View Full Version : OT: Thermometers

John Foster
03-07-2004, 10:59 PM
OK, how do they make alcohol or mercury thermometers? I'm OK until it comes to heating the top and sealing it off. Well maybe a little troubled by figuring out how much "stuff" to put in one.

03-08-2004, 12:49 AM
Per Her Majesty's Meteorological Office Handbook of Met. Instruments: with mercury you should produce a vacuum above the mecury column before sealing, or less desirable - fill with an inert gas, e.g. nitrogen. Below -37.9F should use spirit thermometr, e.g. ethyl alcohol, and seal it at the lowest temperature of its range.
Unfortunately the handbook sheds no light on methods for doing that. Sorry.

J Thornton
03-08-2004, 02:59 AM
Wouldn't it seal fairly easily just by twisting the tube while maintaining the vacuum? You would need to start with a tube a couple of inches longer than your finished product.

Where's the spellchecker when I need it?
[This message has been edited by J Thornton (edited 03-08-2004).]

[This message has been edited by J Thornton (edited 03-08-2004).]

John Foster
03-08-2004, 10:26 PM
I thought about using a vacuum but then what about vapor pressure especially on alcohol? Seems the fluid would boil off with the vacuum. An inert gas would have to be compressed, in a sealed tube, as the temp rises. In any case it seems to me the bulb would be chilled as much as possible while heating the top of he tube to seal it off.
How do they get the liquid into the bulb. Come on you guys, this is the science lesson for this week!

03-08-2004, 10:32 PM
Take an open ended tube, put some mercury in it, top it off with the gas of your choice and seal it. Then set up a graph by taking it to temperatures of known existances. (water is 212 when boiling at sea level) water freezes at same elevation at 32) Mark off the distance in between the 2 marks (freeze and boil) with the appropriate number of marks and you have it.
Just my guess, David from Jax


03-09-2004, 12:05 AM
I assume you are using pyrex tubing. I would think that starting with a piece of pyrex tubing you close one end and ball it a little and then draw the tubing down in size to get the capillary thermometer dimension. The best way to do that is to use a "drop tower" where the hot glass tubing is dropped and draws the glass in a very straight line when it is dropped. I guess you could pull it but I would worry that it might not be as constant wall and bore size as by dropping. Since pyrex has lousy thermal conductivity I think you could pull a vacuum on the closed tube then tip it up to let mercury in at the tip and then vent to drive the mercury to the bulb. You could calibrate the thermometer with the top open to the air and then pull vacuum after that and heat and pinch off the glass when vacuum was established.

03-09-2004, 02:52 AM
This doesn't entirely answer the question but is apropo..