Fiber Optic Sensing Vs. Strain Gauges

Fiber Optics Sensing vs. Strain Gauges – Installation Effort/Complexity

Any experienced experimental stress analysis engineer would agree that the quality of a strain gauge installation greatly influences the accuracy of the measurement.

Simply put, a strain gauge can only function as intended if the substrate strain is transferred to the gauge properly, which requires comprehensive expertise from the installer.

As a matter of fact, the British Society of Strain Measurement (BSSM) has offered a formal strain gauge certification program since 1964 to formally qualify individuals who demonstrate the ability to competently install strain gauges.

The most basic qualification (Level 1) offered for technicians or engineers requires 2 full days of training and typically 2 months of practical installation experience. In other words, proper strain gauge installations are not easy and precision is required during every step of the installation.

Large aerospace organizations often provide technicians with lengthy self-inspection checklists to ensure each step of the process is correctly followed. A typical strain gauge and FOS installations are shown in Figure 1.

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Figure 1: Typical strain gauge and FOS installations.

Due to the laborious nature of the installation process, strain gauges are often deployed in limited numbers at probable critical points throughout a structure. FOS sensors are installed using similar methods, but at a significantly faster pace.

The reduced installation effort is an enormous advantage for large structures where hundreds or thousands of strain sensors are required. In the following subsections, a comparison of installation time and cost for both traditional strain gauges and fiber optic strain sensors is provided.

Traditional Strain Gauges Installation Time

Install time for a single gauge varies greatly depending on the application (bridge configuration used, substrate material, difficulty of access, routing requirements, etc.). As a baseline reference, the time allotted (180 minutes) during the BSSM Level 1 certification assessment to install a general-purpose quarter-bridge strain gauge is used.

The following assumption is made to determine the per sensor installation time:

An experienced strain gauge installer only requires 15% of the 180 minutes allotted during the BSSM Level 1 certification assessment. Thus, 27 minutes of installation time is required per strain gauge.

Required Installation Material Cost for Traditional Strain Gauges

Strain gauges are procured in hundreds of different configurations depending on the application (backing material, gage length, pattern type, resistance value, etc.).

The price per strain gauge varies depending on the rarity of gauge and the ordered quantity. As a baseline reference, the price of a common general purpose axial strain gauge (CEA – XX – W250A-350) is used.

This commercially available strain gauge is available for $13 when ordered in quantity (>250). The following assumptions are made to determine the material costs required to install a traditional strain gauge:

  • The costs associated with electrical wiring varies based off the length of wire required and the strain gauge configuration (i.e. 2 wire vs. 3 wire vs. 4 wire). Commercial strain gauge wire can be procured for $1.50 per meter. It is assumed that required electrical wiring costs $10 per strain gauge.
  • The costs of surface preparation materials and adhesives are omitted.

Cost Summary for Traditional Strain Gauges

Using the assumption that the labor rate for a skilled technician is $50/hour, the per sensor cost breakdown for traditional strain gauges is summarized in Table 1.

Table 1: Per sensor cost breakdown of traditional strain gauges

Material Cost Labor Cost
Per individual sensor $231 $22.52

Traditional strain gauges are discrete point sensors that are installed on a per sensor basis. Multiplexing multiple sensors together is not possible, therefore, there is no cost break when using multiple sensors.

Required Installation Time for Fiber Optics Sensing

A representative FOS installation is shown in Figure 2. The installation consists of 4’ of fiber bonded to the surface of a uniaxial carbon fiber beam.

With the operational gauge length set to 0.0625”, the installation comprised of 798 individual strain sensors spaced uniformly along the length of the fiber.

plate 2

Figure 2: Representative fiber installation using similar procedures to a strain gauge installation. Fiber is installed at the locations and directions where a strain measurement is desired. 72 minutes were required to install the 798 individual strain sensors.

72 minutes were required to complete the installation including fiber connectorization, material surface prep, fiber routing, and application of the adhesive. The following observations and assumptions are used to determine the per sensor installation time:

  • Although the installation included 798 individual strain sensors, some of the sensors are not at locations/orientations of interest. It is conservatively assumed that 80% of the sensors are useful (638 sensors).
  • The 72 minutes required to complete the installation breaks down as follows:
    • Install prep: 22 minutes
      • Fiber connectorization: 7 minutes
      • Substrate surface prep: 15 minutes
    • Fiber routing: 40 minutes
    • Adhesive application: 10 minutes

Based off these observations and assumptions, each individual fiber optic strain sensor within this installation required approximately 7 seconds1 of installation time.

Required Installation Material Cost for Fiber Optics Sensing

In contrast to strain gauges, FOS sensors are procured in only a few different varieties because the sensor gauge length is software selectable and not physically inherent to the fiber.

Sensors are available in a few different diameters and coating options. As a baseline reference, 195  ORMOCER coated fiber is used which is procured at $30/ft.

The following observations and assumptions are made to determine material costs required to install a fiber optic strain sensor:

  • If only a single fiber strain gauge is desired, a minimum 6” length of sensing fiber ($15) is required to practically connectorize the fiber and perform the installation. Additionally, a $5 fiber optic pigtail is required to connectorize the fiber.
  • The costs of surface preparation materials and adhesives are omitted.

Cost Summary for Fiber Optics Sensing

FOS technology allows over 2000 individual strain sensors to be multiplexed onto the same fiber, thus providing some significant savings.

The costs for individual sensors and multiplexed sensors are summarized in Table 2 using the following assumptions and observations:

  • The labor rate for a skilled technician is assumed to cost $50/hour.
  • If only a single fiber optic strain gauge is desired, the full 22 minutes of install prep is still required (conservative) as well as 5 minutes (conservative) to bond the sensor.
  • When several sensors are multiplexed on the same fiber, each additional sensor requires an additional 0.0625” of sensing fiber ($30/ft) and an extra 30 seconds of installation time (conservative).

Table 2: Per sensor cost breakdown of FOS strain sensors

Material Cost Labor Cost
First sensor $20 $22.5
Per additional multiplexed sensor $0.16 $0.42

 Cost Comparison between strain gauges and Fiber Optics Sensing

As shown in the Figure 3, the per sensor cost for traditional and fiber optic strain gauges are comparable for a single sensor.

figuretable 3

Figure 3: Installation costs associated with a single traditional strain gauge compared to a single fiber optic strain sensor.

However, significant savings are realized when using multiple sensors due to the cost benefits associated with multiplexing several FOS sensors. For the same amount of money required to install two quarter bridge strain gauges, approximately 85 FOS strain gauges can be installed (11.25” of fiber discretized at 0.0625” gauge length).

As illustrated in Figure 4, significantly more spatial coverage is accomplished via FOS as a result.

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Figure 4: 85 FOS sensors can be installed for the same costs associated with 2 quarter bridge strain gauges, providing increased spatial coverage and significantly more insight into the behavior of the test article.

The increased spatial coverage is invaluable for complex structures, especially in areas where high strain gradients exist.

Thom Rollins, Sr. Principal Engineer at Northrop Grumman said it best: “A single fiber allows us to replace thousands of strain gauges, saving us significant man-hours of effort on a single project and providing us with new insight we would not have gotten by using legacy sensing technology.”

For applications that require or can benefit from the use of several strain sensors, FOS technology is clearly attractive due to cost effectiveness, shown graphically in Figure 5. However, the reduced installation time is also advantageous for keeping projects on schedule.

graph 5

Figure 5: Cost effectiveness of FOS strain sensors.

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