Post by Professor Lake Shore on Mar 29, 2017 14:28:16 GMT -5
Short of upgrading to a “dry” cryogenic probe station, what are some things that I can do to reduce ongoing LHe consumption in my station?
Here are steps you can take to help minimize LHe consumption in a “wet” cryogenic probe station:
- Ensure that you have good vacuum in the probe station. In short, poor vacuum equates to a higher cooling load. Proper vacuum is important because it reduces heat transfer to the cold stage via the residual gas in the chamber as well as the thermal load due to the solidification of the gases on the cold surfaces. To operate efficiently, the probe station should be able to achieve a vacuum of <10-3 Torr at room temperature using the vacuum system and with the gauge on the chamber. Chapter 6 in the user manual provides diagnostic procedures for verifying that the vacuum pumping system and probe station vacuum chamber are functioning properly.
- Ensure that you have good vacuum in the transfer line. If you see that you are requiring more and more LHe to cool the probe station (as shown in the cryogen consumption spec listed in the user’s manual), the problem may be a loss of vacuum in the jacket of the transfer line resulting in a “soft” transfer line. A tell-tale sign is frosting of the transfer line during operation. Soft vacuum can be caused by leaks that have developed in the jacket and O-ring seal as well as the buildup of outgassing in the absorbent charcoal filters within the line. The solution is to attach the line to a turbo vacuum pump and evacuate the line at room temperature, typically over the course of a day*. This will release what is stored in the charcoal and “recharge” the vacuum space. (See Chapter 6 in the user manual, which provides more details on troubleshooting for vacuum integrity.)
- Ensure proper maintenance of storage Dewars. Typical storage Dewars boil-off roughly 1 to 2 L of liquid helium per day; excessive boil-off rates point to faulty or poorly maintained storage Dewars where loss of vacuum in the Dewar jacket is often to blame. Contact your helium supplier or Dewar manufacturer for maintenance procedures.
- Regulate the pressure on the Dewar during operation. The correct amount of pressure creates efficient cryogen flow (see Chapter 3 in the user manual for recommended gas pressure rates). Excessive pressure in the Dewar can result in higher transfer rates in which more hot gas flows into the storage Dewar and increases boil-off. Additionally, with higher transfer rates, the enthalpy of the cold helium is not fully utilized in the heat exchange mechanisms of the probe station before being vented to atmosphere – effectively wasting cooling power.
- Limit the flow rate during cooldown and at higher temperatures. At the expense of cryogen consumption, higher cryogen flow rates are conventionally used for faster cooldown and increased measurement throughput. When cooldown time is not a significant factor, helium can be conserved by slowing the cooldown with a reduction in the foot valve opening – typically 2 to 3 turns. Additionally, when you’re experimenting at higher temperatures, you don’t need as much cooling capacity. In this situation, adjust the flow as needed to conserve helium usage. (Chapter 4 in the user manual details this procedure. Pay particular attention to the recommended heater output power.)
* A compact turbo pump is required to perform all of the evacuation procedures listed above. It is also recommended that you contact Lake Shore Cryotronics for assistance before evacuating lines.