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From Gifu! Hot Spring Museum Episode 21: Observe Hot Springs Yourself

Recently, hot spring magazines and other hot spring information have started to be published with accurate information by editors who understand hot springs. However, there is still a tendency for incorrect information to be published on websites and pamphlets.
If you cannot rely on information about "what kind of hot spring this is," why not observe it yourself?
Points to observe in hot spring water include the temperature, smell, texture, taste, color, and the sound of the water gushing out. These are things that can be observed using the five senses.

First is the temperature of the water. Dip your index finger into the bathtub and guess the temperature. Then, actually measure the temperature with a thermometer and continue learning "what degree this heat is." As you get used to observing, you will be able to roughly know the temperature just by dipping your finger into the bathtub, which is very convenient. However, it has been found that "the more concentrated the hot spring components, the cooler the water feels," so in the case of concentrated hot springs, a correction of +α°C to the actual finger sensation is necessary.
Next is the smell of the water. By smelling the water, you can determine whether it is a hydrogen sulfide smell, a metallic smell, or a petroleum smell, and understand the components such as hydrogen sulfide, iron, and oil. Furthermore, you can even tell the presence and concentration of chlorine disinfection. Chlorine disinfectants themselves do not have a strong smell, but when the water becomes dirty with sweat, the ammonia nitrogen in the sweat reacts with the chlorine to produce combined chlorine (chloramine). The smell of this combined chlorine is the cause of the stimulating smell remembered as the "smell of pool disinfection." When chlorine changes to combined chlorine, its bactericidal power decreases significantly, so by smelling the water, you can sometimes understand the state of the bactericidal effect.
In terms of the texture of the hot spring, you can distinguish sensations such as slipperiness, stickiness, and tingling. You can also roughly predict the components contained. The "slippery water" popular with women is often a hot spring containing baking soda as a component. After bathing for a few seconds, you will feel a slippery sensation on your skin. Even if there is a lot of baking soda, you may not feel the slipperiness depending on the balance with other components such as salt. When comparing the slipperiness with other hot springs, it is good to compare the sliminess at the same place on your body after some time has passed since bathing. Why not challenge the ranking of "slippery hot springs" that seem to exist but don't?
If the hot spring is drinkable, it is recommended to taste the fresh water that has just gushed out by rolling it on your tongue rather than drinking it. By doing so, you can use all the sensory parts of your tongue to observe bitterness, astringency, saltiness, sourness, etc. Each component has a unique taste, and by getting used to it, you can roughly guess the components and concentration.
The color of the hot spring is a clue to know the freshness and type of components of the water. Except for black-brown hot springs called moor springs, most colored hot springs are colorless and transparent when they first gush out, and over time, they come into contact with oxygen in the air and begin to show color. In the case of milky white hot springs, they are colorless and transparent when they first gush out, but over time, the hydrogen sulfide in the hot spring combines with oxygen to generate sulfur, causing Mie scattering due to sunlight, and turning white. There are also hot springs known to change from colorless and transparent to blue and then to cloudy white over a week. Therefore, in such hot springs, the color can tell you the freshness of the hot spring water.
What you observe with your eyes is not just the color. The presence, shape, and color of hot spring flowers in the water, and the type and amount of scale attached to the bathtub and bathroom floor are also important information to determine the quality of the spring.

After observing the hot spring in this way, it is recommended to take notes immediately afterward. It is reliable information obtained by actually observing it yourself. It is sure to become a valuable treasure.
At the Gero Hot Spring Museum in Gero Hot Spring, an original souvenir "Hot Spring Notebook" is sold for 500 yen. It is designed to easily record the hot springs you visit, and I think it is very useful for those who are touring hot springs.