Edited by Robert Repas
Countries worldwide increasingly seek renewable-energy sources that are also environmentally friendly. Hydrogen is the most common clean fuel and has the highest energy content of any fuel by weight. In fact, it has 3× more energy than the amount contained in an equivalent weight of gasoline.
Hydrogen can come from a variety of sources including water, fossil fuels, and biomass. It is a commonly generated by-product of other chemical processes. Unlike electricity, large quantities of hydrogen are easily transported or stored for future use. Hydrogen also finds applications in places where the use of electricity is limited. It’s increasingly looked upon as an alternate clean, pollution-free fuel to gasoline and diesel.
While hydrogen is the lightest element, it poses two major corrosion problems for pressure sensors: embrittlement and permeation. Both corrosive actions lead to pressure-sensor failure. In addition, hydrogen is compressed and stored at pressures of 13,000 psi and higher.
Pressure measurements made in extreme environmental conditions such as high pressure, radiation, temperature, and vibration have historically been practical only with pressure-switch technology. Basically electromechanical devices, these switches don’t need power or integral electronics to trigger their switch mechanism. In most of these switches a thick-walled diaphragm bulges with the application of pressure. A push rod or spring attached to the center of the diaphragm trips a snap-action contact when pressure reaches a preset value.
Typically the center of the diaphragm must travel between 0.02 and 0.075 in. to provide sufficient force for switch actuation. Such a large movement eventually leads to fatigue while pressure cycling over time may let the switch drift from its original setpoint until the system malfunctions. The single on/off condition of switch status means it cannot be used in closed-loop systems for trend monitoring.
Critical systems in alternative energy, nuclear plants, engine controls, and braking systems need pressure sensors that go beyond mere on/off control. Such systems need sensors with accurate and linear output to track changes in operation before those changes become critical. Additional demand for electronic pressure sensors over pressure switches is fueled by the rapid development of microprocessor and microcontroller-based systems.
Early pressure sensors were little changed from their switch counterparts. Again, a thick-walled diaphragm held back the pressure. Metal-foil strain gages, inductance, or capacitance devices measured the amount of bulge in the diaphragm when pressure was applied to generate a corresponding output signal. The sensors were typically large, bulky, and expensive. Though they offered better performance than the on/off pressure switch, they suffered long-term stability problems due to material quality.
In the late 1970s, the emergence of silicon as a sensing material started to influence pressure measurement in industrial and commercial applications. Silicon-based sensors used a technology that has since become known as MEMs, or microelectromechanical systems. MEMs sensors were quickly employed in automotive and medical applications within benign environments. Compact in size, they were relatively inexpensive in high-production volumes. Despite development of other pressure sensing technologies, such as thin film, thick film, and ceramic capacitive, MEMs is still the most widely used and destined to be the driver in emerging markets such as alternative energy.
The basic construction of a MEMs high-pressure sensor consists of a thin corrugated diaphragm, typically made from stainless steel no thicker than a few thousandths of an inch. The diaphragm seals the chamber where the silicon sensing material is located, isolating the sensor from the process fluid whose pressure is being measured. The chamber is filled with a liquid such as silicone oil. While the liquid-filled chamber helps prevent additional sensor contamination by the process fluids, its main purpose is to transfer the pressure applied to the diaphragm to the sensing element.
Permeation and embrittlement
Hydrogen is normally found as a diatomic molecule, H2, that contains two hydrogen atoms. Molecular hydrogen cannot penetrate the voids found between the molecules of the metal diaphragm used in hydrogen sensors. When permeation occurs the H2 molecule splits into two H+ ions. Each ion is much smaller than an H2 molecule, essentially being a single proton nucleus with no orbiting electron. It is so small that it can pass through the voids between the molecules of the diaphragm into the fill liquid behind it. There the H+ ions can recombine with stray electrons forming H2, eventually creating trapped hydrogen bubbles within the fill liquid. The bubbles lead to span and zero shifts, degrading sensor operation.
Embrittlement occurs when the hydrogen atoms recombine within a void of the material they’re passing through. This traps the molecular hydrogen, which is now too big to diffuse through the material. The large molecule stresses the cavity walls by exerting an internal pressure. When enough hydrogen penetrates the material, pressure builds to the point where the material cracks. The crack propagates through the grain boundary of the material, weakening it, and letting hydrogen escape. For this reason high strength martensitic and precipitation hardening steels with large grain size should never be used in rich hydrogen and hydrogen sulfide environments, either at room or elevated temperature.
The European Integrated Hydrogen Project (EIHP) has defined the design and development of pressure sensors as stand-alone components for use in hydrogen-powered vehicles and filling systems. To meet EIHP standards, pressure sensors must undergo extreme testing such as 150,000 full pressure cycles, 2,000 pressure cycles at 1.2 times the rated pressure using pure hydrogen, chemical tests, and many other environmental tests from −40 to 185°F (−40 to 85°C). In a typical hydrogen-based system, stored hydrogen from the tank is regulated down to a lower pressure where it is mixed in a fuel cell with oxygen from the air to produce electricity. The by-product exhaust of this reaction is pure water. The output of the fuel cell, electrical current, is used as a power source. These fuel cells can range from 100 W to 150 kW and are more efficient than conventional gasoline or diesel engines. The pressure ranges associated with a typical hydrogen-driven system can range from ±15 psi (±1 bar) up to 13,000 psi (900 bar).
Oil-filled, silicon MEMs pressure sensor
Oil-filled, silicon MEMs technology has been around for 25 years and is widely used in low-to-medium pressure (15 to 3,000-psi) industrial applications. The MEMs sensing element is made by diffusing P-type silicon strain-sensing resistors in an N-type silicon sensing diaphragm. It is isolated from the media via a thin-metal corrugated diaphragm and silicone oil. The thickness of the metal diaphragm ranges from 0.001 in. (0.025 mm) for low pressure to 0.002 in. (0.05 mm) for medium pressure. As the metal diaphragm is quite thin and is typically made of stainless steel, it suffers from hydrogen permeation in both pure and nonpure hydrogen applications. The hydrogen bubbles trapped in the fill liquid eventually build up enough pressure to change the span and zero offset, and can eventually lead to the rupture of the diaphragm. Failure mode accelerates as temperature and pH levels vary.
Polysilicon MEMs pressure sensors
Polysilicon technology came onto the market about 20 years ago as an alternative technology to metallic thin-film sensors. The sensing element, designed in a form of a small cup, is typically made of precipitation-hardening stainless steel such as 17-4, with polysilicon strain gages grown using a chemical-vapor-deposition (CVD) process. The sensing element is then welded to a pressure port for use in the final application. This design suffers from hydrogen embrittlement as the hydrogen atoms migrate through the sensing diaphragm grain boundary like a termite, reducing the tensile strength of the material leading to its failure. Hydrogen embrittlement occurs at all temperatures. However, embrittlement accelerates as the temperature rises from 25 to 85°C.
Krystal Bond MEMs pressure sensor
Krystal Bond technology came into existence about 10 years ago primarily for tough applications. It uses the properties of special bulk-silicon strain resistors with precise doping, along with proprietary inorganic bonding techniques between silicon and a thick metal substrate. Special doping permits high output from the silicon strain gages that is stable and offers a high degree of compensation over a wide operating temperature. The inorganic bonding can be thermally matched to a host of stainless steels, titanium alloys, and high-nickel alloys with ease and flexibility. With the high output from the silicon strain gages and a flexible bonding process, the sensing elements can be made from thick, one-piece 316L stainless steel that offers zero permeation and embrittlement resistance due to the small grain size of 316L. This technology also meets the requirements of EIHP for safety and reliability. It has been used extensively on worldwide hydrogen platforms from vehicular to stationary hydrogen systems operating from 15 to 13,000 psi (1 to 900 bar).
In addition to its use in hydrogen applications, Krystal Bond technology is being applied in other tough applications such as mild radiation, seawater, down hole, diesel fuel-combustion differential-pressure measurements, aerospace, and high-pressure steam. It is the only technology that can deliver performance from 15 to 60,000 psi (1 to 4,000 bar) over a temperature range of −65 to 275°F (−55 to 150°C).