Making a Secchi disc

Some years ago, I was researching a book on the way science changed in 1859, the year that Charles Darwin published On the Origin of Species.  In many ways, 1859 was an annus mirabilis, a year of miracles, and one of the threads I pursued was the matter of weather observation and weather forecasting.  This science had been gathering pace during the 1850s, and really came into its own around 1858–1860.

That was when I came across a well-known name, but in an unexpected context. First, I learned that the scientist I knew only as Secchi, who invented the Secchi disc, was Angelo Secchi, but more than that, he was Father Angelo Secchi.  He was a Jesuit priest who had been made director of the observatory at the Collegio Romano at the Vatican in 1849, after two years studying physics at Georgetown, Washington D.C.

By 1859, he had made the interesting observation that there were "atmospheric waves".  Think of the situation where a "high" moves.  It was this movement that Secchi said could be detected.  He added that these waves could travel from Rome to London in about 36 hours.

He is remembered today for suggesting the classification of stars by their spectra and for his Secchi disc, used to assess turbidity (if you like, you can say in water.  I have to confess that I knew him only for the Secchi disc.

Turbid water, muddy water, is a problem for living things.  Plants need light to grow, and all animals depend, directly or indirectly on plants for food.  The problem in the 19th century was to find a reliable way of comparing turbidity in water.

Creating a scale was too hard, but at least, with a Secchi disc, you can get a comparative measure of different turbidities.  One lot of water, you can still see the disc when it is three metres down, but in muddy water, the disc may disappear when it is only 10 or 20 cm below the surface.

The completed Secchi disc. home-made variety, Mark I and only.

The basic idea was to lower a standard object (the disc) into water until it just disappeared, note the depth at which the happened, using marks on the line with which it is being lowered.  Then as a control, the standard object is lowered further, and then hauled in until it is just visible.  Once again, the depth is measured, the two depths are averaged, and there is your turbidity measure.

There are a few catches there: you need to have shade over you to cut out dazzle that might make it hard to see the disc.  An umbrella is good for that: look to the right.

And in shallow water, you may need to use a mirror on a stick, like the illustration on the left-hand side.  Just improvise as you see fit!


People looking for science projects: think about comparing the animals and plants in turbid water and clear water. It won't be easy, but you will most certainly need a Secchi disc, and as they are hard to buy, it is easier to make one.

You need an old paint tin lid (or something similar), a drill, a ring-bolt, some string, and either a pen or some bits of cloth to mark the depths.

I made mine many years ago by drilling a hole in an old paint tin lid that I had painted white. I spray-painted two quarters of it black before drilling a hole in the middle before fitting a ring-bolt in place.

The red colour just happens to show
that the lid came from a 4-litre (or
1-gallon) paint tin. Only the black and
white pattern on the top matters.

While the official design (see the diagram on the left) has a dangling weight below the disc, I found that all I needed to do was put a couple of extra nuts on the base, and everything worked perfectly.  You can see those on the right.

Then I tied some reliable twine to it, with pieces of coloured rag tied into the line as reference markers, one at each metre from the surface of the disc. You can vary this design as you wish.