3. Water & Environmental / Turbidity

Why is my turbidity reading higher/lower than expected?

For an accurate measurement of turbidity a sample vial is placed in an nephelometer meter called a turbidimeter or turbidity meter. The meter emits a known intensity of light through the sample cell and the scattered light is measured at 90 degrees to the light source to give a turbidity reading in units NTU (nephelometric turbidity units) or FTU (formazin turbidity units).

There are a range of different meters available such as the portable hand held Hach 2100Q turbidimeter which makes it easy to make measurements on the go or the Hach Benchtop Turbidity TL23 series which, with it’s large colour touch display and intuitive user interface and guided procedures, will give you highly accurate results.

Turbidity meter
Hach 2100Q Portable Turbidimeter

Turbidity is a simple test but errors can be introduced which will lead to inaccurate results. Click more below to see a list of the most common issues when making turbidity measurements.

Are your sample vials clean? Clean vials are critical because the light source passes in and out of the sample just as the sample we are measuring. Any dirt, dust or finger prints on the vials you are using will alter the measurement results. Use a lint free cloth to remove any smudges and contaminants before placing the sample vial into your turbidity meter to prevent increasing your turbidity reading. Replace scratched or stained sample cells to avoid them causing falsely high results.

Oil your sample cells – silicone oil is used to mask minor imperfections which can affect your results. As this oil has the same refractive index as glass it will not affect your turbidity readings. Take care to only add a couple of drops of oil (too much will interfere with results) and wipe with a lint free cloth.

Turbidity standards – calibration is always vital to accuracy and must be done before each sample turbidity measurement. Be aware that all turbidity standards will have a shelf life and need to be replaced when required.

Vial condensation – It is common that condensation forms on the glass sample vials, especially when samples are cold. Prevent this from affecting your turbidity results by wiping down the sample cell before placing in the turbidity meter.

White light or Infrared? Turbidity meters either emit white light which comply to EPA 180.1 or Infrared which adhere to the ISO standard. The main differences of the two light sources are that Infrared meters have less sensitivity at low turbidity compared to a white source, but are ideal for coloured samples which a white light source will give inaccurate results for.

Need higher sensitivity at low turbidity? Hach have engineered a unique optical design that sees more of your sample than any other turbidimeter, delivering the best low level precision and sensitivity while minimising variability from test to test. The Hach TU5 Benchtop Turbidimeter measures turbidity by directing a laser into a sample to scatter off suspended particles. The light that is scattered at a 90° angle from the incident beam is reflected through a conical mirror in a 360° ring around the sample before it is captured by a detector.

For more information on turbidity measurements visit www.camlab.co.uk, or contact one our technical team by phoning 01954 233120 or emailing support@camlab.co.uk