[JOHN Gamel]

JOHN Gamel  asked the Naked Scientists:
   
I recently bought an electric steamer that allows me to cook from one to seven eggs at the same time, and they can be cooked soft, medium or hard.

There is no timer - rather, you add a precise measure of water, cover the container with a lid, and turn it on. When the eggs are done, a light flashes, an alarm sounds, and the power turns off. You're meant to immediately immerse the eggs in cold water for a minute or two before eating.
 
As you would expect, the smallest amount of water is added for soft eggs, a medium amount for medium eggs, and the largest quantity for the hard eggs.

BUT HERE'S THE CHALLENGE: the FEWER the eggs you place in the cooker, the MORE water you add. For example, to hard cook a single egg you have to add approximately a third MORE water than for hard cooking all seven eggs.
 
How does this work?
 
John Gamel

What do you think?

[tommya300]

Looking at the operation of a similar described unit, I read this...

 http://www.cuisinart.com/share/pdf/manuals/cec-7.pdf

from my interpretation
If the eggs are submerged into the water they will displace the water.
If you fill the unit to cook only on egg the water level need to be high enough to cover a good portion of the egg.
This will take the maximum amount of water the instruction level recommends.
If you suddenly decide at that moment to add more eggs the water displacement will be higher then recommended use and may overflow or as it boils will mess up the works and you maybe mopping up a mess before your feast. 

[wolfekeeper]

I've got one of these types of steamers. I don't think the eggs actually touch the water, and even if they did that wouldn't change the amount of water there was.

I think it's because the steam condenses on the eggs and drips back into the tray; that's how they cook, the steam has to give up its heat to the eggs, but the eggs can never reach boiling because the steam is wet steam, which is always at 100C, and so the eggs must be colder than that, so the steam is always condensing on the colder eggs.

So the more eggs you add, the more water drips and the less steam escapes through the hole, and the less water you need to use to start with to ensure that the eggs are cooked.

If I'm right, when you're cooking more eggs the cooking time should be the same or slightly longer than if you're cooking one egg, even though you use less water for more eggs.

[Geezer]

Sounds like it operates on a thermostat that turns off the element when it reaches a certain temperature. The clue is that it turns back on automatically if you don't unplug it - presumably when the thermostat cools again.
 
If that's the case, the heat input is constant, so you add more water with fewer eggs to prevent the eggs from cooking too quickly. This probably has something to do with the specific heat of eggs!
 

[wolfekeeper]

But the eggs don't start cooking until the water boils, so adding more water would cook them for longer if your theory was right; because for constant heat input it would take longer to boil off the greater volume of water you use when you cook fewer eggs, and then the eggs would be overcooked!

[CliffordK]

I had one years ago...
But, you measured the water in the lid...  and dumped it in.  Although, I don't think one counted the number of eggs, just so much for hard, less for soft.  A little pin was in the lid to (p r i c k) the fat end of the eggs.  Sorry, one word was censored!!!

It would boil water until the water was all gone, then it would shut off.

My interpretation of the variable amount of water is this.
Hmmm
Hmmm
I like the idea of wolfekeeper that the multiple eggs help condense the water.  I was thinking more like something in the microwave.  2 cups of water take longer to boil than 1 cup.  So, if you wished to cook an item for the equivalent time as cooking 2 cups, you would add more water. 
If you put in 1 tablespoon of water with 1 egg, the 1 egg wouldn't provide enough of a heat sink to keep the water from flashing off.  You might well get the outer layers of the egg cooked, but not bring the core temperature up high enough.  With two eggs, you have more to heat up, and thus more likelihood to cook both the outer part, but uniformly heat the egg to the center. 

Nonetheless, the variable amount of water does seem odd, and I think we need some more details of exact procedures to make a determination.

[Geezer]

Yes, I think Wolfkeeper is right. The eggs will act as condensers while they are coming up to temperature. If you have fewer eggs, you need to add more water to compensate for the lack of recycling, otherwise it will boil dry too quickly and the egg would be undercooked. Once the thing boils dry the temperature will shoot up and open the thermostat.

If that's how it actually works, it's using the time to convert a mass of water into steam with constant power input to time the cooking of the eggs. It might work with a steam generator and a time-switch to control the power, but a thermostat is a lot cheaper!