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Sensor Probe Installations with Tilt

Sensor probes are to be generally mounted normal (perpendicular) to a target surface. With some applications, such as displacement of a cantilever beam, the target surface rotates as deflects. In other applications, due to installation constraints, the probe can not be mounted normal to the surface. In applications where tilt is unavoidable, what sensors can be used?

Our latest application note Sensor Applications With Tilt provides guidance on sensor performance and selection.


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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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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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Wireless Sensors for Turbine Blade Tip-to-Casing Cold Clearance Measurements

Gas and steam turbine manufacturers of fossil power plants have long used hand-held feeler gages to measure turbine blade tip clearances and inspect the roundness of the TBC linings at access ports provided for that purpose. With new construction and at overhaul, turbine blade tip-to-casing clearances at room temperature must be properly set.

Philtec has developed the wireless non-contact sensor system CMS3400 to replace manual feeler gage methods. As a technician slowly jogs a rotor thru 360° of rotation, Gap and Angle data pairs are simultaneously recorded from 4 rows of blades and saved to a USB flash drive for offloading and data analysis. CMS3400 is a four channel wireless sensor system that digitally captures the full 360° picture of  blade tip-to-casing roundness and clearance data. This product represents a major advancement in the state-of-the-art.

Sensor ModulesTip Clearance Data

MORE INFO AT  Wireless Sensors for Blade Tip Clearance

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USB Powered Sensors

µDMS-RC22 & USBIntroducing our smallest and most powerful digital sensor series with USB output: the MicroDMS series (µDMS). Powered via any standard USB port, or multiplexed via any standard USB hub, this new sensor series boasts a maximum data sample rate of 16,000 samples per second in a 145 gram enclosure of LWH dimensions 75 x 50 x 23 mm. With Philtec’s fully-featured DMS Control freeware, this sensor is a powerful solution to many measurement problems.

With its miniaturized size and USB power, it is an ideal solution for high density workstations where space is at a premium. Sensors can be stacked together to form the most compact arrangement. For example, 6 sensor modules can be stacked together in a space of 132 mm.

Contact our application engineering staff to explore the possibilities the µDMS series offers for you.

6 sensor boxes

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New Sensor Model for 75 mm Operating Range

Philtec adds the model D240 sensor having an overall operating range of 75 mm.

Sensor with 75 mm Measurement Range

Sensor with 75 mm Measurement Range

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