Introduction: What is Cryogenics?
This is another one of those areas that people have heard about, but have a tough time defining. It is a very important area in basic science, engineering, food, metallurgy, manufacturing, to name but a few.
It deals with cold and colder, essentially everything below about -150°C or 123 K. In this range, the International Temperature Scale of 1990 (ITS-90) has seven fixed, primary defining points. Indeed, one of the features of ITS-90 was the extension of the low end of the scale from the previous lower limit of 13.81 K (-259.43°C), the low limit of IPTS-68, to 0.5 K (-272.65°C).
If you wish more detail on the scales you should check the Scales page on this site or visit the web site dedicated to the 1990 scale, www.ITS-90.org.
Most people associate this region with liquid nitrogen and liquid oxygen, some even with liquid hydrogen and liquid helium, for these are some of the gases used as cooling agents or propellants in many space vehicle rocket engines.
Much of the work reported in the popular press does indeed revolve around work in these areas, e.g. 77 K to 4 K.
There are many many faces to cryogenics from the liquid oxygen plant next to a steel mill (that condenses oxygen from the air and supplies to the Basic Oxygen furnaces for making steel) to the special research apparatus used in studying materials properties at temperatures below that of liquid Helium.
A Wikipedia (The Open Encyclopedia) article entitled "Cryogenics," provides an overview of the technology.
Special Sensors for Cryogenic Temperatures
Many of the more commonly used temperature sensors are also used for work at cryogenic temperatures, e.g. most thermocouples, platinum RTDs, silicon diodes and special thermistors. However, in the very low reaches of the temperature scales, there are some unique measurements made.
Not only are they all very cold, but there are situations when the temperature sensor is immersed in a strong magnetic field and/or a radio-frequency field.
A big step in cryogenic thermometry improvements was produced in 2003. Based on novel doping technologies for Silicon (Si) and Germanium (Ge) semiconductor thermistors, AdSem, Inc. (Palo Alto, CA) developed both unique ultra-low temperature Ge thermistors with operating temperature range down to 0.9 mK and Si and Ge thermistors with very wide operating temperature ranges of: 700K - 1K, 300K - 1K, 300K - 20 mK.
This gives rise to a need not only for sensors that can measure at very, very low temperatures, but also sensors that are not affected by the presence of a magnetic field or a radio-frequency electromagnetic field superimposed on a magnetic field.
Excellent reviews of the scientific literaure on thermometry and sensors in the region from 160 K to 0.3 K (about -113°C to -272.85°C) are covered in great detail in three review papers by L.G. Rubin in the journal "Cryogenics" in 1970, 1982 and 1997. The references are detailed on the References page.
In addition, a special issue of the journal "Metrologia" in 1996 included significant articles on cryogenic thermometry as did the various volumes of the International Temperature Symposia, already listed.
The LakeShore Cryotronics, Inc. and Scientific Instruments,Inc. web sites include lists of the major sensor types arrayed in a table along with the key features and use conditions .
Below the table on the Lakeshore site is a brief summary of all the types listed there along with a description of key properties.
Some of the sensors covered are:
- Silicon Diodes
- Platinum RTDs
- Gallium Aluminum Arsenide Diodes
- Germanium RTDs
- Ruthenium Oxide RTDs
- Gallium Arsenide Diodes
- Rhodium-Iron RTDs
- Capacitance Sensors
- Chromel®/Gold Thermocouples
- Proprietary RTDs
CHROMEL ® is a registered trademark of Hoskins Manufacturing Company
Some of these temperature sensors are different than the types listed in the relevant pages as common industrial and scientific sensors. These are special versions, tailored for use in cryogenic applications. As a rule they are significantly more expensive than "regular" temperature sensors of similar type.
Some uses of Cryogenics
- Liquified gases, generation, storage and handling
- Physics research
- Hall Effect/Magnet Studies
- Materials and metallurgical research
- Thermal Gravimetric Analysis
- Radio Astronomy
- Ceramics research
- Carbon research
- Fuels research
- Nuclear Magnetic Resonance (NMR)
- Cooling of special optical, x-ray and electron detectors
- Flash freezing of food products
- Vacuum degasing, processing, and testing
- Low temperature research
- Semiconductor laser development
- Superconductor research & development
- Tritium liquification
- Nuclear physics detectors
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