LVDT pressure sensors offer reliability and long life in harsh environments

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 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.

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 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 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. For further information, e-mail: info@impress-sensors.co.uk   Refer to page 55

Shielding cans with selective non-conductive coating

Precision Micro, the Birmingham based specialist manufacturer of bespoke EMI/RFI board level shielding solutions has developed its own dielectric coating process for the inside of shielding cans where the possibility of arcing exists between circuitry and the grounded shielding can. Arcing can occur as a result of turn-on spikes or surges (electromagnetic pulses) caused by external stimuli and can be very damaging to active electronics devices.

The insulative coating can be applied selectively to inside surfaces of the shielding can considered most susceptible to arcing, enabling cans to be designed to fit in with reduced profile requirements favoured increasingly by electronics designers.

The ability to reduce the overall height of a can makes it better able to meet the miniaturisation requirements of modern electronics assemblies and can also improve the attenuation by minimising the size, which in broad terms governs the shielding efficiency of the shielding can.

Branded MicroSafe this exceptionally even coating is pinhole free and has no detrimental effect on the shielding efficiency of the formed can. Adhesion to the substrate metal (usually nickel silver) is excellent and the upper operational temperature is claimed to be in excess of 260°C.  Selective application of the Micro-Safe coating allows pins and SMT lands or feet to be left bare to maintain the can's naturally high solderability.

The insulation resistance of the MicroSafe coating, even under high humidity conditions is reported to be greater than 1 x 10¹¹ ohms. It has outstanding adhesion properties and a flammability rating of UL 94 V-0. The coating surface is smooth and non-hygroscopic and SIR testing has been carried out with excellent results.

For further information, and a sample of a Micro-Safe dielectric coated EMI/RFI shielding, e-mail: westonl@precisionmicro.com    Refer to page 20   

New differential pressure transmitter ideal for level and flow measurement

The XMD range of smart differential pressure transmitters for process industry applications, from Impress Sensors & Systems, can be used for level measurement of closed, pressurised tanks, pump and filter control, or for closed pipe flow measurement in conjunction with an orifice plate, Venturi or nozzle.

The XMD range from Impress Sensors & Systems Ltd is ideal for flow applications due to the device's ability to switch the output from a standard linear output to a square root extraction where flow rate can be output. Target industries include chemicals, paper and pulp, petrochemicals, energy & power, food and dairy, and pharmaceuticals.

The XMD has a differential pressure range from 75mbar up to 2 bar and with a static over-pressure capable of 130 bar line pressures. The calibrated pressure range can also be turned down 10:1 times, which means that a 75mbar range can give a full scale output at 7.5mbar, whilst still maintaining the 130 bar line pressure. A 5:1 turn down ratio maintains the accuracy of the transmitter, which is ±0.1% FSO.

Outputs available are 4-20mA as standard with the option of having HART® communication. If this option is selected, the XMD is ATEX-approved as standard. The device is intrinsically safe for Zone 0/1 and an Ex d version is available with a flameproof enclosure for Zone 1 applications.

The XMD range is designed for harsh process manufacturing environments and so the electrical enclosure is a two-chamber aluminium die cast, powder coated with the option to include an integrated smart display. As Sam Drury, Sales & Marketing Director at Impress Sensors & Systems comments: "The integrated display enables the user to programme the device, including electronic damping of the signal, offset adjustment [of up to 90 per cent of full scale] and a turndown ratio of 10 to 1. The two-chamber aluminium die cast housing is prepared for chemical seals to extend the application possibilities even further."

The optional display is an LCD with a visible range of 32.5mm x 22.5mm. There's also a textual display and a 52-segment bar graph that runs along the bottom of the display for easy interpretation of the output from the device. Even with this display option, the XMD is still rated to IP67.

For further information, e-mail: info@impress-sensors.co.uk   Refer to page 40