How to measure turbidity
Before we get into the many ways of measuring turbidity, let’s understand the main idea behind the methods.
We know that turbidity refers to the cloudiness of a liquid caused by suspended particles in it. These particles scatter light, which makes the liquid lose transparency. The more light scattered, the higher the turbidity value and the cloudier the liquid.
So it’s clear (pun intended) that turbidity directly relates to this light scattering. Thus, every turbidity meter works on that principle.
Now that we know how the measuring methods work, let’s dig into them, from simple to complex.
The first people to measure turbidity didn’t have computers or much technology at all. They all relied on the same sensing element: the human eye.
Angelo Secchi invented the first method of measuring turbidity in 1865, using a disk attached to a rope. To measure the turbidity of a body of water, you lower the disk into the water until you can’t see it. As soon as the disk disappears, you pull up the rope and read the depth, also known as Secchi depth, from the rope. It’s amazing that people still use this method today, but it works.
This method also uses the human eye as a sensing element. It consists of a special candle and a glass vessel with a scale in Jackson turbidity units (JTU).
The idea here resembles the Secchi depth method. However, instead of lowering a disk in the water, you fill the vessel with the liquid until you can’t see the candle flame. When you get to this point, you can read the turbidity value from the vessel scale.
Similar to the Jackson candle, the turbidity tube has a visual reference and a tube with a turbidity scale. But instead of a lit candle, a turbidity tube has a yellow or white disk at the bottom with a black cross or circle on it.
Again, you pour the liquid into the tube until you can no longer see the cross/circle. You then read the turbidity from the scale on the tube. The scale here is usually a Nephelometric Turbidity Unit (NTU) scale.
Now we’ll go over the electronic methods. These have two things in common, a light source and a light detector.
The U.S. Environmental Protection Agency (EPA) Method 180.1 uses NTUs. And the European ISO 7027 uses the Formazin Nephelometric Unit (FNU) standard. Both use nephelometric devices to measure a liquid’s turbidity. These methods differ mainly in light source. Method 180.1 uses tungsten lamps with color temperatures between 2000 and 3000 Kelvin (K). ISO 1027 uses a light source within the range of 830-890 nanometers, which means near-infrared light.
This method, widely used in industries such as water and wastewater, has its detector set at a 90-degree angle from the light source. Why 90 degrees? Because it’s the angle considered the most sensitive to light scattered by suspended particles, regardless of size.
To measure a sample’s turbidity, the light source sends a beam through the sample. When this beam hits particles, the detector senses the scattered light. The more light the detector senses, the higher the turbidity. But these measurements are accurate only in the range between 0 and 40 NTUs. Within this range, turbidity and light scatter have a linear relationship, which disappears above 40 NTUs.
Total solids sensors
In applications with higher turbidity, measuring scattered light at a 90-degree angle won’t do. More turbid solutions contain so many suspended particles that the light beam becomes backscattered. But we can detect differences in turbidity better in the backscattered light!
Suspended solids or total solids sensors use this method. Instead of having one light detector at 90 degrees to the incident light, we have another light detector at a 135-degree angle.
Last but not least, we have the absorption sensors. These sensors use the attenuation method. This method also uses a light source and a detector. However, instead of measuring scattered light from the suspended particles, the sensor detects how much the particles attenuate the emitted light.
In an absorption sensor, the light emitter and detector sit directly opposite each other, with the liquid between them. Depending on the turbidity of the medium, the particles will absorb more or less of the light emitted. The sensor reads this absorption and calculates the turbidity with it.
You need the right turbidity sensor to get accurate readings. Before researching the market, make sure you know the range of turbidity values in your application and the precision you need for the measurements.
Knowing the needs of your process before going shopping can save you both money and time.