Turbidity Meter – TurbSense®

Why does the market need another turbidity sensor when so many are already available?

The simple answer to this question is that current sensors cannot overcome all of the common issues relating to online turbidity measurement. Pi’s patented TurbSense^® has a solution for all these problems.

Did you know that…
…the TurbSense^® uses a patented measurement method which means background light and electronic drift have no effect?
…Pi’s TurbSense^® provides a resolution of 0.001NTU up to 1000NTU using a single point calibration with no zero required?
…the TurbSense^® flow cell ensures the measurement is unaffected by both entrained and nucleated bubbles?

The problems with turbidity sensors and the TurbSense^® solutions

Bubbles

All turbidity sensors are severely affected by bubbles because they affect the optical properties of the water they are in. To a sensor these bubbles look like turbidity. Bubbles can become entrained in the sample water and, when pressure is reduced, dissolved gases in the water can also nucleate on any surface as bubbles, including the sensor window. Both nucleated and entrained bubbles can result in erroneous readings, affecting the reliability of the measurements.

Pi’s CRONOS^® and CRIUS^®4.0 TurbSense^® can utilise a flow cell, in which the sample flow passes through a baffle system, forcing the entrained air out of the sample. Furthermore, the TurbSense^® can regularly drop the level of the water in the flow cell, causing any nucleated bubbles on the sensor to burst. These simple, yet highly effective features ensure the turbidity sensor is free of interference due to entrained and nucleated bubbles.

Calibration

Most nephelometric turbidity analysers either need a ‘factory set’ zero which can drift over time or a ‘zero NTU’ sample to determine the ‘zero’ during a calibration. It is almost impossible to source a ‘zero NTU’ sample due to the ease of contamination, so determining the ‘zero’ of a turbidity analyser is a big problem… but not for the TurbSense^®!

Pi’s CRONOS^® and CRIUS^®4.0 TurbSense^® has a very clever and simple calibration system, removing the need for a ‘zero’. Pi’s TurbSense^® takes advantage of the linear nature of 90° nephelometric turbidity to determine the zero. By halving the output light from the sensor the TurbSense^® halves the detected light at a constant turbidity. Therefore, the zero can be back calculated which removes any drift and ensures the most accurate zero achievable.

To see a technical discussion on the measurement techniques employed by the TurbSense^®, click here.

Fouling of optics

Turbidity probes are used in a wide variety of water applications from potable to waste waters. In many applications, the sample water can contain materials which can foul the optics preventing consistent accurate measurements. Both particulates and bubbles could partially or completely block sensor optics. Taking the sensor out of a sample to manually clean is an inconvenience to an industrial process as it may need shutting down to do this. Pi’s TurbSense^® ensures fouling is never a problem and manual cleaning is rarely required by utilising the autoclean functionality in which water can be jetted across the optics, effectively keeping the sensor clean.

Poor accuracy at low level turbidities

Often as a result of manufacturers setting a zero, any subsequent drift from electronics results in small errors. At low level turbidities, small errors can lead to a significant impact on the accuracy of the measurements. Due to the patent applied for optical calibration method used by the TurbSense^® this is never an issue. Any drift is removed when a new zero is established on calibration. Using this approach the end user can always have confidence in the measurements made by TurbSense^®.

To summarise, Pi’s TurbSense^® has all the functionality required to provide a highly accurate and reliable system. The instrument is free from the issues so many of the current online turbidity instruments are plagued by, and with full functionality the instrument remains competitive on price.

For a technical discussion on the effect of bubbles or on the measurement techniques employed by the TurbSense^®, click here.

A Water Treatment Works in Georgia, US, recently trialled a TurbSense^® online turbidity analyser from Pi (supplied by Pi’s US technology partner, Chemtrac Inc.).

The instrument was installed on the raw water inlet alongside a different manufacturer’s surface scatter turbidity meter. The results of the instruments were compared against a HACH 2100N Laboratory Turbidimeter.

The Plant

The trial plant had used the competitor’s surface scatter for some time on their raw water, however they found that the accuracy of their results vs their HACH 2100N Laboratory Turbidimeter was poor, and they therefore trialled the TurbSense^® as a potential alternative

Auto-Cleaning

TurbSense^® was equipped with its autoclean capability, which actively cleans the sensor with a water jet to remove dirt from the sensor tip on a regular basis, however this wasn’t used during the trial so the customer could see how quickly the sensor fouled.

Results

Measurements from the 6th of January 2017 to the 31st January 2017 have been used to demonstrate the variation between the two analysers. This data set was chosen to be representative of the whole trial.

The trial found that the CRIUS^® TurbSense^® from Pi (HA4 from Chemtrac in the USA) provided the most accurate and reliable turbidity meter. Graph 1 shows the correlation between the HACH Laboratory Turbidimeter and the Pi TurbSense^® online turbidity meter. The graph clearly shows that online results and lab results have a high correlation. The three results circled are anomalous and are probably due to a sampling error.

Graph 2 continues the correlation between the online and laboratory results.

Graphs 3 and 4 both show the same data set, but this time for the online surface scatter results plotted against the laboratory results. An x-y plot of laboratory results vs the surface scatter turbidity meter results shows a very poor correlation between them.

Conclusion

The data from the trial clearly shows a highly reliable correlation between the actual turbidity (as determined by the bench top lab unit) and the TurbSense^® online turbidity meter from Pi. Perhaps surprisingly, and counter intuitively, the results show a much better correlation with the sample contacting TurbSense^® than with a competitor’s non-contacting surface scatter turbidity meter.

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.

A solution for each application

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.