Pressure sensors - A222

Pressure sensors
Document Type	Article
Document Identifier	222
Relevant Class	Equipment
Tags	Equipment Test lab
Factory Cells	Deposition Thermal transformation Test lab
Prerequisites	Equipment

Introduction

Pressure sensors are widely used in composites manufacturing processes to monitor and control pressure conditions which are critical to part quality and structural performance. Pressure affects resin flow, consolidation, void formation, heat transfer coefficients and curing behaviour. Accurately measuring pressure during production runs can be beneficial in ensuring consistency in part quality and also provide essential data for process optimization and control.

This page provides an overview of pressure sensors commonly used in composite manufacturing and testing applications. It highlights the types of sensors, typical uses, advantages and limitations, and key considerations for their effective use.

Scope

This page focuses on the use of pressure sensors in manufacturing processes such as vacuum bagging, resin infusion, resin transfer moulding (RTM), autoclave curing, and mechanical testing of composites. Basic sensor principles, selection factors, and practical application notes are provided.

Significance

The mechanical performance and quality of composite materials are significantly affected by the pressure conditions during processing. In manufacturing, accurate control and monitoring of pressure are critical for ensuring proper resin flow, fibre consolidation, and void reduction. Pressure sensors are commonly used to verify integrity of vacuum bags during materials deposition steps as well as to record real-time data in processes such as resin infusion, and autoclave curing. In mechanical testing equipment, pressure sensors are used to measure the applied load by converting mechanical force to electrical signals. Hence, reliable pressure sensors are critical for quality assurance and certification of composite manufacturing processes.

Prerequisites

Recommended documents to review before, or in parallel with this document:

• Equipment
• Sensors

Overview

Pressure sensors are devices that detect the force exerted by a fluid (liquid or gas) or surface contact and convert it into an electrical signal, typically through resistive, capacitive, piezoelectric, or optical mechanisms. In manufacturing, pressure sensors can be integrated into autoclaves, vacuum bags, resin infusion systems, and automated fibre placement (AFP) heads, where they monitor compaction forces at the roller-substrate interface. In mechanical testing systems, they help quantify applied loads for testing of composite parts. A range of sensor types are available, each suited to specific environments, pressure ranges, and dynamic response requirements.

Types of pressure sensors

Flexible Film Pressure Sensors

These sensors measure pressure through resistive or capacitive changes across a flexible surface, typically used to map compaction pressure in AFP. However, the applicability of these sensors are limited by low pressure range and long-term drift.

Load Cells

Using strain gauges to measure deformation under load, load cells are commonly applied in mechanical testing setups such as compression tests. These sensors however cannot capture spatial pressure distribution.

Piezoelectric Sensors

Piezoelectric materials generate electric charge when stressed, which can be calibrated to measure the applied pressure making these sensors ideal for capturing fast dynamic events like impact testing. However, the biggest disadvanage of piezoelectric sensors is their inability to measure static pressure.

Pressure Transducers

These sensors convert fluid or gas pressure into electrical signals via strain gauge or capacitive elements, widely used in autoclaves or RTM setups. Depending on the material, pressure transducers might require signal conditioning and protection from harsh environments.

Capacitive Pressure Sensors

Capacitive pressure sensors operate by detecting changes in capacitance due to diaphragm deflection. These are particularly useful in low-pressure environments like vacuum leak monitoring, but are prone to electromagnetic interference and drift.

Optical Fibre Pressure Sensors

Fiber Bragg grating based fibre optic sensors can be used to detect pressure-induced strain via wavelength shifts in the grating. Due to the small diameter of the optical glass fibre, they are well suited for embedded or high-temperature cure environments. A large number of sensors can be multiplexed on a single fibre. The sensors themselves are inexpensive, however require expensive optical interrogation systems to implement.

MEMS-based Pressure Sensors

MEMS (Micro-Electro-Mechanical Systems) sensors work by detecting the mechanical deformation of a micromachined silicon diaphragm under applied pressure. Tese sensors can be used for compact sensing in moulds or tooling, but are limited in durability under extreme conditions.

Comparison of Different Pressure Sensors

The table below summarizes the typical applications, advantages, disadvantages, and cost ranges for commonly used pressure sensors in composites manufacturing and testing.

Sensor Type	Typical Application	Advantages	Disadvantages	Typical Cost (CAD)
Flexible Film	Monitoring compaction pressure in AFP and ATL Pressure distribution in vacuum bagging	Conformable to complex surfaces Enables spatial pressure mapping Thin and lightweight	Limited pressure range Signal drift over time Sensitive to handling and calibration	~$300–$1,500
Load Cell	Force measurement during mechanical testing (e.g., CAI) Press consolidation monitoring	High accuracy and repeatability Wide force range Robust industrial designs available	No spatial resolution Requires mechanical integration Bulky in some configurations	~$250–$2,000
Piezoelectric	Impact testing High-speed dynamic pressure measurement Real-time process event detection	Excellent for dynamic and transient loads High-frequency response Compact form factor	Not suitable for static measurements Requires charge amplifiers Fragile sensing elements	~$800–$3,000
Pressure Transducer	Monitoring pressure in autoclaves, RTM, and vacuum bags Process control feedback	Suitable for liquids and gases Wide operating pressure range Good accuracy and durability	Requires electrical signal conditioning Can degrade with heat or chemical exposure Installation-dependent accuracy	~$200–$2,500
Capacitive	Leak detection in vacuum bagging Low-pressure resin front tracking	High sensitivity at low pressures Compact and low power Suitable for static measurements	Prone to electromagnetic interference Thermal drift without compensation Limited dynamic range	~$300–$1,200
Optical Fibre (FBG)	Embedded pressure monitoring during curing Structural health monitoring (SHM)	Immune to EMI and high temperatures Suitable for harsh or embedded environments Multiplexing capability along a single fibre	High system cost Requires optical interrogation units Complex integration	~$2,000–$10,000+
MEMS	Localized sensing in tools or mould cavities Compact process monitoring applications	Miniaturized and low power Integrates easily into electronics Suitable for mass production	Lower durability in harsh conditions May have limited range or resolution Requires protection from contamination	~$50–$500

About	Help

For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

Resin transfer moulding (RTM) involves loading a preform into a two (or more) piece, matched tool, closing it, and injecting resin under pressure (~15-100 psi, or ~1-7 bar).

Well suited to small to medium sized parts, limited to large sizes due to injection pressure loads and tool cost.