Archive | March, 2017

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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Glass Shape Gaging

The shape of curved glass windows in automated production lines is typically measured using LVDT contact sensors. When the glass is very thin and light, normal pressure of the LVDT probes tend to move the glass from the holding fixture and corrupt the measurement. Philtec’s non-contact FODS (Fiber Optic Displacement Sensors) solve the problem.

This month Philtec has delivered 100 sensors for a gaging station at a European automotive glass window manufacturing plant (50 for the driver side, and 50 for the passenger side). These sensors are very accurate, and prevent movement or scratches on the glass.


The model RC171 FODS is a reflectance compensated optical sensor with 15 mm operating range. It is insensitive to part-to-part reflectance variations as well as to ambient lighting interference. Data from a set of 50 probes is read in less than 1 second.

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