Keep the Heat B/Thermodynamics C

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Re: Keep the Heat B/Thermodynamics C

Postby Schrodingerscat » March 22nd, 2012, 6:23 am

I have found when I kept a probe in the water at each step of transfer, that the graph was fairly flat while the beaker was sitting or being moved. The drops came when I transferred to a new container, supporting that it is the heat capacity of the glassware that is the main component of temperature loss.

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Re: Keep the Heat B/Thermodynamics C

Postby smartkid222 » March 23rd, 2012, 6:10 pm

There was much discussion previously in the thread about how supervisors should run the event but I'm not sure if we came to a consensus on loading the device. Imagine the following set up: A large 1L beaker containing water is heated up until the water reaches the desired temperature, say 90oC. Then the some water form here, say 100ml, is poured into the 250ml beaker, which is at room temperature. Last, the beaker is loaded into the device.
In this situation, due to the time it takes and heat capacity of the glassware, the first temperature reading that is humanly possible after the device is loaded is 70-75oC.
Is this how most teams test their devices? The beakers are at room temperature? Are others are finding that the temperature drops significantly (such as 90 to 73) by the time they are able to take a first temperature measurement or are there other methods that prevent this?
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Re: Keep the Heat B/Thermodynamics C

Postby harryk » March 23rd, 2012, 7:24 pm

Well when I do my own testing I just heat the water in the beaker
Starting at 75 it is 56 after 5 minutes, so the water itself is not catastrophically losing heat as some people say, though as schrodingerscat suggested it may be the transfer between beakers/graduated cylinders that causes some drop on temperature, though that is how they have done it at competition and my predictions haven't been thrown off
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Re: Keep the Heat B/Thermodynamics C

Postby smartkid222 » March 24th, 2012, 3:40 pm

Yes, that is one of my questions. Do most competitions have the water transfered from a large hot beaker to the cool 250ml beaker, or is the 250ml beaker already heated?
Also, if you are heating in the water in the 250ml beaker, have you considered evaporation? Some of the water will evaporate by the time it reaches the desired temperature, especially if you are going to 90oC. How do you account for this?
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Re: Keep the Heat B/Thermodynamics C

Postby foreverphysics » March 24th, 2012, 5:22 pm

Yes, that is one of my questions. Do most competitions have the water transfered from a large hot beaker to the cool 250ml beaker, or is the 250ml beaker already heated?
Also, if you are heating in the water in the 250ml beaker, have you considered evaporation? Some of the water will evaporate by the time it reaches the desired temperature, especially if you are going to 90oC. How do you account for this?
We bring our own temperature probe and measure the temperature when it's actually inside the device. Then you predict from there. Usually, evaporation's not a huge issue for us. We were 0.5 degrees off our prediction.
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Re: Keep the Heat B/Thermodynamics C

Postby obstinator » March 25th, 2012, 12:02 pm

Is wax paper an allowed material?

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Re: Keep the Heat B/Thermodynamics C

Postby adrenalynn » March 25th, 2012, 12:49 pm

So it ended-up going off pretty much without a hitch. Measured losses from my preheated volumetric flasks into the student's beakers were a consistent 10C. I heard from two DivC teams that that was consistent with their experiments, and they were able to very closely predict the final outcome.

I ended-up building a PID-controlled water bath. It maintained the temp within +/- 0.5C from 7AM to 2PM.

Here's a photo from its first light on the floor of my lab. Someone needs to mop that floor. ;)

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If anyone wants to know how to build their own for around $100, let me know.

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Re: Keep the Heat B/Thermodynamics C

Postby adrenalynn » March 25th, 2012, 1:10 pm

Oh - and here's a word of advice for event coordinators, if any are reading:

An electric blanket makes an outstanding inexpensive glassware warmer. :)

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Re: Keep the Heat B/Thermodynamics C

Postby cupcakefacex » March 27th, 2012, 6:57 pm

we have a barrier (with a 1.5 cm hole) that we place into the beaker. it is removable so it doesn't modify the beaker. Could this barrier be placed in before the water is poured, though? it would help save set-up time.

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Re: Keep the Heat B/Thermodynamics C

Postby JTMess » March 28th, 2012, 10:30 am

we have a barrier (with a 1.5 cm hole) that we place into the beaker. it is removable so it doesn't modify the beaker. Could this barrier be placed in before the water is poured, though? it would help save set-up time.
From an earlier clarification I read, I don't think it's allowed for parts of your device to extend below the lip of your beaker.
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