Conductivity measurement in natural waters & other liquids

Conductivity measurement in natural waters & other liquids

by | Wednesday, 1 July, 2020 | Water

When measuring conductivity, various factors have to be taken into account, depending upon the liquid being tested. Particular attention is to be paid to temperature as the major influencing factor.

Conductivity is expressed in microsiemens and indicates a substance’s ability to conduct electric current. The conductance is the inverse of resistance, as expressed in ohms. It thus follows that the higher the conductance, the lower the resistance.

Conductivity in natural waters

Pure water is practically non-conductive (0.055 µS/cm, compared to drinking water at 500 µS/cm). It becomes conductive only through dissolved substances such as chlorides, sulfates and others. The purity of a water body can thus be determined via a conductivity measurement, where the higher the conductivity, the more substances are dissolved in the water. Typical applications for conductivity measurement include, for example, landfills for testing groundwater contamination and the monitoring of salt water ingress into groundwater sources. This makes conductivity an important factor for monitoring tasks in environmental technology in order to draw conclusions about possible impurities. Although conductivity is only an indicator of pollution, the composition of the substances entering the water must subsequently be chemically analyzed. In addition, not all substances that can be dissolved in water are also conductive (e.g. hormones or fungicides).

Another common application is the determination of flow direction and flow velocity. For this purpose, salt is added to the water and its conductivity accordingly increased. Flow velocity and direction can be precisely determined by measuring the conductance at specific points.

As already mentioned, the conductivity of a substance is highly temperature-dependent. Two samples of a substance can therefore produce different conductance values at different temperatures. Without temperature compensation, there is practically no possibility of comparing two substances if they cannot be examined at exactly the same temperature. For this reason, conductivity measurement and temperature measurement go hand in hand. Usually, therefore, both the conductivity and the temperature are measured in a single conductivity measurement. Temperature compensation is then used to calculate the conductance at a reference temperature, which is normally set at 25 °C.

Temperature compensation function: The substance decides

Which temperature compensation function is used to determine the conductivity at reference temperature depends entirely on the liquid being examined. For natural waters, the non-linear function according to the DIN EN 27888 standard  for water quality is used.

Linear functions are used for salt solutions, acids and alkalis. To calculate the percentage change in conductivity (K) per °C temperature change (∆T), we use the following formula:

α = (∆K(T)/∆T)/K(25°C)*100

∆K(T) = Conductivity change from the selected temperature range
∆T = Temperature change from the selected temperature range
K(25°C)= Conductivity at 25°C

Finally, let us consider an example calculation for determining the conductivity of a quick descaler. To obtain the necessary figures for the calculation, three measurements must be carried out:

122.37 mS/cm at 20°C
133.10 mS/cm at 25°C
135.20 mS/cm at 26°C

∆K(T) = 135.20 mS/cm -122.37 mS/cm = 12.83 mS/cm
∆T = 26°C – 20°C = 6°C
K(25°C)= 133.10 mS/cm

α = ((135.20 – 122.37)/(26 – 20))/133.10*100 = 1.60 %/°C

Contact
High Accuracy Pressure Measurement at High Temperatures

High Accuracy Pressure Measurement at High Temperatures

by | Tuesday, 30 June, 2020 | Pressure Measurement

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.

Contact
Temperature compensation: The key to precision

Temperature compensation: The key to precision

by | Tuesday, 30 June, 2020 | Pressure Sensor Technology

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.

Contact
How pressure transmitters also work reliably in the cold

How pressure transmitters also work reliably in the cold

by | Tuesday, 30 June, 2020 | Pressure Sensor Technology

Ambient temperatures have a major influence on the functionality and accuracy of pressure transmitters, with Arctic temperatures representing a particular challenge here.

In pressure measurement on a piezoresistive basis, semiconductors diffused onto a silicon membrane serve as strain gauges. When pressure acts on the membrane, these strain gauges deform and a change in resistance occurs. It is this change that ultimately gives the determined pressure. These resistors, however, are also temperature-sensitive. The sensitivity of pressure sensors is therefore decreased with sinking temperature. The pressure transducer is thus no longer as precise as at room temperature.

Because of this property, manufacturers of pressure transmitters always indicate the behavior of their products under certain temperature conditions. To achieve the most linear behavior possible, pressure transmitters are nowadays electrically compensated over a relatively wide temperature range (temperature compensation). This implies that temperature errors are automatically calculated. As a result, piezoresistive pressure transmitters can provide precise measurements over a relatively wide temperature range. Temperature effects, however, cannot be completely eliminated. For this reason, the manufacturer datasheets are generally specified with accuracy values for different temperature ranges.

Extreme cold: Pressure transmitters without O-rings

The cold not only affects the resistances in the semiconductors employed. There are four other factors that should be considered when looking for a suitable measuring instrument for outdoor applications in cold regions. Among these is the use of sealing rings. Temperatures below -20 degrees Celsius cause the sealing materials between the pressure port and the membrane to become brittle. Leakage will then render the sensor useless. No pressure transmitters with O-rings should therefore be used in regions of extreme cold. A compact pressure sensor in which the pressure port and measuring cell are directly fused together would be the right choice here.

Icing: Beware of overload pressure

Freezing over can also affect the functionality of a sensor. If we take, for example, natural gas drilling in Arctic regions, then water can also be present in the gas-conducting pipes. When this water freezes, the pressure acting on the pressure transmitter may increase to a degree for which it has not been constructed. The consequence here can be tearing of the membrane. If there is a risk that the sensor is iced up, then a corresponding overload pressure must be watched out for.

In piezoresistive pressure measurement, the pressure is applied indirectly to the silicon membrane via a transfer medium. This usually consists of an oil. As the temperature drops, the viscosity of this oil will increase. Depending upon the oil and the actual temperature, it can gel or even harden up. This change also adversely affects the functionality of the pressure transducer.

Also to be considered is the resistance to condensation: If there is damp air in the housing of the pressure sensor, condensation will form at cold ambient temperatures, which can damage the electronics and destroy the sensor.

Summary

Users employing pressure sensors in cold temperatures should ensure that the individual components are directly fused without O-rings and are also resistant to condensation. It is also to be evaluated whether the pressure transmitter can freeze over if, for example, it comes into contact with water. In this case, a pressure transmitter with a corresponding overload pressure should be selected. As with any application, the pressure transducer should, of course, be compensated for the expected temperature range.

Contact
How to select the right pressure sensor?

How to select the right pressure sensor?

by | Tuesday, 30 June, 2020 | Pressure Sensor Technology

Extensive testing is essential in the development of new technology. To achieve reliable results, measurement instruments are required which precisely meet the requirements. We show you which factors play a role here.

Pressure range

An initial indicator in the search for a suitable measurement technology is the pressure range to be measured and whether a measurement of the relative or absolute pressure is anticipated.

Depending upon application, special features have to be considered. Particularly in test and measurement applications, individual measurement ranges are required which standard sensors with ISO pressure ranges cannot deliver. In this case, sensors are needed which display the appropriate pressure range and thus attain the desired precision.

Precision

In engine development for racing cars, the smallest of measured readings are the deciders between victory and defeat on the track. In this case, the utmost in precision is demanded and in specific applications developers will opt for a sensor with ±0.05% FS.

Within this question of precision, the factors of necessity and cost are balanced against one another. The pressure range to be measured is usually a good decision-making aid. If this were extremely broad, then no exceptional precision would be necessary. Those who nevertheless decide for the most precisely available sensors should be aware that this precision comes at a price.

Temperature

The temperature factor in some cases is difficult to determine. Developers are often not exactly aware over which temperature ranges the pressure sensor employed is to deliver its service. Many pressure transmitters from STS , for example, are optimized for operating temperatures from -25°C to 100°C. In this way, the common areas of application are all covered. In principle, all sensors can be optimized and ordered to a special temperature range so that even at temperatures of -40°C or 150°C accurate results can be attained.

Process interfacing

The subject of process interfacing can quickly become an exclusion criterion for developers, since many companies use standardized connections. Even the location where the sensor is to be mounted can be an important factor here.

There are a multitude of optional electrical connections, whether it be M12, DIN, MIL or others, which should also be offered by manufacturers in a variety of lengths and materials.

STS itself provides a broad range of connectors. A multitude of connection options are possible due to the modular construction principle of these measurement instruments.

Output signal

Equally decisive is the question of whether the measured pressure is to be carried as an analog signal or over a digital interface such as Modbus. With an analog signal transmission, the pressure is converted into an analog signal that still needs to be measured. In a digital signal transfer, the value of the measured pressure is directly expressed across an interface.

Space requirements

In various applications, only a little space is available for the mounting of pressure sensors. For this reason, the size of the sensor combined with the available process interfaces can become an important selection criterion. The form of pressure measurement also plays a role here. Piezoresistive pressure sensors are particularly suited to miniaturization. For this reason, STS can offer sensors of only a few millimeters in diameter.

Materials

Where will the sensor be deployed? Which ambient conditions will it encounter? Will it come into contact with steam, gasoline or particular gases? The housing material determines which media the sensor will be exposed to. For applications on the test bench, stainless steel housings are mainly used. Upon contact with saltwater, the material selection shifts to titanium.

A major influence upon the appropriate sensor is also played by the sealant material. The sealing material remains dependent upon the fluid used in the pressure system. Temperatures to be anticipated must also be expressly included during these considerations.

Certifications

When using in particularly dangerous applications, such as the possibility of explosion, certain certifications are essential which supply information about safe operation of the instruments. Within the STS portfolio, there are sensors like the ATM.ECO/IS, which carries the FM, Fmc, IECEx, ATEX certification, whose use is authorized in explosive areas

Delivery period

Long delivery periods can delay prototype testing and ultimately jeopardize product introductions. It should thus be established in advance whether the required sensors are available or what delivery period is to be anticipated for custom production.

Click on the graphic to download it

The right pressure sensor – Summary

Sensors do not necessarily meet all of the required specifications. In some cases, the required sensor from one manufacturer is not available in the company’s standardized connection option. Considerable additional costs could arise in this case. Delivery periods could also be correspondingly delayed.

To make the choice of the right sensor as easy as possible for customers, our pressure measurement instruments are based on a modular principle. This means that all of our pressure sensors can be calibrated to the required temperature range. Our products are also exceptionally flexible in terms of process interfacing, sealant materials and pressure measurement ranges. Due to the modular construction of our measurement technology, it is possible to deliver pressure sensors to the exact required specifications within the shortest of times.

Contact

Subscribe To Our Newsletter

Join our mailing list to receive the latest news and updates from our team.

You have Successfully Subscribed!