Sam Drury, Sales and Marketing Director at Impress Sensors & Systems Ltd, discusses the advantages of LVDT pressure sensors for harsh environments, including those in the nuclear industry.
Pressure sensors are used in a wide variety of applications for monitoring and control, plus they can be used to measure other variables indirectly, such as liquid or gas flow (in conjunction with an orifice plate), speed, fluid level and 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.
Some specific advantages can be gained from using pressure transducers that operate on the linear variable differential transformer (LVDT) principle. Here, a metallic, pressure-responsive element is coupled directly to the core of a linear LVDT, hence the core displacement of the LVDT is caused by the movement of the pressure-responsive diaphragm.
Some LVDT pressure transducers are fitted with a single, precision metallic diaphragm with over-range pressure protection stops. This arrangement can be used for differential, gauge and absolute transducers, which all employ a common design philosophy (illustrated here is a sectioned differential transducer).
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.
LVDT designs also lend themselves very well to modification in order to fulfil a range of different applications in both research and process engineering.
An LVDT gauge-type pressure transducer can also be readily protected from the risk of damage caused 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 improve rupture integrity, supported by an over-pressure stop. In addition, the diaphragm material can be relatively thick, which offers 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, hence 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 hygienic application such as a dairy or food processing application, a low-cost silicon-filled sensor will require a barrier of some sort to eliminate the risk of contamination. In contrast, the design of an LVDT pressure sensor makes it inherently suited to hygienic, FDA-compliant applications.
Zero and span
LVDT pressure and level transmitters enable users to adjust both zero and span settings. Analogue and digital signal processing types are available; most analogue transmitters offer zero and span adjustment, a 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 application such as: 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 decommissioning, 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 transducers provide high immunity to radiation and can be stable to 10exp6 rad, with some manufacturers offering versions that allow up to 10exp12 rad without damage to the sensor. LVDT sensors can also withstand higher temperatures, with high radiation continuous working options typically available up to 200degC. Furthermore, LVDT sensors benefit from the way in which the signal conditioning electronics can be sited at a distance of 1000m or more from the sensor.
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.
Follow the link for more information about LVDT pressure sensors from Impress Sensors & Systems.