Isotech Nitrogen Boiling Point Apparatus

The Isotech 18205 Nitrogen Boiling Point Apparatus provides a simple, reliable and highly stable way to realize the liquid–vapour equilibrium of liquid nitrogen or liquid argon. This equilibrium forms a well‑defined reference point at –195.798 °C (liquid nitrogen) or –185.8468 °C (liquid argon) and is widely used for thermometer calibration under the International Temperature Scale of 1990 (ITS‑90). The apparatus is completely self‑contained, requires no electrical power, and is cooled solely by the liquid cryogen supplied by the user. Liquid nitrogen is typically preferred due to its low cost and easy availability.

A practical alternative to the Argon Triple Point

Although ITS‑90 defines the Argon Triple Point (–189.3442 °C) as the lowest fixed point for long‑stem SPRTs, realizing this point is expensive, time‑consuming and technically demanding. For this reason, most laboratories calibrate thermometers in this region by comparison with a reference thermometer immersed in a liquid nitrogen bath.

Even national metrology institutes routinely use this method. As the National Physical Laboratory (NPL) notes:
“Most thermometers submitted for calibration will involve measurements by comparison with NPL standards in a bath of liquid nitrogen (about –196 °C).”

The Isotech 18205 is specifically designed for this type of comparison calibration.

Design and operation

The apparatus consists of a stainless steel dewar containing an internal equalizing block with wells for three thermometers. The block provides a highly uniform temperature field, with a distribution of less than 2 mK across the copper mass. Top‑mounted connections allow the user to fill the dewar, monitor the liquid level and ensure safe operation via a pressure blow‑off. An optional helium gas manifold can be used to thermally couple the thermometers to the block, improving stability during comparison measurements. As long as the boiled‑off gas is replenished, the liquid–vapour equilibrium remains indefinitely stable, enabling long, drift‑free calibration sessions.

Accuracy and performance

Because SPRTs have very similar dR/dT slopes at these temperatures, comparison calibrations can achieve uncertainties of ±0.002 K, at only a fraction of the cost of an absolute triple‑point realization.

The final uncertainty depends on:

• > the uniformity of the equalizing block,
• > the uncertainty of the reference thermometer,
• > and any mismatch between the sensors being compared.

Applications

The 18205 is ideally suited for:

• > calibration of SPRTs and industrial PRTs near –196 °C,
• > comparison calibrations against a known reference,
• > laboratories seeking a practical alternative to Argon Triple Point realization,
• > national institutes performing routine low‑temperature calibrations.

Practical features

The apparatus is bench‑mounted, compact and easy to operate. With no electrical components, it is silent, vibration‑free and inherently stable. The only requirement is to replenish the liquid nitrogen or argon as it boils off. The Isotech 18205 Nitrogen Boiling Point Apparatus offers a stable, cost‑effective and highly practical solution for low‑temperature ITS‑90 comparison calibrations. Its simplicity, reliability and excellent thermal performance make it an ideal choice for laboratories that require accurate calibrations around –196 °C.