Exploring the levels of control
08 January 2013
Modern level measurement components offer engineers the potential to protect and increase the quality of plant output. Charlie Sorbo, level product manager at Gems Sensors and Controls, looks at the development of level measurement and some of its recent innovations.
Modern engineering, with its increasingly sophisticated processing systems, demands ever-tighter degrees of control, and this has the quest for more reliable level measurement systems.
Boosting the accuracy and performance of level measurement helps to reduce variability of the process and product output, resulting in the measurable benefits of higher output quality, reduced cost and less waste. While these benefits should be attractive enough on their own, it is also in the interest of manufacturers to stay up-to-date with level measurement developments from a defensive point of view – increasing legislation right across industry can set stringent requirements for reliability and sustainability and the latest level measurement technologies can offer powerful tools to help plant engineers meet these goals.
Level measurement has evolved since the original sight glass, which suffered from a number of limitations, not least that the transparent material was vulnerable to failure – a particular problem when the process material was hazardous. These devices were rapidly supplanted by some more advanced technologies. A variety of level-detection devices have emerged, based of several principles, each with its own unique advantages and disadvantages depending upon the application and environment. Some level measurement tools use the simple force of gravity – using a float to detect the level of the process fluid –while more complex sensors incorporate transducers that can output signals for computer automation. More recently, wireless capabilities have enabled the transmission of level data to be sent over longer distances without signal degradation.
A range of methods
Among the most familiar level measurement methods employed are RF capacitance, conductance (conductivity), hydrostatic gauging and ultrasonics. It is important to consider the benefits offered by each of these techniques to ensure the correct specification.
RF capacitance and conductivity, for example, work in entirely different ways. RF capacitance works on the principle that the electrical capacitance between two separated conductors changes when the space between them is interrupted by a quantity of non-conducting material. A conductive, sensing electrode can be lowered into a metal tank to act as one conductor, while the wall of the tank acts as the other. As the level of process material rises, covering a greater area of the suspended electrode, a change in capacitance is measured between the two conductors and this is transmitted to an RF level transmitter on the exterior of the tank, providing a direct measurement of tank level.
In some applications, this method can be compromised by a steady build-up of process material residue that begins to coat the level-sensing probe, causing errors of measurement. Conductive level measurement can avoid this problem. Here, liquid level measurement is achieved via the electrical conductance of the process material, typically a liquid with a low-voltage source, and offers a relatively low-cost, simple method of detecting and controlling level.
In a tank of liquid, conductive sensors are positioned at high and low points in the tank to detect maximum and minimum levels. When the level of process material rises to reach the upper probe, a switch closes to activate a discharge pump; when the process material makes contact with the lower probe, the switch opens to stop the pump. Conductive level measurement is not always the best option, but the above example illustrates the importance of carefully considering the properties of the process material before specifying the sensors.
Conductivity-based sensors are now also available that can detect the level of water in fuel and lubricating oil and prevent mechanical damage as a result of lubricant property degradation. Gems Sensors & Controls work in this area has resulted in the launch of the WIF-1250, for example, a no-moving-parts solution for use with fuel filters, and in compressor crank cases to determine if water is present in the lubricating oil.
The unit contains integral, high-temperature-rated electronics that generate an alternating voltage to a probe tip. The presence of water completes the circuit, which, in turn, changes the condition of the transistor output. The output options of the sensor can be used to actuate relays, indicator lights or LEDs, as well as to interface with CMOS/TTL logic, PLCs or microprocessors. Solutions such as this add reliability to fluid measurement. Historically, the presence of water in fuel would more likely have been discovered after some damage had been done, whereas today’s solutions prevent the problems from occurring.
As with measurements for particle contamination in fuel, which were once conducted by means of a simple visual inspection of a sample transferred into a clear vessel, judging the presence of water in fuel is haphazard without sensor technology. It has even been known for visually inspected fuel to be judged contaminated and rejected when, in fact, fibres of lint that have become detached from the cloths used to wipe clean the sample jars were, in fact, the only contamination present. This only goes to underline the importance of sensor technology as a reliable means of measurement.
Side mount level switches in engineered plastic for high temperature applications have also proved to be a powerful innovation. These versatile components are currently available in a form that is compatible with a range of challenging fluids such as oils and solvents, with resistance to temperatures of up to 148.9°C. As an affordable solution for handling high temperature applications and corrosive fluids, these switches have proved invaluable for use within methylene chloride and anti-freeze tanks, and are suited for low coolant, low hydraulic monitoring within off-highway vehicle and transportation applications.
Electro-optic sensors offer another method for measuring level. An electro-optic sensor contains both an infrared LED and a light receiver. Infrared light from the LED is directed into a prism at the tip of the sensor and, when no liquid is present, the light is reflected within the prism to the receiver. However, when liquid rises in the vessel to immerse the prism, the light from the infrared LED is refracted out into the liquid. At this point, there is no longer any light reaching the receiver, which actuates electronic switching to operate an external alarm or control circuit. There are now electro-optic components on the market with the potential to operate within extreme temperatures ranging between -40 and +125°C, offering ingress protection ratings of IP66 and upwards, which can be utilised even within extreme environmental conditions.
All these level measurement innovations offer huge potential. However, to get the best from any component you need to ensure the right specification for your particular operation. This could involve supplying a sample of the process material for evaluation, along with a diagram of the vessel, when working with your sensor supplier.
There are plenty of opportunities for operators and engineers to reinforce product and process quality with level measurement components. A degree of care in the specification, installation and maintenance of these sensors will always be required but the effort will pay dividends by significantly enhancing plant performance.
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