xDevs.com | LakeShore 370 AC Resistance bridge for cryogenics thermometry

Introduction

Let’s imagine application or a physical experiment require measurement of extremely cold temperature, such as 4 Kelvin (-269 °C or -452.5 °F). You cannot use your usual thermonuclear thermometer for that. Even high-end top of the line DMMs such as Keithley 2002 wouldn’t be suitable for this job. Such extremely cold temperatures require special characterized probes such as silicon or germanium diodes or special resistive elements designed to operate at cryogenic temperatures. Such sensors often cost hundreds or even thousands of dollars, but its only beginning of the story. Cryogenic sensors need to operate with very low exitation power to limit self-heating, because most of cryogenic refrigerators have very limited cooling power available. Typical 2-stage GM cryocooler like ones used in the MRI machines consume 8000-10000 Watts of electricity to provide just 1W of cooling at 4 Kelvin temperature.

Few manufacturers in the world mastered designs and production of special thermometers for cryogenics work to support those special sensors. One of such manufacturers is Lake Shore Cryotronics, advancing cryogenics industry for five decades since 1968. Today I’ve got lucky to look at and experiment with Lake Shore Model 370 AC Resistance Bridge . This instrument can accurately digitize temperatures even below 0.05 Kelvin.

Some key features presented below:

Measurement type : AC Sinusoidal AC current source, 4-lead differential resistance
Number of inputs : 1 (or more with external optional scanner)
Measurement units : Ω or Kelvin (with temperature response curve calibration)
Resistance ranges : 2 mΩ to 2 MΩ (excitation dependent)
Supported cryogenic sensors : Any resistive sensor including NTC resistors (e.g., germanium, carbon-glass, Cernox™, ruthenium oxide [Rox™]) and PTC resistors (e.g., rhodium-iron RTD)
Temperature units : K
Minimum temperature range : From below 20 mK and up to room temperatures
Temperature resolution : Up to 6 digits, depends on sensor
Temperature curve memory : Space for twenty 200-point calibration correction curves
Accuracy : Range/sensor dependant, best is ±4.2 mK
Excitation frequency : 13.7 Hz / 9.8 Hz / 16.2 Hz
Excitation currents : 21 ranges from 3.16 pA to 31.6 mA RMS
Minimum excitation power : 10^-18 W into 100 kΩ (see 370 performance specification table)
Input impedance : Approaches 5 × 10¹² Ω
Range selection modes : Manual, Voltage Excitation, Current Excitation, and Autorange
Outputs/heater control : High-resolution closed-loop or open-loop PID
Heater power available : from 0.1 µW up to 1 W into resistive heater
User interface : 8 × 40 graphical vacuum fluorescent display, up to 8 parameters, keypad
Bus interfaces : RS-232C and GPIB
Operation temperature : +15 °C to +35 °C at rated accuracy
Power requirement : 100, 120, 220, 240 VAC, +6% -10%, 50 or 60 Hz, 50 VA
Size : 435 mm W × 89 mm H × 368 mm D (17 in × 3.5 in × 14.5 in), full rack
MSRP : Around $8000 USD, today 370 is replaced with newer Model 372

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