Strain gauges in pressure measurement technology

Strain gauges in pressure measurement technology

Strain gauges are measuring devices that change their electrical resistance through mechanical deformation. They are used in a variety of measuring instruments, which, besides scales and load cells, also includes pressure sensors.

Pressure sensors rely on several physical variables, including inductance, capacitance or piezoelectricity. The most common physical property by which pressure transmitters operate, however, is the electrical resistance that can be observed in the metallic deformation, or piezoresistive effect, of semiconductor strain gauges. The pressure is determined by a mechanical deformation, where strain gauges are attached to an elastic carrier. It is important here that the strain gauges can follow the movements of this carrier. If a pressure acts on the carrier, the deformation arising brings about a change in cross-section of the conductor tracks, which in turn causes a shift in the electrical resistance. It is ultimately this change in electrical resistance that a pressure transducer records and from which the pressure can then be determined.

Figure 1: Strain gauges deform under pressure

The deformation acting upon the conductor will thus cause it to change in length (Δl). Since the volume remains the same, it is the cross-section and thus the resistance R that changes:

ΔR/R = k • Δl/l

The change in resistance (ΔR) is proportional to the change in length (Δl), and the proportionality factor (k) will depend on both the geometry and the material properties. While ‘k’ will be 2 for metallic conductors, it can also be very high in semiconductors. Because of these relatively high ‘k-factors’ for semiconductors, these are more sensitive and can therefore measure even the slightest of pressure changes. Temperature dependency, however, also increases as a result of this.

The change in resistance in metallic strain gauges results from dimensional changes (geometry). In semiconductor strain gauges, however, the change is due to alterations in the crystal structure (piezoresistive effect).

The evaluation of the resistance change triggered by a pressure-induced deformation then takes place via a bridge circuit. For this purpose, the strain gauges are connected up to form a Wheatstone bridge (Figure 2). Two of the strain gauges are placed in a radial direction and two in a tangential one. It is thus so that two become stretched and two become compressed under deformation. In order for temperature effects to be compensated and for the signal to be as linear as possible, it is important that the strain gauges have the exact same resistances and are arranged in an exact geometry.

Figure 2: Bridge circuit

Metallic strain gauges

Among metallic strain gauges, we must differentiate between the foil and thin-film varieties.

Foil strain gauges consist of rolled foil, only a few microns thick. Constantan is normally used as the material here, but Karma and Modco can also be employed, especially if a larger temperature range is needed or the temperatures are below -150 °C. Constantan has a very low ‘k-factor’ of 2.05 and is therefore not very sensitive. Considering this, the material displays a lowered temperature dependency, which is also why it is most often used in foil strain gauges.

Foil strain gauges are more likely to be used in load cells. Often they are not sensitive enough to be pressure transducers, since values of less than one bar cannot be recorded with them. Their temperature range is also relatively limited, and, depending on the version, temperatures of even 80 °C should not be exceeded.

Thin-film strain gauges are produced by the so-called thin-film technique, by, for example, vapor deposition or sputter coating. The manufacturing process is more complex here and also more expensive than for foil gauges. On the other hand, however, a temperature range of 170 °C is possible, with their long-term stability also being very good.

Metallic thin-film strain gauges provide for stable over the longer term, but also quite expensive, measuring instruments. It holds true that the lower the pressures to be detected are, the higher the manufacturing cost will be. Low pressures of less than 6 bar can only be detected at a poor accuracy.

Semiconductor strain gauges

Semiconductor strain gauges operate by the piezoresistive effect. The material used in most cases is silicon. Semiconductor strain gauges tend to be more sensitive than the metallic variety. They are also usually separated from the medium by a separation membrane, with the pressure being passed on via a transfer fluid.

Figure 3: Piezoresistive measuring device

In semiconductor materials, the piezoresistive effect is about fifty times more pronounced than with metallic strain gauges. The semiconductor strain gauges are either glued to a carrier or directly sputter-coated onto it. The latter enables an intense bonding and assures freedom from hysteresis, as well as a resistance to aging and temperature stability. Although the piezoresistive effect is not exclusive to the semiconductor strain gauge, the term “piezoresistive pressure sensor” has come to be used for instruments where the elastic structure deforming under pressure and the resistors are all integrated into one chip. Piezoresistive pressure transducers can be made small in size and (apart from the membrane) without any moving parts. Their production is based on normal semiconductor fabrication methods. At the same time, there is the possibility of integrating the resistors with the elastic membrane deforming under pressure all into one chip and thus produce a full pressure measurement cell in the size of just one chip.

Piezo thin-film strain gauges are attached to a silicon carrier and separated from the carrier by an insulating layer. This increases the manufacturing requirement and thus also the price, but temperature ranges from -30 °C to 200 °C are possible here. Thanks to the highly elastic properties of silicon, only a low hysteresis can be expected with these. It is the high ‘k-factor’ that achieves the high sensitivity, making piezoresistive pressure transmitters the first choice for the smallest of pressure ranges on the mbar scale. In addition, devices of tiny dimension can be produced, which has a positive effect on the scope of potential applications. Also, the long-term stability and EMC compatibility is very good, with the latter, of course, depending upon carrier material. Temperature compensation, however, requires a little more effort, but even this challenge can also be overcome quite easily. You can read more about temperature compensation here.

Thick-film strain gauges are printed onto ceramic or metallic membranes. With a thickness of 20 microns, they are up to 1,000 times thicker than thin-film strain gauges. Because of their low production requirements, these are cheaper in price, but not very stable longer term due to the aging of their thick film.

Summary: The type of strain gauge used has a major influence on the measuring instrument. Factors such as price, accuracy and long-term stability play an important role in choosing the right pressure transmitter. In our experience, pressure transmitters with piezo thin-film strain gauges have proven to be the most efficient, because, thanks to their sensitivity, they can record wide pressure ranges at high accuracy, whilst also exhibiting good long-term stability.

Fouling as a cause of drift in pressure sensors

Fouling as a cause of drift in pressure sensors

We all know the saying that ‘you can’t make an omelet without breaking eggs’. In the development of new combustion engines specifically, this means that soot particles or oil residue can contaminate the sensors employed.

The consequence of such soiling amounts to increasingly inaccurate readings. When, for example, the exhaust system of a new combustion engine is being monitored with pressure transducers, more and more fine dust will settle on the diaphragm of the sensor over time. The membranes of piezoresistive pressure sensors are very thin so that they can deliver high-precision measurement results. But when a layer of soot forms on this over time, it reduces the overall sensitivity of the pressure transducer.

Protecting pressure transmitters from particulate matter

End users make note of this drift in the sensor by performing reference pressure measurements. They will find considerable differences between the values of this reference pressure gauge and the soiled sensor itself. Often, however, the readings experienced by users reveal when the measured signals deviate too far from the expected results. Strong fluctuations in these measured values can also be an indicator of contamination.

STS generally recommends that users whose sensors are exposed to dirt should service them after a maximum of 100 operating hours. In addition, users can also try to protect the sensor as much as possible from contamination. There are two common methods used here.

Method 1: Protective foil

The first method does not replace maintenance of the sensor after a maximum of 100 hours, but it does simplify cleaning and also preserves the membrane. In this case, a very thin, metallic protective foil is applied to the membrane to protect it from soiling. After a maximum of 100 operating hours, this film is then simply peeled off and replaced with a new one.

Method 2: Cooling adapter

This method allows users to kill two birds with one stone. By screwing a cooling adapter or a climatic valve to the front end of the pressure port, the membrane is now largely protected from soiling. The climatic valve opens only when there is actually something to be measured. Where no permanent pressure monitoring is required, this can be a good method for minimizing the degree of contamination to the sensor employed.

At the same time, a constant sensor temperature can also be ensured via this cooling element. Besides membrane contamination, temperature also has an effect on the measuring accuracy of piezoresistive pressure transducers (More on the influence of temperature on the accuracy of pressure sensors can be found here).

Cleaning of pressure sensors from oil contamination

Contamination with heavy oil particularly comes about in the development of marine engines. The additives incorporated become especially deposited on the membrane and can even damage it. These residues reduce the sensitivity of the pressure transducer and regular servicing must be observed here also.

To keep soiling and the consequences thereof as minimal as possible, consideration should be given to the nature of the pressure sensor at its selection. A stainless steel membrane is recommended, which is front-flush and has no channels in which even the smallest of deposits can gather. The smoother the better also applies here, because on a rough membrane undesired particles will actually deposit faster and these are also more difficult to clean.

To clean a soiled pressure sensor, it must be removed from its application. Isopropanol (IPA) is recommended as the cleaning agent here. While the sensor housing requires no special caution, the membrane should be treated without any firm pressures by using, for example, cotton swabs. Under no circumstances should compressed air be used, since the membranes are very thin and, when too much pressure is exerted, deformations can occur.

High Accuracy Pressure Measurement at High Temperatures

High Accuracy Pressure Measurement at High Temperatures

In some applications, pressure transmitters have to work reliably when exposed to very high temperatures. Autoclaves used to sterilize equipment and supplies in the chemical and food industries are certainly one of these demanding applications.

An autoclave is a pressure chamber used in a wide range of industries for a variety of applications. They are characterized by high temperatures and pressure different from ambient air pressure. Medical autoclaves, for example, are used to sterilize equipment by destroying bacteria, viruses and fungi at 134 °C. Air trapped in the pressure chamber is removed and replaced by hot steam. The most common method for achieving this is called downward displacement: steam enters the chamber and fills the upper areas by pushing the cooler air to the bottom. There, it is evacuated through a drain that is equipped with a temperature sensor. This process stops once all air has been evacuated and the temperature inside the autoclave is 134 °C.

Very accurate measuring at high temperatures

Pressure transmitters are used in autoclaves for monitoring and validation. Since standard pressure sensors are usually calibrated at room temperature, they cannot deliver the best accuracy under the hot and wet conditions encountered in autoclaves. However, STS has recently been approached by a client in the pharmaceutical industry that requires a total error of 0,1 percent at 134 °C measuring -1 to 5 bar.

Piezoresistive pressure sensors are rather sensitive to temperature. However, temperature errors can be compensated so that the devices can be optimized for the temperatures encountered in individual applications. For example, if you use a standard pressure transmitter that achieves 0,1 percent accuracy at room temperature, the device would not be able to deliver the same degree of accuracy when used in an autoclave with temperatures of up to 134 °C.

Users who know that they require a pressure sensor that achieves a high degree of accuracy at high temperatures hence need a device that is calibrated accordingly. Calibrating a pressure sensor for certain temperature ranges is one thing. However, the client who inquired about the autoclave application with very high accuracy demands had another challenge for us that was even trickier to realize than a properly calibrated sensor: not only the sensor element was to be in the autoclave at 134 °C, but the complete transmitter including all electronics had to go in there, too. Unfortunately, we cannot go into specifics as to how we were able to assemble a digital transmitter that both delivers the desired accuracy of less than 0,1 percent total error at 134 °C but whose other components can handle the hot and moist conditions as well.

In short: Piezoresistive pressure sensors are sensitive to temperature changes. However, with the right know-how, they can be optimized for the requirements of individual applications. Moreover, not only the sensor element can be calibrated accordingly, the whole transmitter can be assembled in a way that even hot and wet conditions can be managed.

Correctly interpreting accuracy values for pressure sensors

Correctly interpreting accuracy values for pressure sensors

In the search for a suitable pressure transmitter, various factors will play a role. Whilst some applications require a particularly broad pressure range or an extended thermal stability, to others accuracy is decisive. The term “accuracy”, however, is defined by no standards. We provide you with an overview of the various values.

Although ‘accuracy’ is not a defined norm, it can nevertheless be verified from values relevant to accuracy, since these are defined across all standards. How these accuracy-relevant values are specified in the datasheets of various manufacturers, however, remains entirely up to them. For users, this complicates the comparison between different manufacturers. It thus comes down to how the accuracy is presented in the datasheets and interpreting this data correctly. A 0.5% error, after all, can be equally as precise as 0.1% – it’s only a question of the method adopted for determining that accuracy.

Accuracy values for pressure transmitters: An overview

The most widely applied accuracy value is non-linearity. This depicts the greatest possible deviation of the characteristic curve from a given reference line. To determine the latter, three methods are available: End Point adjustment, Best Fit Straight Line (BFSL) and Best Fit Through Zero. All of these methods lead to differing results.

The easiest method to understand is End Point adjustment. In this case, the reference line passes through the initial and end point of the characteristic curve. BSFL adjustment, on the other hand, is the method that results in the smallest error values. Here the reference line is positioned so that the maximum positive and negative deviations are equal in degree.

The Best Fit Through Zero method, in terms of results, is situated between the other two methods. Which of these methods manufacturers apply must usually be queried directly, since this information is often not noted in the datasheets. At STS, the characteristic curve according to Best Fit Through Zero adjustment is usually adopted.

The three methods in comparison:

Measurement error is the easiest value for users to understand regarding accuracy of a sensor, since it can be read directly from the characteristic curve and also contains the relevant error factors at room temperature (non-linearity, hysteresis, non-repeatability etc.). Measurement error describes the biggest deviation between the actual characteristic curve and the ideal straight line. Since measurement error returns a larger value than non-linearity, it is not often specified by manufacturers in datasheets.

Another accuracy value also applied is typical accuracy. Since individual measuring devices are not identical to one another, manufacturers state a maximum value, which will not be exceeded. The underlying “typical accuracy” will therefore not be achieved by all devices. It can be assumed, however, that the distribution of these devices corresponds to 1 sigma of the Gaussian distribution (meaning around two thirds). This also implies that one batch of the sensors is more precise than stated and another batch is less precise (although a particular maximum value will not be exceeded).

As paradoxical as it may sound, accuracy values can actually vary in accuracy. In practice, this means that a pressure sensor with 0.5% error in maximal non-linearity according to End Point adjustment is exactly as accurate as a sensor with 0.1% error of typical non-linearity according to BSFL adjustment.

Temperature error

The accuracy values of non-linearity, typical accuracy and measurement error refer to the behavior of the pressure sensor at a reference temperature, which is usually 25°C. Of course, there are also applications where very low or very high temperatures can occur. Because thermal conditions influence the precision of the sensor, the temperature error must additionally be included. More about the thermal characteristics of piezoresistive pressure sensors can be found here.

Accuracy over time: Long-term stability

The entries for accuracy in the product datasheets provide information about the instrument at the end of its production process. From this moment on, the accuracy of the device can alter. This is completely normal. The alterations over the course of the sensor’s lifetime are usually specified as long-term stability.  Here also, the data refers to laboratory or reference conditions. This means that, even in extensive tests under laboratory conditions, the stated long-term stability cannot be quantified precisely for the true operating conditions. A number of factors need to be considered: Thermal conditions, vibrations or the actual pressures to be endured influence accuracy over the product’s lifetime.

This is why we recommend testing pressure sensors once a year for compliance to their specifications. It is essential to check variations in the device in terms of accuracy. To this end, it is normally sufficient to check the zero point for changes while in an unpressurized state. Should this be greater than the manufacturer’s specifications, the unit is likely to be defective.

The accuracy of a pressure sensor can be influenced by a variety of factors. It is therefore wholly advised to consult the manufacturers beforehand: Under which conditions is the pressure transmitter to be used? What possible sources of error could occur? How can the instrument be best integrated into the application? How was the accuracy specified in the datasheet calculated? In this way, you can ultimately ensure that you as a user receive the pressure transmitter that optimally meets your requirements in terms of accuracy.

Temperature compensation: The key to precision

Temperature compensation: The key to precision

When selecting the right pressure transducer, knowledge of the temperatures that may arise is of the utmost importance. If the measurement technology used is not adequately temperature-compensated, serious inaccuracies and other risks will be the net outcome.

This is why end users need to know in advance which temperatures are to be expected within their own specific application. There are two values to consider here: The media temperature and the ambient temperature. Both of these values are important. The media temperature is the value at which the pressure port makes its contact. The ambient temperature, however, is the value arising in the environment surrounding the application and ultimately affects the electrical connections. Both values can be very different from one another, yet each also has differing consequences.

Why is temperature an important factor?

The materials used in piezoresistive pressure transducers display a certain temperature dependency (read more about the thermal characteristics of piezoresistive pressure transmitters here). The measurement behavior of the pressure transducer thus also shifts with temperature. As a result, temperature-related zero offsets and span errors will now arise. Expressed in simple terms, if a pressure of 10 bar is approached at 25 °C and then for a second time at 100 °C, different measured values will be obtained. For users viewing a data sheet, this means that excellent accuracy values are really of little use when temperature compensation itself remains insufficient.

Apart from avoiding serious measurement errors, the mechanical functionality of the measuring instrument also depends upon the existing temperature. This mainly affects components such as electrical connections and the cables used for the transmission of measured values. Very few of the standard materials can withstand temperatures around, yet alone above, 100 °C. The cable sockets and cables themselves can melt or even catch fire here. Besides measuring accuracy, temperature also has an influence on operational safety.

Fortunately, users need not to live with these risks, since pressure transducers can be optimized for different temperature conditions – on the one hand through temperature compensation, and on the other using additional cooling elements and particularly heat-resistant materials.

Temperature errors can be avoided

The manufacturers of pressure sensors employ temperature compensation. Products from STS, for example, are optimized as standard for operating temperatures from -0 °C to 70 °C. The further the temperature deviates from these values, the greater the measurement inaccuracy becomes. A measuring instrument optimized for a range from 0 °C to 70 °C but used at temperatures around 100 °C will no longer achieve its specified accuracy values. In this case, a sensor must be deployed, which is actually compensated for temperatures of around 100 ° C.

There are two forms of temperature compensation:

  • Passive compensation: Temperature-dependent resistors are activated in the Wheatstone bridge
  • Active compensation (polynomial compensation): Various pressures are approached at rising temperatures within a heating cabinet. These are then compared with the values from a calibration standard. The temperature coefficients determined from this are next entered into the electronics of the pressure transmitter so that the temperature errors in actual practice can now be compensated for “actively”.

Active temperature compensation remains the preferred method because it leads to the most accurate of results.

Temperature compensation itself, on the other hand, does have its limitations. As previously mentioned, temperature affects not only the precision of a pressure transmitter. The mechanical components of the measuring cell also suffer at temperatures above 150 °C. At these temperatures, contacts and bondings can become loose and the sensor itself suffers damage. If exceptionally high media temperatures are to be expected, then additional cooling elements will be required to ensure the functionality of the sensor.

Cooling elements at very high media temperatures

To protect the pressure transmitter from very high temperatures, there are four variants which can be employed depending upon the application and the temperature involved.

Variant A: Media temperatures to around 150 °C

In this variant, a cooling fin element is integrated between the measuring cell and the amplifier. It is a matter here of separating the electronics from the actual application, so that these remain undamaged by the elevated temperatures.

Variant B: Temperatures above 150 °C

If the medium is very hot, a cooling element is screwed in front of the pressure port (cooling fins, for example, that can be screwed from both sides). The pressure port thus comes into contact now solely with the cooled medium. These forward-attached cooling fins have no effect at all on the accuracy of the sensor. Should the medium be extremely hot steam, however, then a siphon would instead be employed as the cooling element.

Variant C: Extremely high temperatures (up to 250 °C)

When the media temperature is extremely high, a forward-facing isolating system incorporating a cooling section can now be used. This variant, however, is quite large in size and does affect the accuracy negatively.

Pressure transducer with forward isolator and cooling section for media temperatures up to 250 °C

Variant D: Special case of a warming cabinet or climatic chamber

When pressure measurements are necessary inside a warming cabinet at ambient temperatures of up to 150 °C, the electronics of the pressure transmitter cannot be exposed to these temperatures without suffering damage. In this instance, only the measuring cell (with pressure port and stainless steel housing) is located within the cabinet, with this connected to the remote electronics outside the cabinet (also housed in a stainless steel housing) via a high-temperature FEP cable.

In summary: Consultation is king

The precision of piezoresistive pressure sensors is influenced by temperature conditions. The temperatures acting on the pressure port can be compensated for passively or actively so that the pressure sensor used meets the requirements upon accuracy over the anticipated temperature range. Furthermore, the influence of ambient temperature on the mechanical components of the measuring instrument must also be taken into consideration. Using forward-mounted cooling elements and heat-resistant materials, this can also be brought under control. Users should thus always rely on the comprehensive advice offered by the manufacturer and ensure that the pressure transducers available can be optimized to their very own specific applications.

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