A cup of coffee, please

Warning: high levels of nostalgia

There is a quite well-known urban myth regarding the ritual of drinking coffee. Some say that it's better to add milk right away if you want to cool the drink faster, others say it's better to wait a bit.
Almost two years ago I decided to clear all doubt concerning this argument. It was August 2009 when I got my first pair of the (well known for regular readers) digital thermometers, and this experiment was perfect to make them useful.

So I gathered all the required instruments: two identical digital thermometers, two identical ceramic cups (0.15 L each), two identical stainless steel spoons and a digital camera.

I cooked a large pot of coffee and poured equal amounts (0.1 L) of the boiling liquid into each cup wherein I have already put the probes of the digital thermometers. Then I immediately poured two spoons of refridgerated milk (kept at 8 °C) in the first cup and after approximately five minutes I did the same to the second cup. After every addidion of milk I stirred the mixture with the spoon for two full turns (clockwise).

An artist's impression; the original image is probably lost forever but the reproduction is pretty accurate.

Temperature of the room was 23.5 °C. The temperature changes were recorded in real time with a digital camera and the data was then processed for easier understanding. The thermometers were not calibrated, so there was a small difference between the initial temperatures. Also, the coffee was poured in the second cup with a small delay. I adjusted the data of the second cup accordingly.
The results:

The plot of the temperatures.

As you can see the temperature differences are minimal. The initial temperature drop after the addition of milk was about 5 °C for both cups. On average the coffee with the milk added right away was cooler for 2 °C before the addition of milk to the second cup and about 3 °C hotter immediately after. Because the thermodynamic phenomenon that a hotter body cools more quickly and the poor thermal conductivity of ceramic the first cup was actually cooler after seven minutes from the addition of the milk in the second cup. Unfortunately I was still a rookie scientist at the time and I did not have a control cup.
In conclusion it's pretty irrelevant when will you put the milk in because you probably won't even notice the difference. But it was still a fun experiment and all doubts about the myth are cleared.
Some possible extensions of the experiment: measuring the temperatures in plastic cups; measuring the temperatures when adding ice cubes into the drink immediately and after 5 minutes; measuring the temperatures with different mixing techniques; and so on.

Digital thermometer modification: halfway through

Expect loads of pictures!

After I got that idea about modifying a digital thermometer I couldn't stop thinking about it. So I decided to do it right away. First I had to disassemble the thermometer. The procedure is simple and easy, only tool needed is a small flat screwdriver to press some pins and the case opens. The insides can be seen in the picture below.

1 - two LR44 button batteries; 2 - LCD screen; 3 - case, upper half; 4 - plastic spacer between the LCD screen and the circuit board;
5 - circuit board; 6 - case, bottom half

Next I had to find the correct pair of connections that I could use to determine the temperature. After some testing I found out that I have to connect the + source to the connection marked on the picture below.

Once I soldered all the wires and a female 3.5 jack connector I reassembled everything and the physical work was done. Results:

Multimeter reading is in Volts.
After some experimenting I found out that when temperature increases, voltage drops.

As you can see on the multimeter, I can measure the voltage without any problems. The maximum voltage would be 1.50 V so there is no harm to the computer.

The problem occours when I actually plug the jack into the computer Line-In. Suddenly the voltage drops to about 5 mV and there is no way to increase it back to the previous normal level. Voltage measurement of the computer's Line-In input:

No explanation for this yet

If you have any suggestions what might be causing the voltage drop and how to fix it, please let me know.

Digital thermometers

I've been sick and didn't have much strength to think or write. But today I saw some of my chilli plants have sprouted (yeah, I also grow chillis) and I noticed that the temperature in the room was over 33 °C when there was sun shining.
And it got me thinking that every time that I want to do a temperature measurement, I have to look at the tiny LCD display of the digital thermometer, write down the temperature on a piece of paper, and then look at my watch and write down the time the temperature was taken. That's because I only have the cheapest thermometers available, those for $2 off ebay.

This is so cheap, you should buy hundreds!

So I decided to do something about it. I want to modify one of these thermometers so I can connect it to my computer and record temperatures automatically. I am not sure how I'm going to do it yet. The most simple option would be to just connect it to the Line-In audio jack and then record the raw voltage input. A more advanced option would of course be the USB connector, but this will most likely require some kind of a driver.