Low Pressure Control of Cryogenic Tank
30 June 2021
By Matthias Bogar
The challenge: Research set-up under cryogenic conditions
Our customer, the Quantum Photonics Group at ETH Zürich (a university centre of innovation and knowledge in Switzerland), presented us the following situation: The group runs an experiment inside of a cryogenic tank that is connected to a large gas recovery system. The pressure in that tank was fluctuating, as a consequence of events related to this gas recovery system, that took place in a room adjacent to the research area. These fluctuations had a negative impact on the research measurements. They asked PCS to supply a solution that would protect the application from these fluctuations, to ensure more reliable measurements. With our passion for precision process control, obviously our Matthias Bogar was keen to help the customer crack this nut!
The approach: Exploring the process conditions
To meet all the process requirements of this cryogenic application, PCS presented a combination of an Equilibar ® Low Flow LF1 back pressure regulator (LF Series) and an electronic pressure controller from PCS’ very own ERC Series; both customized specifically to the process conditions of this particular application in order to be robust during operation under the very low temperatures that are characteristic for cryogenic applications. Phd student Gian-Marco Schnüriger – part of the Quantum Photonics Group at ETHZ – explains how stable pressure control is important to help avoid and correct mechanical fluctuations in his research application:
“Being able to stabilize mechanical fluctuations is a crucial ingredient when working with microcavities, which are by nature very susceptible to vibrations. Noise is often not preventable in a research environment, so finding a way to stabilize the environment is very important.”
Schnüriger continues to explain how the cryogenic temperatures pose a challenge in his particular set-up:
“In our case, the experiment requires cryogenic temperatures. For that end, a metal tube containing the sample is submerged into a bath of liquid helium. To minimize the loss of helium, the evaporated gas, about 0.3m3 a day, is returned to a large-scale recovery system spanning the whole institute. So our sample is coupled to all the other cryogenic systems in the proximity, which inevitably represents a large source of noise and vibrations.”
Just as in many other research fields and applications, optical resonators play an important role in the Schnüriger’s research set-up. He explains how the research environment further challenges the process stability, that PCS was challenged to optimize:
“In our case, a fibre-based microcavity is coupled to a semiconductor quantum well structure, allowing the study of the thereby created hybrid particles. Related conditions are:
the narrow cavity linewidth (30ueV);
the requirement for in-situ tunability of the cavity length;
the subsequent necessary degree of freedom
Meeting these conditions comes at the cost of a large susceptibility to the mechanical fluctuations in the direct environment. These fluctuations mainly affect the length of the cavity and thereby the energy of the resonance. In turn, this influences the measured properties and may even lead to a shift out of the parameter range of a given measurement.”
In other words: These fluctuations put the reliability and reproducibility of the research outcomes at risk and are very much unwanted.
Why is stable pressure control so important in this case?
To illustrate the impact of fluctuations in the environment on the process stability in Schnüriger’s application, he shares this example: The filling of another cryostat at the end of the corridor in Schnüriger’s research facility leads to fluctuations in the transmission through the cavity as a consequence of the changes in helium pressure. Figure 1 shows us how this causes a disturbance, consisting of two spikes. One at the beginning and an even larger spike at the end of the filling, when the cryostat volume is already full and all the excess helium is fed into the recovery line.
The solution provided by PCS
In response to the customer’s request, PCS supplied a product solution to stabilize Schnüriger’s research process. This solution consists of a low flow back pressure regulator (model LF1), placed between the cryostat and the recovery system. The LF1-unit is controlled through an electronic reference pressure controller that is capable to maintain a very precise and stable pressure level in the dome of the back pressure regulator. This enables us to control the pressure at a setpoint that is high enough to compensate for the fluctuations in the recovery system. The pressure in the recovery system is around 10mBar, and the pressure for the cryostat is set to 80mBar by the electronic regulator.
How did this solution work for Schnüriger and his cryogenic tank filling application?
“Indeed, looking at the cavity transmission during the filling times of the cryostat along the corridor, we now see that the impact of these fluctuations is minimized. The system is mostly decoupled and external effects are kept to a minimum.”
The transmission through the cavity over the course of a 1-hour-measurement, showing the influence of external events, in this case filling of a bath-cryostat in a different room on the same floor. The chart illustrates 2 larger fluctuations, matching the start and the ending of this filling process, increasing the pressure in the helium recovery line and shifting the transmission through the cavity by 60%.
The effect of the solution designed by PCS becomes clear when we compare Figure 1 with Figure 2; the latter is an illustration of the measurement stability during filling times, after the installation of PCS’ product solution. The difference between these 2 graphics clearly demonstrates the strong performance of the product solution provided by PCS.
Like in Figure 1, the transmission through the cavity over the course of a one hour experiment, while the backpressure regulator is set up and running. Indicated by the black line is the start of filling of a cryostat in the same room as in Figure 1 and it shows that no disturbance of the system is visible.
Matthias Bogar of PCS comments:
“The product set-up supplied by PCS to ETH Zürich [shown in the picture] is a good example of the strong performance of Equilibar regulators under unusual or demanding process conditions; it is precisely what these regulators are designed to do and what sets them apart from traditional regulators.”
In this case, the combination of low pressures and cryogenic temperatures were a challenge that conventional products were unable to solve. Conventional back pressure regulators (usually spring loaded) valves tend to have a high hysteresis that make it almost impossible to achieve a stable tank pressure control at only a few mbar gauge. The Equilibar dome loaded diaphragm sealed technology works frictionless and is capable to control down to very low pressures.
The LF Series back pressure regulators, as used in this set-up, can reach a turndown ratio of more than 1:1.000.000 and can be used for a wide variety of process fluids, flows and pressures, thanks to the available materials (SS316, Hastelloy, Monel, PEEK, PTFE, PVDF etc.) and thanks to its astonishingly broad Cv range from 1E-8 all the way up to 0.07.
The ERC Series electronic reference pressure controller used in this particular application is a single-valve pressure regulator. This means that it has a very small bleed, either into or out of the controller. This ensures that the proportional control valve is continuously actuated, which in turn leads to a control stability within 0,01%, resulting in extraordinary and precise and stable control of the Equilibar back pressure regulator.
Want to know more?
Do you also run an application where stable process control is of paramount importance? Do you want to learn how precision, accuracy, stability & reproducability can be improved in your process lines? Contact our engineers now to talk about your particular process challenge. We will help with expertise and adequate product solutions!
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