# Tag Archives: heat transfer

## Mythbusting: Cooling a Drink with a Wet Paper Towel

While reading one of the many pages claiming to have "15 Amazing Life Hacks" or something similar, I found a claim about quickly cooling a drink that deserved some investigation.  The post claimed that to quickly cool your favorite drink you should wrap the bottle/can in a wet paper towel and put it in the freezer.  Supposedly this would quickly cool the drink, faster than just the freezer.  My guess is that the thought process says evaporative cooling is the culprit.  This is why we sweat, evaporating water does indeed cool the surface.  Would water evaporate into the cold, but dry freezer air? Below we'll look at a couple of experiments and decide if this idea works!

We will attack this problem with two approaches.  First I'll use two identical pint glasses filled with water and some temperature sensors, then we'll actually put glass bottles in and measure just the end result.  While the myth concerns bottles, I want to be able to monitor the temperature during the cooling cycle without opening the bottles.  For that we'll use the pint glasses.

First I had to build the temperature sensors.  The sensors are thermistors from DigiKey since they are cheap and relatively accurate as well.  To make them fluid safe, I attached some three-lead wire and encapsulated the connections with hot-glue.  The entire assembly was then sealed up with heat-shrink tubing.  I modified code from an Adafruit tutorial on thermistors and calibrated the setup.

To make sure that both sensors had a similar response time, we need to do a simple control test.  I placed both probes in a mug of hot water right beside each other.  We would expect to see the same cooling at points so close together, so any offset between the two should be constant.  We also expect the cooling to follow a logarithmic pattern.  This is because the rate of heat transfer is proportion to the temperature difference between the water and the environment (totally ignoring the mug and any radiative/convective transfer).  So when the water is much hotter than the air, it will cool quickly, but when it's only slightly hotter than the air it will take much longer to cool the same amount.

Plotting the data, we see exactly the expected result.  Both sensors quickly rise to the water temperature, then the water cools over a couple of hours.  The noisy segments of data about 0.25 hrs, 0.75 hrs, and 1.75 hrs in are likely interference from the building air conditioning system.

If we plot the temperature difference between the sensors it should be constant since they are sensing the same thing.  These probes look to be about dead on after calibration.  Other than the noisy segment of data, they are always within 0.5 degrees of each other.  Now we can move on to the freezer test.

I used two identical pint glasses and made thermocouple supports with cardboard.  One glass was wrapped in tap water damped paper towel, the other left as a control.  Both were inserted into the freezer at the same time and the temperature monitored.  The water was initially the same temperature, but the readings quickly diverged.  The noisy data segments reappear at fixed intervals suggesting that the freezer was turning on and off.  The temperature difference between the sensors grew very quickly, meaning that the wrapped glass was cooling more slowly than the unwrapped glass.  This is the opposite of the myth!

Next I placed two identical, room temperature bottles of soda in the freezer, again with one wrapped and one as a control.  After 30 minutes in the freezer, the results showed that the bottles and their contents were practically identical in temperature.  The wrapped bottle was slightly warmer, but it was within the resolution of the instruments (thermocouple and IR sensor).  I did this test multiple times and always got temperatures within 1 degree of each other, but not consistently favoring one bottle.

So what's happening here? Well, I think that the damp paper towel is actually acting as a jacket for the beverage.  Much like covering yourself when it's cold outside, the damp paper towel must be cooled, then the beverage can cool.  Adding that extra thermal mass and extra layer for the heat to diffuse through.  To provide another test of that hypothesis I again tested bottles with a control and a foam drink cooler around the base.  The foam cooler did indeed slow the cooling, the bottle being several degrees warmer than the control.

The last question is why did the test with the glasses show such a pronounced difference, but the bottle test show no difference? My best guess is that the pint glass was totally wrapped vertically and that bottle had the neck exposed still.  Another difference could be the thickness of the towel layer and the water content of the towels.

The Conclusion: BUSTED! Depending on how you wrap the paper towel it will either have no effect or slow down the cooling of your favorite drink.

Let me know any other myths I should test! You can also keep up to date with projects and future posts by following me on twitter (@geo_leeman).

Arduino Code:
``` // which analog pin to connect #define THERMISTOR1PIN A0 #define THERMISTOR2PIN A1 // resistance at 25 degrees C #define THERMISTORNOMINAL 10000 // temp. for nominal resistance (almost always 25 C) #define TEMPERATURENOMINAL 25 // how many samples to take and average, more takes longer // but is more 'smooth' #define NUMSAMPLES 15 // The beta coefficient of the thermistor (usually 3000-4000) #define BCOEFFICIENT 3950 // the value of the 'other' resistor #define SERIESRESISTOR1 9760 #define SERIESRESISTOR2 9790```

int samples1[NUMSAMPLES];
int samples2[NUMSAMPLES];

void setup(void) {
Serial.begin(9600);
analogReference(EXTERNAL);
}

void loop(void) {
uint8_t i;
float average1;
float average2;

// take N samples in a row, with a slight delay
for (i=0; i< NUMSAMPLES; i++) {
delay(10);
}

// average all the samples out
average1 = 0;
average2 = 0;
for (i=0; i< NUMSAMPLES; i++) {
average1 += samples1[i];
average2 += samples2[i];
}
average1 /= NUMSAMPLES;
average2 /= NUMSAMPLES;

//Serial.println(average);

// convert the value to resistance
average1 = 1023 / average1 - 1;
average1 = SERIESRESISTOR1 / average1;

average2 = 1023 / average2 - 1;
average2 = SERIESRESISTOR2 / average2;
//Serial.print("Thermistor resistance ");
Serial.print(average1);
Serial.print(',');
Serial.print(average2);
Serial.print(',');

float steinhart;
steinhart = average1 / THERMISTORNOMINAL; // (R/Ro)
steinhart = log(steinhart); // ln(R/Ro)
steinhart /= BCOEFFICIENT; // 1/B * ln(R/Ro)
steinhart += 1.0 / (TEMPERATURENOMINAL + 273.15); // + (1/To)
steinhart = 1.0 / steinhart; // Invert
steinhart -= 273.15; // convert to C

Serial.print(steinhart);
Serial.print(',');

steinhart = average2 / THERMISTORNOMINAL; // (R/Ro)
steinhart = log(steinhart); // ln(R/Ro)
steinhart /= BCOEFFICIENT; // 1/B * ln(R/Ro)
steinhart += 1.0 / (TEMPERATURENOMINAL + 273.15); // + (1/To)
steinhart = 1.0 / steinhart; // Invert
steinhart -= 273.15; // convert to C(steinhart);

Serial.println(steinhart);
//Serial.println(" *C");

delay(1000);
}

## Liquid Cooled Laptop Stand

This is going to be a short post that was requested during the LifeHacker "How I Work" feature.  In the post (here) I had mentioned my custom laptop stand that has an automotive transmission cooler and there was some interest in its construction.  Since moving I haven't hooked everything (fluid and such) up, so I did make any thermal profiles of the stand, but maybe at some point I'll attach some thermocouples and so just that.  Regardless, here are a few photos and some construction notes.

First off I should state the purpose and design requirements of the stand.  I wanted a stand to that the laptop monitor would line up nicely with my second monitor and wasn't made of books.  At the time I was running lots of rather intensive thermal models and gridding some large data sets, so that my laptop would be running very hot with the fans full blast for anywhere from 5-20 hours straight.  To keep it running a bit cooler I decided to build the stand of something thermally conductive, Aluminium was a good choice since that's what the laptop case is made of and it looks nice.  It's also not bad at conducting heat!
 The stand was designed to hold the laptop screen at the same level as my second monitor and give a nice angle of viewing.

I bought some Al sheet a Lowe's, as well as a small strap of metal, and some "L" shaped material.  The channel makes the supports for the sheet and the runners on the desk.  I left them long incase I decide to mount the fluid tank and pump back there.  So far I haven't found a setup that is quiet and that fits in the space.  I will try again soon, but I've played with pumps and small aquarium tanks in the past.

Using a sheet metal shear and brake I cut and bent the top plate to hold my laptop.  Be sure that the rubber feet on the bottom of the computer are off the stand, we want metal-metal contact for the best heat transfer!
So there were no screw heads to scratch my laptop, I used adhesive to mount the top plate to the frame.  The frame was assembled with nuts and bolts, then set on plastic feet to prevent scratches to the glass desktop.
Next I made the stand match the computer a bit better by giving it a brushed Al finish instead of shiny metal.  A wire polishing wheel attached to the drill gave a nice, but time consuming finish to the entire stand.
To further the cooling I wanted to mount a heat-sink to the bottom of the stand.  It so happened that I found a great solution at the automotive store that would allow for liquid cooling! A small generic automotive transmission cooler add-on kit (about \$25 at the time) provides lots of surface area and a nice look.  The cooler is mounted with JB-weld and seems to get nice and warm when I'm working the laptop.  I'll probably inject some thermal grease to increase the coupling even more.

 The transmission cooler on the bottom of the stand.

 The surface where the computer sits.
This was a really fun little afternoon project and its not done yet! Eventually I'll run onto a tank/pump combo that I like and will fit onto the stand.  I'll mount it and use some colored water to give a nice effect when I'm cooling.  The easiest control mechanism is a small temperature sensor that turns the pump on and off as necessary to maintain a set-point.  When that happens, I'll be sure to post and update.
To William (the commenter that requested some details of my stand): Sorry this took so long! The LifeHacker article went live not long before I took my candidacy exam!
As always feel free to comment/email questions!