Turbidity tube (50 cm)

This parameter is part of the newly defined UK National Protocol as part of CaSTCo. This description is preliminary and will be updated shortly.

The turbidity tube (sometimes known as a Secchi tube) measures the optical clarity of water from between 240 and 12 ‘turbidity units’.

To make a measurement first make sure you are standing with your back to the sun (so that the tube is in your shadow) and remove any sunglasses. Fill the tube slowly from your sampling container, checking to see when the disc at the bottom of the tube disappears.

As soon as you can no longer see the disc, stop and read the number from the scale on the side that is closest to the water level in the tube. If the water level is exactly half way between two numbers then record the smaller number.

If the disc disappears before the first line (‘240’) is reached record ‘>240’ – this is a highly turbid sample and will be visibly very murky. If you can fill the tube to the top and still see the disc then record ‘<12’ or ‘less than 12’.

When you have completed the measurement, discard the sample and shake out any excess water from the tube. If turbidity levels were high then it is a good idea to rinse out the tube with tap water when you return home.