Understanding the Function of a Constant Volume Thermometer

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Discussion Overview

The discussion centers on the functioning of a constant volume thermometer, exploring its principles, measurement techniques, and specific applications such as determining the boiling point of water and the triple point of water. Participants express curiosity about the underlying processes and the use of different gases in temperature measurement.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant inquires about how a constant volume thermometer operates, specifically regarding the use of different gases and the reference point of the triple point of water.
  • Another participant explains that the thermometer uses a constant volume of gas and measures pressure to derive temperature using the ideal gas law, noting potential errors due to real gas behavior.
  • A participant expresses confusion about the practical measurement process, questioning how to measure the boiling point of water with the thermometer.
  • It is clarified that the gas bulb should be placed in boiling water or its vapor, and pressure is measured using a gauge, with references to both traditional and modern measurement tools.
  • One participant seeks information on measuring the triple point of water, highlighting the specific conditions required for this measurement.
  • A response describes the use of a triple point cell, detailing the process of achieving the triple point with ice, water, and vapor in equilibrium.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and curiosity about the measurement techniques and principles involved, but there is no consensus on the clarity of the process or the specifics of using different gases.

Contextual Notes

Some participants mention limitations in understanding the practical application of the thermometer and the conditions required for accurate measurements, such as the specific pressure and temperature at the triple point.

Who May Find This Useful

This discussion may be of interest to students and professionals in physics and engineering, particularly those studying thermodynamics and temperature measurement techniques.

orthovector
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Does anybody know how a Constant volume thermometer works?

I know that one of the reference points is the triple point of water, but I'm not sure how different gases are used along with this reference point to get different values of temperature for certain known processes. for instance, if oxygen gas is used to measure the BP of water...one gets a certain temperature that is close to 373.15 degrees kelvin. if nitrogen gas is used, one gets a value that is also close to 373.15 degrees kelvin...but it is different from the value from using oxygen gas.

I cannot picture what is going on...
 
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It uses a constant volume of gas and measures the pressure so from PV=nRT you get the temperature. But the equation assumes a perfect gas so for a real gas you will get a small error because of a, the molecules of oxygen occupy real space so at 0K there is still a volume and b, there is an interaction between molecules so there is more pressure than you would expect.
 
I know that... I just can't imagine how you would measure a process. For instance, if you want to measure the bp of water, do you place the gas bulb into the water and hear the water? And how would u take a reading of the pressure?
 
Yes - you put the bulb in the boiling water (or ideally in the vapour just above it)
Then you measure the pressure with a separate pressure gauge attached to the bulb - normally a mercury manometer but probably with some sort of digital pressure gauge nowadays. I must admit I've only seen 'lab demonstration' type constant volume gas thermometers, not a real industrial one.
 
how would one measure the triple point of water? as u know, triple point occurs ar specific pressure and temp.
 
You use something called a triple point cell.This is evacuated and pure water added but the cell is not filled up., It is then left in the fridge so that some of the water freezes,you now have the triple point with ice water and water vapour in equilibrium.
 

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