I would like to know what the principal methods and the relative instrumentation to check the purity level of helium gas stream in the purification system of a common helium cryogenic plant are. What principles are these instruments based on? And what are their principal characteristics?
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Goran Perinic
August 22, 2012I assume that you would like to measure the purity at the outlet of the purifier. For a standard helium refrigerator/liquefier your will usually want to know the air and moisture content in the ppm range with a precision of say 0.1 ppm. (Depending on the cooling application and helium sources you may also need to consider other impurities.)
There are several possibilities to measure these impurities in helium. Some instruments are for several impurities in the same time and some for one single impurity:
1. Gas chromatography with a Thermal Conductivity Detector (TCD) will measure O2, N2 and possibly hydrocarbons and other gases depending on the configuration (other detector types possible) – expensive. Search the internet for gas chromatographs.
2. Photoluminescence analysis will measure N2, H2O and O2 depending on the configuration – from mid price for single component detector to expensive for multicomponent detector.
Search internet for helium multi-component detector, trace nitrogen in helium analyzer or pure gas analyzer helium.
3. Capacitance measurement for H2O measurement – from low to mid price.
Search the internet for dewpoint meter.
4. Electrolytic measurement of oxygen – mid to high price instruments cover good to very good precision. Search the internet for trace oxygen analyzers.
5. Zirconium oxide analyzers (ionization based) – mid priced instruments.
These are the instruments that come to mind first, but there are certainly others that I missed.
Each measurement principle and analyzer has its advantages and disadvantages as more or less maintenance (calibration) and consumables may be required, some require a reference gas, etc., and the market is moving. The precision and autonomy of the instruments is improving all the time and they become more and more compact.
As a general rule you should prefer a fail-safe analyzer i.e. one that indicates a high impurity if something is wrong (no gas flow, detector fault, analyzer cell polluted …) as the measurement is critical for your process.
A question is often also the output for the process control (+ alarms).