Edible Scale Student Materials

"THE INCREDIBLE EDIBLE SCALE"

WHY DOES THIS PROBLEM MATTER?

We are all crew members on "Spaceship Earth". Therefore, we must eventually consume and/or recycle all of our products. This engineering design project is intended to focus on the significance of this recycling requirement when designing useful things. Engineers of many types get involved in the design process. Some engineers, like Environmental Engineers, focus specifically on the long term consequences of waste disposal and recycling.

Conventional weighing devices are easier to design and construct if there is no thought to eventual recycling. But what if you must design the scale to weigh accurately and to be completely recycled? This project gives you an opportunity to explore this task. For today, total recycling may sound strange, but tomorrow it may be routine.

WHAT ARE WE SUPPOSED TO DO?

For this project, you are to design, build, test, and eat a weighing device, or scale, that measures a weight between 5 and 30 grams (approximately one ounce). The object you will be given to weigh may be in any shape and any material but it will fit inside a cube 10 centimeters (approximately 4 inches) to a side and will be solid.

WHAT EQUIPMENT AND MATERIALS CAN WE USE?

You are free to use any tools or equipment which you have available. Your teacher should give you use of either known weights or a lab scale to let you calibrate your scale. Calibration is the process of using known weights to adjust your scale to indicate the correct weight.

Your Incredible Edible Scale (IES) may be made only of materials which are sold locally for the purpose of consumption by humans.

HOW WILL WE KNOW HOW WELL WE HAVE SUCCEEDED?

After constructing your IES, you will have 2 minutes to make your estimation of the weight of the object. You will then eat (including chewing and swallowing) the IES while being timed. You may drink water with your IES. DO NOT EAT FASTER THAN NORMAL. Your eating time is a minimum of 240 seconds (4 minutes). In other words, you will always be assessed 240 seconds of eating time even if you are able to eat your scale in less time.

The Basic Performance Requirement for the IES is that it provide an estimate of the unknown weight, and that you are able to consume the entire IES.

The Extra Performance Index (EPI) will be calculated by multiplying the amount of time (in seconds) it takes you to eat your IES times the difference between the actual weight and the weight your scale reads (in grams). The lowest index value represents the highest performance design.

EPI = |[[ estimated weight(grams)] X [actual weight (grams)]]| X [eating time(seconds)]

WHAT ELSE MIGHT BE USEFUL TO KNOW?

There are at least three physics principles you could employ in your scale (perhaps you can think of others). One involves balancing two objects on opposite ends of a beam with the weight of one being known. A second is to use an elastic object (such as a spring or rubber band), allowing the weight to stretch or compress the elastic object and measuring the deflection. A third method is similar to measuring how far a boat sinks into a river when weight is added.

Balancing Principle

For the balancing principle, let's think of a see-saw. If a large person sits on one end of the beam, then in order to be in balance, a lighter person should sit farther away from the pivot point (Fig. 1).

Figure 1. See-saw.

Realizing this, you may construct your scale in several different ways. You may have several objects of known weight and place them an equal distance from a pivot point as the unknown weight. Then, simply subtract or add known weights until the beam balances (Fig. 2).

Figure 2. Balance scale with equal distances.

If the distances are equal then the unknown weight must equal the known weights. Another way to construct it would be to have the unknown weight close to the pivot point and have a small weight be able to move along the beam (Fig. 3). The unknown weight is then calculated using the formula:

[unknown weight]X[distance 1] = [known small weight]X[distance 2 ]

Calibration is essential to the success of your scale. You can calibrate your scale by marking the position of the known small weight with different known weights on the other side.

Elastic Principle

For the elastic principle, we only need to think of a rubber band on a hook. To calibrate this scale, you take different known weights and hang them on the rubber band, noting the distance that the rubber band stretches. You could also stack known weights on top of a spring and note the distance the spring compresses. Roughly, the distance stretched (or compressed) is directly proportional to the weight and doubling the weight would double the distance stretched (Fig 4). The equation can be written as:

known weight/ known stretch = unknown weight/ unknown stretch

Figure 4. Spring scale.

Archimedes Principle

The principle of using the depth a boat sinks into the water when loaded is called Archimedes Principle. Think of a glass of water with a "boat" floating on the top of the water (Fig. 5). To calibrate this scale note the height of the water on the "boat" when the "boat" is loaded with different known weights, we could have a very accurate scale. The weight of the water displace is equal to the weight of the object on the "boat".

Figure 5. Archimedes scale.

With these principles of physics, you can easily build a scale; the challenging task is doing it with foods that you like to eat and can eat quickly.

SAFETY CONSIDERATIONS UNIQUE TO THIS PROJECT

First, it is very important that you eat your scale at your normal eating rate. This is to insure that you do not choke on your IES. Please be very careful while eating so you don't get carried away with doing well and get a piece of your product stuck in your throat.

Cleanliness is also important (at least to us). While building your IES, you should take care to have clean hands and clean equipment to be sure your scale is fit for eating. We don't think it would be pleasant to eat a carrot covered with dirt. Although this is not a safety consideration, please keep in mind, there are combinations of foods that are not pleasant to eat.

Bon Appétit!