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School project involving freezer

Posted: Mon Aug 26, 2019 3:51 am
by DrG123
Hi

I recently acquired a Raspberry Pi and had an idea for my son's science project (he's only 7 so will need some help!).

I want to use it to demonstrate the Mpemba effect (https://en.wikipedia.org/wiki/Mpemba_effect) where, under certain conditions, hot water freezes faster than cold water.

My idea it to get a box with two cups of water (one hot, one cold) and have the Pi connected to a camera (to get a time lapse video of the freezing) and 2 thermocouples to measure the temperature of each cup of water during the freezing. Then put the whole thing in the freezer compartment (with a power bank to power the pi)

A quick search reveals the pi can withstand extreme cold temperature (so long as I take precautions to avoid condensation). However, I am concerned that the amount of heat generated by the pi could disturb the temperature of the freezer compartment.

Question 1. Has anyone put a pi in a typical freezer compartment? Did it generate more heat than could be removed by the freezer?!

Question 2. Any general advice on components and electronics to use (especially for the thermocouples / temperature sensors)? I image I would also need illumiation for the camera.

Many thanks

Re: School project involving freezer

Posted: Mon Aug 26, 2019 9:45 am
by takyon
A1: Yes. No. https://www.youtube.com/watch?v=rtWF53Czz_M

A2: If you don't want to submerge the Pi in mineral oil, you might not want to stick it in a freezer. Maybe you want to try what the above vid did except have the Pi outside of the freezer and run wires through an ice machine.

Re: School project involving freezer

Posted: Mon Aug 26, 2019 2:10 pm
by W. H. Heydt
Pis have been run outdoors in Antarctica. I doubt your freezer will hit -42'C.

Re: School project involving freezer

Posted: Mon Aug 26, 2019 3:16 pm
by jbudd
However, I am concerned that the amount of heat generated by the pi could disturb the temperature of the freezer compartment.
I suspect that say 100g of water at 100 degrees will heat the freezer up more than running a Pi in there for a couple of hours.
I did work it out, juggling Joules, Watts, grammes and specific heat capacity but I have no faith in the reproducibility of my calculations.

There are waterproof temperature sensors on a 1m cable eg DS18B20 so you could manage with the Pi outside the freezer, unless it's the appearance of ice on the surface that you want to define as "when it freezes".

Are you ready with the answers for ...
"Why is it below zero but still liquid?"
"So why did it freeze faster?" (Assuming it does!)
"Can I play minecraft now?"

Good luck!

Re: School project involving freezer

Posted: Wed Aug 28, 2019 4:42 am
by DrG123
Thanks for the replies.
Mineral oil and making holes in the freezer sounds like fun but are not really an option - my wife will kill me!

At least it sounds like it should work OK the freezer. I do want to have a camera so I can see visibly when it freezes. It would be interesting to see if it occurs during the 4C plateau due to the anomalous density of water or as it plunges below 0C (see below). It hopefully will provoke some questions and highlight that water is actually quite strange stuff. As for minecraft - I'd probably best delete it from my pi before we get started!

Image

Re: School project involving freezer

Posted: Wed Aug 28, 2019 1:55 pm
by Paul Hutch
An experiment of a poorly described hypothesis that has had many inconsistent results due to the huge number of confounding variables is about the worst possible choice for an educational experience for a student.

Re: School project involving freezer

Posted: Wed Aug 28, 2019 2:49 pm
by jbudd
Paul Hutch wrote:
Wed Aug 28, 2019 1:55 pm
An experiment of a poorly described hypothesis that has had many inconsistent results due to the huge number of confounding variables is about the worst possible choice for an educational experience for a student.
Never mind the Mpemba effect, surely the fact that the water stops getting colder until it's all frozen is valuable scientific knowledge.
Edit - though if it's boiled water you are likely to get supercooling followed by instantaneous freezing.

Re: School project involving freezer

Posted: Wed Aug 28, 2019 7:09 pm
by cruster
As suggested, why not place your Pi outside the freezer? With just a run of thin wires to a pair of thermocouples. That way, your wife will be happy, the heat from the Pi won't disturb the equilibrium of your freezer and you won't need to immerse it in mineral oil either :)

Perhaps drop the idea of the "in freezer" camera until you've got some results first?

Code up a data logger for each thermocouple (a nice project in itself) and you'll be good to go.

Interesting! Never heard of this effect. Let us know how you get on?

Re: School project involving freezer

Posted: Fri Aug 30, 2019 2:04 am
by DrG123
Paul Hutch wrote:
Wed Aug 28, 2019 1:55 pm
An experiment of a poorly described hypothesis that has had many inconsistent results due to the huge number of confounding variables is about the worst possible choice for an educational experience for a student.
On the contrary, I think it's an excellent choice. It will teach:
1. The importance of having a well defined hypothesis in the first place.
2. The importance of good experimental design.
3. The reason for doing experimentation in the first place. Sometimes you get an unexpected result that goes against you original hypothesis (poorly defined or not)
4. The need for follow-up studies to better understand the nature of the phenomenon.
5. That the real world is complex with many variables.

You are right, the Mpemba effect is messy, poorly understood, of variable reproducibility and with a range of differing theoretical explanations. This is what most scientific experimentation is like in the real world!
I'd rather my son learn this now and have fun doing so!

Re: School project involving freezer

Posted: Fri Aug 30, 2019 2:10 am
by DrG123
cruster wrote:
Wed Aug 28, 2019 7:09 pm
As suggested, why not place your Pi outside the freezer? With just a run of thin wires to a pair of thermocouples. That way, your wife will be happy, the heat from the Pi won't disturb the equilibrium of your freezer and you won't need to immerse it in mineral oil either :)

Perhaps drop the idea of the "in freezer" camera until you've got some results first?

Code up a data logger for each thermocouple (a nice project in itself) and you'll be good to go.

Interesting! Never heard of this effect. Let us know how you get on?
Yes- that would be the better option. I just need to find components with cables long and thin enough - the gap on the freezer door is not very big but maybe it would be OK.
Do you think there is much benefit using thermocouples (which would need an ADC) verus the one-wire temperature sensor suggested in another post?