Isotech 785 Libra Parallel Tube Liquid Bath

The Isotech 785 Libra is a high‑performance stirred liquid bath designed for temperature calibration with uncertainties as low as < 1 mK. It uses the proven parallel tube architecture, originally developed by H.N. Irving & Sons and refined by Isotech in the 815 and 915 bath systems. The Libra brings this pedigree into a compact, efficient design with integrated refrigeration for low‑temperature operation.

Parallel Tube Design — physically separated temperature zones

A defining feature of the Libra is the physical separation between the heating/cooling chamber and the calibration chamber. This prevents thermal disturbances from propagating into the calibration volume. The geometry of the parallel tubes — including their curvature and angle — is optimized to create a highly uniform flow pattern, resulting in an exceptionally isothermal calibration zone.

Three models for a wide temperature range

The Libra is available in three versions:

• > 785L: –80 °C to 125 °C
• > 785M: –40 °C to 125 °C
• > 785H: 30 °C to 300 °C

All models share the same calibration volume: 98 mm diameter, 485 mm deep.

Efficient fluid usage

Despite its deep immersion capability, the Libra requires only 8.5 litres of fluid. This significantly reduces operating costs, especially when using specialty fluids such as ethanol or silicone oils.

Thermal performance

The Libra delivers outstanding performance:

• Absolute stability:
  • ±0.0025 °C (water, 50 °C)
  • ±0.006 °C (ethanol, –80 °C / –40 °C)
  • ±0.006 °C (C10‑oil, 125 °C)
• Vertical uniformity:
  • < 1 mK (volgens evaluatierapport)
• Heating times:
  • 25 → 125 °C in < 50 min (C10‑oil)
  • 30 → 250 °C in < 150 min (VH‑oil)
• Cooling times:
  • 0 → –80 °C in < 300 min (ethanol)
  • 10 → –40 °C in < 180 min (ethanol)

Lowest comparison calibration uncertainties

For best‑practice comparison calibration, the unit under test is compared directly to an SPRT using a thermometry bridge such as the microK. Because both sensors track the same temperature fluctuations, the resistance ratio remains extremely stable.

The Libra’s isothermal calibration zone ensures that bath stability contributes minimally to the overall uncertainty, making it one of the most accurate comparison baths available.

Construction and operation

The Libra uses a carefully engineered fluid circulation system involving:

1. controlled flow through an orifice plate
2. a circular heating/cooling chamber
3. a physically separated calibration chamber
4. a weir for full immersion of liquid‑in‑glass thermometers
5. an expansion tray
6. high‑efficiency insulation

This design ensures uniformity, stability and repeatability.

Accessories

A wide range of accessories is available, including:

• > Aluminium or copper equalising blocks
• > LIG thermometer support assemblies
• > Monocular magnifiers
• > ITS‑90 fixed‑point cell baskets

These options allow the Libra to support both industrial calibration and high‑accuracy metrology. The Isotech 785 Libra delivers exceptional stability, uniformity and efficiency. Its parallel tube design, separated temperature zones and low fluid volume make it one of the most capable comparison baths available, ideal for laboratories seeking the lowest possible uncertainties.