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Article - SilverBullet PR
23rd June 2010
Ref. IMP21
Latest silicon pressure sensors show huge improvements
The historical problems associated with silicon oil-filled pressure sensors are well
documented. However, recent advances in sensor design has led to improved
versions that overcome many of these disadvantages, says Sam Drury, Sales &
Marketing Director at Impress Sensors & Systems Ltd.
Historically, oil-filled pressure sensors that use silicon sensing elements have suffered in
terms of their measurement performance and reliability due to a number of different
factors. However, the latest versions now coming onto the market benefit from muchimproved mechanical designs and are therefore able to overcome these drawbacks.
Most silicon pressure sensors are of the media-isolated type, i.e. they operate by using an
isolated diaphragm. When measuring pressures, this diaphragm flexes and transfers the
pressure via the oil onto a silicon chip. Compared to the latest sensor designs, traditional
oil-filled, isolated diaphragm, silicon pressure sensors used a much higher volume of oil.
Whilst the cost of this oil is generally not an issue for users, the effects this oil has on the
overall performance and thermal stability of the sensor is a critical design factor. With
traditional designs, air often penetrates the housing and becomes trapped within the oil,
which can cause problems. As the sensor’s operating temperature flucuates, the physical
propoerties of the oil change as the air within it expands or contracts, which leads to
measurement errors. Over time, as the sensor becomes less airtight, more air will enter
the chamber and the problem worsens.
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In order to solve these issues, the latest silicon pressure sensors use much-improved
mechanically-sealed designs, which use only a fraction of the volume of oil compared to
previous versions. This means that even if any air does get through to the oil, there is less
oil and so the effects of temperature fluctuations on the oil are much less, which results in
more reliable measurement performance and improved thermal stability. The effects are
not completely eliminated but are greatly reduced compared to conventional versions. This
means the latest sensors with improved mechanical sealing are now suitable for use in
vacuum conditions.
Higher overpressures are also now possible. Typically, the latest silicon pressure sensors
offer five times overpressure when operating at 1bar and above, which is much higher
than devices currently on the market, therefore offering customers added security that the
sensors will last even in an over-pressure state on their system. Similarly, for pressure
ranges below 1bar, the latest sensors typically provide 10 times overpressure, therefore
protecting the sensitive low range diaphragms.
Accuracy is also improved to less than ± 0.1%, which compares very favourably with
traditional sensors. There are similar improvements in resolution, repeatability and thermal
stability of the sensor.
Oil-filled silicon pressure sensors with an all-stainless steel construction can also be used
in food and beverage processing. The latest sensors operate with food-grade oils and due
to their improved sealing, are resistant to frequent washdowns, chemicals and other
cleaning detergents.
In the past, oil-filled silicon pressure sensors have suffered from orientation issues. For
example, if a sensor is factory calibrated to 4mA in the horizontal position, but in the
application, is then mounted at an angle or becomes slightly offset, the changes to the
physical properties of the oil (due to gravitational effects) may affect the sensor’s
measurement performance. The latest versions, which use significantly less oil, do not
suffer to the same extent and the effects of gravity on the oil is greatly reduced.
There are many reasons why users select silicon pressure sensors over alternative
pressure sensing technologies. Here are some of the technical advantages:
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Perfect Elasticity
With silicon pressure sensors, the base diaphragm is typically made from ‘n’ type monocrystalline layer silicon, which is 100 per cent elastic up to its breaking point. This is what
makes silicon the ideal material to use as the sensing diaphragm.
Low Hysteresis
The combination of a high elasticity material and almost perfectly bonded and integrated
strain gauges produces a structure that has very little hysteresis. This ensures that
pressure readings are precise, regardless of the direction of changing pressure.
Repeatability
Strain gauges are key elements for converting pressure into electrical signals. As these
gauges are chemically altered areas of a uniform piece of material, they will deform almost
perfectly (without lag or separation) with the stresses of the diaphragm. This means that
the repeatability of pressure measurements is excellent over numerous pressure cycles
(i.e. the sensor does not suffer from fatigue).
High Gauge Factors
High levels of voltage drop can be achieved across semiconductor strain gauges for a
given pressure change, which means that better use can be made of analogue to digital
resolution and improving signal to noise ratios.
Compact and Lightweight
Silicon chip and silicon piezoresistive elements are manufactured using a combination of
chemical etching, micro-machining, doping and masking, which creates mechanical
structures with microelectronic components, which enables the production of miniature
sensing elements. As the sensing diaphragm is often the limiting factor that controls the
diameter of the finished sensor design, a more compact sensing element will help the
engineer design a more compact, lightweight pressure sensor.
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Acceleration
The compactness and reduced weight of the silicon sensing elements means that they are
less susceptible to acceleration forces. These sensors are therefore preferred for use in
applications where there are high G force levels, high vibration or impact shocks.
Stability
Semiconductor strain gauges are not glued, printed or electroplated to the surface of the
sensing diaphragm, but instead are a chemically changed part of the same material. Over
time, strain gauge bonds can deteriorate or change with pressure and temperature cycling,
which leads to instabilities in sensor performance.
High Overpressures
The higher gauge factors of semiconductor strain gauges compared to other types of
strain gauge, make it possible to use stiffer diaphragms for a given pressure range to allow
a high overpressure rating to be specified, without compromising too much sensitivity. The
high elasticity of silicon also makes it possible to strain silicon further than other less
elastic diaphragm materials, with almost no resulting stress.
Dynamic Response
Due to high sensitivity of semiconductor strain gauges, the thickness to cross-sectional
area is relatively high for a sensing diaphragm. Also, the small size results in a very high
natural frequency. This, combined with the elasticity and atomically bonded strain gauges,
makes silicon strain gauge diaphragms highly responsive to rapid changes in pressure.
– ENDS – [1,011 words]
Note to Editors:
About Impress Sensors & Systems Ltd
Impress Sensors & Systems Ltd (www.impress-sensors.co.uk) is a rapidly expanding UK manufacturer and
distributor of instrumentation products based in Aldermaston, Berkshire. The company prides itself on its
expertise in pressure measurement and its knowledge of general instrumentation. The company manufactures
its own range of standard and custom-designed pressure instruments in the UK, and distributes a wide
portfolio of instrumentation products from its European partners - BD Sensors in Germany and Metallux in
Switzerland. Impress specialises in offering a customised service in applications where standard products
may not satisfy the technical requirements and price aspirations of customers. The company boasts the
necessary technical expertise in mechanical, electronic and application engineering that enables it to supply
instruments tailored to the customer’s requirements.
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Issued by:
Dean Palmer
Director
SilverBullet PR Ltd
The Grey House, 3 Broad Street, Stamford,
Lincolnshire PE9 1PG
Tel: 01780 753 000
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Email: [email protected]
Website: www.silverbulletpr.co.uk
Reader Enquiries/Advertising:
Sam Drury
Sales Director
Impress Sensors & Systems Ltd
Unit 6B, Mercury House
Calleva Park, Aldermaston,
Berkshire RG7 8PN
www.impress-sensors.co.uk
Tel: +44 (0) 118 981 7980
Fax: +44 (0) 118 981 7990
Email: [email protected]
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