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Say you have a cooler of frozen food and ice to last for several days or weeks of river-trip / car camping. To keep things as a cold as possible for as long as possible, is it better to leave the cold melted ice-water in, or to drain it out on a regular basis?

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So most of the answers here are pointing to what I believe to be the right answer: that you should keep the water in. However, if you really want some hard data on this, I can do a small experiment with some friends to get a definitive answer. This would also quantify how much of a difference there is between the two methods, and you’d get those graphs you wanted. Is this something you’d be interested in? –  Big General Feb 8 '12 at 22:20
    
@BigGeneral Give us anything you got ;) Though the problem with an experiment is for it to be conclusive, you would need many replicates... and this bounty only has a 6 days left. –  LBell Feb 9 '12 at 1:04
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12 Answers

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From a thermodynamics point of view, I'd say you should leave the water in.

Temperature is a measure of the active kinetic energy of the molecules in a substance. Warming up is essentially the surrounding environment imparting some of its kinetic energy into the object being warmed up. Simply thinking about that, the more you have that needs warming, the more energy it requires to warm, and so the slower its temperature will rise (given the same rate of exchange of thermal energy - the water submerges at least some of the other contents, so if anything this is an overestimate). Now consider that water has a relatively high specific heat, meaning that it takes more energy to warm it up. The food inside the cooler won't be warmed up faster than the surrounding water, so since it takes more energy to heat the food and water than just the food, so the food will stay cooler longer.

The Igloo (yes, the cooler company) FAQ supports this view:

During use, it is not necessary to drain the cold water from recently melted ice unless it is causing contents to become soggy. The chilled water, combined with ice, more readily surrounds canned and bottled items and will often help keep contents colder more effectively than the remaining ice alone.

Coleman agrees:

Don’t drain cold water – Water from just-melted ice keeps contents cold almost as well as ice and preserves the remaining ice much better than air space. Drain the water only when necessary for convenient removal of cooler contents or before adding more ice.

I think the key point a lot of people forget is that what matters isn't how long the ice lasts, but how long the contents remain under some temperature.


Assuming convection is fairly significant relative to the rate of energy input from the outside (a good assumption, I think), it doesn't matter how good an insulator air is, the inside temperature will be the same throughout, necessitating that the rate of heat energy coming into the cooler is the same in both cases, so the ice will melt at the same rate, and once the ice is gone, the cooler with water will take much longer to warm. I'm still working on the no-convection theory (which would provide at best an extreme overestimate), but in the meantime if anyone wishes to posit that convection is tiny enough to bridge the enormous gap (assuming I find the upper bound does eclipse water), please explain why you believe so.


Some math/physics to back this up for the quantitatively inclined. (This would be so much easier with the MathML markup from the math and physics sites.)

The cooler will be very near perfect convection, the heat is entering the cooler slowly enough that the contents - air, water, and ice - are at the same temperature (namely 32°F/0°C/273.15K). Heat conduction, H, as far as our coolers are concerned depends only on ΔT: H = kAΔT/x, where k is the thermal conductivity of the cooler, A is the area of the cooler through which thermal energy is flowing, x is the thickness of the cooler, and ΔT is the temperature difference between the inside and outside of the cooler (T_out - T_in). Notice that all these are the same for both coolers. Now, melting the ice requires energy of Q = Lm where Q is the total energy required, L is the (latent/specific) heat of fusion, and m is the mass of ice in the cooler. Since the total energy is Q = Ht, we can calculate the time required to melt all the ice: Q = Ht = kAtΔT/x -> t = Qx/(kAΔT) = Lmx/kAΔT. Since all variables are the same for both coolers, it will take the exact same amount of time for the ice to melt.*

*: We are ignoring the air replacing the ice, which would actually give a (very) slight advantage to retaining water. Draining the water in that cooler requires adding heat - the excess heat of the air that replaces the melted water. Fortunately, that excess heat is fairly easy to calculate: m = Vρ -> V = m_ice/ρ_ice = m_air/ρ_air -> m_air = m_ice * ρ_air/ρ_ice. The air comes in with an excess energy of Q = m_air*C_air*ΔT = m_ice*(ρ_air/ρ_ice)*C_air*ΔT. This reduces the energy required from the cooler heat influx, reducing the time required: Q = Lm_ice = Q_cooler + Q_air = kAtΔT/x + m_ice*(ρ_air/ρ_ice)*C_air*ΔT -> t = (L/ΔT - ρ_air/ρ_ice)*C_air)mx/kA. The fractional difference this will make ends up being about 4e-6*ΔT, or about 0.016% on a rather hot (40°C) day, coming to 2min 18s over 10 days. So we were right to ignore it.

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There is a lot of debate about this issue among rafting guides. It's about a 50-50 split, but I take to the draining doctrine. Nice explanation overall for the "bathing" option. –  Dangeranger Jan 26 '12 at 18:17
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+1 for bringing up specific heat of water. It takes twice the energy to raise the temperature of water by 1C (or about 2F) that is required to raise the temperature of ice by 1C. This is why the water will stay cool for so long even after the ice has all melted. –  Clare Steen Feb 8 '12 at 3:54
    
"what matters isn't how long the ice lasts, but how long the contents remain under some temperature" - Yup (and +1) –  Russell Steen Feb 8 '12 at 4:31
    
Mythbusters explored this very topic, and came away with conclusion that ice in water is better than ice alone, and that ice in salted water is even better. –  theUg Jun 25 '12 at 4:32
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@LBell water is great at thawing things (e.g. turkey) because of its high specific heat - the same reason it is good at keeping things cool. It doesn't matter which way the temperature difference goes, water is hard to heat or cool. Turkeys need to be thawed quickly and at a low temperature difference, and water is one of the only things that can do that. –  Kevin Dec 5 '12 at 4:07
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We (Kent and Deny) did an experiment in order to shed some light on this debate. We found that keeping the water in the cooler along with the ice kept the overall temperature of the cooler below 5 degrees Celsius for approximately 4 hours longer than when the water was removed.

Experiment. We filled a Coleman cooler with 12 341mL bottles of Waterloo Dark beer and 2.7 kg of store-bought ice cubes and sealed the cooler. Beer bottles were kept at 4 degrees Celsius in a refrigerator overnight before both experimental trials. The ice and thermometer were both kept in a freezer, which is at -10 degrees Celsius, before the experiment. Temperature was monitored using an Omega OM-62 temperature logger which recorded temperature every 5 minutes over the course of the experiment. The thermometer was kept in a Tupperware container to avoid water damage.

We setup two separate trials of the experiment. In the first, water was removed from the cooler approximately every 8 hours using a siphon. Excluding the first removal of water at 8 hours into the experiment when there was still little water in the cooler, approximately 450 mL of water was removed from the cooler every 8 hours.

In the second case, where all water was kept in the cooler,we opened the cooler for 2 minutes every 8 hours in order allow the same amount of warm air to enter the cooler, as it would in the case of draining the water.

Results. Temperature data is shown in the figure below.

Cooler Results Figure

From the figure, we can see a few points. Excluding the first draining, when there was little water, each time the water is drained, the temperature of the cooler immediately rises. After 24 hours, draining the water from the cooler preceded a linear growth in temperature, which grew in slope at each subsequent draining. Finally, we can clearly see that keeping the water in the cooler keeps the temperature below 5 degrees Celsius for approximately 4 hours longer.

We conclude that this experiment gives support to the argument for keeping the water in the cooler.

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Awesome - great work - definite +1 for putting the effort in. 2 questions: 1) where was the thermometer? (Near the beer? On the bottom (submersed) or floating on the water?) 2) What was the ambient temperature doing outside during this period? And 3) Any interest in replicating about a dozen times or so? :) Thanks for the contribution –  LBell Feb 16 '12 at 1:58
    
I'll upload some photos of the setup soon. The thermometer was in a tupperware container half submerged in the ice, touching the bottom of the cooler, in the same way the bottles did. I put the cooler in a sink in my basement for the experiment. I don't have data for how the temperature of the room varied (my only thermometer was in the cooler), but our house has been kept consistently at 18 degrees Celsius for the past while. –  Kent Fisher Feb 16 '12 at 4:01
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Never remove cold water from a cooler so long as the water is cooler than the outside temperature.

  1. Opening the lid allows more warm air in, but assuming the lid is on the top and air disturbance minimal, this could be a small loss of cooling / small entry of heat. Opening a drain will have to let warm air in to replace whatever cool water leaves the cooler.
  2. While ice remains, water will circulate and keep all the objects in closer thermal contact with the ice than will air alone - this will prevent a thin spot in the insulation from warming some of the food that stays in contact with the cool water. This won't have an effect on the long term temperature in the cooler, though.
  3. Once all the ice is melted, the cool water will continue to be a heat sink, absorbing more of the heat that is leaking in through the walls of the cooler. This keeps your actual contents inside the cooler more chilled than if you remove this cold mass from the system. Without the cold water, all that heat will go to the food and warm it up instead of warming both the combined mass of the food + cold water.

You don't need any fancy graphs when you look at the heat flow into the cooler. Keeping cool water in the cooler delays the melting of the remaining ice and once the ice is gone, delays the warming of the food by absorbing it's share of the incoming heat.


Say you have two coolers with one gallon of beer each in a perfect container that absorbs no heat - just keeps the beer together. Also, assume that you have one pound of ice in each cooler and the beer and cooler are chilled to 0°C (32°F). One cooler also has a gallon of 0°C (32°F) water.

So the cooler on the left has heat leaking into the insides - warming everything inside. It warms the air, the beer, the inside of the cooler and the beer's container.

The cooler on the right has the exact same amount of heat leaking inside. It warms the air (but less air due to the extra water), the beer, the inside of the cooler and the beer's container.

The only variable is does water absorb more heat per degree rise in temperature than air. The answer to that is yes - extremely so.

Air's specific heat is 1.007 J/(gK) - Joules is energy, gram is mass and K is degrees Kelvin. Water's specific heat is 4.18 J/(gK) - so for a fixed amount of Joules added, water goes up less than 1/4 a degree in temperature compared to a gram of air. If water and air were equally dense (they are not) then you have a 4 to 1 advantage. Water takes four times as long to warm up as air, so your beer stays cooler longer.

Now - what about your typical 70 quart cooler? It is 66.24 Liters or 66,245 cm^3 in volume.

Air's density is 0.001275 g/cm^3 and water's density is 1.00 g/cm^3. Here's where water really owns the show in terms of cooling capacity since water is 784 times denser than air. That 70 quart cooler could contain either 84.5 grams of air or 66,245 grams of water (or 3 ounces of air versus 146 pounds of water).

Now we have water with a 4 to 1 advantage on heat absorption and a 784 to 1 advantage on packing mass into the same space, so water is over 3000 times better than air for absorbing heat while raising the temperature inside the cooler one degree. Whether you have a thimbleful of water or the cooler is mostly water, you want that amount of cold water staying inside the cooler to absorb its share of the heat leaking in.


Since it's easy to agree that the amount of heat entering the cooler is basically the same whether you drain or don't drain, leaving cool water inside the cooler slows the time it takes to both melt the remaining ice and warm the contents since that water will absorb heat if it is left in the cooler up to the point where the water in the cooler is equal in temperature to the outside air.

You do need formulas to calculate the rates of temperature increase, but the which is better scenario can be decided quite easily by considering where the heat leaking into the cooler goes and whether draining the water also has a side effect of allowing more heat in. Both of these fall on it being better to leave the container closed and with the melt water inside.

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Small nitpick -- This doesn't take into account the insulative affect of the negative space in air due to it being less dense. Yes, water is more dense, but that also means it will transfer heat faster and more efficiently from one molecule to the next. (still +1) –  Russell Steen Feb 8 '12 at 6:02
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Hmm - air only insulates when it is trapped and immobilized, so I don't see how having melt water raising up inside the cooler or draining it changes the heat transfer inside the cooler. Natural convection will be the dominant heat transfer mechanism within the cooler no matter what proportion of air or water is present. –  bmike Feb 8 '12 at 6:30
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EDIT: The more I consider this, the ambient air temperature around the cooler is the largest factor. Replacing water with 95F (35C) degree air will have a much larger impact than replacing water with 40F (4.4C) degree air.

Actually, the answer is very simple because you asked longer, not colder.

If you drain all the water, then when the ice all melts...you're done. Because whether or not the ice is better than the water at keeping your food cold, the water is going to be better than nothing. The ice is going to melt (effectively) at the same rate either way, because you can drain the water, but you can't really dry the ice. So the ice will melt at roughly the same rate regardless of when you drain the water.

Also, when you drain the water, something has to replace it. That something will be air at the temperature of the environment you are in... which will destroy the equilibrium. Basically everything I read online ignores this point. It talks about the coefficients and thermodynamics, etc, and, neglects to even consider that the water is being replaced with something and that something is air which must then be cooled. When you drain the water, you are actively replacing cold with heat.

It should be noted that if you have infinite ice, you should probably drain the water. However if you have infinite ice it doesn't seem like this would matter.

More here

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Actually I asked "as cold as possible for as long as possible"... I want it all ;) –  LBell Feb 8 '12 at 4:13
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+1 for "replacing the water with warm air" -- never really considered that either. –  LBell Feb 8 '12 at 4:17
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Even if you replaced water with equivalently chilled air, water is 3000 times more resistant to raising the temperature a degree than air. You can't add air cool enough to be better than keeping cold water in the cooler. –  bmike Feb 8 '12 at 5:33
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I sometimes leave melted ice water in my cooler, which then gets into the food, making the food inedible.

If my "food" is a can of beer, fine, leave the water in the cooler. If it's a sandwich, drain the water if it might get the sandwich wet. Or have the cans of beer at the bottom and put the sandwich on top to keep it out of the ice water.

Another "outside the box" answer is if it gets cold enough at night, I'll leave the lid open. Leaving all the melted ice water inside is good thermal mass to keep it cool during the day. You have to remember to open and close the lid at the right time. If critters might get in this doesn't work so good.

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The thermal conductivity of air is 0.000057

The thermal conductivity of water is 0.0014

Therefore water is 24.5 times more conductive than air, and has a temperature above 32 degrees Fahrenheit. The reason this is a problem is that bacteria can grow in that water quite quickly (within 6 hours) and start to make your food unhealthy to eat.

Additionally all that warm 32+ degree water will assist in melting the remaining ice.

So do yourself a favor, drain the water that melts out the bottom of the cooler with the spout that is provided. It's safer, and your food will last longer.

See this fine article about cooler packing and draining.

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I'm not sure bacteria would grow 'quickly' in relatively clean, zero-degree water. –  mendota Jan 27 '12 at 10:57
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Mythbusters sort of touched on this one: kwc.org/mythbusters/2005/03/mythbusters_cooling_a_sixpack.html. They were trying to determine the fastest way to cool a six-pack of beer. Ice+salt+water was faster than ice+water, which was also faster than just ice. –  Clare Steen Feb 8 '12 at 3:38
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I humbly suggest you are looking at this the wrong way. Adding water that is warmer than the ice may or may not be good depending on many factors. You don't want to add anything that is warmer than the ice if your contents are already chilled. You do want to add anything that is cooler than the contents of the ice chest since that will draw heat out of the food and start you off cooler than if you didn't add that water (or whatever else). Once you separate that, the answer to not drain chilled water is self evident. Anything cooler than the outside air keeps the cooler more cool. –  bmike Feb 8 '12 at 5:32
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"and has a temperature above 32 degrees Fahrenheit" - Wrong. Water with ice in it will remain at 32F until all the ice is melted (assuming it's not being heated so rapidly convection can't even things out - and if that's the case, you have bigger problems with your cooler). –  Kevin Feb 10 '12 at 21:37
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Also, the fact that the water is still at freezing will inhibit bacteria from growing. –  Kevin Feb 10 '12 at 21:46
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As a rafter I have taken part in lots of discussions about this and couldn't resist any longer so have set up an experiment to test this. I hypothesize that the drained cooler will hold ice longer due to the insulating effect of air--as Snitse has described above. Convection will reduce air's effectiveness but, as Snitse points out, it is still far superior to water. I have set this experiment up with identical styrofoam coolers loaded with equal amounts of crushed ice (in the second series of experiments I am going to substitute identical blocks of ice). The drained ice chest has a drain tube in place to allow it to drain continuously. These are placed side by side (with a gap between) at room temperature (in my lab in a large building where temperature is fairly constant). In the first replicate, the drained ice chest did, indeed hold ice longer, but not by much and the undrained ice chest contained water that reached room temperature at nearly the same time the ice melted! I am replicating this experiment and will begin the ice block experiment after replicating this three times.

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Here is a different twist on the question of how to best use block ice. Before refrigeration northern states used to saw block ice fron frozen lakes and store for the summer in "ice houses", log houses that used only the thermal inertia of massive amounts of ice stacked together.

We just returned from a 6 day canoe trip in which we used block ice to keep fresh food cold. On Friday, july 19 we bought 200 lbs of block ice (20 blocks of 10 lbs). these almost completely filled two 12" x 12" x 30" low tech garage sale ice chests. We kept these two ice keepers drained of water because water is a conductor, not an insulator. Then we used single blocks of ice from these to supply as needed four smaller chests which we did not drain.

We then traveled from oklahoma City to the Current River in Missouri and floated 100 miles over six days and returned to OKC on Fri Jul 26 with about 15 lbs of block ice remaining. Eight days and 15 hours! We fed 18 Scouts and adults on fresh food that was previously frozen ar cooled for a week. I know that this is a completely unscientifec "experiment" , but is a practical example that worked for us. I hope this is useful for your wilderness trip.

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As Dangeranger stated,

water is 24.5 times more conductive than air

The idea is that water will conduct heat energy throughout the cooler much more effectively than air. This means that water will conduct heat energy from the leaks and seams in the cooler to the ice more effectively than air.

Say we have two hypothetical coolers, Cooler A and Cooler B. Both leak heat at the same rate, and both started with a identical large block of ice in it, and equal air temperature occupying the rest of the cooler. Every hour you drain the water from A, and leave it in B. For the first hour they both have the same amount of water and the same amount of ice, and then you drain the water from A. Now, in the second hour because the water left in B conducts heat from the walls to the ice faster than the air in A, more of B's ice will melt than of A's ice.

Clearly, B's ice will be completely melted before A's ice is. Let's call the moment that ice B completely melts 'meltB'. At meltB ice A has x percent of its original mass remaining.

We can all agree that at meltB there is less heat energy in cooler B than in cooler A, since you've been replacing things in cooler A with other things that had more heat energy (replacing cool water with air temperature air).

Where it gets tricky is that after meltB, the heat energy in cooler B will increase faster than the heat energy in cooler A because of all that water conducting heat from the walls.

So from start until meltB, A gains energy faster than B, but after meltB B gains energy faster than A.

Graph

What this all comes down to is the exact numbers. If you want to eat your food at time a or b you are better off with cooler B, but if you want to eat your feed at time c or d you are better off with Cooler B.

As for how to calculate meltB, I have no idea. So I guess its somewhat impossible to figure out.

EDIT:

To clarify, the key point here is that different parts of the cooler are different temperatures. The wall is warmer than the area immediately around the ice. However, if the cooler is full of water, the water brings heat energy from the wall to the ice much more effectively than air would.

So the coolers leak at the same rate, meaning the walls gain energy from the outside at the same rate. However, if there is no water, the walls gaining energy does not necessarily mean that the ice will gain that energy and melt immediately. It will take time for that energy to get to the ice, and the time it takes depends on the medium the energy transfers through.

In summary, walls gain energy at same rate, ice in the middle does NOT gain energy at the same rate because of what is between them and the walls.

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"Both leak heat at the same rate," and "at meltB there is less heat energy in cooler B than in cooler A," but "B's ice will be completely melted before A's ice is." Those can't all be true. In fact, the amount of ice in the cooler depends entirely on how much heat energy there is in the cooler, so by acknowledging that there will be more heat energy in the drained cooler, you are actually saying that the ice in it will melt faster. –  Kevin Feb 12 '12 at 18:35
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Oh, and I should mention, the temperature doesn't increase linearly, they will both be at a constant 32 until all ice is melted (except small jumps for draining the ice in the appropriate cooler), after which the temperature difference will decay exponentially (with the water-filled cooler rising much more slowly). –  Kevin Feb 12 '12 at 18:38
    
At meltB there is less heat energy in cooler B than in cooler A, and there is NO ice because there is quite a bit of rather cool water. –  Snitse Feb 12 '12 at 18:38
    
The graph is the amount of energy in the coolers, not the temperature. The energy increases as the temperature stays the same because energy goes into melting ice. You are also assuming that the whole cooler is the same temperature. The whole point is that different parts of the cooler are different temperatures (the wall vs. the area immediately around the ice) and water is better at bringing that heat from the wall to the ice than air is. –  Snitse Feb 12 '12 at 18:41
    
E=(3/2)kT, so T=(2/3)(E/k). If k were constant, the temperature would be directly proportional to your (still incorrect) graphs. However, the average k of the drained cooler is dropping, so the temperature is increasing even faster. (Though, as I said, not until the ice is all melted.) –  Kevin Feb 12 '12 at 18:48
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Ok, put away your calculators people. Draining the cool water causes the warm air to enter the cooler. Cool water is COOLER than warm air, so leave the cool water in the cooler. If I had a compass, some graph paper a protractor handy I'd draw you a picture.

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Your ice will keep longer if you drain the water. If you want to prove it to yourself, get 2 cups. Fill one with cold fridge water and leave the other empty. Put an ice cube in each and see which one is melted first. Even with the ice cube in the water being surrounded by cold water it still melts much quicker than the other ice cube being surrounded by the hot ambient air.

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Your experiment is flawed. "Cold fridge water" is warmer than water that just changed states from solid to liquid. Since water has a higher heat capacity than air, the fridge water, depending on it's temperature, can quickly melt the ice, even when it is significantly colder than the surrounding air. The higher heat capacity of water is also what can slows down the melting when you don't remove the melt-water. A better experiment would be to actually conduct the experiment the OP proposes, at different ambient temperatures and in containers with different insulating properties. –  DudeOnRock Oct 12 '13 at 5:57
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It's really simple. The universe wants everything to be the same temperature so if we left the cooler in a sufficiently large, constant temperature environment, the cooler and all its contents would eventually end up at the same temperature, that being the ambient temperature of the environment.

Let's first begin by noting that we want to keep something cool rather than make it cooler. In that case, we want to stop it gaining heat energy. The heat energy is coming from the walls of the cooler.

If our food is packed in the cooler such that the ice touches the walls, then the ice is going to be subject to warming before the food. The ice touching the food will not impart any heat energy because it's the same temperature.

However, add some water to the system and there is path of conduction from the wall to the food since the water is at higher temperature than the food.

Air has 1000 times less heat capacity than water, so it's not actually that interesting to take into account replacement of cold air with ambient temperature air.

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