One of the most important tools you will ever need to grow Spirulina, regardless of whether you’re growing it with or without electricity, organically, or not organically, is the Secchi disk, or as we like to call it, the Spirumeter.
So What it The Spirumeter?
A secchi disk is a plain circular disk of about 30cm in diameter (12 inches), which is connetcted to a pole or a string from its center, and is used to measure density in bodies of water. In the case of Spirulina culture, the secchi disk allows us to observe the concentration of our Spirulina culture, indicating the right timing for adding culture medium and for harvesting.
How to Make Your Own DIY Secchi Disk in No Time:
Take a wooden skewer and stick it inside a round plastic lid, like the lid of a mayonaise jar. Using the skewer, pop a tiny hole in the center of the lid and leave the skewer inside. Next, mark the following measurements on the skewer, starting from the disk and going upwards: 2cm, 3cm, 5cm.
How Does it Work?
We use the Secchi disk on several occasions:
Before adding culture medium
Simply dip the Spirumeter vertically inside the water until you reach the 3cm mark. If the disk is still visible from above the tank, it means the concentration is too low and you should not add any culture medium. If the disk disappears at 3cm, and all you see is the deep blue-green color of Spirulina, then the concentration is high enough for diluting with culture medium. Remember that too much culture medium can cause sedimentation and harm your Spirulina, so its important to use the Secchi disk every time.
After adding culture medium
After adding your culture medium, recheck the concentration with the Spirumeter. Ideally it should reach the 5cm mark before becoming invisible again. If you still see the disk at 3-4cm after adding culture medium, it means you haven’t added enough, and your Spirulina will not have enough to feed on.
Before and after harvseting
Once your tank has reached its full capacity, it is advisable to let the concentration rise before harvesting for the first time. In this case, we use the 2cm mark, and only harvest when it becomes invisible. Again, following the harvest, we would replace with appropriate nutrition and then recheck with the Spirumeter to see that our concentration has lowered to 5cm. From this point onwards, you can harvest every time Spirumeter reaches 2-3cm, and replace with appropriate feeding as described in our eBook.