There is no insulating material inside the wall of a vacuum insulated cryogenic storage tank. There is almost nothing at all, and that is the point. The performance you paid for is a volume of near vacuum a few centimetres thick, and it is the only thing standing between your samples and a room at 20°C.

Why nothing insulates better than something

Heat reaches your liquid nitrogen by three routes. Conduction through solid material, convection through gas, and radiation. Remove the gas and you remove convection almost entirely and knock conduction down to whatever the neck, the supports and the pipework carry. What is left is radiation, and radiation is handled separately.

That is why the annular space between the inner and outer vessels is pumped down and sealed. It is also why a vessel that has lost its vacuum does not perform slightly worse. It performs catastrophically worse, because you have restored the convection path the design deleted.

Superinsulation, and what the layers are actually doing

Pumping down alone does not deal with radiation. Multilayer insulation, usually called superinsulation, wraps the inner vessel in many alternating layers of reflective film and low conductivity spacer. Each reflective layer bounces radiant heat back, and the spacers stop the reflective layers touching each other and forming a conduction bridge.

Larger bulk tanks often use perlite in the annulus instead, a fine expanded volcanic glass powder, again under vacuum. Both approaches attack the same problem. The distinction matters at service time, because a perlite filled annulus cannot simply be opened, re-wrapped and pumped down on site the way some smaller vessels can.

The getter, the quiet component nobody specifies

A sealed vacuum does not stay perfect. Materials inside the annulus outgas slowly for years, and hydrogen and helium migrate through steel. So manufacturers fit a getter, a chemically active material that absorbs stray gas molecules, sometimes with an adsorbent for the same job at cryogenic temperature.

The getter has a finite capacity. This is the real reason vacuum insulated vessels have a service life rather than lasting indefinitely. When the getter is saturated, the annulus pressure creeps up, and the vessel performance follows it down. Nothing visibly breaks.

The number to buy on, and the number to monitor

The specification that matters is the static evaporation rate, sometimes called the normal evaporation rate or NER, expressed as litres lost per day or as a percentage of capacity per day, measured under defined conditions with the vessel closed and undisturbed. It is the single honest comparator between two vessels.

Treat the manufacturer figure as a laboratory condition baseline, not a promise about your store room. Real consumption is always higher, because your vessel gets opened. Every lid lift, every cane retrieval, every warm object introduced adds heat that the static figure does not include. A busy IVF store can run at several times its static rate, and that is normal rather than a fault.

What you monitor is the trend. Establish your own baseline in your own room, in litres per week, and watch it. A rising trend at constant workload is a vacuum question.

How vacuum loss announces itself

Frost or condensation on the outer shell where there was none before is the headline sign, and it usually appears first near the bottom or around a support point. Any cold spot on an outer surface that should be at room temperature means heat is being conducted from the inner vessel and the annulus is no longer doing its job.

Other signs are less obvious. Consumption rising steadily over months at unchanged workload. Hold time falling short of what the same vessel managed last year. A vessel that has been dropped, struck or transported roughly and has behaved differently since, because a knock can crack a weld or fracture an internal support without leaving a visible mark.

Take a suspect vessel out of service and transfer the contents. A degrading vacuum does not recover, and the failure rate accelerates once it starts.

The standards that sit behind the design

Static vacuum insulated vessels are covered by the BS EN 13458 series, which runs to three parts covering fundamental requirements, design and fabrication with inspection and testing, and operational requirements. Storage at users premises is addressed by BCGA Code of Practice 36, which also sets out the safety device arrangement and the requirement for a written scheme of examination under the Pressure Systems Safety Regulations 2000 where the vessel operates above 0.5 bar gauge.

Open neck storage dewars used in IVF laboratories generally sit outside the pressure regime because they are vented to atmosphere, but they are still vacuum insulated vessels and everything above about vacuum degradation applies to them in full.

What this means when you are choosing a vessel

Compare static evaporation rates, not capacities. A larger vessel with a poor rate can cost more to run than a smaller one with a good rate. Look at neck diameter, because a wide neck makes retrieval easier and increases heat ingress, and the right compromise depends on how often you go in. Ask about the warranty specifically on the vacuum, since that is the component that determines service life. Ask what happens at end of life, because a vessel with a saturated getter is not a repair job on most designs.

Then plan for monitoring from day one. Continuous level and temperature monitoring with alarms is what turns a slow vacuum failure into a maintenance job rather than an incident, and it is considerably cheaper than the alternative.

Cryolab supplies vacuum insulated cryogenic storage vessels, dry shippers and monitoring systems to IVF clinics, NHS trusts, biobanks and research facilities in the UK and internationally. For help matching a vessel to your capacity, workload and room conditions, contact Cryolab.