Making accurate measurements at ultra-low temperatures is no easy feat, especially when working in the ranges seen by modern dilution refrigerators. The Model 372 has many features specifically developed for dilution refrigerator applications.
Dedicated temperature control input
Taking measurements at ultra-low temperatures deserves uninterrupted attention from measurement devices. The Model 372 uses a dedicated temperature control input that is designed specifically for connection to a negative temperature coefficient resistive sensor. This input is designed to continuously monitor the temperature of the dilution refrigerator sample holder, while the measurement input scans through the multiple other temperature sensors placed throughout the dilution refrigerator.
The dedicated control input ensures uninterrupted dilution refrigerator temperature control
Multiple heater options
Three separate heater outputs are available on the Model 372:
- Sample heater—for fine control of the sample stage at ultra-low temperatures with up to 1 W of power available.
- Warm-up heater—supplying up to 10 W of power and featuring a warm-up mode specifically for the purpose of bringing the system temperature up to allow work to be performed on the sample stage.
- Still heater—an additional 1 W heater is available for the purpose of controlling the temperature of a dilution refrigerator’s still. Alternatively, this output can provide an analog out signal to other devices if required.
The sample and warm-up heaters have many powerful control options, including PID control that allows both the setting of fixed temperature setpoints as well as ramp rates.
Stable temperature control
When operating at ultra-low temperatures, even small amounts of added energy can cause unwanted spikes in system temperature. The Model 372 heater outputs implement several protection mechanisms to reduce or eliminate this potential:
- The circuitry for the sample and still heaters are electrically isolated from other instrument sections
- Multiple power range settings allow extremely fine or coarse power transitions, depending on the need
- Heater outputs are shunted during power up and power range changes, eliminating the potential for unwanted power surges
- Terminal connections allow twisted pair cabling to be easily used for heater wiring; additional shielding of these wires can also be added to further reduce the potential of injecting noise into a system via the heater cabling
Temperature zone control
Thermal response characteristics of a dilution refrigerator system can change quite dramatically over the useful range of operation, particularly down towards the lower temperature limit of a system, where cooling power is reduced. To accommodate these system variations, different PID values can be set for different temperature ranges (zones). This allows for more aggressive transition settings to be used at higher temperatures where system response is faster, and less reactive settings at low temperatures when temperature overshoots result in long recovery times.