My son and I are planning to play junior scientist this summer, starting by building a weather station. The plans that I have call for a sealed chamber barometer (a jar with a balloon stretched across the top; it looks like it might be somewhat accurate.)
I’m not sure whether to place the unit inside, in the air conditioning, or put it outside in the shade. Putting it inside will, of course, flatten out the changes in pressure as things heat up outside, but will still show the changes. Do the metorologists factor out the effects of temperature on pressure, or do they just record it as the meter reads outside?
If you are comparing an air conditioned room to the outside, there is no difference in pressure. There can’t be any pressure gradient on your door/window/wall - if there is, air will leak in or out to equalize the pressure. If you are thinking pressure must depend on temperature because PV=NRT, it’s the N/V (number density) that differs between indoors and outdoors.
Now, as for your sealed chamber barometer - I have no idea how it works. Are you merely creating a sealed container and measuring the volume change? Then in this case, the volume depends both on temperature and pressure. If you want the volume to be dependent on pressure alone, you must keep the temperature very stable. You’d need a heater controlled by a thermostat. Alternatively, you can measure both the temperature (with a thermometer) and the volume of the chamber, and calculate the pressure from there.
Also, be sure to take into account the leakage of air through the rubber membrane. If it is simply a jar with a rubber membrane on top, I think the membrane will become flat in a day or two of constant pressure. It would be better if you could make a system where there is no pressure across the moving surface. Maybe a well lubricated cylinder? (Assuming evaporation of the lubricant isn’t a problem)
Thanks scr4. For some reason, I overlooked the obvious on this one. By the way, since the meter uses an air sample as a reference, I will need to keep it inside.
The design is simple. A balloon is stretched across the top of a jar, creating a flat diaphragm. A straw is glued on top, radially, from the center of the jar to a point a few inches away and next to a scale. Pressure changes will cause the tip of the straw to move up and down as the ballon is pushed up and down on the jar mouth. Since the reference (the air inside the jar) is at atmospheric pressure when the meter is created, it probably won’t leak very much. All the same, I may add a layer of saran wrap or some other material to further seal the jar.
OK, that should work. As I said, you can’t read the pressure straight off the barometer, you have to take a temperature reading and compensate.
However, I’m not convinced that it will seal well enough. If it doesn’t work, here’s an alternative design: Take a jar, drill a hole in the lid and put a straw through the hole, almost reaching the bottom of the jar. Make sure the gap around the straw is sealed (epoxy or RTV should work, maybe even candle wax), as well as the jar lid itself. Put a small quantity of non-volatile liquid in the jar. That way, the only way the air inside the jar can expand is by pushing the liquid up the straw. I think straw is translucent enough that you can put markings on it and read the level of the liquid inside. I’m not sure what kind of liquid would work - vegetable oil or motor oil is probably fine, but if you can find something colored, it’d be better. Water and water based ink is too volatile. I haven’t actually built this thing (since I came up with the design only 2 minutes ago) but I don’t see why it shouldn’t work.
To add a little bit to SCR4’s idea: I would make sure to pick a day with high atmospheric pressure. Remember, you are comparing outside pressure to your internal standard inside the jar, which is fixed based on the weather of the day you set it up. It’s much easier to read lower pressures (which cause the column of liquid to rise in the straw) rather than higher pressures (which force liquid out of the straw. Plus, if all of the liquid is forced out of the straw, you have a problem, since you would have introduced some air into your closed system, which in turn makes all of your pressures after that point null and void. Other than those few caveats, I would say that SCR4’s set up is quite sound…
Something to keep in mind as well. When calibrating your system, remember that weathermen give pressure in inches of mercury. You need to correct this measurement based on which liquid you are using by correcting for the density difference.
scr4, The straw idea will work, but the incremental change will be so small that it will be difficult to see it. There were instruments made in the old days that were similar but they had a tapered glass tube that in effect amplified the level change. The rubber diaphragm technique does work quite well - at least in the short term, don’t know about long term calibration. By using a lever arm you get the amplification that makes the change easy to see.