[Esr-users] [New Logentry] Superconducting Helium Level Probe - Pressure sensitivity

[kashy at jlab.org]

Logentry Text:
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During the current Hall A Cooldown, there are questions if the dipole helium level sensors are working. 

One phenomenon that I've seen in the past and even recently is that this probe can loose its ablity to read properly at elevated pressures. The cause of this can be explained by the small differerence in between saturated liquid and gas. At our normal operating pressure of 1.2 atm, the density ratio of liquid to gas is 5.8 see 
https://userweb.jlab.org/~kashy/Cryo%20Lectures/Cryo_basics_L1_v3.pdf  (pg 28)

As the pressure increases these densities get closer together until they become the same at the critical pressure ~2.2 atm.

To function a Superconducting level probe partially submerged in liquid helium requires that the liquid touch part of the wire. if the gas is pressurized enough, then its density increases and bubbles do not float out of the liquid fast enough and the probe can become "vapor locked" this can cause it to read zero helium level.

The attached plot is from 8/13/2018 during a recent upset of the ESR when the Hall B buffer dewar pressure rose above normal. This is a fairly long probe ~36". One can see that at ~1600mbar the level probe started to indicate a decline in the level. At 1790mbar the level went to zero indicated. When the pressure wend back down to 1500 the level recovered to the value before the pressure event started.

A similar thing was observed this morning during a test with C. Perry and myself of the Hall A right Dipole
At 23.1 psi (1.57atm) the probe was reading 1 to 4%, by limiting the supply flow Chris dropped the pressure in the magnet (while on warm return) to 1.444 atm and the probe incidated a peak helium level of 29%. This level then slowly fell due to reduced supply flow.  Chris is working with Hall A staff to put the Right dipole D  on cold return so it can be at lower pressure becasue the cold return line has much lower pressure drop that the warm return line. Hopefully this will allow filling of the magnets and reading of the filling with superconducting level probes running in helium below 1.5 atm.




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This is a plain text email for clients that cannot display HTML.  The full logentry can be found online at https://logbooks.jlab.org/entry/3590085
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