Hydrogen: source of hope

Hydrogen: source of hope

Pressure transmitters with gold-coated stainless steel diaphragms master special gas pressure measurements

Many experts are seeing hydrogen as the ideal substitute for coal, oil and natural gas in industry and transport, as it leaves practically no exhaust gases when burned. This versatile element is already used successfully in various industrial sectors. However, the handling of hydrogen gas puts high demands on the technical components used, and specifically on pressure transmitters.

The energy transition gets another pillar with hydrogen – in addition to renewable energies and energy efficiency. Hydrogen produced with renewable energies is a sustainable, flexible and easily transportable energy carrier. In addition to the German government’s current support programs, seven billion euros are invested to ensure that hydrogen becomes established on the market. A further two billion have been allocated for international partnerships. The focus is on so-called green hydrogen, which is produced exclusively with renewable energy. Only by means of green hydrogen, the CO2 emissions can be reduced using low-carbon energy sources. In Europe, 9.8 million metric tons of hydrogen are currently produced annually using mostly fossil fuels. Therefore, the EU Commission has set itself the goal of increasing the production of clean hydrogen to one million tons per year by 2024 and to ten million metric tons by 2030.

The production process of hydrogen

Hydrogen occurs in nature in combined form and is not easy to obtain. If it is used as a gas, the combination of hydrogen and oxygen has to be split up. But this electrolysis process, which chemically separates hydrogen and oxygen, requires a lot of energy. If electricity from solar plants or wind turbines is used, it is called “green hydrogen”. If the electricity comes from fossil fuels, the resulting hydrogen is called “gray hydrogen”.

Hydrogen is already used on a large scale by industry. In this case, however, it is not used as an energy carrier, but primarily in basic chemistry and petrochemistry in the context of stoichiological production processes. The hydrogen used in these applications is mainly referred to as gray hydrogen, which is produced by electrolysis processes or mostly as a by-product, e.g. in refineries.

Pressure sensors for hydrogen: what needs to be considered?

Regardless in which way hydrogen is produced and used, the handling of this element is very demanding in terms of technical solutions. Above all, working with hydrogen in its gaseous state is a challenge. Hydrogen is the element with the lowest density and the smallest atomic radius. This results in a fundamental problem in the handling of the gas: its extremely high permeation rate. Metallic materials are permeated by hydrogen, which has a negative effect, for example, on the use of pressure sensors. Piezoresistive transducers operate with an oil-filled housing with a thin steel diaphragm. If the hydrogen diffuses through this membrane and it accumulates in the transducer, the latter will be damaged or even destroyed in the long term. In the worst case, the hydrogen can even penetrate the entire sensor, creating an acute explosion hazard.

“Even doubling the thickness of the membrane leads at best to a doubling of the diffusion time,” knows our expert, founder of STS Sensor Technik AG. “However, the standard gold coating of the stainless steel membranes of our pressure transmitters in contact with hydrogen allows us to increase the time until a critical volume of hydrogen is reached in the transducer by a factor of 10 to 100. In this way, we significantly increase both the safety and the service life of the sensor.” This is due to the fact that the hydrogen permeability of gold is 10’000 times lower compared to steel. 

Gold coating of the membrane – the slight difference

STS develops, manufactures and sells application-specific solutions in pressure measurement technology – from the manufacturing of the individual parts to the calibration of the sensor and the final inspection of the end product. The applications range from machine and plant engineering to maritime applications, gas applications, life sciences and hydrogen applications. In applications, where the media to be measured has a significant hydrogen content, STS company uses gold-coated stainless steel diaphragms. Thereby, a significant optimization of the service life can be achieved. 

How does it work?

The permeability of gold is about 10,000 times lower than that of stainless steel. With a 1μm gold coating on a 50 μm steel membrane, hydrogen permeation can be reduced more effectively than by doubling the membrane thickness to 100 μm. In the first case, the time to reach a critical volume of hydrogen gas accumulated inside the pressure sensor can be increased by a factor of 10 to 100, in the second case only by a factor of two. The prerequisite for this is a completely closed system and a defect-free coating.

The piezoresistive pressure transmitter ATM.1ST  is suitable for static and dynamic pressure measurement in hydrogen applications. Its measuring ranges are between 0 … 50mbar and 0 … 1000 bar, the accuracies range up to 0.05%FS, hysteresis and repeatability are better than 0.01%. Due to its modular design, the pressure transmitter ATM.1ST can be individually adapted to many applications.

Hydrogen effect on piezo transducers (bio fouling)

Hydrogen effect on piezo transducers (bio fouling)

BIOFOULING

Biofouling or biological fouling is the accumulation of microorganisms, plants, algae or animals on wetted surfaces, devices such as water inlets, pipework, grates, ponds and of course on measuring instruments, causing degradation to the primary purpose of those items.

ANTIFOULING

Antifouling is the process of removing or preventing these accumulations from forming. There are different solutions to reduce / prevent fouling processes at the ship hulls and in sea or brackish water tanks.

Special toxic coatings that kill the biofouling organisms; with the new EU Biocide directive many coatings were forbidden due to environment safety reasons.

  • Non-toxic anti-sticking coatings that prevent attachment of microorganisms on the surfaces. These coatings are usually based on organic polymers. They rely on low friction and low surface energies.
  • Ultrasonic antifouling. Ultrasonic transducers may be mounted in or around the hull on small to medium-sized boats. The systems are based on technology proven to control algae blooms.
  • Pulsed laser irradiation. Plasma pulse technology is effective against zebra mussels and works by stunning or killing the organisms with microsecond duration, energizing of the water with high voltage pulses.
  • Antifouling via electrolysis
  • Organisms cannot survive in a copper ions environment.
  • Copper ions occur by electrolysis with a copper anode.
  • In most of the cases, the tank housing or the ship hull serves as cathode.
  • A copper anode installed in the configuration generates an electrolysis between the anode and the cathode.

Electrolysis can appear due to ballast water treatment systems (electrolysis and UV-sytems), corrosion processes or differences of electric potential between different materials.

EFFECT OF ELECTROLYSIS ON THE PIEZO RESISTIVE TRANSDUCER

  • A result of the electrolysis are positive hydrogen ions
  • Because of their polarization, the hydrogen ions move towards the cathode (tank housing or ship hull) where the transducer is installed.
  • In case of direct contact between tank and transducer, the hydrogen ions will permeate through the thinnest component of the anode, which is the diaphragm of the transducer.
  • After permeation of hydrogen ions through the diaphragm, the hydrogen ions grab an electron and transform into molecular hydrogen (H2). The hydrogen accumulates in the fill fluid of the transducer.
  • If this effect lasts for a longer period, the concentration of hydrogen in the fill fluid will increase and the diaphragm will be bloated. As a result, the sensor drifts and issues an incorrect value.

FINDINGS

Stainless steel pressure transmitters used during 2-3 years in ballast tanks of ships were investigated by the Swiss Federal Laboratories for Materials Science and Technology in Zurich.

Findings

The formation of deposits on stainless steel membranes cannot be prevented in practice. As long as corrosion processes can take place on the membrane under anaerobic conditions, the formation of hydrogen and its penetration into the sensor must always be expected.

For this reason, under such conditions, the membrane should be made of a more corrosion-resistant material such as titanium.

Gap corrosion occurs on metal parts in presence of a corrosive medium in narrow, unsealed gaps such as overlaps and in welds that are not through-welded. The driving force is concentration differences between the medium in the gap and the area outside the gap, which are caused by the inhibited diffusion of the reactants in the gap. The potential difference associated with the concentration difference leads to electrochemical corrosion in the gap (hydrogen type) or its immediate surroundings (oxygen type).

For this reason, the membran should be welded to the housing.

RECOMMENDATION

According to this findings, STS Sensor Technik Sirnach AG has been successfully using piezo-resistive elastomer-free sensors with housing and membrane in titanium for applications in marine, brackish water and sea water applications for over 10 years.

More information about this topic

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Pressure sensors with a current loop: What to consider in case of self-heating?

Pressure sensors with a current loop: What to consider in case of self-heating?

When pressure sensors equipped with a current loop are used, self-heating may occur due to their inherent design. This heat is produced when electric current flows through an electrical conductor or semiconductor. The effect of heat formation is based on Joule’s first law, whereby a voltage is generated through the electrical resistance of the conductor. The entire electrical conductor then becomes affected by this temperature rise, where that electric heat created is also known as “Joule heating.”

A corresponding investigation at STS has shown that self-heating can lead to accuracy fluctuations in measurements. The extent of these fluctuations depends on the quality of the respective sensor, as well as the specific application environments and conditions.

In applications where pressure is rapidly exerted across the entire pressure range of the sensor, a maximum error rate of < 0.1 % FS may occur. Depending on the sensor design, however, this measurement error also typically disappears after a period of two minutes. With a constant, uniform energy supply and elevated temperature, a state of equilibrium then prevails, where the heat created is now equal to the electrical power consumed.

To avoid any temporary measurement inaccuracies, however, STS recommends the following procedures:

  • Reduce the supply voltage from 24 V to 12 V, since a lower voltage also entails a lower power input.
  • Increase the load resistance.
  • Switch to sensors with a voltage output.

The advantages of following these tips are obvious. By reducing power input, you will immediately achieve more accurate results, improving both the efficiency and reliability of the entire measurement process. Once any temporary measurement inaccuracies have been eliminated, a precise and reliable dynamic measurement can also be applied.

We would be happy to assist you further with any questions or problems arising.

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