Industrial Pressure Measurement
Industrial Pressure Measurement
A major portion of all industrial measurement relates in some way or other to pressure in its several forms. For example, flow is often measured by determining the pressure which exists at two different points in a system. In a Bourdon system, pressure changes are used to produce the mechanical motion of a recording stylus. Pressure can also be used to measure temperature in a filled system through changes produces by an expansion of the liquid or fluid in the filled system.
Next to temperature, pressure is the most important physical state variable in the technological processes, since it gives information about the pressure situation of liquids and gases in processing lines as well as about the load on the equipment. Pressure measurement application has a wide range. It ranges from simple set point monitoring for ensuring sufficiently high or low pressure levels to continuously monitoring as a part of a complex automation system for a technological process.
Accurate and reliable pressure measurement is a requirement for the safe operation of all the technological processes. It is probably the measurement parameter most applied in process instrumentation. The object of the pressure measurement is to produce a dial indication, control operation, or a signal, typically the standard 4–20 mA, representing of the pressure in the process.
Pressure measurement is obtained from the effects of pressure which cause position movement, change in resistance, or other physical effects which are then measured. The most common pressure sensors or primary pressure elements employ a Bourdon tube, diaphragm, bellows, force balance or variable capacitance arrangement. Some other methods are also used.
Pressure units and terminology
There are six terms applied to pressure measurements. These are as given below.
Total vacuum – It is zero pressure or lack of pressure, as generally experienced in outer space.
Vacuum – It is a pressure measurement made between total vacuum and normal atmospheric pressure.
Atmospheric pressure – It is the pressure on the earth’s surface due to the weight of the gases in the earth’s atmosphere and is normally expressed at mean sea level, for example, as 14.7 psi or 101.36 kPa. It is however, dependant on atmospheric conditions. The pressure decreases above the mean sea level.
Absolute pressure – It is the pressure measured with respect to a vacuum and is expressed, for example, in ‘psi a’.
Gauge pressure – It is the pressure measured with respect to the atmospheric pressure and is normally expressed, for example, in ‘psi g’.
Differential pressure – It is the pressure measured with respect to another pressure and is expressed as the difference between the two values. This represents two points in a pressure or flow system and is referred to as the ‘dp’.
Process pressure is defined as the force applied to a surface area, for example, kilogram per square centimeter (kg/sq cm). The SI (Système international d’unités) unit for pressure is Pascal (Pa) which is newton/square meter (N/sq m), but bar is more commonly used for process measurement. Fig 1 shows pressure terminology and the relationships for the more common pressure units.
Fig 1 Pressure units and terminology
Pressure is a relative measurement defined as either gauge or absolute. A pressure measurement can further be described by the type of measurement being performed. The three methods for measuring pressure are absolute, gauge, and differential. Absolute pressure is referenced to the pressure in a vacuum, whereas gauge and differential pressures are referenced to another pressure such as the ambient atmospheric pressure or pressure in an adjacent vessel.
Gauge pressure varies with atmospheric pressure, which in turn varies with the altitude above the mean sea level and the weather conditions. The relationship between these definitions is shown in Fig 1. Absolute pressure (Pa) is the pressure above a total vacuum, and gauge pressure (Pg) is the pressure above or below atmospheric pressure (Patm) giving Pa =Pg + Patm for all Pg, where Pg is negative if less than Patm. To avoid sign confusion, pressures below atmospheric pressure are referred to as Pvac giving Pa = Patm – Pvac for Pg less than Patm. It is desirable that absolute pressures are stated, for example, as ‘psi a’ or ‘bar a’ and gauge pressures are stated, for example, as ‘psi g’ or ‘bar g’ to avoid confusion.
Gauge pressure is the commonly used unit. A gauge pressure instrument indicates zero pressure when vented to the atmosphere. Absolute pressure includes the effect of atmospheric pressure with the gauge pressure. An absolute pressure instrument indicates atmospheric pressure (not scale zero) when vented to atmosphere. Fig 2 shows the pressure measurement and a dead weight tester, the primary standard for pressure measurements.
The tester uses calibrated weights on a piston to generate pressure in the tester volume. The test pressure is increased until it just supports the weights and piston, and the pressure is known very accurately (P = weight/area).
Fig 2 Pressure measurement
The dead weight tester uses the most fundamental pressure measurement technique. It is favoured for the primary calibration of pressure sensors, or piston gauge. This device uses calibrated weights (masses) which exert pressure on a fluid (usually a liquid) through a piston. Dead weight testers can be used as primary standards because the factors influencing accuracy are traceable to standards of mass, length, and time. The piston gauge is simple to operate. The pressure is generated by turning a jack-screw which reduces the fluid volume inside the tester, resulting in increased pressure. When the pressure generated by the reduced volume is slightly higher than that generated by the weights on the piston, the piston rises until it reaches a point of equilibrium where the pressures at the gauge and at the bottom of the piston are exactly equal. The pressure in the system is then P = W/A where W is the weight of piston plus the used weight.
The height of a column of liquid, or the difference between the heights of two liquid columns, is used to measure pressure head in devices called U-tube manometers (Fig 2). If a fluid is installed in an open U-shaped tube, the fluid level in each side is same. When pressure is applied to one side, that level goes down and the level on the other side rises until the difference between the heights is equal to the pressure head. The height difference is proportional to the pressure and to the density of the fluid. The U-tube manometer is a primary standard for pressure measurement.
Although many manometers are simply a piece of glass tubing formed into a U shape with a reference scale for measuring heights, there are many variations in terms of size, shape, and material. If the left side is connected to the measurement point, and the right is left open to atmosphere, the manometer indicates gauge pressure, positive or negative (vacuum).
Static, dynamic and differential pressure
A pressure measurement can be described as either static or dynamic. The pressure in cases with no motion is static pressure. Often, the motion of a fluid changes the force applied to its surroundings. A thorough pressure measurement is to note the circumstances under which it is made. Many factors including flow, compressibility of the fluid, and external forces can affect pressure.
Static pressure is uniform in all directions, so pressure measurements are independent of direction in a stationary (static) fluid. Flow, however, applies additional pressure on surfaces perpendicular to the flow direction, while having little impact on surfaces parallel to the flow direction. This directional component of pressure in a moving (dynamic) fluid is called dynamic pressure.
An instrument facing the flow direction measures the sum of the static and dynamic pressures and this measurement is called the total pressure. Since dynamic pressure is referenced to static pressure, it is neither gauge nor absolute and it is known as the differential pressure. While static gauge pressure is of primary importance in determining the net loads on pipe or vessel walls, dynamic pressure is used to measure flow rates and air/ gas speed. Dynamic pressure can be measured by taking the differential pressure between instruments parallel and perpendicular to the flow. The presence of the measuring instrument inevitably acts to divert flow and create turbulence, so its shape is critical to the accuracy and the calibration curves are often non-linear.
Dynamic pressure can be expressed as Pd = 0.5.d.(v2) where Pd is the dynamic pressure, d is the density of fluid (kg/cum) and v is the velocity (m/s). It can be seen that the dynamic pressure is related to the square of the velocity.
Differential pressure (dp) is the pressure difference between two points of measurement. Typical applications include pressure drops in ventilation systems, across both vented and pressurized vessels and for gas pressure measurement on low pressure vessels where a dp transmitter with the low-pressure side vented to atmosphere gives more accurate results than a pressure transmitter.
Different measurement conditions, ranges, and materials used in the construction of a sensor lead to a variety of pressure sensor (transducer) designs. Often one can convert pressure to some intermediate form, such as displacement, by detecting the amount of deflection on a diaphragm positioned in line with the fluid. The sensor then converts this displacement into an electrical output such as voltage or current. Given the known area of the diaphragm, one can then calculate pressure. Pressure sensors are packaged with a scale which provides a method to convert to engineering units. The three most universal types of pressure transducers are the bridge (strain gauge based), variable capacitance, and piezoelectric.
Of all the pressure sensors, Wheatstone bridge (strain based) sensors are the most common because they offer solutions that meet varying accuracy, size, ruggedness, and cost constraints (Fig 3). Bridge-based sensors can measure absolute, gauge, or differential pressure in both high-pressure and low-pressure applications. They use a strain gauge to detect the deformity of a diaphragm subjected to the applied pressure.
When a change in pressure causes the diaphragm to deflect, a corresponding change in resistance is induced on the strain gauge, which one can measure with a conditioned data acquisition (DAQ) system. The foil strain gauges can be bond directly to a diaphragm or to an element which is then connected mechanically to the diaphragm. Silicon strain gauges are sometimes used as well. For this method, it is essential to etch resistors on a silicon-based substrate and use transmission fluid to transmit the pressure from the diaphragm to the substrate.
Mechanical and electro-mechanical pressure gauges
Mechanical pressure gauges and electro-mechanical pressure sensors (Fig 3) incorporate an elastic element called a force-summing device which changes shape when pressure is applied to it. The shape change is then converted to a displacement. Of the wide variety of mechanical pressure sensing devices, the most common are Bourdon tubes and diaphragms. Bourdon tubes provide fairly large displacement motion which is useful in mechanical pressure gauges. The lesser motion of diaphragms is better in electro-mechanical sensors.
Fig 3 Mechanical pressure gauges and electro-mechanical pressure sensors
The motion of the pressure sensing devices can be linked to a linear variable differential transformer, which acts as the electro-mechanical transduction element. Alternatively, it can be linked, usually through a motion amplifying mechanism, to the wiper of a potentiometer. To reduce acceleration error, a balancing mass can be provided.
Mechanical pressure gauges – In mechanical gauges, the motion generated by the pressure sensing device is converted by mechanical linkage into dial or pointer movement. The better gauges provide adjustments for zero, span, linearity, and sometimes temperature compensation for mechanical calibration. High-accuracy mechanical gauges take advantage of special materials, balanced movements, compensation techniques, mirror scales, knife-edge pointers, and expanded scales to improve the precision and accuracy of readings. The most accurate mechanical gauges, test gauges, are used as transfer standards for pressure calibration, but for applications requiring remote sensing, monitoring, or recording they are impractical. Their mechanical linkages also limit their frequency response for dynamic pressure measurements.
Electro-mechanical pressure sensors – Electro-mechanical pressure sensors, or pressure transducers, convert motion generated by the pressure sensing device into an electrical signal. These sensors are much more useful and adaptable than mechanical gauges, especially when applied in data acquisition and control systems. In well-designed pressure sensors, the electrical output is directly proportional to the applied pressure over a wide pressure range. For rapidly changing or dynamic pressure measurement, frequency characteristics of the pressure sensor are an important consideration.
Principles of electronic pressure measurement
For electronic pressure measurement, a sensor is needed to detect the pressure and / or its change, and to convert it into an accurate and repeatable electrical signal utilizing a physical operating principle. The electrical signal is then a measure of the magnitude of the applied pressure or change in pressure.
The most critical mechanical component in any pressure sensor is generally the pressure sensing structure. The function of the structure is to serve as the reaction for the applied force, and in doing so, to focus the effect of the force into an isolated uniform strain field where strain gauges can be placed for pressure measurement (Fig 4). While there are various types of pressure sensing technologies, four key measuring principles and their technical understanding are described below.
Resistive pressure measurement – The principle of resistive pressure measurement is based on the measurement of the change in resistance of electric conductors caused by a pressure-dependent deflection. The equation applied for the resistance of an electric conductor is R = r. (l/a) where ‘R’ is electrical resistance, ‘r’ is resistivity, ‘l’ is length, and ‘a’ is the cross-sectional area. If a tensile force is applied to the conductor, its length increases and its cross-sectional area decrease. The resistivity of a metallic conductor is a (temperature-dependent) constant for a particular material and, hence, independent of the geometry, so the electrical resistance increases as a result of the elongation. In the case of compression, the opposite applies.
The principle of resistive pressure measurement is achieved using a main body which shows a controlled deflection under pressure. This main body frequently has a ‘thin’ area referred to as the diaphragm, which is weakened intentionally. The degree of deflection caused by the pressure is measured using metallic strain gauges.
Normally, four strain gauges are applied to a diaphragm. Some of them are located on elongated and others on compressed areas of the diaphragm. If the diaphragm deflects under the action of a pressure, the strain gauges are deflected correspondingly (Fig 4). The electrical resistance increases or decreases proportionally to the deflection (elongation or compression). To accurately measure the resistance change, the strain gauges are wired to a Wheatstone measuring bridge.
Fig 4 Pressure sensing mechanisms
Piezo-resistive pressure measurement – The principle of piezo-resistive pressure measurement is similar to the principle of resistive pressure measurement. However, since the strain gauges used for this measuring principle are made of a semiconductor material, their deflection due to elongation or compression results primarily in a change in resistivity. As per the equation of the resistance of an electric conductor, the electrical resistance is proportional to the resistivity. While the piezo-resistive effect in metals is negligible and thus effectively insignificant within resistive pressure measurement, in semi-conductors such as silicon it exceeds the effect of the variation of length and cross-section by a factor between 10 and 100.
Unlike metallic strain gauges, which can be attached to nearly any material, the semi-conductor strain gauges are integrated into the diaphragm as micro-structures. Hence, the strain gauges and the deflection body are based on the same semi-conductor material. Normally four strain gauges are integrated into a diaphragm made of silicon and wired to a Wheatstone measuring bridge. Since the micro-structures are not resistant to many pressure media, for most applications the sensor chip is to be encased. The pressure is then to be transmitted indirectly to the semiconductor sensor element, e.g. using a metallic diaphragm and oil as a transmission medium.
Due to the magnitude of the piezo-resistive effect, piezo-resistive sensors can also be used in very low pressure ranges. However, due to strong temperature dependency and manufacturing process-related variation, individual temperature compensation of every single sensor is needed.
Capacitive pressure measurement – The principle of capacitive pressure measurement (Fig 4) is based on the measurement of the capacitance of a capacitor, which is dependent upon the plate separation. The capacitance of a dual-plate capacitor is determined using the equation C = e. (A/d), where ‘C’ is the capacitance of the dual-plate capacitor, ‘e’ is permittivity, ‘A’ is the area of the capacitor plate, ‘d’ is the plate separation.
The principle of capacitive pressure measurement is achieved using a main body with a metallic diaphragm, or one coated with a conductive material, which forms one of the two plates of a dual-plate capacitor. If the diaphragm is deflected under pressure, the plate separation of the capacitor decreases, which results in an increase in its capacitance while the plates’ surface area and permittivity remain constant. In this way, the pressure can be measured with high sensitivity. Hence, capacitive pressure measurement is also suitable for very low pressure values, even down in the single digit milli-bar range. The fact that the moving diaphragm can be deflected until it reaches the fixed plate of the capacitor ensures a high overload safety for these pressure sensors. Practical restrictions on these sensors arise from the diaphragm material and its characteristics, and also from the required joining and sealing techniques.
Piezo-electric pressure measurement – The principle of piezo-electric pressure measurement is based on the physical effect of the same name, only found in some non-conductive crystals, e.g. mono-crystalline quartz. If such a crystal is exposed to pressure or tensile force in a defined direction, certain opposed surfaces of the crystal are charged, positive and negative, respectively. Due to a displacement in the electrically charged lattice elements, an electric dipole moment results which is indicated by the (measurable) surface charges (Fig 4). The charge quantity is proportional to the value of the force. Its polarity depends on the force direction. Electrical voltage created by the surface charges can be measured and amplified. The piezo-electric effect is only suitable for the measurement of dynamic pressures. In practice, piezo-electric pressure measurement is restricted to specialized applications.
The three most common sensor principles are shown in Fig 5. Metal thin-film and ceramic thick-film sensors are the two most common implementations of resistive pressure measurement. The significant differences between them result from the different materials used and their properties.
Metal thin-film sensor – The main body and the diaphragm of a metal thin-film sensor are normally made of stainless steel. The sensor can be produced with the required material thickness by machining the diaphragm in automatic precision lathes and then grinding, polishing and lapping it. On the side of the diaphragm not in contact with the medium, insulation layers, strain gauges, compensating resistors and conducting paths are applied using a combination of chemical vapour deposition (CVD) and physical vapour deposition (PVD) processes and are photo-litho-graphically structured using etching. These processes are operated under clean room conditions and in specialized plants, in some parts under vacuum or in an inert atmosphere, in order that structures of high atomic purity can be generated. The resistors and electrical conducting paths produced on the sensor are significantly smaller than a micrometer and are thus known as thin-film resistors.
The metal thin-film sensor is very stable because of the materials used. In addition, it is resistant to shock and vibration loading as well as dynamic pressure elements. Since the materials used are weldable, the sensor can be welded to the pressure connection. It can be hermetically sealed without any additional sealing materials. As a result of the ductility of the materials, the sensor has a relatively low over pressure range but a very high burst pressure.
Fig 5 Types of pressure sensors
Ceramic thick-film sensor – The main body and the diaphragm of the ceramic thick-film sensor are made of ceramic material. Alumina (Al2O3) is widely used due to its stability and good processability. The four strain gauges are applied as a thick-film paste in a screen-printing process onto the side of the diaphragm which is not to be in contact with the pressure medium, and then burned in at high temperatures and passivated through a protective coating. No impurities are permitted during the screen-printing and the burn-in processes. Hence, production is normally done in a clean room. For the maintenance of high process stability, proper segregation is important in order to avoid any cross-contamination.
The ceramic used for the sensor is corrosion resistant. However, installation of the sensor into the pressure measuring instrument case needs an additional seal for the pressure connection, which is normally not resistant against all media. In addition, the ceramic is brittle and the burst pressure is hence lower in comparison to a metal thin-film sensor.
Piezo-resistive sensor – A piezo-resistive sensor has a far more complex structure. The sensor element is made of a silicon chip. This chip consists of a diaphragm, structured with piezo-resistive resistors, which deflects under pressure. The chip has a surface area of only a few square millimetres and is thus much smaller than, for example, the diaphragms of metal thin-film or ceramic thick film sensors.
The piezo chip is very susceptible to environmental influences and, hence, is to be hermetically encased in most cases (Fig 5). For this reason, it is installed into a stainless steel case which is sealed using a thin flush stainless-steel diaphragm. The free volume between the piezo chip and the external diaphragm is filled with a transmission fluid. Synthetic oil is normally used for this. In an encased piezo-resistive sensor, the pressure medium is only in contact with the stainless steel diaphragm, which then transmits the pressure through the oil to the internal chip’s diaphragm.
For minimizing the influence of the thermal expansion of the transmission fluid on the pressure measurement, the sensor design is to be optimized in such a way that the free internal volume for the given contour of the stainless steel diaphragm is minimal. Among other things, special displacement bodies are used for this purpose.
A header is normally used for mounting and electrical connection of the sensor chip. It has integrated glass-to-metal seals for the electrical connection between the inner and outer chambers and can be hermetically welded to the case. The sensor element, glued to the rear side of the header, is connected to the pins using bond wires and transmits the electrical signals from the sensor element to the connected electronics in the external chamber of the sensor. A ventilation tube, which leads to the rear side of the sensor diaphragm, is located in the centre of the header. If the chamber behind the sensor element is evacuated and the ventilation tube is closed, it is possible to use such a piezo-resistive sensor to measure absolute pressure, since the vacuum of the hollow space serves as a pressure reference.
In sensors designed for gauge pressure measurement, the ventilation tube remains open and ensures continuous venting to the rear side of the diaphragm, so that the measurement is always performed relative to the local atmospheric pressure. The venting is achieved either through the outer case or through a ventilated cable to the outside. This ventilation tube is to be carefully protected against contamination, especially moisture ingress, since the sensor is very susceptible to this and can even become inoperative.
There is no ideal sensor principle since each of them has certain advantages and disadvantages. The comparison of the sensors is in Fig 6. The sensor which is most suitable for an application is primarily determined by the requirements of the application. It is not only the basic sensor technology which is the key for the suitability of the sensor, but above all the practicalities of its implementation. Depending on the application, the sensor principles described can indeed make the implementation either easier or more difficult. The material in contact with the pressure medium (wetted parts) and its suitability for certain media are of fundamental importance. One of the disadvantages of the ceramic thick-film sensor in comparison with the metal thin-film sensor is that it needs additional sealing between the non-metallic diaphragm material and the case. This almost always prevents universal applicability.
Fig 6 Comparison of sensors
Pressure measuring instruments
The normal instrument types are pressure transmitters, level probes, pressure switches, and process transmitters. Basically, these electronic pressure measuring instruments consist of a pressure connection, a pressure sensor, electronics, an electrical connection and the case (Fig 7). In addition, there are also simpler instrument types known as pressure sensor modules which are often consisting of no more than a pressure sensor and simple mechanical and electrical interfaces. These types are particularly suitable for complete integration into users’ systems.
Fig 7 Components of a pressure measuring instrument
Pressure transmitter – A pressure transmitter (Fig 7) has standardized interfaces, both on the process side and on the electrical output signal side, and converts the physical pressure value to a standard industrial signal. The pressure connection is used to lead the pressure directly onto the sensor. It has a ‘standardized’ thread and an integrated sealing system to enable easy connection of the pressure transmitter simply by screwing it in at the relevant measuring point. A suitable case protects the sensor and the electronics against environmental influences. The electronics transform a weak sensor signal into a standardized and temperature compensated signal (e.g. the common industrial signal of 4-20 mA). The output signal is transmitted though a ‘standardized’ plug or cable for subsequent signal evaluation.
Level probe – The level probe, sometimes also referred to as a submersible transmitter, is a special type of pressure transmitter used for level measurements. For this purpose the level probe measures the hydrostatic pressure at the bottom of the vessel. Particularly important is the choice of material for the case and cables, and also the seals at connection points, because of the complete and permanent submersion into the medium. Venting of the sensor system, needed for the gauge pressure measurement, is achieved through a ventilation tube passed through the cable.
Pressure switch – In many applications electronic pressure switches replace the mechanical pressure switches which were earlier very commonly used. This is because they offer, as a result of their design principle, additional functions such as digital display, adjustable switch points and considerably higher reliability.
An electronic pressure switch is based on an electronic pressure transmitter and hence offers the entire functionality of a transmitter. With the integrated electronic switch, which can close or open an electrical circuit, it is able to perform simple control tasks. The switch point and the reset point can be set individually. By default, a pressure switch only outputs binary signals such as switch point or reset point ‘reached’ or ‘not reached’ but it does not output how far the measured pressure is from the switch or reset point. Hence, many pressure switches have a display and additionally an analog output signal. The set parameters and measured pressure can be read off the display. In addition, the measured pressure can be transmitted by the analog output signal to a downstream control unit. Thus, the widely adopted type of electronic pressure switch includes a switch, a pressure transmitter and a digital indicator, all in one instrument.
Process transmitter – The process transmitter is a pressure transmitter with a pressure range which can be set within a predefined pressure range (turndown). It is mainly used in technological processes, since in this application area it is necessary to adjust every single measuring point to a multitude of specific requirements which are to be individually set by the operator on site. The process transmitters have very high measurement accuracy within the entire pressure range. In addition, the pressure range, the zero point and further parameters can normally be set individually. For this purpose many process transmitters have both digital display and additional operating elements and extensive operating software directly within the instrument.
Pressure transducer – Pressure transducers are normally available in a multitude of sensor modules which can be directly matched to the requirements of the process. They have, for example, a user-specific pressure connection and / or a user-specific electric interface. Only very few suppliers of electronic pressure measurement technology even offer the so-called ‘bare’ pressure sensor as a module. For these, the users are to develop their own design solutions in order to get the pressure to the sensor and evaluate the sensor signal. For pressure transducers, it is normally the case that their correct function is to be ensured by the user’s design-related measures. Hence, this option is generally only suitable for mass-produced equipment.
Environmental influences and special requirements
The following are the environmental influences and special requirements for the pressure sensing devices.
Temperature influence – Since temperature influences many properties of a material, it also affects the proper operation of measuring instruments. Very high or very low temperatures can damage or even destroy parts of the measuring instrument. In particular, plastic parts and sealing materials age much faster under the influence of constant high or low temperatures. For example, if the temperature is too low, they lose their elasticity. To ensure proper function of the pressure measuring instruments, some producers specify temperature ranges in their data sheets for the pressure medium, ambient conditions and during storage. Other producers define an operating temperature range which includes both the medium and ambient temperature range. The measuring instrument is not damaged provided the specifications are adhered to. The data specified in the data sheets regarding the measuring accuracy, on the other hand, are only valid for the temperature-compensated range which is significantly smaller and are also be specified in the data sheets.
Compatibility with the pressure medium – The pressure media are as many and diverse as the applications of pressure measurement technology. In pneumatics, it is mostly air mixed with residues of compressor oil and condensed water. In level measurement, it is mostly fuel, oils or chemicals. In hydraulics, the pressure of the hydraulic oil is to be measured while in refrigeration technology, the pressure of the refrigerant is to be measured. All physical and chemical characteristics of the pressure medium is to be considered when selecting the material and other properties of those parts of the pressure measuring instrument in contact with the pressure medium.
Special attention is required to be paid to the fact that the diaphragms are only a few micro-meters thick. Material abrasion due to the corrosion cannot be accepted since it not only erodes the diaphragm, but also affect the measurement characteristics which changes continuously. Due to the small material thickness, there is a risk of pressure medium diffusing through the diaphragm and reacting with the materials behind it such as filling media and adhesives. To prevent chemical reactions initiated by aggressive media, measures such as having a flush stainless-steel diaphragm with a highly resistant coating made of special plastic, ceramic materials or noble metals are often taken. As an alternative, the wetted parts can be made of titanium or other special materials such as alloys based on nickel, molybdenum or cobalt.
The reactivity of the pressure medium is, however, just one aspect from a whole range. If, for example, the water used as a pressure medium does not drain completely and subsequently freezes, it can damage the internal sensor diaphragm as a result of expansion. Lime deposits can also choke the pressure port. Some media, such as those with high viscosity or high solids content, require a pressure connection without a pressure port. For this purpose a flush variant of the sensor diaphragm is used.
Protection against soiling and water – The electronic components and electrical connections are to be protected against the ingress of any foreign objects or water in order to ensure that they continue to operate. The IP ratings specify the level of protection which is provided by an electrical or electronic instrument at room temperature against contact with, and intrusion of, foreign objects (first digit) as well as against ingress of water (second digit). A higher IP rating does not automatically imply an improvement in protection. As an example, IP67 (total dust ingress protection, protection against temporary immersion) does not necessarily cover IP65 (total dust ingress protection, spray water protection), since the load due to spray water can be significantly higher than the load during temporary immersion. For the IP68 rating (total dust ingress protection, protection against permanent submersion), the producer is always to specify additionally the duration and depth of immersion. These conditions are not specified in the standard.
Sealing problems can also be caused through temperature variations. Hence, some producers utilize different testing procedures to verify that their measuring instruments remain functional and measure within the specified accuracy limits even after temperature variations.
The use of pressure measuring instruments outdoors places especially high demands on them. A combination of high ambient humidity and low temperature can lead to condensation or even icing. Large cyclic climatic fluctuations can lead to the accumulation of water within the instrument if the instrument is not sealed (pumping effect). Intensive moisture accumulation (continuous condensation) on the measuring instrument, and partially inside it, occurs regularly if the ambient humidity is high and the temperature of the pressure medium is much lower than the ambient temperature. In this case, a special case design is required, which can only be achieved for certain instruments optimized for such operating conditions.
Mechanical load capacity – In many applications the pressure measuring instruments are sometimes exposed to significant shock and vibration loadings. Vibration loads are oscillating mechanical loads of longer duration. In contrast, shock is considered as an impulse wave which decreases quickly compared to vibration. Strong vibrations have an effect when using pressure measuring instruments. Shocks occur, for example, during its application in machines with high accelerations during operation, such as solid forming presses or drop forges.
For the pressure measuring instrument to be used safely in applications with strong vibrations and / or shocks, it is to withstand these loads. The vibration resistance of industrial pressure transmitters is usually in the range of 10 to 20 times the acceleration due to gravity (10 g to 20 g). Nowadays, the shock resistance of industrial pressure transmitters is at several hundred g.
Electro-magnetic radiation – Every electrically operated device can potentially emit electro-magnetic radiation. However, since an electronic circuit can also be influenced by electro-magnetic radiation, such instruments can also influence (interfere with) each other. The requirements for electro-magnetic compatibility (EMC) cover both interference emission and immunity. EMC problems frequently occur if many electronic devices are located within a small space. With increasing automation, this is also the case in many applications of electronic pressure measurement technology. EMC problems occur more and more frequently, because of the increasing operating frequency and electrical power of electronic devices, plants or systems. The normal protection requirements which are stipulated by the EMC directive and its implementations refer to the corresponding harmonized standards. Mandatory limit values for the interference immunity and emitted interference are specified in the standards. Only instruments developed and manufactured in accordance with these standards have labels with the CE mark. However, in certain applications the end-users place much higher demands on the electro-magnetic compatibility and, in particular, on the interference immunity, in order to ensure safe operation even under unfavourable conditions.
Explosion protection – For electronic measuring instruments used in the hazardous areas, it is necessary to ensure thorough technical measures which, in accordance with the classification of the hazardous area; no ignition source can have an effect. There are several technical approaches to achieve explosion protection for an electronic instrument. The corresponding design concepts are referred to as explosion protection types. In electronic measurement technology the most frequently used is the concept of limitation of the ignition energy which is generally referred to as intrinsic safety (abbreviation ‘i’). For this, the current and voltage of the electrical power supply are limited in such a way that neither the minimum ignition energy nor the ignition temperature of an explosive mixture is ever reached.
Another explosion protection type is enclosing the measuring instrument in a flame-proof enclosure (abbreviation‘d’), where all components which are likely to cause ignition are installed within an enclosure that can withstand the internal explosion pressure. The escaping ignition energy is reduced by means of gaps between the enclosure parts to the extent that no ignition or external transmission of it is possible.
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