Troubleshooting Abnormal FBG Temperature Sensor Readings on Hyperion

Product Family: Luna Innovations Hyperion Platform / ENLIGHT Software

Overview

When an absolute temperature sensor connected to a Hyperion interrogator returns an obviously incorrect reading, there are several common causes. This article walks through each one in order from the simplest to check to the most specialized.

The directions below reference ENLIGHT specifically, but the underlying troubleshooting logic applies equally to custom software that reads from the Hyperion. In any software environment, the same fundamentals hold: verify that your sensor coefficients and equation are entered correctly, confirm that your formula references the correct FBG/peak, ensure there are no spurious peaks interfering with the measurement, and account for distance-dependent wavelength offset on long fiber runs.

Common symptoms

The sensor returns a temperature that is clearly wrong — for example, 86°C when the actual ambient temperature is approximately 24°C. The sensor may have been reading correctly in the past, or it may have never read correctly since initial setup.

Cause 1: Incorrect sensor coefficients or equation

This is the most common and quickest issue to check. A single mistyped coefficient or an error in the formula structure will produce incorrect readings.

How to check in ENLIGHT

Navigate to the Sensors tab and locate the sensor that is reading incorrectly in the bottom “Sensors” table. Double-click the sensor to open its definition. Compare the equation and every coefficient value against the sensor calibration sheet that shipped with the sensor. Pay particular attention to:

• Polynomial coefficients (order and sign matter — a missing negative sign or a transposed digit will shift the result significantly)
• Nominal wavelength
• Any unit conversions in the formula

If any value does not match the calibration sheet, correct it and re-zero the sensor.

Figure: ENLIGHT Edit Sensor window showing the sensor expression and calibration coefficients

Cause 2: Sensor definition references the wrong Peak/FBG

If the sensor definition points to the wrong FBG, the formula will calculate temperature using a wavelength from a different sensor entirely — producing a reading that may look plausible but is incorrect.

How to check in ENLIGHT

1. Open the sensor calibration sheet (or sensor information sheet) and note the nominal wavelength for the temperature sensor.
2. Read the serial number on the sensor information sheet and physically verify which channel the sensor fiber is connected to on the Hyperion interrogator.
3. In the Sensors tab in ENLIGHT, click the radio button that corresponds to that physical channel.
4. In the top-left FBG table, identify which FBG has a current wavelength close to the nominal wavelength on the calibration sheet.
5. Confirm that this FBG name matches the FBG referenced in the sensor definition.

For example, a sensor with a nominal wavelength of 1551.2 nm that is physically connected to channel 4 might appear in ENLIGHT as FBG_D1 with a current wavelength of 1541.876 nm (if there are long fiber distances between the instrument and the sensor, you may need to skip to Cause 4 — the distance-related wavelength shift can greatly increase the current wavelength reading). The sensor definition should reference FBG_D1 — not an FBG from a different channel or a different wavelength region.

Figure: ENLIGHT Sensors tab showing the FBG table (top left), sensor table (bottom), and spectrum graph with FBG bin windows (top right)

Cause 3: Multiple peaks present in the FBG bin window

If more than one optical peak falls within the wavelength bin assigned to your FBG, ENLIGHT may track the wrong peak or jump between peaks, causing erratic or offset readings.

How to check in ENLIGHT

1. Navigate to the Sensors tab and click the radio button for the channel the affected sensor is connected to. You should see FBG names with wavelength values appear in the top-left table. (If no FBGs appear, click the button to auto-create FBGs, then follow the steps in Cause 2 above to verify you have the correct FBG assigned.)
2. In the top-right section of the tab, locate the graph showing peak locations within the instrument’s wavelength range.
3. Click the button at the far right of the graph toolbar to show FBG limits (bin windows). These appear as pairs of dashed vertical lines around each FBG.
4. Click the FBG name for the sensor you are troubleshooting in the left-hand table. It should highlight in blue, and its corresponding peak in the graph should also highlight.
5. Check that there is only one peak present between the two dashed lines defining that FBG’s bin window.

If multiple peaks are present in the bin:

• Adjust the bin widths so that only the correct peak falls within the window.
• If you are unsure which peak is correct:
  • Navigate to the Acquisition tab and view the full spectrum trace for the channel the sensor is connected to.
  • Enable peak tracking using the buttons at the right side of the trace display.
  • Look for dashed tracking lines near the sensor’s expected wavelength.
  • Identify whether any side lobes, noise floor spikes, or other artifacts are being tracked as peaks alongside the true sensor peak.
  • If spurious peaks are present, refer to the knowledge article on peak detection settings for guidance on adjusting detection thresholds to filter them out. 

Cause 4: Distance-dependent wavelength offset

When an absolute temperature sensor is located far from the Hyperion interrogator — particularly over distances of several hundred meters or more — a systematic wavelength offset can occur that shifts the reported wavelength away from the true Bragg wavelength. Because absolute temperature calculations depend on the actual wavelength value (not just a relative shift), even a small offset produces a significant temperature error.

How to identify this cause

If you substitute the nominal wavelength from the calibration sheet directly into the temperature formula and the result matches the expected “Temperature @ nominal wavelength” value on the calibration sheet, but the live reading is wrong, the issue is likely a distance-dependent offset.

How to resolve

1. Note the total fiber distance between the sensor and the interrogator.
2. Use the Luna distance compensation tool to calculate the appropriate wavelength correction.
   1. Refer to TN 775-2 Hyperion Distance Compensation for steps on enabling distance corrections
3. Apply the correction in your sensor configuration.

Still seeing incorrect readings?

If you have verified the equation, coefficients, FBG assignment, bin window, and distance compensation and the sensor still reads incorrectly, contact support at support.lunainc.com. Include the sensor serial number, calibration sheet, and a screenshot of the sensor definition in ENLIGHT so the support team can diagnose the issue quickly.