# Constant Volume Thermometer

#### orthovector

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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#### mgb_phys

Homework Helper
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.

#### orthovector

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?

#### mgb_phys

Homework Helper
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 guage nowadays. I must admit I've only seen 'lab demonstration' type constant volume gas thermometers, not a real industrial one.

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#### orthovector

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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