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LVDT pressure sensors offer reliability and long life in harsh environments

12 August 2009

Pressure sensors vary considerably in their design technology used, performance and application. Sam Drury, sales & marketing director at Impress Sensors & Systems, considers the advantages of using LVDT technology for pressure sensors used in harsh environments, including the nuclear industry.
Pressure sensors are used in a wide variety of applications for control and monitoring purposes.

LVDT pressure sensors offer reliability and long life in harsh environments
LVDT pressure sensors offer reliability and long life in harsh environments

Pressure sensors can also be used to indirectly measure other variables such as liquid or gas flow (in conjunction with an orifice plate), speed, fluid level and also altitude.

However, due to the wide range of technologies available, pressure sensors vary considerably in their design, performance, application and cost. Every technology has its own benefits and reasons for selection within an application.

When used directly to measure pressure, applications include meteorology instrumentation, aerospace and defence, research and development, automotive and other machinery or equipment that has pressure functionality implemented. Other applications for pressure sensors include hydraulic and pneumatic systems, water depth, offshore & marine, waste water & sewage, oil & gas exploration, nuclear, medical, food and beverage processing, tank level/contents, HVAC systems, agricultural equipment, environmental monitoring and chemical & processing plants.

Some specific advantages can be gained from using pressure transducers that operate on the Linear Variable Differential Transformer (LVDT) principle. Here, a pressure responsive element is directly coupled to the core of a linear LVDT.

An LVDT is an electro-mechanical device that produces an electrical output that is linearly proportional to the displacement of a moveable core. It consists of a primary coil with two secondary coils placed on either side of the primary coil. A rod-shaped soft magnetic core inside the coil assembly provides a path for the magnetic flux linking the coils.

When the primary coil is energised by an alternating current, source voltages are induced in the two secondary coils. The secondary coils are connected in series with the start of each winding being connected together. This arrangement produces a net zero signal output from the secondaries when the induced voltages are equal in each coil. This condition occurs when the core is centrally disposed between the two secondaries. A movement of the core leads to an increase in magnetic coupling to the coil in the direction of movement and a reduction in of magnetic coupling to the other coil producing a net output signal from the connected secondaries. Movement in the opposite direction produces an identical signal output but of opposite phase.

To form a pressure transducer, the core displacement of the LVDT is produced by the movement of a metallic pressure responsive diaphragm.

Some LVDT pressure transducers are fitted with a single, precision metallic diaphragm with over range pressure protection stops as the pressure-responsive element. This arrangement allows the manufacture of differential, gauge and absolute transducers, which all employ a common design philosophy.
The distinct advantage of using an LVDT transducer is that the moving core does not make contact with other electrical components of the assembly, as is the case with other types. This means an LVDT transducer offers high reliability and long life.
The LVDT design also lends itself very well to easy modification in order to fulfil a whole range of different applications in both research and process engineering.

An LVDT gauge-type pressure transducer lends itself very well to being protected from damage by positive over-pressure. The sensor’s safe limits are normally much greater than those specified by the manufacturer and unrivalled by alternative technologies. Often, the sensor will still operate above the specified over-pressure limit, but at a reduced accuracy. In contrast, silicon and thick-film pressure sensors do not exhibit this level of over-pressure capability.

Unlike silicon and thick film pressure sensors, LVDT pressure transducers provide process containment for applied static pressures of up to 400bar or higher. Special welding techniques are used to improve rupture integrity, supported by an over-pressure stop. In addition, the diaphragm material can be relatively thick, offering enhanced durability and improved resistance to pin-holing (corrosion).

LVDT pressure transducers can be impact shock-loaded in all three axes without sacrificing the performance of the sensor. The diaphragms are not made from brittle materials and so failures due to shock loads are rare.

Process compatibility is also a key requirement when sourcing a suitable pressure transducer. With LVDT pressure sensors, flush diaphragms can be provided rather than fluid-filled units. This offers enhanced process compatibility and does not limit the temperature range. In addition, if the pressure sensor is required to perform in a hygiernic application such as a dairy or food processing application, a low cost silicon-filled sensor will require a barrier of some sort to prevent contamination. In contrast, the design of an LVDT pressure sensor makes it inherently suited to hygienic, FDA-compliant applications.

LVDT pressure sensors open up a wide range of process interface and wetted material options for the user. With sufficient understanding of the application, the sensor manufacturer is able to optimise the measurement solution at the lowest cost.

LVDT pressure and level transmitters enable the user to adjust both zero and span settings. Analogue and digital signal processing types are available. Most analogue transmitters will offer zero and span adjustment, square root option, time constant and ±100 per cent offset adjustment.

Digital electronic types offer local configuration of zero and span, along with the ability to turn on or off the instrument preset non-linear function. Digital types can normally be configured via an integral communication port.

Submersible type LVDT pressure sensors normally use digital signal processing and have the option of either a simple single wire configuration port that allows zero and span calibration together with the ability to turn on or off the instrument preset non-linear function, or full RS485 communication that enables full configuration of the transmitter.
In the nuclear sector, LVDT pressure transducers are utilised in reactor research and development work; leak detection on nuclear transport flasks; detection of leakage from Magnox storage ponds; monitoring material storage pond levels; storage room pressure monitoring; level measurement in effluent treatment works; and glove box gas handling systems. LVDTs are even used in weapons de-commissioning, where the sensor must withstand highly aggressive chemicals such as Hydrobromic Acid and where radiation immunity is critical.

LVDT pressure transducers are generally favoured by the nuclear industry because they offer distinct advantages over alternative pressure sensor designs.

LVDT tranducers provide high immunity to radiation and can be stable to 10 exp6 rad, with some manufacturers offering versions that allow up to 10 exp12 rad without damage to the sensor.
LVDT sensors can also withstand higher temperatures, with high radiation continuous working options typically available up to 200 deg C.

LVDT sensors also benefit from the fact they can have remote electronics up to 1,000 metres or more of cable between the sensor and the signal conditioning electronics. This allows the sensor to operate in extreme radiation, temperature and high magnetic fields, conditions that would normally damage the conditioning electronics.

In LVDT sensors, the segregation of the transducer from the pressure-responsive element enables many specialist materials to be used for compatibility with the process fluid. Manufacturers can therefore produce sensors with Tantalum, Hastelloy, stainless steel, Monel, Inconel and PTFE sintered coatings.

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