A turbidity value is a way of quantifying the cloudiness, or haziness of a sample. It is calculated, using an instrument called a turbidimeter, by measuring the amount of light scattered or blocked by suspended particles in a water sample. We often think of turbidity in terms of things like microorganisms, silt or other materials - but turbidity can literally be anything present in the sample that causes the light beam to scatter. Turbidimeters are available as portable, benchtop or online instruments.
The Origin of the Turbidimeter
Humans have long used turbidity as a direct indicator of overall water quality. Long before modern instrumentation was available, people inherently understood that the clearer the water was (low turbidity), the safer it was to consume. The first standardized turbidity measurements were conducted using visual systems – such as the Jackson Candle Method. While rudimentary by today’s standards, these systems did provide a way to quantify the clarity of a water sample. While visual systems are still used today, they are mostly associated with environmental applications.
How Turbidimeters Work
While there are multiple turbidimetric measuring principles, all turbidimeters have three main components:
- An incident light beam
- A sample to analyze
- A detector
When the incident light beam is projected into the sample, light it will be scattered in all directions by the particles present in that sample. The detector is the mechanism that calculates exactly how much light is scattered by the sample and coverts it to a useful NTU/FNU value for the user.
Turbidimeters which use a 90° angle are known as nephelometers. While other geometries, such as 180° or forward scatter, are used in some industries, most “general purpose” turbidimeters – and those used for regulatory reporting purposes in drinking water – use turbidimeters with a 90° detection angle.
When particles are hit by the incident light beam, depending on the nature of that particle, the light is going to scatter differently. For example, particles larger than the wavelength of the incident wavelength will scatter predominantly in the forward direction. Real-world sample matrixes will be composed of particles that are a mix in sample sizes and shapes. The 90° angle is used, because it is the angle that is most sensitive to the widest variety of particle sizes and shapes.
Which Turbidimeter is Right for Your Application?
On the surface, turbidity analysis is a simple measurement. However, there are many factors that can affect the accuracy of readings or the dependability of an instrument.
When considering a turbidimeter, it’s important to balance technical requirements and feature options offered by different manufactures. Here are some things to think about as you narrow down your options.
1. Determine your general needs:
- Compliance – do my results need to in accordance with the guidelines set by a governing body for reporting purposes?
- Primary Use Model – where will you predominately use this instrument?
• Portable / Field use
• Benchtop / Laborator
• Process / Online
2. Determine your Technical Needs
- Measurement Specifications – think about things like:
• Total measurement range needed vs. primary measuring range
• Limit of detection – especially important for low level turbidity samples
• Displayed resolution - Reliability
• Accuracy
• Repeatability
3. Once you’ve thought through all of the “must haves” from a use and technical standpoint – be sure to evaluate all of the features available to you – and how they can enhance your workflow and usability of the instrument.