Temperature calibrator 7196B series

Product overview
The nominal boiling point of nitrogen is –196 °C at one atmosphere. The defining triple point of argon is –189.3442 °C. The difference between nitrogen boiling point and the argon triple point can be corrected mathematically, enabling comparison calibrations with an achievable uncertainty of less than 2 mK relative to the actual argon triple point.

Design and models
Fluke Calibration LN₂ Comparison Calibrators comprise a vacuum-sealed stainless steel Dewar flask, a high-purity copper block and a precision-fit lid. The Dewar is charged with liquid nitrogen and the copper block is suspended inside; a standard platinum resistance thermometer (SPRT) is inserted into the block and compared to the user’s calibrated SPRT for calibration.

• 7196B-4: four 8 mm (0.32") I.D. thermal wells.
• 7196B-13: five 8 mm (0.32") I.D. wells and eight 6.35 mm (0.25") I.D. wells.

Key features

• Enables low-cost comparison calibrations at nominal –196 °C.
• Measurement uncertainty typically < 2 mK versus the argon triple point.
• Mechanically simple: vacuum Dewar, high-purity copper block and precision lid for repeatable immersion depths.

How it is used
The device uses a copper block immersed in liquid nitrogen; an SPRT placed into a thermal well in the block is compared to a calibrated reference SPRT to perform cryogenic comparison calibrations. The unit is intended for laboratory comparison work and requires a calibrated reference SPRT and appropriate handling of cryogens.

Technical specifications

• Model family: 7196B LN₂ Comparison Calibrator (variants: 7196B-4, 7196B-13)
• Nominal temperature: –196 °C (at 1 atm; varies with atmospheric pressure)
• Thermal wells: 7196B-4 = four 8 mm (0.32") I.D. wells; 7196B-13 = five 8 mm (0.32") I.D. wells + eight 6.35 mm (0.25") I.D. wells
• Immersion depths: 275 mm from top of lid to bottom of well; 150 mm immersion into copper block
• Outer dimensions (O.D. x height): 168 mm x 406 mm (6.6 in x 16 in)
• Temperature stability: typically < 2 mK over 20 minutes (depends on atmospheric pressure stability)
• Temperature uniformity: <0.0004 °C
• Volume: 5.47 liters (1.45 gallons)
• Operation principle: vacuum-sealed Dewar with high-purity copper block; SPRT inserted into block for comparison calibration
• Note: equilibrium temperature varies with atmospheric pressure (dT/dp ≈ 0.085 mK/Pa); actual stability depends on pressure stability