There are no clocks in the tea room, the kama is one of your timing devices. Depending on how you lay the charcoal — farther apart, the air gets in and the fire burns hotter faster — closer together, there is less circulation, and the longer it takes the kettle to come to a boil. By experience, you want to adjust your timing so the water in the kettle is at just the right temperature when you are making tea.
In the tea literature, there are different stages of boiling water, including poetic descriptions of the size and frequency of the bubbles as the water comes to a boil. When you are boiling water in an iron kettle with a lid on it, you cannot see the bubbles as the water comes to a boil.
In a lecture at Midorikai, Tsutsui-sensei gave us a description of the sounds of boiling water.
At the beginning, when you put the kama on the fire, the water is cold and kettle is wet. At this stage it is silent. You may hear slight hissing as the water is evaporating from the surface of the kama, but then the kama is silent.
As the kama heats up, you will hear the beginnings of occasional sounds (the equivalent of crab eyes bubbles around the rim).
The next stage, he called to nami, or the sound of distant waves. The kettle is heating up and you can hear something coming from the kama like the sound of distant waves.
Next, gyo gan, (the equivalent of fish eyes – larger bubbles coming from the center of the kama) the sounds are getting louder and closer together, more regular.
At the next stage, kyu in mimizu, he called worms crying. Sometimes you can hear a squeaking from the kama which is what I think this means. He likened this stage to the string of pearls bubbles coming from the bottom of the kama to the top.
The next stage is what we are familiar with, matsu kaze or the wind in the pine trees.
And then raime or thunder rolling, the kettle is at full boil, and the steam is escaping from the lid.
Koicha is made perfectly at 80º C (matsu kaze), and usucha at about 70º C. If the kettle is boiling, you can always add water from the mizusashi to adjust the temperature, but if the water is not hot enough, you cannot turn up the charcoal to make the water hotter. So please pay attention to the sound of the water in the kama as it boils.