[Unknown008]

I'd like to how how a thermometer can work. The mercury (or tinted alcohol) expands and finds its way through the capillary tube. However, its expansion will compress the air in the capillary tube. Doesn't that air create a pressure on the mercury (or alcohol) and affect the readings while being compressed? Also, the gas has to heat up upon compression, and will tend to make the mercury (or alcohol) rise more.

Any sensible way to help me understand this is welcomed :)

I actually have a hypothesis, it's that the heating due to the compressed air 'compensates' the pressure it creates on the liquid. But that seems 'too good to be true' since both factors exactly compensate cancels each other.

[Perito]

You evacuate the thermometer before putting anything in it. There isn't any air to compress -- just alcohol or mercury vapor. As you compress the vapor, some of it condenses to maintain the same vapor pressure.

[Unknown008]

Ok, I've done some research before posting this question, but what I obtained was that thermometers do contain gases, either argon or nitrogen for their inert properties. Ok, just forget that.

Now, if there were no air at all, wouldn't there be a tendency for the liquid to move up?

That may be a stupid question from me... :(

Anyway, I've got another question:

If you heat a metal rod, it will get longer by expansion (neglecting the diameter increase)

If you heat a metal sheet, both its length and width will increase.

Taking this idea, if you heat mercury, you will not have it increased linearly. Since an increase caused by an atom of metal will add up in a volume.

Thus, a rise of 1 degree previously producing x change in height will then produce an x^3 change in height. :confused:

[Perito]

Actually, I didn't know they contained gases. Interesting. We built a thermometer in a lab once -- just evacuated it.

Alcohol thermometers do have problems with the liquid evaporating and depositing elsewhere. The usual fix is to heat up the thermometer until all of the liquid collects in the small enlargement at the top of the thermometer. The liquid segments merge and you cool it down. You have to be careful not to overheat the thermometer or it will break. Maybe that's why they put some inert gas in there -- though I don't see why air wouldn't work as well. What's going to react?

Taking this idea, if you heat mercury, you will not have it increased linearly. Since an increase caused by an atom of metal will add up in a volume.

Thus, a rise of 1 degree previously producing x change in height will then produce an x^3 change in height

You're probably right, you get an x^3 change in volume. When you force that volume into a capillary, you get a change in height. It doesn't matter whether it's proportional to v^3, it's still linear when confined to the tube's dimensions.

[Unknown008]

Yeah, I've got some difficulty to express myself when I don't understand something. :o

That means therefore that the capillary is not constant in diameter?

And we can say I have some experience in 'overheating a thermometer. I thought that a thermometer would break at the top when overheated, but it happens to be the bottom part, at the bulb. It was my physics teacher who told that first. After considering, I realised that it was true since the walls were thinner there.

EDIT: I think that oxygen can react with mercury it it were exposed for a long time.

[Perito]

The capillary is constant in diameter except at the bottom where the liquid reservoir is, and at the very top there is an enlargement.

[Unknown008]

I'm still not convinced... :(


I'll continue the discussion tomorrow. I'll try to upload pictures to show what I mean.

[Perito]

Here's a link to a Cole Parmer page where they talk about reuniting separated columns in thermometers:

Methods For Reuniting Separated Columns

In the "Heating Method", they mention that it is only applicable to thermometers "with expansion chambers sufficiently large to accommodate the separations plus a portion of the main column". Not all thermometers have expansion chambers at the top.

I've used the cooling method a few times, but I've never tried to centrifuge columns back together.

In the Omega catalog, Omega Catalog - Temperature, they mention:

"Expansion Chamber– an enlargement of the capillary bore at the top of the thermometer to prevent build up of excessive pressures in gas-filled thermometers". The picture doesn't go high enough to show it, however.

So maybe it's only in thermometers which contain gases.

Here's a diagram of a thermometers with an expansion chamber. They also show a contraction chamber, and I don't believe I've ever seen one of those -- at least not like the one in the picture.

Thermometer Book

[Unknown008]

Nor I have seen Perito. I found you edited your post, there was more information that time, and the book was very good. There is another source of error (from the book) in the expansion of the glass relative to the mercury. Also, pressure too affects the bulb.

Anyway, here is what I picture in my head:

The little atoms occupy more and more space while expanding. Here, you can only see the linear expansion. If I had put more on the sides of each column, there would have been an increase there too. Now, considering the fact that diameter of the tube remains constant, the column of liquid cannot expand to the sides. The atoms would therefore tend to move up, creating an additional change in length.

But that is not the case (well, I've been told) in a thermometer... :(

[KISS]

Found this:

The Encyclopedia Britannica: A ... - Google Books

[Unknown008]

Hmm... I didn't find anything so much interesting there. There were three books, and none could be previewed... :(