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Fiber Optic Displacement Probes in Liquid Oxygen

When the cryogenic medium is Liquid Oxygen (or Liquid Natural Gas), calibration in water is a better approximation than calibration in air:
Air Refractive Index = 1.000
Liquid Oxygen Refractive index = 1.221
LNG Refractive Index = 1.286
Water Refractive Index = 1.332

A sensor’s operating range is extended in proportion to the ratio of the refractive indices.

For example, to calculate a sensor’s response in LOX, first calibrate the sensor submerged in water. Then remap the displacement data by the ratio 1.221/1.332.

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Cryogenic Turbopumps

Space launch vehicles use liquid propellant rocket engines with high-pressured turbopumps to deliver extremely low temperature propellants of liquid hydrogen, liquid nitrogen, liquid oxygen or liquid natural gas (methane) to a combustion chamber in the engine. During developmental testing of advanced turbopump designs, engineers monitor the radial and axial displacements of the rotors over the entire speed range of operation.

Philtec’s FODS (Fiber Optic Displacement Sensors) are being successfully used for rotor dynamics measurements in cryogenic fluids. When a fiber optic probe is submersed in a fluid, light rays diverging from the probe tip are more collimated than they would be in air. This increases the operating range of the sensor. The degree of collimation is proportional to the index of refraction of the fluid.

Refractive Index of Common Cryogenic Fluids

1.0002926   Air

1.0974   Liquid Hydrogen

1.2053   Liquid Nitrogen

1.221      Liquid Oxygen

1.286     Liquid Natural Gas (Methane)


Philtec provides the rocket engineers with a calculated best estimate of sensor performance following this example:

  • calibrate the displacement sensor in air (or water), then
  • rescale the gap data to derive an estimate of the sensor output in the cryo fluid.

For example, a sensor is calibrated to a target surface in Air with Refractive index of 1.0002926

For a probe immersed in Liquid Hydrogen with Refractive index of 1.0974, the sensor’s calibration gap data is scaled by the ratio of the Refractive Indices:

1.0974 ÷ 1.0002926 = 1.0971

Two calibration charts are provided: One for operation in Air; one for operation in LH2

Example Chart_LH2


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