Science Buddies: "Ask an Expert"

Hello, I first asked a question which you helpfully answered several months ago. I have now decided to try for a later regional fair instead of in June due to time constraints. For this project, I would like to know how much a surface would heat up in degrees centigrade or K when water condenses on it in drops. I know when the water condenses, it will heat up the surface and I would like to know by how much. Do you know how would I calculate this and what parameters would I have to measure to be able to do it? I would be grateful for any advice. Thank you.

Hello squnkus,

To begin, you will need a sensitive thermometer to measure the temperature change. Common electrical thermometers used are based on thermocouples or thermistors. The thermocouple generates a voltage that changes with temperature, and the thermistor's electrical resistance changes with temperature (see http://www.alliedelec.com/images/produc ... 012600.pdf). Thermocouple thermometers can be bought ready-to-go. I don't think thermistor devices are as standardized, but the electrical circuit needed to operate one is not very complicated. I think the thermistor is more sensitive. Both the thermocouple and thermistor are small, so they can be attached to almost anything.

When the water vapor contacts the condensing surface, the vapor has to cool to the condensing temperature; then it has to condense and release its latent heat or heat of vaporization ( see http://en.wikipedia.org/wiki/Enthalpy_of_vaporization). Finally it has to cool down to the condensing surface temperature.

Untangling these effects should be fun. You will need to understand the basics of heat and temperature and the concepts of specific heat and latent heat, and you will have to do the calculations for your setup. The calculations can range from simple algebra to calculus depending on how sophisticated you want to be.

You would need to measure the temperature of the vapor, the temperature of the condensing material, and the amount of vapor condensed. The amount condensed could be measured by weighing.

Okay, that's a quick survey of what you need. Still interested? Get back to us with details if you proceed.

Good luck, WW

Thank you very much for your reply WW. I have read the Wikkipedia article now. I find some of it a bit confusing but overall I understand. I have another question if you wouldn't mind. You said that I would have to measure the temperature of the water vapour. Would this be the same as the temperature of the ambient air or is this something else that might be difficult to measure? I am not sure if I want to proceed with this but I hope I can. Thank you again.

squnkus,

Yes, the water vapor will be mixed with the surrounding air. The water vapor and the other gases making up the air will all be the same temperature except in very unusual circumstances.

Since I don't know the details of what you plan to do, I can only guess at additional issues to consider, but I'll mention a few others. Condensation will occur only if the surface is colder than the air and the surface is colder than the "dew point" of the air/vapor. In your experiment, what keeps the surface cold? If the surface is part of a large object, then any heat from the condensation will flow into the whole object over time. The overall heating of a large object might be very small. Also, the object will be warmed by the surrounding air in addition to the condensation.

If you want to discuss more details, please give me a verbal description of how you plan to set up your experiment. A sketch would also be very helpful.

WW

Hi Wendell,
Thank you again for your reply. Sorry it has taken me a while to get back to you. My school exams are over now and I finally have a chance to try to get on with my project. I agree that cooling will be one of the most important parts of it. I hope to use the night sky for this. If I have a surface facing the night sky and this is connected to my device, I am hoping that the device will cool faster than without this surface. I am not sure if this will cool it enough and keep it cool. I have purchased a thermometer to measure surface temperature. I am just going to do the
designs now. You mentioned calculations that should be done before set up. Should I do calculations before I do the design? If I have to, which ones should I do? The basic design will be a rectangular prism with the small ends open and the long axis perpendicular to the ground. The construction material will be metal. After what you said regarding a larger object heating up more slowly than a smaller one, I am wondering if a larger device would be better than a smaller one. I’m also not sure what size to make the surface facing the sky relative to the rest of the device in
order to have any chance of cooling it. Would I have to figure it out by trial and error or could there be a simple mathematical way I could do this? I haven’t done calculus yet and my parents can’t remember how to do it. I would try it if it’s the only way.
Thanks again,
Squnkus

Hello,

I am just wondering if anyone would have answers to any of my questions. I have started the project but I would appreciate any advice. Thank you.

Hi squnkus,

Sorry to be so late in answering your post of 6/28, but I missed it and only saw your more recent post.

Here are some comments about the device described in your recent post.

Using the night sky to cool your device is a "cool" idea. From your description, I envision a thin metal plate (maybe aluminum) exposed to the night sky and insulated on the bottom and around the sides. A thermocouple junction attached to the plate would measure temperature.

If you live at high altitude where the humidity is low, you should see large cooling. If you live at low altitude and with high humidity, the effect will be smaller.

See http://asterism.org/tutorials/tut37%20R ... ooling.pdf.

The "emissivity" of the plate will affect the cooling strongly. A high emissivity will cool more than a low emissivity. Painting the plate flat-black will give a high emissivity, and a polished "mirror" finish will give a low emissivity.

See http://en.wikipedia.org/wiki/Emissivity.

Air movement around the device will affect the heat conduction between the plate and the surrounding atmosphere. Be sure to arrange what you can to reduce air movement. Also, the temperature of the surrounding air will affect its heating/cooling. Be sure to measure the air temperature and compensate for changes in it.

Set up the device in a particular configuration and wait for the temperature to settle to constant values. It may take quite a while before everything settles. You don't want to be measuring while unknown parts of the device are still cooling off.

I suggest that you start with simple measurements and go to more complicated ones as suggested by the simple experiments. A good start would be to measure the temperature with the plate exposed to a clear night sky versus the plate covered with a styrofoam sheet. Another measurement would be the plate temperature with a second metal plate covering it (but not touching) to block radiation, but allow conductive cooling.

Please stay in touch with your results and more detail about your device. Pictures would be nice if you can post them on the web.

I'll try to watch for any further posting and get back to you promptly.

Good luck, WW

Hi Wendell. Thank you for your reply. I have just started the project and I will use all your suggestions. My first problem is to set up a testing system that will not be destroyed by animals during the night ( we have a lot of racoons and squirrels here) and rain as it has to be open to the sky - any suggestions welcome.... I will let you know how I am doing. Thank you again.

squnkus,

While I mentioned that you have to set up the device and then wait for it to reach a stable temperature, that doesn't mean all night. The air temperature will change a lot during the night and a single reading won't capture what happened. I think you will need to monitor it every few minutes and record the results. Your periodic appearance will discourage the beasts.

Also, I didn't mention measuring any water that condenses. Do you have a method to measure condensed water?

WW