Avoiding Blocked He Impedances

Service Note 1070-315
Recommendations for Avoiding Blocked Helium
Impedances
(Systems: MPMS, PPMS, SQUID VSM)
Introduction
If you have difficulty in setting temperatures below 10K or are experiencing reduced performance
when cooling the sample space from high temperatures, this behaviour is most likely caused by a
blockage in the secondary and/or primary helium impedance tube. This blockage can be caused by
frozen air or other contaminants that can enter the impedance. The impedances are at the bottom of
the probe and pump helium liquid from the dewar. The blockage will reduce the flow of helium and
lead to the symptoms mentioned above.
For diagnosing and clearing a blocked impedance on the PPMS please follow the procedure in the
PPMS service note 1070-306 on the website www.qdusa.com.
Possible Sources of the Blockage
A blockage can occur for diverse reasons and we will discuss in this note the most common.
I.
Blockage during liquid helium transfer
A full description of the procedure for cooling down your system is at the section “Initial
Helium Transfer/ Consecutive Transfer” of the relevant system manual.
In addition to the instructions presented in the manual, there are some recommendations to
prevent blockages during the transfers:
1. Avoid lowering the transfer line to the bottom of the storage dewar. Keep the end of
transfer line at least 5 cm above it. This will avoid collecting all the frozen particles
accumulated on the bottom of the dewar.
2. If the system already contains helium, minimize the time between removing the dewar
pop-off valve and inserting the transfer line. This reduces the amount of air that is
cryopumped into the liquid helium space.
3. If the system already contains helium, do not insert the transfer line inside the system
until you observe the helium “flame” coming out of the extension. This will prevent the
presence of air inside the transfer line, which will then enter the dewar.
4. In case you need to use more than one storage dewar to cool down your system, warm
up and dry the transfer line before inserting it in the second dewar and resuming the
transfer. To dry the transfer line faster it is useful to flow warm helium gas through it.
Quantum Design
Service Note 1070-315, Rev. A0
January 17, 2012
1
Recommendations for Avoiding Blocked Helium Impedances
5. Do not wait until the helium level is below 40% before refilling the system. Below this
level the helium liquid may fall below the impedance inlet. This increases the risk that
air enters and blocks the impedance when one opens the dewar for inserting the transfer
line.
II.
Blockage related to the vacuum pump
The vacuum pumps are a basic element of the temperature control and gas handling system
and if not properly maintained can cause problems in reaching low temperatures.
1. The oil of the oil pumps will need a periodic refilling to ensure it is inside the
operational limits. Failure in this basic maintenance will produce gas backflow that can
produce a blockage over time.
2. Perform maintenance on the pumps without previously taking the required steps to
isolate the system will allow air to enter the system and block the impedances. If you
have questions about the pump maintenance, please contact your Quantum Design
representative in order to receive the proper instructions.
3. In the case of scroll pump maintenance for systems with EverCool or Reliquifier option,
it is very important to contact your Quantum Design representative to receive the
appropriate instructions.
III.
Blockage because of poor quality of the liquid helium or gas used
during the transfer
Some commercial suppliers do not make a regular cleaning and warm-up of their storage
dewars. Consequently the dewars accumulate frozen contaminants in the liquid helium.
During the helium transfers these contaminants can be pushed inside the system dewar and
block the impedances. We recommend you to contact your local Quantum Sales/Service
Representative in order to receive information about the recommended suppliers in
your region. The recommended suppliers will be chosen relying exclusively on the positive
experience of regular users of this supplier in the region. Quantum Design will not take
responsibility in the malfunction of the system caused by helium bought from suppliers that
are not specifically recommended. This recommendation is subject to change, therefore we
recommend contacting your Quantum Service Representative as soon as you suspect poor
quality helium. The feedback of our customers is very important to select the right supplier.
The helium gas used to pressurize the storage dewar has to have a purity of at least 99,996%.
Do not use helium of lower quality or unspecified quality, such as the one from a recovery
system, to perform the transfers.
One way to recognize blockages produced by dirty helium is when one or more QD systems
block simultaneously right after a helium transfer. Please inform your supplier in case this
happens in order to prevent this incident in the future.
The use of a filter at the transfer line inlet in the storage dewar might reduce the amount of
contaminants, however since nitrogen tends to form a slush it seems to be difficult to filter
100% of contaminants when you use helium with frozen impurities. A filter made of a
sintered block of fine metal particles, such as the 1 micron sinters used at the inlet of the QD
impedance assembly, may be a good candidate for such an inlet to the transfer line. Contact
Quantum Design if you would like more information about such filter designs.
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Service Note 1070-315, Rev. A0
January 17, 2012
Quantum Design
Recommendations for Avoiding Blocked Helium Impedances
IV.
Blockage produced by a leak in the system
When, despite taking the above precautions, an impedance blockage reappears during
cooldown or after some time of system operation then it is necessary to look for a possible
leak in the dewar or gas handling system. To locate such a leak may require you to contact
your Quantum Design representative for further assistance.
Quantum Design
Service Note 1070-315, Rev. A0
January 17, 2012
3