The Care and Keeping of the Cold Head
NORMAL COLD HEAD PERFORMANCE
As the cold head ages it may develop some different noises, most commonly there is a chirp, a high pitched noise that does not last long (less than a cycle). Some chirps are soft and some are louder. These can happen at any time in the cool down and the occurrence of the chirps may change over time. The reason for these chirps is that the cold head is a very clean environment, with minimal grease used, so as the motor wears the moving parts produce squeaking. The cold head will continue to cool down to the base temperature as normal.
Figure 1: Front view of cold head without lid and front panel
ABNORMAL COLD HEAD PERFORMANCE
If the cold head grinds or cogs, for instance sounds like metal grinding against metal and lasts for a full cycle or more, then it is best to stop the cold head. Please report to a Montana Instruments representative the nature of the issue. It is also an important data point to know when the noise started to occur (at the start of the cool down or when it was approaching base temperature). These issues could indicate contamination or a broken mechanical part inside the cold head.
Montana Instruments uses a Sumitomo RDK-101 cold head which has 100mW of power at 4K (RDK-101D Capacity Map). The warranty is 1 year after receipt. The manufacturer recommends maintenance every 10,000 hours of use, but we have found that with the variable speed compressor the lifetime of the cold head typically extends to 20,000 hours. If the user needs to operate the system at the highest speed to get the lowest possible temperature, then it should be expected that maintenance may need to be performed prior to the 20,000 hours. You will know when to contact your local Montana Instruments representative for maintenance if you have a decrease in performance. If the system cannot reach base temperature then you’ll need maintenance. You may also need maintenance if anything mechanically begins to fail which prevents the system from getting cold.
The software automatically controls the speed of the compressor as it cools down; it is not recommended that the user manually control the speed of the compressor as it cools down. Once at the target temperature, the compressor and cold head switch into a slower speed, which extends the lifetime of the system. If your target temperature is the lowest that the system can go, then your system will continue to run hard at 25Hz/70Hz. Note, this is not bad for the system, but the cold head may need maintenance sooner than 20,000 hours.
HOW TO EXTEND THE LIFE OF THE COLD HEAD
Standby or Standby-Equivalent
The lowest power mode for the compressor and control unit is 14Hz/40Hz and this mode is used in a few instances. If you are running experiments during the daytime and do not want to wait the next day for a full cool down, then you can run the system in Standby. This will de-activate the PID, so all heaters will turn off and the temperature will not be controlled. The system will run at 14Hz/40Hz. The next morning the user can set the target temperature and press cool down. The system will quickly arrive at the base temperature, which prevents the user from having to wait an entire cool down cycle before taking measurements. Standby only works below 77K; however, this is not really a problem because if your target temperature is greater than 10K the system will run at 14Hz/40Hz anyway. If your system is already running at 14Hz/40Hz there is no harm in keeping it actively at the target temperature at this speed.
Other Compressor Speeds and Implications for Cold Head Wear
When the system gets to the target temperature and the target temperature is between the base temperature and 10K, the compressor will run at 22Hz/50Hz. This is a medium level speed and running at this speed also reduces wear on the cold head.
If you are going to be pausing your experiment for a couple of days, for instance over a weekend, it is better to warm-up the system and cool down again the following week. In most instances, if there is not a strict firewall system at your facility, you can use the Team Viewer program and start the system cooling down the night before you would like to begin taking data.