LVDT Pressure Sensors
Pressure sensors vary considerably in their design technology used, performance and application. Sam Drury, Sales & Marketing Director at Impress Sensors & Systems Ltd, considers the advantages of using LVDT technology for pressure sensors used in harsh environments, including the nuclear industry.
Pressure sensors - commonly referred to as pressure transducers, pressure transmitters, pressure indicators or pressure switches - are used to measure the pressure of gases and liquids (fluids).
Pressure is an expression of the force and is normally stated in terms of force per unit area. A pressure sensor will generate an electrical signal relating to the pressure imposed. This signal is either analogue or digital in more modern designs, although optical, visual and auditory signals are also common.
Industrial pressure transducers normally have a diaphragm type design that uses strain gauges, which are either bonded to, or diffused into it, with the strain gauges acting as resistive elements. Under the pressure-induced strain, the resistive values change. In capacitive technology, the pressure diaphragm is a single plate of a capacitor that changes its value under pressure-induced displacement. These are probably the most common technologies used in industrial applications for their favourable price/performance ratio.
Pressure is measured against a reference, which greatly depends on the application and installation of the sensor. Depending on the relevant pressure, the term 'absolute' is used when the reference is vacuum; 'gauge' is used where the reference is atmospheric pressure; and 'differential' is used where the sensor has two ports for the measurement of two different pressures.
Pressure sensors are used in a wide variety of applications for control and monitoring purposes. 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.
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 proprtional 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 arranagement 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.
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 conatmination. In contrast, the design of an LVDT pressure sensor makes it inherently suited to hygienic, FDA-compliant applications.
Process Interface Options
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.
LVDT Pressure Transducers for the Nuclear Industry
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.
LVDT pressure sensors offer significant features and benefits to the user, most of these deriving from the fundamental physical principles of operation and from the materials and techniques used in its construction:
Because an LVDT operates on electromagnetic coupling principles in a friction-free assembly, it can measure infinitely small changes in core position. This capability is limited only by the 'noise' in an LVDT signal conditioner and the output display's resolution. These same factors also give an LVDT its excellent repeatability. In LVDT pressure sensors, the limits of accuracy are defined by the diaphragm (pressure-responsive element).
Extended Mechanical Life
Due to the fact that there is no contact between the LVDT core and coil, an LVDT sensor is wear-free. This means the sensor offers unlimited mechanical life. This is particularly important in high reliability applications such as aerospace, defence and nuclear installations, the life being limited by the fatigue life of the diaphragm, which by careful design can be extended significantly.
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.
The materials and construction techniques used in assembling an LVDT result in a robust, durable sensor that is protected from a variety of environmental conditions. Bonding of the windings is followed by encapsulation into the housing, which means superior moisture and humidity resistance, as well as high resistance to impact shock loads and high vibration levels in all axes. Furthermore, the internal high-permeability magnetic shield minimises the effects of external AC fields.
Because the only interaction between an LVDT core and coil is magnetic coupling, the coil assembly can be isolated from the core by inserting a non-magnetic tube between the core and the bore. In doing this, a pressurised fluid can be contained within the tube, in which the core is free to move, while the coil assembly is non-pressurised.
Conventional LVDTs can operate over a very wide temperature range, but, if required, they can be produced to operate down to cryogenic temperatures, or, using special materials, operate at the elevated temperatures and radiation levels found in many nuclear reactors.
The absence of friction during ordinary operation permits an LVDT to respond very quickly to changes in core position. The dynamic response of an LVDT pressure sensor is limited only by the inertial effects of the core's slight mass. More often, the response of an LVDT sensing system is determined by the characteristics of the signal-conditioning unit.
IMPress Sensors and Systems
Tel : +44(0)118 981 7980
Web : www.impress-sensors.co.uk
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