# Wet towel trick - are we just fooling ourselves?

### Help Support Homebrew Talk:

#### Grinder12000

##### Well-Known Member
We all know that putting a wet towel or t-shirt around your carboy and a fan will lower the temp of the fermentation . . . or does it??

The typical measuring instrument is a plastic thermometer on the outside of the carboy.

Of course it will be cooler. The glass is cooler, and the thermometer is right next to the towel.

What is the temp 2 inches away from the glass in the carboy.

This has probably been discussed a LOT but I could not find an answer for a study on the inside temps.

Anyone?

#### polarbearbrewing

##### Well-Known Member
as long as the outside of the carboy stays cool it will eventually distribute to the inside...just watch how much that beer moves around while its fermenting

#### MayDayBrewing

##### New Member
I don't know the scientific term for it, but there is a phenomenon where evaporating water cools its surroundings. For example, you can spray a fine mist of water into the air on a hot day in your yard and, even without the water hitting you, it will cool down the yard. There are some neat "air conditioner" designs that use some type of water evaporation near a fan (usually a towel or something that passes through water).

#### Philip1993

##### Well-Known Member
Simple answer: It takes heat to turn water into vapor.

#### Stevorino

##### Well-Known Member
It does cool down the carboy-- but it's not a suitable means of controller fermentation temperature if you are trying to make the best beer possible.

OP

#### Grinder12000

##### Well-Known Member
I understand all of that but perhaps a 5 degree drop on the outside of a carboy where the temp is taken is less then 1 degree two inches into the carboy?

I know in THEORY it looks good but does a 5 degree drop = 5 degrees in the middle?

The reason I ask is I am fermenting as I type - will a 3 degree drop on the OUTSIDE of a carboy make that big of a deal to the 99% of the rest of the beer.

I know I'm sounding anal but all I have heard or read is theory and I'm seeing holes in it.

#### Stevorino

##### Well-Known Member
I understand all of that but perhaps a 5 degree drop on the outside of a carboy where the temp is taken is less then 1 degree two inches into the carboy?

I know in THEORY it looks good but does a 5 degree drop = 5 degrees in the middle?
On the Brewing Network, Jamil talked about an experiment that said that the wort closest to the glass in a fermenter was within 1 degree from the wort in the middle of the fermenter.

For this reason, he justified taping a probe to the outside of the glass fermenter (but protected from fridge fan).

For the record-- this experiment wasn't something he did-- it was a published experiment given credibility by commercial & big-time homebrewers.

#### BrianP

##### Well-Known Member
It's call evaporative cooling. That's why sweating cools you off.

#### CGengo

##### Well-Known Member
There are some neat "air conditioner" designs that use some type of water evaporation near a fan (usually a towel or something that passes through water).
That's called a swamp cooler and they work well in drier climates - we use to have one on our house when we lived in CA. If I remember correctly, you have to leave your windows open so that the moisture will exit the building. Anyway, I did this on a couple batches of beer before buying a Johnson control and it worked well in keeping the fermenter cool although I would imagine it works considerably better on glass carboys than plastic fermentation buckets due to glasses better conductivity.

http://en.wikipedia.org/wiki/Evaporative_cooling#Evaporative_cooling

#### BierMuncher

##### ...My Junk is Ugly...
...I know I'm sounding anal but all I have heard or read is theory and I'm seeing holes in it.
Good question. Here's my finding on a 10 gallon batch of my SMaSHed Nugget Pale Ale...one glass carboy in a water bath with a t-shirt and fan, one plastic bucket standing dry.

I dropped a probe into the middle of both beer 24 hours in and the carboy was chugging along at 66 degrees. The plastic fermenter was fermenting at 75 degrees.

A 9 degree differential between a simple water bath and a dry fermenter.

Done.

#### olllllo

##### []-O-[]
The dew point has to be below 55°F for adiabatic cooling to be effective (ie. DRY).

#### sonetlumiere85

##### Well-Known Member
It does cool down the carboy-- but it's not a suitable means of controller fermentation temperature if you are trying to make the best beer possible.
Unfortunately not all of us have money or room for extra refrigeration. If it weren't for living in a tiny apartment I'd definitely have a better system.

OP

#### Grinder12000

##### Well-Known Member
Thanks - I always like to hear hard facts behind solid theory's.

Like baseball there are a lot of theory's that sound great but in real life are wrong. I just wanted to hear that this had actually been looked into for the real facts.

#### Schlenkerla

##### Supporting Member
HBT Supporter
Good question. Here's my finding on a 10 gallon batch of my SMaSHed Nugget Pale Ale...one glass carboy in a water bath with a t-shirt and fan, one plastic bucket standing dry.

I dropped a probe into the middle of both beer 24 hours in and the carboy was chugging along at 66 degrees. The plastic fermenter was fermenting at 75 degrees.

A 9 degree differential between a simple water bath and a dry fermenter.

Done.
I did the same thing a few years ago. I got the same results as Biermuncher's. I try to make beers that can withstand the higher temps during the summer and cooler in the winter so I don't have to resort to this method. Occasionally I'll use the brew belt to heat.

My basement is ~66-70F in the summer & 58-62F in the winter. Spring & Fall is between 62-66F.

#### Coastarine

##### We get it, you hate BMC.
I've been experimenting with my carboy wet t-shirt contest (that what I call it when talking to SWMBO) Here's what I've tried:

PROBLEM: shirt isn't staying damp all the way to the top. I felt I'd get better cooling if it did.

ok...so I came up with a theory. If the wicking can't keep up with the evaporation, and the air temp is warmer than the carboy temp, then having the ceiling fan set on high is just going to be blowing 77* air over a 70* carboy and then I'm warming it up with convection. I took a few days to try the fan on high, medium, and low, and saw no difference in temperature.

SOLUTION: at first I just poured water on it whenever I walked by but I wanted a better solution. I bout a 6\$ "sweat wicking" t-shirt from wal-mart (black to block light). It does seem to wick the water up better but no discernible temperature difference.
SOLUTION II: I have a camelbak bladder and I put a little splinter of a toothpick in the opening so it would drip like an IV. set up to drip on the t-shirt. No discernible temperature difference.

CONCLUSION: Just set the f***ing thing up and stop thinking about it.

#### Schlenkerla

##### Supporting Member
HBT Supporter
BTW - I used thick fluffy towels. I don't know about the temp difference but they stayed wet.

#### beerthirty

##### big beers turn my gears
I don't know the scientific term for it, but there is a phenomenon where evaporating water cools its surroundings. For example, you can spray a fine mist of water into the air on a hot day in your yard and, even without the water hitting you, it will cool down the yard.
swamp coolers AKA evaporative coolers work well on the inside of a building with proper ventilation to vent the spent cooling moisture. Outside they are called cooling towers. Patriot park in Phx comes to mind. They have towers open at top and bottom. Water is misted inside the tower and reverse convection draws the cooled air down and out the bottom openings where the people are. They can drop the ambient temp 15* within a 50 foot radius of the tower. Does it work? Well would you rather be standing in 120* dry, dead air or 105*slightly moist and moving air? you make the call.

OP

#### Grinder12000

##### Well-Known Member
I use a thick fluffy towel - after 10 hours it is still dampish and I rewet it. Takes the temp down 4-5 degrees.

#### Stevorino

##### Well-Known Member
Unfortunately not all of us have money or room for extra refrigeration. If it weren't for living in a tiny apartment I'd definitely have a better system.
I hear ya man-- Homebrewing hasn't been easy for me in an apartment either. Just look forward to the day when things change

#### david_42

##### Well-Known Member

Yes, members have done the test and the temperature inside the carboy is very close to the outside wall temperature.

An active ferment keeps the wort mixed and the temperature consistant.

##### Well-Known Member
The dew point has to be below 55°F for adiabatic cooling to be effective (ie. DRY).
I think a lot of people are missing the fact that it doesn't work well in high humidity.

#### Laughing_Gnome_Invisible

##### Well-Known Member
Hey, someone send this question to Mythbusters! I would love to see them tackle it! They might even come up with a new idea for us!

#### niquejim

##### Burrowing Owl Brewery
The dew point has to be below 55°F for adiabatic cooling to be effective (ie. DRY).
The dew point drops below 55 maybe a dozen days all year down here, so it doesn't work too good. A deep waterbath does work well though.

#### olllllo

##### []-O-[]
By way of example, in PHX, when you get out of a 95 degree pool and step out into the 110°F air (<5% humidity- I'll let some weather genius figure out the dewpoint), the water dries off of you so fast that it feels like you are freezing.

#### Austin_

##### Well-Known Member
The dew point drops below 55 maybe a dozen days all year down here, so it doesn't work too good. A deep waterbath does work well though.
I agree with you there. The wet towel set-up doesn't work for people in climates like you and me. I tired it once and it lowered the temperature maybe 1-3 degrees, and when it's 80 degrees in my apartment that doesn't do much. I saw other peoples coolers with deep water and ice jugs and decided to try that. With one frozen gatorade bottle every 12 hours or so I can keep the deep water right at 66 or so (I could even lager if I wanted to. I got the water down in the mid 40's last week right before I bottled). While I have not measured the internal temperature of the carboy, I am pretty certain it is the same as the water around it.

#### Beerrific

##### Well-Known Member
So the question: if the temperature on the outside is 68°F, what is the temp in the center?

Well, you could get a probe and measure it, or you could look up the appropriate heat transfer and thermal conductivity constants, and density for glass and wort. Then all you have to do is solve the heat equation:

With a few assumption about the geometry and symmetry, I think this would end up being a 1-D problem. You might even be able to solve it analytically, if you are into those things.

#### AZ_IPA

##### PKU
By way of example, in PHX, when you get out of a 95 degree pool and step out into the 110°F air (<5% humidity- I'll let some weather genius figure out the dewpoint), the water dries off of you so fast that it feels like you are freezing.
and the opposite is true during the monsoons. get out of 95 degree pool and step into 110 air with a dew point of 60 and you actually feel like your in a sauna.

#### Cugel

##### Well-Known Member
It's call evaporative cooling. That's why sweating cools you off.
<pedant mode on>
Note that evaporation does not actually produce any cooling i.e. a temperature decrease. Evaporation occurs when water (on your skin in this case) absorbs the latent heat of evaporation (from sunlight) and whaddya know, evaporates.

This energy would otherwise heat your skin. Evaporation prevents your skin from heating, but does not produce a temperature decrease.

Water vapor transforming to liquid water during precipitation events releases the energy it absorbed while being evaporated and this provides a heat source for clouds.

When a cool water mist is sprayed onto you, your body welcomes this on a warm summers day as it cools you down, but typically not through evaporation - rather the mist is at a cooler temperature than your skin.

</pedant>

#### Cugel

##### Well-Known Member
So the question: if the temperature on the outside is 68°F, what is the temp in the center?

Well, you could get a probe and measure it, or you could look up the appropriate heat transfer and thermal conductivity constants, and density for glass and wort. Then all you have to do is solve the heat equation:

With a few assumption about the geometry and symmetry, I think this would end up being a 1-D problem. You might even be able to solve it analytically, if you are into those things.
That equation does not hold as heat is being generated through fermentation. Add a "-q" to the LHS of your equation to reflect the diabatic heating.

#### zoebisch01

##### Well-Known Member
I know in THEORY it looks good but does a 5 degree drop = 5 degrees in the middle?
A fermometer is reading the outer edge of the profile, so in essence if you were to get a thermal image of the actively fermenting bucket it would most likely be focused around the yeast activity and drop off steeply from there. It is greatly dependent on a bunch of factors though. But like david mentions, it is really splitting hairs. Plastic is a fair insulator so I never worry about it.

I have one lid that I drilled a small hole into through which I can insert a long stemmed thermometer. What I found is that during the most active phase of fermentation, you only see a very small differential. But that happens quickly and you pretty much come back towards equilibrium with the room temperature fairly fast. Where I think a lot of the concern stems from is rooted in commercial applications. Some of the large breweries in Belgium are working with such massive quantities of specialized yeast, the exothermic reaction is enough to drive the temperature of the fermenting wort into the high 80's even with the ambient being in the mid 60's.

#### olllllo

##### []-O-[]

http://permaculturetokyo.blogspot.com/2006/11/passive-cooling.html

Its called a zeer pot, or the pot-in-pot and was developed by Mohammed Bah Abba, who realized that he could put the second law of thermodynamics and transpiration to work for him. The zeer pot, is actually two earthenware pots ...

The smaller pot is put inside the bigger pot, and the space in-between them is filled with sand. The sand is made wet with water (twice a day) and a wet towel is put on top of the two pots to keep warm air from entering the interior. As water in the sand evaporates through the surface of the outer pot, it carries heat, drawing it away from the inner core, thus cooling the inside of the inner pot which can be filled with soft-drinks, water, fresh fruit, vegetables or even meat. A damp cloth placed on top keeps the inside pot away from hot air. In this way, fresh produce can be kept for long periods of time without the need for electricity, or camping coolers made high embodied energy.

Tomatoes and peppers will last for up to three weeks, and African spinach, or rocket, which normally would spoil after just a day in the intense African heat, can and will remain edible for up to twelve days. Eggplants will keep for up to 27 days instead of three. It can even be used for storing sorghum and millets for a long time since it protects from humidity, thus preventing fungi from developing.

The zeer will keep water (and other liquid beverages) at about 15 degrees Celsius [59° F] (maybe acceptable for Guinness), and even meat can be kept fresh for long periods.

#### GilaMinumBeer

##### Half-fast Prattlarian
Never worked too well for me.

#### Stratotankard

##### Well-Known Member
<pedant mode on>

Water vapor transforming to liquid water during precipitation events releases the energy it absorbed while being evaporated and this provides a heat source for clouds.

</pedant>
My meteorology professor used to talk about this. He loved to talk about the 680 calories* per gram of water vapor in latent energy or as he put it "2 Snickers Bars per gram." That's a LOT of energy and one reason why thunderstorms are so powerful.

*I know we're talking different calorie measurements here, but it was a good analogy and helped me remember it for a test.

Terje

#### Cugel

##### Well-Known Member
My meteorology professor used to talk about this. He loved to talk about the 680 calories* per gram of water vapor in latent energy or as he put it "2 Snickers Bars per gram." That's a LOT of energy and one reason why thunderstorms are so powerful.

*I know we're talking different calorie measurements here, but it was a good analogy and helped me remember it for a test.

Terje
That's right - food calories are different from regular "energy" calories bizzare as it may seem.

One "food calorie" = 1000 calories, or a kilocalorie. Our 2000 "calories" a day diet is actually a 200,000 calorie per day diet.

Try and see what happens if Nestle put "2,000 calories per serving" on a package of chocolate

#### Schlenkerla

##### Supporting Member
HBT Supporter
I think the OP bought into it....

The tribal wisdom of the Dakota Indians, passed on from generation to generation, says that when you discover that you are riding a dead horse, the best strategy is to dismount.

The Federal Govt. management policy dictates that a wide range of much more advanced strategies be employed, such as:
2. Changing Riders.
3. Threatening the horse with termination.
4. Appointing a committee to study the horse.
5. Arranging to visit other countries to see how others ride dead horses.
6. Lowering the standards so that dead horses can be included.
7. Reclassifying the dead horse as "living, impaired."
8. Hiring outside contractors to ride the dead horse.
9. Harnessing several dead horses together to increase the speed.
10. Providing additional funding and/or training to increase the dead horse's performance.
11. Doing a productivity study to see if lighter riders would improve the dead horse's performance.
12. Declaring that as the dead horse does not have to be fed, it is less costly, carries lower overhead, and therefore contributes substantially more to the bottom line of the economy than do some other horses.
13. Rewriting the expected performance requirements for all horses.
14. Promoting the dead horse to a supervisory position.