For those of you who have determined the precise temperature at which your favorite tea gives off your favorite aromas and flavors, but want to get to the perfect temperature more quickly than allowing the water to cool off the boil naturally, I’ve got a refresher course for you from high school chemistry. Now, don’t run–it’s pretty easy. Well, it’s easy mainly because we’re talking about adding water to water (so there’s no worrying about different thermal properties of two different liquids). We’ll also ignore for the moment the effects of surface area on cooling and the heat transfer coefficients of different materials you might hold your water in.
See? We’re ignoring all the confusing stuff and making it simple.
Let’s say you’ve got some amount of boiling water (212F) and another amount of room-temperature water (68F) and you’re going to add the cold water to the hot water to make this perfect tea-brewing temperature. Now, 194 isn’t much below 212, so we probably don’t need to add too much of the colder water. We can use an equation to figure out the resulting temperature based on the amounts of each of the two waters we combine.
Here’s the equation. Don’t be scared.
Tx= Vc*Tc + Vh*Th
Where T=temperature, and V=volume of water, expressed as a decimal percentage of the combined total. The “x” refers to the combined water, and “c” is “cold” while “h” is “hot.”
For the moment, let’s pretend we’ve got a teapot or cup or gaiwan or whatever that holds 8 fluid ounces. And to make the math easy, we’re going to measure in 1-ounce increments. Since we only need a little cool water, let’s do 1 ounce cold, and 7 ounces hot. What temperature would that make?
Let’s plug in our numbers, remembering that “V” is the percentage of the total. One ounce is 12.5% of 8 ounces, and 7 is 87.5%. Converted into decimals, that’s .125 of cold, and .875 of hot:
Tx = .125*68F + .875*212F
So doing the multiplication first: Tx = 8.5F + 185.5F
Or Tx = 194F
Ta da! If you were to add 1 fluid ounce of room temperature (68F) water to 7 fluid ounces of boiling water (212F), they’d mix to make a lovely 194F. By the way, for those that are more scientifically minded or live outside of the US, all this works in Celsius as well. Tx = .125*20C + .875*100C = 2.5C + 87.5C = 90C. And 90C=194F
Once you’ve used this equation to figure the right proportions, you can make the right temperature all the time without resorting to the use of lots of thermometers or devices that heat water and shut off at preset temperatures or whatever. If 194F is your ideal, you just need to remember that if your cold water is really at 68F, it’s a precise ratio of 1 part cold, 7 parts boiling.
Clearly, you can also adjust based on whatever the situation is. If you keep bottled water in the fridge and the fridge stays at a more constant temperature than “room temp” you can use that number for your cold and find your new perfect ratio.
If you want to get more complicated, recognize that different materials absorb heat at different rates. However, the closer the water vessel used to combine your two temperatures is to the final temperature, the less the effect will be. So your calculations will be more accurate by pre-warming your vessels. Or if you’re geeky about temperatures even beyond what I’ve given you here, you can research the thermal properties of whatever material your teaware is made of.
Of course, I usually use boiling water for everything, and drastically lower brewing time if it’s a “low temperature” tea. But that’s just my personal method. Do tea the way it tastes best to you, and experiment with other ways to see if you can improve on what you already like.