davidkallman wrote:Hi Jennifer,
Some possible bad news, in:
http://answers.yahoo.com/question/index ... 649AAyfA6E
(How can you make a homemade calorimeter?),
it's stated that "measuring the calories of a lemon would be very difficult with any type of homemade calorimeter. This page gives two issues (difficulty in burning the sample and accuracy) with the calorimeter. These issues would apply not just to lemons but to any food item.
I’m not sure if the first issue is valid, but the second issue appears valid.
The question above was generated as a result of the previously referenced sciencebuddies query:
http://www.sciencebuddies.org/mentoring ... p017.shtml
(Burning Calories: How Much Energy is Stored in Different Types of Food?).
I hate to be posting a message that raises a problem with a sciencebuddies write-up. So, please do your experiment and let us know how it works out. Note: everything you read on the internet could be suspect. And, I welcome other thoughts.
Louise wrote:In this case, better engineering of the insulation will help with the second issue Dave raises (preventing heat loss should improve the accuracy for a given sample (assuming complete combustion)), and clever sample preparation will help with the first (incomplete combustion). In other threads on this project, several recommendations have been made for sample prep- particularly drying the sample and grinding it up to assure more complete combustion. Validating your instrument over many trials with a known quantity of food (with a specific calorie content) is important to determine the accuracy and precision of the instrument.
In this case, if I understand Jennifer correctly, she is just measuring the heat change of water, and not any tricky food sample. Furthermore, she isn't actually combusting anything, just heating water with a candle (so I am not sure how the teacher can evaluate that no heat is lost). So, focusing purely on the insulation is a good strategy, if you want to start with something like the coffee cup calorimeter. Craig gave you some great suggestions, and his apparatus sounds extremely non-flammable- so you wouldn't risk the 'fire-extinguisher'!
The second issue, accuracy, however, was not related to the design of the calorimeter. Instead, it related to the measured quantities. Quoting from the referenced page re:accuracy:
"The second problem is that lemons contain very few carlories so that your potential error will be on the order of the number of calories in your sample."
davidkallman wrote:Hi Louise,
You got it almost correct; my interpretation of the referenced webpage was related to one paragraph in your post:
"The point you should take away from all this, is that if you are building an apparatus, you need to decide what order of magnitude number you need to measure, and what your tolerance for error is. If you are measuring the calories in a lemon or in a block on TNT, your design parameters are going to be very different. Your problem seems a little easier than either of these extremes."
It's my understanding of the referenced webpage is that is difficult to measure numbers at the lower extreme, and it becomes easier to measure a quantity as the measured number becomes bigger. So, the easiest number to measure is at the high end of the measurement interval.
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