Pressure sensors with modular construction ease the task of plant planners. The process determines the connection, the environment, the housing and the evaluation of the type of signal.
In industrial measurement, the demands made of pressure sensors are almost as numerous as the applications themselves. Here industry-specific standards for installation, housing variations, calibration and display, chemical resistance, mechanical robustness, measurement ranges, approvals, electrical connections ... adaptable sensors are all in demand.
Vega is offering the integration of absolutely oil-free measuring cells in a modular system of process connections, housings, types of signal transmission, calibration methods, all combined with the ease of use provided by an intelligent system of operation.
Of course, users would like to have 'one sensor for everything'. With the technologies now available, this goal is now being, at least partially, approached. But, if only for reasons of chemical resistance, 'one for all' is foredoomed to failure. Graduated measuring ranges help to achieve good accuracy, high-quality materials ensure reliability, and appropriate approvals permit application in critical fields.
Pressure sensors
The capacitive pressure sensors made by Vega operate completely dry - they depend only on the variations in an electrical capacitor caused by pressure variations. They cannot be made as small as one would sometimes like, but bring a list of advantages in their train:
* They can be made with metallic or ceramic sensing diaphragms.
* Where there is nothing inside, nothing can run out - contamination of the medium is impossible.
* Self-monitoring begins at the diaphragm.
The metallic measuring cell
Metal diaphragms are thin, formed foils. Diaphragms used in the MKD measuring cells are 80-300 mm thick, depending on the pressure range. All metallic diaphragms are naturally more or less sensitive to any type of contact with foreign bodies.
A brief profile: measuring ranges 0 ... 0,1 bar to 0 ... 20 bar, resistant to static overloads up to 25 times the range, very good thermal shock characteristics, medium temperatures up to 200°C and no elastomeric seal in contact with the medium.
The ceramic measuring cell
Ceramic is an ideal spring material. Ceramic does not creep under load, is free from hysteresis, has excellent long-term stability, and is very resistant to abrasive and chemically aggressive media.
Similarly, depending on their measuring range and on the type of sensor, the thickness of ceramic diaphragms can vary, for the CERTEC measuring cell, for example, between 0,21 and 2 mm. That makes them much more robust than their metallic counterparts.
Measuring ranges from 0 ... 0,1 bar to 0 ... 60 bar, static overload resistance up to 80 times. Ceramic measuring cells are made for both relative and absolute pressure measurement.
Chemical resistance
The chemical resistance of measuring diaphragms is important, but the adapter used for mounting the sensor must be made at least of an equivalent material. The choice here is between, on the one hand, high-grade, extremely expensive stainless alloys, and on the other, ceramic, which as a non-metallic material offers a quite different perspective. The base material, the manufacturing process, and the secondary materials all play an important role.
Alpha ceramic is a ceramic especially developed for corrosive media. The main component of this aluminium-oxide ceramic is a fine-grain matrix of highly resistant sapphire. The cement necessary to hold the grains of sapphire together also fulfils stringent demands - it contains almost no glass, a weak point when it comes to corrosion.
Flush mounting
Suitability for flush mounting is determined by the location of the process seal. If the outside diameter is not critical, the seal can be located on the external diameter. Such a sealing belt can be accommodated if the ceramic body is made thick enough. It can take the form of a seal protected against mechanical damage by being recessed within the housing, or be a flush-mounted moulded seal that fills even the minutest remaining gap to exclude moulds and bacteria.
Cleaning
Why does the thing need cleaning at all? Is it not possible to avoid dirt in the first place? Disregarding for the moment the need for cleaning on hygienic or aseptic grounds, the danger of contamination can be eliminated at the design stage. With absolutely flush mounting, diaphragm and container wall form a single plane, there are no 'nooks and crannies' for all manner of build-ups. This technology is also available for waste water and surface drainage water in sewage pipes.
Process connections
Here the user demands the greatest possible variety of industry-typical connections. From process pressure measurement with a simple manometer connection, through special chemical resistance, to sterile adapters for pharmaceutical sensors.
Examples are:
* Compressed air - Here, low-cost sensors can often be used. The build-up of dirt and harsh conditions are not normally to be expected.
* Water containing soil and sludges - If the sensor is installed in moving water, a flush-mounting diaphragm and the flow of water combine to eliminate build-up on the diaphragm. Integral overvoltage protection guarantees availability even when there are lightning strikes in the vicinity.
* Filter monitoring - Robust ceramic diaphragms and seals protected within the housing do not leave much for abrasive materials to destroy. Even sharp metallic particles, such as are found when recycling paper, cause no damage.
* Paint manufacture - Complete freedom from silicones through metallic diaphragms. Gap-free installation means effective cleaning.
* Foodstuffs processing - SIP/CIP-cleanable, gap-free flush mounting.
* Aggressive liquids in a plastic tank - Ceramic diaphragm, high-quality seals, and PVDF threaded or flanged fittings. Cost-effective installation with full chemical resistance. Can be used at tank pressures up to 10 bar.
* Hot bitumen - Temperature 160-180°C. Metal diaphragm with cooling elements. Robust and no liquid filling.
No isolator required
The metallic capacitive measuring cell MKD can operate at medium temperatures up to 200°C without separating diaphragms and isolation systems. Such accessories are only needed for higher temperatures.
Measuring range
In level measurement, the kind of 'hair-raising' pressure one would find in hydraulic systems simply do not occur. Ranges from 100-2000 kPa are typical, up to 6000 kPa or even 60 MPa are other possibilities.
Turn-down
With ceramic diaphragms, the permissible range turn-down is significantly greater. For metallic diaphragms, a factor of 5 is typical, whereas for ceramic diaphragms up to a factor of 10 is possible without difficulty. As an emergency measure, a ceramic diaphragm can even be used at a turn-down factor of 30. Of course, this also affects the accuracy, but to a degree which, in a service emergency, would still be acceptable.
Accuracy
Sometimes there is a difference between the accuracy quoted in a catalogue, and the accuracy that can be achieved in practice. The determining factors are:
* Accuracy of the measuring instrument (Lin).
* Temperature coefficient (TK).
* Long-term stability (LZS).
* Turn-down (TD).
When defining the accuracy of an instrument, the hysteresis and reproducibility must normally be taken into account. For the CERTEC, these are so small that they can be neglected.
The approximate formula for calculating accuracy is:
Example:
CERTEC ceramic measuring cell
Range: 0,4 bar
Lin 0,1%, TK 0,05%/10K,LZS 0,1% p.a
Temperature change DT= 50°C
Turn-down 4 m to 3,2 m = 1,25
= 0,32%
This is the 'worst case', the greatest error that can be expected. In practice, the error will normally be about half of this, because the tolerances will not all be taken up in the same direction.
With an isolator system, the accuracy of the pressure isolator must be added to that of the instrument. It is determined by the oil volume and the diameter of the process diaphragm. Larger diaphragms can absorb a greater volume-change due to temperature fluctuations. Unfortunately, temperature compensation only helps to a limited extent.
Operation and transmission of readings
The type of local operation distinguishes between basic functions such as zeroing, measurement range and integration period, menu-guided operation offers, in addition, pressure-free calibration, scaling of the optional display on the instrument in all units, and a choice of language for the operating menu.
The local display can also be in a separate external housing near the measuring point. In that case the operating elements are in the same housing. The user then has direct access to the display (and operation).
Remote control is by analog transmission on 4-20 mA lines using the HART protocol. In a network using VBUS, besides all the typical parameters that are available with HART, there is also service information, for example a peak value memory for min and max pressures and min and max temperatures (telltale function). Interrogation is carried out using VVO (Vega Visual Operating) operating software. VVO can set parameters on HART and VBUS sensors, and can carry out linearisation calculations. This provides a simple way of generating a linearisation curve for almost any shape of tank (see illustrations) and transmitting it to a sensor. The sensor then supplies a volume-compensated signal.
The measurement signals can be provided to four different standards.
* 4-20 mA.
* 4-20 mA HART.
* VBUS, digital network.
* Profibus PA.
Housings
A housing must provide protection against external influences. Plastic housings have been developed primarily for use in environments that are generally dry. Aluminium and stainless steel housings are better suited to the often rougher demands of level measurement. For use in really wet conditions, the IP68 versions in stainless steel are to be preferred.
Advantages of the modular system
The user orders. The multitude of possibilities enables maintenance-free, easy-to-connect sensors to be assembled to unified products without a long search.
Plastics provide toughness for cost, a housing for every need, measurement signals to international standards, an extensive range of approvals. Out of building blocks, professional tools are created. Standard tasks and solutions to practical problems become simpler.
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