Sensor Technology
Digging deep
Typical factory set pressure switch
Karmjit S. Sidhu looks at how pressure sensors are extending their reach into extreme environments but without comprising their performance
H
istorically, pressure measurements taken in extreme environments have been limited to pressure switch technology due to environmental conditions such as radiation, temperature and vibration. Electromechanical in construction, these switches do not require power or integral electronics to activate the switching mechanism. Typically, a diaphragm with a push rod or spring will actuate a snap action contact when pressure is reached. To provide sufficient force for switch actuation, the diaphragm needs to travel between 0.02 to 0.075 inches, a large movement that can lead to fatigue. Pressure cycling over time causes the switch point to drift from its original set
Early pressure sensors used metal foil strain gages, inductance and capacitance, as the core sensing mechanisms to provide the output signal with respect to applied pressure. These devices were large, bulky and expensive and suffered from long- term instability due to material quality. The emergence of silicon as a sensing material, also known as MEMS (Micro Electro Mechanical Systems) based-sensors, is proving a popular technology and is destined to be the driver in emerging markets such as alternative energy and other ‘tough’ applications.
Alternative energy Hydrogen is the most common clean fuel and has the highest energy content of any fuel by weight (3 times more than gasoline). Similar to electricity, hydrogen is an energy carrier and must be produced from another substance. It can be produced from a variety of resources (water, fossil fuels,
High pressure hydrogen storage tanks
point, causing the system to malfunction or fail and since the pressure switch only offers one on/off condition, it cannot be used in closed-loop systems for trend monitoring. To overcome these issues, new generations of pressure sensing technologies have been developed that can provide a reliable linear and accurate output in critical systems including, but not limited to, alternative energy, nuclear, engine controls, and braking systems. The rapid development of microprocessor and micro-controller based systems has also fuelled the growth of electronic pressure sensors over pressure switches.
14 October 2013
biomass) and is a by-product of other chemical processes. Unlike electricity, large quantities of hydrogen can be
easily stored for future use and can be used in places where it’s hard to use electricity. Hydrogen can store the energy, and then be moved to where it’s needed. Hydrogen is gaining popularity in clean, pollution-free transport systems such as buses, cars and light trucks to replace gasoline and diesel. However, hydrogen poses two major
problems for pressure sensors in the form of corrosion that is sometimes ignored by sensor manufacturers and users. The hydrogen corrosion occurs as embrittlement and permeation, both leading to pressure sensor failure if it is not designed correctly (by the sensor
Components in Electronics
manufacturer) or implemented properly (by the user). Hydrogen embrittlement occurs when a crack prorogates through the grain boundary of the material, weakening the material and allowing the hydrogen to escape. High strength martensitic and precipitation hardening steels with large grain
Pollution-free transport systems
size should not 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 the EIHP standards, pressure sensors must undergo extreme testing such as 150,000 full pressure cycles, 2000 pressure cycles at 1.2 times the rated pressure using pure hydrogen, chemical tests and many other environmental tests from -40 to 185°C (-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 with oxygen to produce electricity. This process takes place in a fuel cell where the exhaust of the reaction is pure water. The output of the fuel cell, electrical current, is used to power the equipment. These fuel cells can range from 100W to 150kW and are very efficient compared to conventional gasoline or diesel engines. The pressure ranges associated with a typical hydrogen- driven system can be defined by the following applications:
• ± 15PSIG (± 1 Bar) for Hydrogen/Oxygen fuel ratio differential pressure sensor
• 0 to 36PSIG (2.5Bar) Fuel cell DI water cooling
• 0 to 300PSIG (20Bar) regulator output to fuel cell
• 0 to 6500PSI (448Bar) for car, fork lift, light truck and bus hydrogen storage tanks
• 0 to 10,000PSI (700Bar) for hydrogen filling systems
• 0 to 13000PSI (900Bar) for cars
Oil-filled, silicon sensors: Oil-filled, silicon MEMs technology is used in low (15PSI) to medium pressure (3000PSI) industrial applications. The MEMs sensing element, made by diffusing P type silicon strain sensing resistors in a N type silicon sensing diaphragm, is isolated from the media via a thin metal corrugated diaphragm and silicone oil. The metal diaphragm thickness ranges from 0.001" (0.025mm) for low pressure to 0.002" (0.050mm) for medium pressure. Since the metal diaphragm is very thin and constructed from stainless steel, it suffers from hydrogen permeation, as hydrogen atoms split into a hydrogen ion (two H+ atoms). Hydrogen permeation can occur in both pure and non-pure hydrogen applications. The hydrogen bubbles will be trapped in the oil until they build up enough pressure causing the zero offset to change and diaphragm to rupture. Failure mode will accelerate as the temperature and pH levels vary, causing the diaphragm rupture.
Poly silicon MEMs sensors: Poly silicon technology are an alternative technology to metal thin film sensors. The sensing element, designed in a form of a small cup, is constructed from precipitation hardening stainless steel such as 17-4, with poly silicon strain gages grown using a Chemical Vapor Deposition (CVD) process. The sensing element is, then, welded to a pressure port so that it can utilized for the final application. This design suffers from hydrogen embrittlement as the hydrogen atoms will migrate through the sensing diaphragm grain boundary like a ‘termite,’ reducing the tensile strength of the material and causing it to fail. Hydrogen embittlement occurs at all temperature. However, it accelerates as the temperature increases from 25°C to 85°C. Krystal Bond Technology came into existence about ten years ago primarily for tough applications. It uses properties of special bulk silicon strain resistors with precise doping, along with proprietary inorganic bonding techniques between silicon and thick metal substrate. Special doping allows 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 high output from the silicon strain gages and 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 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 13000PSI (1 to 900Bar). Currently it is the only technology that can deliver performance from 15 to 60,000PSI (1 to 4000 Bar) over a wide range of temperature covering -65 to 275°F (-55 to 150°C). In addition to hydrogen applications, Krystral Bond Technology is being used in other tough applications such as mild radiation, sea water, down hole, diesel fuel-combustion differential pressure measurements, aerospace and high pressure steam over traditional technologies such as metal bonded foil strain gage, ceramic diaphragms with O- ring seals, welded thin film technology and oil filled sensors.
American Sensor Technologies |
www.astsensors.com
Karmjit S. Sidhu is VP, Business Development at American Sensors Technologies
www.cieonline.co.uk
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