Spinning rotor gauges commonly referred to as SRG or viscosity gauges, measures the pressure-dependent molecular drag on a magnetically levitated spinning steel ball.
During the SRG vacuum measurement cycle the spinning of the steel ball is accelerated to a well-defined rotation level and the deceleration time is measured to determine the molecular drag on the steel ball. The molecular drag is mathematically converted to a vacuum gas pressure value. In the high pressure regime the high gas density which will result in a high molecular drag and in the low pressure regime a low molecular drag will be present. A longer measurement cycle is required when measuring low pressure, because of a longer deceleration time. The typical spinning rotor gauge measurement cycle is 3-30 seconds.
The SRG vacuum gauge measurement is depended on the gas type and gas concentration.
The spinning rotor gauge can be used for vacuum measurement in the range from 5E-7 to 1 mbar (1E-5 to 100 Pa).
The spinning rotor gauge sensor device consists of a steel ball inside a tube. The sensing head includes the surrounding electronics and coils to drive and measure the steel ball rotation. During the measurement loop the magnetic driving coil accelerates the spinning steel ball to hundreds rps (rotations per second) after which the driving coil is de-energized. The measurement coil then measures the rotations of the ball during the deceleration time period. The deceleration rate of the ball is a function of the pressure-dependent molecular drag.
The sensor element can be integrated in vacuum systems and requires no electrical vacuum feedthrough. It is hermetically sealed and can be exposed to both high bake-out temperatures and corrosive gasses. The spinning rotor gauge sensor element is passive and does not generate any ions or thermal emission like traditional wire Pirani or hot/cold cathode gauges.
In many industries, cryogenic liquids such as liquid oxygen, nitrogen and argon are stored and transported in specially designed cryogenic tanks. A cryogenic tank is insulated with a double-wall vacuum cavity, where it is critically important to maintain an adequate vacuum level of typically <1×10-2 mbar.
Integrating a spinning rotor gauge sensor head in a cryogenic tank enables verification of the insulation without breaking the vacuum.
Vacuum cavities are used as insulation in a wide range of applications.
Quality check of vacuum pressure inside vacuum insulation panels requires in-situ measurements using embedded sensors. The SRG sensor ball element can be integrated in insulation vacuum panels for verification – ensuring adequate vacuum pressure levels and insulation properties.
Another vacuum insulation application where continuous verification of the vacuum level can be beneficial is vacuum insulated glass (VIG).
In the spinning rotor gauge sensor, the only material exposed to the vacuum gas media is stainless steel and its passive sensor design makes it ideal for outgassing and leak testing by use of the rate-of-rise principle.
The SRG instrument have been used for decades as transfer standard precision instruments in metrology vacuum calibration applications and calibration laboratories.
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