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How Coffee Makers Work


This basic drip coffee maker might not have lots of bells and whistles, but it can definitely whip up a quick cup of coffee.
This basic drip coffee maker might not have lots of bells and whistles, but it can definitely whip up a quick cup of coffee.
HowStuffWorks 2008

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Most people who are dependent on caffeine have breakfast with an old friend -- the coffee maker. Every morning you scoop in the coffee, add some water and flip it on, but have you ever­ wondered what's happening inside. How does the water get from the reservoir over to the coffee grounds in the filter basket? How does everything heat up so quickly, and what on earth is that gurgling noise?

In this article, we'll look inside a typical drip coffee maker so you can understand exactly what's happening when you make coffee. We'll also look at the possible problems that might cause your coffee maker to stop working. By the end of this article, you may look at your old friend in a completely new way.

­Before we get into that, however, let's do a quick coffee rundown. Coffee plants are­ evergreen tropical shrubs and small trees, and they grow best between the Tropic of Cancer and the Tropic of Capricorn (often referred to in the coffee world as the Bean Belt) ­which mak­es sense because the plants enjoy lots of rain and gentle temperatures. Coffee beans as we know them are actually the seeds of the fruit of the coffee plant -- called coffee cherries -- and these popular plants have been cultivated by man for hundreds of years. The two most commonly grown species of coffee plants are Coffee arabica (Arabica coffee) and Coffea canephora (Robusta coffee). For more about details about what's in your morning cup, check out the article How Coffee Works.

There are lots of coffee makers on the market that can arguably make a better pot of Joe, but in this article, we'll be focusing on the trusty drip. If your tastes do run fancier, check out the article How the Clover Coffee Maker Works. On the other hand, if you're gunning for an insider's look at your kitchen's appliance of the year, we'll crack it open and take a peek on the next page.­

Inside a Coffee Maker

A look into a drip coffee maker proves it's a fairly basic gadget.
A look into a drip coffee maker proves it's a fairly basic gadget.
HowStuffWorks 2008

A modern drip coffee maker is a surprisingly simple device. Manufacturers have had more than 30 years to hone their designs, so these coffee makers are pretty straightforward once you open them up.

If you take off the top of the coffee maker, you find three things:

  • There's a reservoir that holds the water when you pour it into the pot at the start of the coffee-making cycle (on the right in the picture above). At the bottom of the bucket there's a hole, and its role will become obvious in a moment.
  • There's a ­white tube that leads up from below the reservoir base, carrying the hot water up to the drip area.
  • There is a shower head (on the left-hand side of the picture). Water arrives here from the white hot-water tube and is sprayed over the coffee grounds. In some coffee makers, the water comes out of the hose onto a perforated plastic disc called the drip area and simply falls through the holes into the coffee grounds.

Looking at this picture, you get your first impression that this isn't a high-tech device. If you take the bottom off the coffee maker, here's what you'll find:

Things are about to start heating up!
Things are about to start heating up!
HowStuffWorks 2008

The depression on the right-hand side of this figure is the bottom of the bucket. The orange tube on the top picks up the cold water coming down from the hole in the reservoir. The orange tube on the bottom is the hot-water tube (it connects to the white tube that we saw in the previous picture). You can also see the power cord coming in as well.

On the left-hand side of the base of the coffee maker is the heating element. This component is comprised of an aluminum extrusion with two parts: a resistive heating element and a tube for water to flow through. The resistive heating element and the aluminum tube heat the water. On the next page, we'll take a closer gander at what this apparatus looks like.

The Heating Element of Coffee Makers

The heating element efficiently serves dual functions.
The heating element efficiently serves dual functions.
HowStuffWorks 2008

The resistive heating element is simply a coiled wire, very similar to the filament of a light bulb or the element in an electric toaster that gets hot when you run electricity through it. In a resistive element like this, the coil is embedded in a plaster to make it more rugged. The heating element has two jobs:

  • When you first put the water in the coffee maker, the heating element heats it.
  • Once the coffee is made, the heating element keeps the coffee warm.

­In the picture above, you can see how the resistive heating element is sandwiched between the warming plate and the aluminum water tube. The resistive heating element presses directly against the underside of the warming plate, and white, heat-conductive grease makes sure the heat transfers efficiently. This grease, by the way, is extremely messy (very hard to get off of your fingers!). You find this grease in all sorts of devices, including stereo amplifiers, power supplies -- pretty much anything that has to dissipate heat.

The coffee maker's switch turns power to the heating element on and off. To keep the heating element from overheating, there are also components such as sensors and fuses. In coffee makers, sensors detect if the coil is getting too hot and cut off the current. Then, when it cools down, they turn the current back on. By cycling on and off like this, they keep the coil at an even temperature. Fuses simply cut the power if they sense too high a temperature. They're there for safety reasons, in the event that the main sensor fails.

The various sensors and fuses of this coffee maker likely lie in the white sheathing bridging the heating element.
The various sensors and fuses of this coffee maker likely lie in the white sheathing bridging the heating element.
HowStuffWorks 2008

Another important part of the coffee maker is the one-way valve. This valve is usually either in the hole in the bucket or in the aluminum heating pipe. If there were no one-way valve, then the boiling water would be just as likely to flow back into the bucket as to rise up the white tube. The one-way valve lets cold water into the aluminum tube, but forces the bubbles of boiling water to flow up the white tube. If you blow on the tube leading into this value, the valve should be open. If you inhale through the tube, the one-way valve should block any air.

Now that we've got the basic elements down, let's turn on the coffee maker and take it for a test drive.

Making Coffee

Sleepy? A cup of HowStuffWorks coffee can get you going.
Sleepy? A cup of HowStuffWorks coffee can get you going.
HowStuffWorks 2008

All this talk of coffee got you thirsting for some java? Well, here's the lowdown on how a drip coffee maker turns a handful of grounds and a couple of cups of water into a steaming hot cup of Joe.

You can see that a coffee maker is about as simple as an appliance can get. Here's how it works:

  • When you pour in cold water, it flows from the reservoir through the hole and into the orange tube.
  • Then the water flows through the one-way valve, into the aluminum tube in the heating element and then partially up through the white tube. This all happens naturally because of gravity.
  • ­When you turn on the switch, the resistive heating element starts heating the aluminum tube and eventually the water in the tube boils.
  • When the water boils, the bubbles rise up in the white tube. What happens next is exactly what happens in a typical aquarium filter: The tube is small enough and the bubbles are big enough that a column of water can ride upward on top of the bubbles.
  • The water flows up the white tube and is dispersed to drip evenly on the waiting coffee grounds.
  • The hot water flows through the ground coffee beans, picking up their oil essence on the way down into the coffee pot. This coffee oil, released during the roasting process, is called caffeol.

Sounds pretty simple, right? This boiling-water pump, by the way, is the same mechanism that drives a percolator-type coffee machine. As you can see, there's no mechanical pump of any type and really no moving parts (except for the moving portion of the one-way valve). This makes coffee machines extremely reliable.

In some drip coffee makers, there are also different advanced features that give you more control over your coffee. On the next page, we'll learn about them, as well as what happens when disaster strikes and the coffee maker goes out of commission.

Drip Coffee Maker Problems: What Can Go Wrong with Your Coffee Maker?

­You're groggy. The sky still looks the same as when you started snoozing the night before and you stumble into the kitchen gung-ho to flip that magic switch. But what's this? You turned it on and nothing happened. No cheerful gurgling, no promising rumbling. No coffee!

Here are some of the most common problems that can cause your drip coffee maker to stop working:­

  • The power cord or on/off switch can go bad. In either of these cases, it's best to get a pro to replace it or buy a new coffee maker -- replacing it improperly can lead to a fire.
  • The one-way valve can get clogged open or closed. You can usually fish out the debris that causes this problem with a toothpick.
  • The tubes can get clogged with calcium. This is especially true of the aluminum heating tube. The normal way to solve this problem is to run vinegar through the machine once, and then run two batches of water through to clean out the vinegar.

There are two problems that are nearly impossible to fix: failure of one of the heat-sensitive switches and failure of the heating coil. Because it's very challenging to get replacement parts, you'll probably have to buy a new coffee maker if one of these problems disables your machine.

­Assuming no calamity has struck your coffee maker, what are some of the advanced features it might offer? Well for starters, many have a programmable timer, meaning you can get everything ready the night before and when your alarm clock starts beeping, the coffee pot starts gurgling. By the time you roll out of bed and trudge to the kitchen, your coffee is ready and waiting. In some machines, a built-in grinder means that cup of coffee will also be nice and fresh, ground just before brewing began. Another interesting feature available is the ability to pause in the middle of a brewing cycle. This way, if you're the first one out of bed making coffee for an entire household, you can grab your first cup before the entire pot is prepared.

Many digital drip coffee makers also allow you to adjust the strength of the brew -- if want a stronger cup of coffee the brewing time slows to accommodate. Some also have self-cleaning cycles and filtration systems. Freshness counters can let you know how long a pot of coffee has been sitting out and automatic shut-off features can ease fears that you forgot to turn the cof­fee maker off when you left for work.

Now, when you make coffee tomorrow morning, you can do so with a new appreciation for exactly what's happening inside. For more information on coffee makers, coffee and related topics, check out the links on the next page.

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Related HowStuffWorks Articles

More Great Links

Sources

  • "All About Coffee." National Coffee Association of U.S.A., Inc. (10/31/2008) http://www.ncausa.org/i4a/pages/index.cfm?pageid=30
  • "Automatic Drip Coffee Makers Reviews and Prices." HowStuffWorks.com Consumer Guide. (10/31/2008) http://products.howstuffworks.com/automatic-drip-coffee-maker-reviews.htm
  • Beller, Deborah. "How Coffee Works." HowStuffWorks.com. (10/31/2008 http://recipes.howstuffworks.com/coffee.htm
  • Coffee Research Institute Web site. (10/31/2008) http://www.coffeeresearch.org/about/overview.htm
  • "Coffee." Encyclopedia Britannica. (10/31/2008) http://www.britannica.com/EBchecked/topic/124337/coffee
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  • "Roasting/Making Coffee." International Coffee Organization. (10/31/2008) http://www.ico.org/making_coffee.asp
  • "What is a French press?" WiseGeek.com. (10/31/2008) http://www.wisegeek.com/what-is-a-french-press.htm