and ensure that this tank never overflows and can automatically drain itself periodically to avoid sediment build up.
As any water engineer can tell you, no matter how well the system is designed, lines can clog, pumps can break and someone on site could fiddle with the settings. Pi recognises these challenges and has engineered solutions into our systems. All Pi membraned sensors have the option to be able to:
For membraned sensors, the best way of housing a membraned sensor is with an open flow cell. There are some occasions where this solution just isn’t practicable and in those instances the closed flow cells from Pi, which can take an overpressure of up to 3 bar, are the best solution.
Seawater contains about 65-80ppm dissolved bromides1 most of which are sodium bromide. When you put chlorine in water it displaces the bromine from the bromide (because it’s more reactive) and becomes a chloride. So for up to about 70ppm of total chlorine dosed, what you actually have in the water is free bromine and combined bromine (NOT free and combined chlorine), so it is the total bromine that actually does the disinfection2. So why does everyone call it chlorination when technically it is bromination? Mainly because most people don’t know this interesting bit of chemistry. So what? Normally it makes no difference as total bromine is an effective disinfectant, however there can be a lot of confusion when it comes to monitoring residuals and controlling dosing. Choosing the correct sensor to control the dosing is crucial as is choosing the correct DPD test.
Because bromine is heavier than chlorine, 1mg/l of chlorine is the same as approximately 1.6mg/l of bromine3 so it is important to understand what you are measuring. DPD kits can be used in three ways for the chlorination of seawater.
In most non-membraned (and some membraned) amperometric sensors, oxidants are detected by a current produced at the working electrode, at a particular voltage. The same technology can effectively be ‘tuned’ to different oxidisers by varying the voltage. Lots of oxidants are measurable over a range of voltages, and sometimes those response curves overlap.
The DioSense membraned chlorine dioxide sensor from Pi is not susceptible to interference by free chlorine. This means that the DioSense sensor can be used in conjunction with Pi’s HaloSense free chlorine sensor, to measure chlorine dioxide and free chlorine independently of each other in the same application, on Pi’s CRONOS® or CRIUS® analyser. The analyser takes the signal from the free chlorine probe, which does have a known interference from chlorine dioxide (1ppm of chlorine dioxide will show up as 0.75ppm of chlorine). The normalised signal from the chlorine dioxide sensor can then be removed.
Our HaloSense Zero chlorine controller measures the water coming out of the carbon filters before it reaches the RO membranes. Any free chlorine breakthrough from the carbon filters is detected by the system, which will set off an alarm and can also be configured to automatically shut off valves to prevent the contaminated water from reaching the RO membranes.
The HaloSense Zero consists of a sensor designed to measure the absence of chlorine connected to a CRONOS® or CRIUS® controller. The sensor can detect even very low levels of free chlorine, while the CRONOS® or CRIUS® controller acts as the brains of the system, interpreting data from the sensor. In the event that any free chlorine is detected, the analyser will send an alarm signal and can automatically shut off valves to prevent chlorine from reaching the RO membranes.
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.
Pi’s AutoClean and AutoFlush systems can give trouble free and fouling free functioning of sensors for weeks, if not months, at a time.
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.
When the DPD kit gives a value, it is often used to calibrate online instruments… and that is where Pi comes in!
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.
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.
In the USA nearly all pools and spas use ORP sensors to control their chlorine dose, yet conversely in the UK and Western Europe most ORP systems have been replaced with systems that measure the concentration of free chlorine in water. Pi provides systems that utilise either or both technologies.
Unfortunately, what ORP sensors measure is tendency and not capacity, i.e. ORP measures the likelihood or the ability of the water to kill bugs, but not how many bugs that water can kill, a subtle but very important difference. A sample with high ORP may be able to kill a small number of bugs very quickly but then not be able to kill future pollution. What’s more, although chlorine affects ORP very strongly it is not the only variable involved. The pH of water affects ORP directly and also affects the concentration ratio of OCl–/HOCl, the two main disinfectant components. A lower pH (higher acidity) will cause an increase in the relative concentrations of HOCl causing an increase in ORP.
These sensors use electrochemistry to measure the free chlorine concentration directly. They tend to be slightly more expensive than an ORP sensor, but are more reproducible and precise, and therefore tend to give better control (and therefore reduced chemical cost). They are specific to free chlorine (the disinfectant) and can be easily calibrated using a DPD test for free chlorine. Whilst the capital cost for a ppm chlorine sensor is higher, total cost of ownership tends to be lower as ORP sensors are typically replaced every year and ppm sensors last for ten years or more.
Advantages |
Disadvantages |
|---|---|
ORP SensorsSimple (no calibration)Inexpensive |
ORP SensorsDoesn’t measure disinfection capacityAffected more by pH than by free chlorine Non-Linear Not reproducible (not the same from site to site) Affected by changing water chemistry Affected by all oxidants Using ORP control normally leads to higher residuals and less stable control |
ppm SensorsMeasure free chlorine directlyResults comparable across different sites Linear response Only affected by free chlorine Using a ppm sensors leads to lower residuals, more stable control and better swimmer experiences |
ppm SensorsRequires calibrationMore expensive – but not much More maintenance – but not much |
| Document | Type | Size |
|---|---|---|
| HaloSense | Brochure | 724kB |
| HaloSense Zero | Technical Note | 606kB |
| HaloSense Hints and Tips | Technical Note | 606kB |
| Potential Savings when Choosing Amperometric Chlorine Measurements over online DPD | Article | 525kB |
| Seawater Chlorination | Technical Note | 710kB |
| ORP vs. ppm | Technical Note | 534kB |
| pH Effects on Pi’s Free Chlorine Sensor | Technical Note | 702kB |
| pH Compensation of a Residual Chlorine Measurement | Technical Note | 540kB |
| Free Chlorine Probe Maintenance | Technical Note | 658kB |
| Total Chlorine Probe Maintenance | Technical Note | 689kB |
| Using Open Flow Cells with Membraned Sensors | Technical Note | 762kB |
| DPD Checklist | Technical Note | 487kB |
| Probe Fouling | Technical Note | 459kB |
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> Technical help with your Analyser
> Help choosing a product
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