Turbidity Meter – TurbSense®

Sapphire is used for the windows as it is tough and scratch resistant (much more than glass or quartz). Its use also allows the windows to be thin (because it’s stronger) and therefore have good optical transmission.

The maximum temperature of the sample being monitored is 50°C. We choose 50°C because it is well within the upper limits of the electronic components that we use and still gives a good range or temperature for measuring online turbidity.

You can if the turbidity is high enough or the pipe is big enough.

To achieve a high level of performance the TurbSense^® uses a powerful light source and sensitive optics. This means that in a pipe, reflections from walls and bends can cause inaccurate readings. The higher the turbidity, the closer the reflective surfaces can be so that at >200NTU putting the TurbSense^® online turbidity meter in a pipe should be no problem.

For optimum performance fit an AutoClean unit to the sensor. This can be used to periodically clean the sensor window to ensure the highest accuracy in readings. Try to avoid placing the sensor in direct sunlight as the sensitive optics can pick up infra-red light emitted by the sun.

The flow chamber removes entrained bubbles to prevent them causing an interference to measurements. Its black polypropylene construction eliminates stray light and the lid removes ambient light, both potential sources of interference in measurements.

Turbidity is a measure of the cloudiness of water. How we measure turbidity is defined in international standard ISO 7027 and several US EPA standards. It is a measure of the amount of light that is scattered at 90 degrees. The more light that is scattered the higher the turbidity.

There are several units that are used for turbidity measurement and all of them are determined by the amount of light that is scattered through 90 degrees by a defined solution of formazin in water. The most common unit is NTU (Nephelometric Turbidity Unit) but there are also other units such as FTU (Formazin Turbidity Unit) and FNU (Formazin Nephelometric Unit). Typically NTU is used when the method is in compliance with a US EPA method such as 180.1 and FNU is used when the method of measurement is in compliance with the ISO standard 7027, but these are not fixed rules and the three units can be used interchangeably.

There is a hard upper limit for the measurement of turbidity which is 4000 NTU. This is the top of the definition of turbidity. Typically turbidities that people measure are much lower, for example drinking water will be less than one NTU and ofter less than 0.1 NTU. River water is typicall below 10 NTU when it is “clear” but can go up to 1000 NTU when it looks “muddy”.

1 NTU (Nephelometric Turbidity Unit) is a measure of the cloudiness of water that equates to a particular amount of Formazin dissolved in water. 1 NTU water looks clear to the human eye and might be found in a swimming pool or in drinking water.

When a turbidimeter (whatever the geometry between the source and the detector) is calibrated against Formazin it has the units FTU (Formazin Turbidity Unit). If the device complies with the US EPA mandated geometry and light source then the unit becomes NTU (Nephlometetric Turbidity Unit). If the device complies with the International standard ISO 7027 then the unit is FNU (Formazin Nepholmeteric Unit), although it is more commonly referred to as NTU. The relationship between FTU:NTU:FNU is 1:1:1.

Every TurbSense^® is factory calibrated and is supplied with a calibration sheet (Certificate of Calibration).

The TurbSense^® turbidimeter uses Modbus to communicate with the control electronics, that means that the sensor cable can be extended or shortened, but it cannot be replaced. This is because it is potted into the sensor to prevent water leaks.

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 MutoLux^® turbidity measurement system revolutionises turbidity measurement and has a solution for all these problems.

Did you know that…
…the TurbSense^® uses a patented measurement method (MutoLux®) 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

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 how easily it can be contaminated, so determining the ‘zero’ of a turbidity analyser is a big problem… but not for the TurbSense^® and its patented MutoLux^® measurement system!

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 varying the output light from the sensor the TurbSense^® varies 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. 

The MutoLux® technology is so effective that TurbSense® users are finding that they never need to calibrate the sensor due to the elimination of drift by MutoLux®.

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 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 electronics drift 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 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^®.

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. Alternatively, the flow cell can be operated up to 6 bar keeping dissolved gasses dissolved and removing the problem of the nucleated bubbles. These simple, yet highly effective features ensure the turbidity sensor is free of interference due to entrained and nucleated bubbles.

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 that 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.