How do they cool liquid helium.

[cragar]

Ok so when they compress the gas and remove the heat until it liquefies then how do they further cool the liquid. I know they can evaporate a little more of the liquid helium to cool it a little bit more. And i couldn't find much on how a pomeranchuk cell works .
Any input will be appreciated . And also could this low temp be measured with a thermocouple.

 

[f95toli]

Cooling liquid helium is easy:all you need to do is to pump on the liquid; i.e. pump away the vapour using a vacuum pump. This will lower the temperature from 4.2K to about 1.2K or so. You can't really go lower than that using only helium-4; you either need to use He-3 as well in a second stage (oumping on liquid He-3 get you down to about 230mK) or use a mixture of He-3 and He-4 in a dilution refrigerator (which will get you down to about 10-20 mK). Or, alternatively, use other types of cooling such as adiabatic demagnetization refrigeration.

Temperatures below 4.2K are usually measured using resistive thermometers (thermocouples don't work very well at these temperature); these are essentially just resistors the component made from materials where the resistivity is strongly temperature dependent meaning you can plot a resistance vs. temperature graph (meaning all you need to do in order to measure the temperature is to measure the resistance), examples of such materials would be RuO2 or germanium.
There are also quite a few other methods (capacitive, various primary thermometers etc) .but resistive thermometers are by far the most common below 4.2K.

 

[cragar]

thanks for your answer , so basically you have a vacuum pump and you pump away the vapor and this cools it , So I imagine this is the same as putting water in a vacuum chamber and pumping the vapor away till the water freezes.

 

[cragar]

And when the Helium reaches the lambda point wouldn't it start to seep out of their container.

 

[f95toli]

thanks for your answer , so basically you have a vacuum pump and you pump away the vapor and this cools it , So I imagine this is the same as putting water in a vacuum chamber and pumping the vapor away till the water freezes.

Exactly

And when the Helium reaches the lambda point wouldn't it start to seep out of their container.

That can happen (and frequently did in e.g. old dilution refrigerators, which is why you needed a so-called film heater); but remember that you need a Rollin film on the outside of the (open) container in order for the He to creep out and "escape".
In most systems this won't happen simply because the outside (or even the top) of the container is warmer than the lambda point.