-40°C to +200°C operating temperature range
Downhole Sensors
A Comparison of Bulk Silicon and Silicon on Insulator (SOI) Pressure Sensor Technologies
Downhole environments demand precision, durability, and stability under extreme conditions. Whether monitoring reservoir pressures, optimizing artificial lift systems, or guiding drilling operations, pressure sensors must deliver reliable, repeatable data — often in high-temperature and corrosive environments.
At Core Sensors, we specialize in developing pressure sensors tailored to these challenges. In this post, we compare two prominent sensing technologies used in downhole applications: Bulk Silicon Piezoresistive Pressure Sensor Technology and Silicon on Insulator (SOI) Pressure Sensor Technology.
Bulk Silicon Piezoresistive Pressure Sensor Technology
Bulk silicon pressure sensors utilize a piezoresistive sensing element manufactured by applying silicon strain gages to a machined process connection using a glass firing process. This approach has long been proven in oil and gas applications due to its excellent mechanical stability, low thermal drift, and high repeatability across units.
What makes bulk silicon sensors especially advantageous in downhole applications is the ability to bond the sensing element to Inconel 718—a high-strength nickel-based alloy with outstanding corrosion resistance and mechanical durability. Inconel’s high yield strength (~1030 MPa) allows it to function as an effective diaphragm material, particularly in high-pressure, high-temperature (HPHT) environments commonly encountered in deep wells.
Silicon on Insulator (SOI) Pressure Sensor Technology
Silicon on Insulator (SOI) technology involves a layered structure where a thin silicon device layer is separated from the substrate by an insulating oxide layer. This architecture offers improved electrical isolation and high-temperature survivability, which has made SOI sensors a compelling choice for certain extreme temperature applications.
However, SOI sensors used in downhole applications are typically paired with Titanium housings and diaphragms. While titanium offers good corrosion resistance and is lightweight, it has a lower yield strength (approximately 880 MPa for Grade 5 Ti-6Al-4V) compared to Inconel 718. This makes it less suitable for high-pressure conditions and may require thicker diaphragms that reduce sensitivity or dynamic range.
Downhole Sensor Material Type Comparison
| Property | Inconel 718 | Titanium (Ti-6Al-4V) |
|---|---|---|
| Yield Strength (MPa) | ~1030 | ~880 |
| Corrosion Resistance | Excellent in sour gas, brines, and high-temp fluids | Good, but not as robust in some downhole chemistries |
| Diaphragm Suitability | Excellent | Moderate |
| Common Use | Bulk Silicon Sensors | SOI Sensors |
Compensation and Sensor Performance
One of the most significant differences between Bulk Silicon and SOI technologies lies in sensor compensation and span tolerance.
- Bulk Silicon Sensors: These are commonly paired with precision resistors, which correct for temperature and pressure-induced shifts. As a result, they offer tight span tolerance and consistent performance between units, enabling better sensor interchangeability and more predictable data for control systems.
- SOI Sensors: SOI-based downhole sensors with a millivolt output often lack compensation, leading to wider span tolerances and greater variation in sensor outputs across the same operating range. This can make system calibration more complex and data consistency harder to achieve without extensive post-processing or software correction.
Conclusion
Both Bulk Silicon and SOI technologies have their place in downhole pressure sensing. However, when your application demands tight performance repeatability, high-pressure endurance, and excellent corrosion resistance, Bulk Silicon pressure sensors mounted with Inconel 718 diaphragms stand out as the more robust and reliable solution.
At Core Sensors, we leverage these advantages to deliver custom pressure sensing solutions built to perform in the harshest downhole environments.
Learn more about Core Sensors’ downhole pressure sensor capabilities and featured product, the CS90 Downhole Pressure Temperature Sensor.
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Our team is committed to manufacturing solutions with standard, semi-custom, and custom sensor designs. Our team consists of some of the most creative, experienced sensor engineers in the industry. For more information or to request a quote, please call us at 862-245-2673 or email [email protected].
About Core Sensors:
Core Sensors designs, manufactures and distributes high quality sensors for measuring pressure and temperature properties for industrial and hazardous applications worldwide. Core offers solutions for industrial, submersible, differential and hazardous applications in both standard and custom packaging. In-house design and manufacturing capabilities along with strategic manufacturing partners allows Core to create solutions to some of the most complex sensor requirements.
