We have learned not to get too confused over suppliers using buzz-words and clever marketing names, but recently it seems the major level measurement system vendors have been introducing new and higher radar frequency systems as their latest development – and therefore, by implication, maybe the best. We were used to 6 GHz, and then 26 GHz radar frequencies, but why should we suddenly go to 80 GHz? Then, perhaps just to add a little excitement to the mix, Endress+Hauser started talking about 113 GHz!
Let’s dispel a few myths. Firstly, in the same way that lasers for fibre-optic communications systems made the technology available to create infrared optical systems for process gas analysers, and mobile phone technology possibly provided the hardware for the first radar level measurement systems; the 80 GHz versions are a result of measurement technology made commercially viable on the back of production investment in the distance measurement systems and parking sensors used in modern cars. So the suppliers take the available sensors and chipsets to create a new industrial product, and then have to find the best applications – in this case, the ones that might benefit from the 80 GHz.
Secondly, E+H do not have a 113 GHz system, this is a marketing statement, made to catch attention – ‘with a wink’ is their expression. They claim a ‘complete radar competence of 113 GHz’ because this is the sum of the many different frequencies their different sensors use! These are 1, 6, 26 and 80 GHz.
So why have different frequencies?
The best explanation for the applications suited to the different frequencies is provided by the Rosemount measurement division of Emerson, whose expertise stretches back many years having acquired the Saab Tank Radar business. Per Skogberg, from its Gothenburg HQ in Sweden, separates the devices into low, medium and high frequency, to generalise.
Radar signals are attenuated, i.e. they lose signal strength as they pass through the air, or vapour, above the liquid. High frequencies are more severely affected. When the air has moisture, steam or liquid droplets (from spray or filling) present, the attenuation is higher. Equally in solids applications, dust particles have the same effect, so low and medium frequency radar are best when there is dust or moisture present.
At lower frequencies, the wavelength is longer (30-50 mm), so surface ripples and disturbances in a tank have a small effect. But the shorter wavelength of the high frequency units (4 mm) allows accurate operation over short distances, for example in small tanks. At higher frequencies, surface ripples and foam on the surface can be a problem.
The higher frequency units can use a smaller sensor construction, so the unit is easier to install. The beam angle is narrower, so it can be aimed at a smaller target area and therefore can be positioned more easily to avoid any obstructions in the tank. But even this can be a disadvantage, as the installation needs to be exactly vertical and any turbulence of the surface during filling or stirring can cause the signal to be lost temporarily, in larger tanks.
When reading these suggestions, it is important to remember that Emerson does not offer an 80 GHz unit yet, so their marketing approach would naturally bias users to look at low and medium frequency units. The suppliers of high frequency units (VEGA, Krohne and E+H) would point out that in many liquid storage tanks the surface is undisturbed, since any foam, turbulence and significant ripples (>2 mm) caused by filling or liquid transfer will only cause short-term interference. Plus the small antenna size and short range performance make 80 GHz units very useful for smaller process vessels and tanks.
Radar system types
There are two types of radar systems, Guided Wave Radar (GWR) and Free Space Radar. The GWR systems use a conducting rod, or similar, extending down into the liquid, often working in a stilling chamber attached to the main process tank. These operate at low microwave frequencies, and are independent of surface turbulence and foam. They are useful for shorter range measurements and interface measurement between liquids, as well as long ranges.
The Free Space Radar systems are more widely used, since they are top-mounted with nothing in the tank: indeed, some can operate through non-conducting windows in the tank roof. Low and medium frequency radar systems generally transmit a signal pulse and measure the liquid distance by the time delay for the returned pulse. High frequency (80 GHz) systems use an FMCW radar measurement, where the frequency of the transmission is swept, and the frequency difference of the returned signal is measured to assess the distance. The FMCW technique is also used at 26 GHz in some recently launched sensors.
Radar systems can transmit their measurement data using 4-20 mA, fieldbus systems like HART, FF, Profibus PA and Modbus, or indeed via wireless systems like Bluetooth. The low and medium frequency pulsed radar systems generally operate over a two-wire interface: some of the higher frequency FMCW systems require more power and use a separate power connection.
Major applications
Simple low-cost radar level measurement sensors have been specifically designed for water industry use, in sewage sumps and flume flow measurement, by VEGA and Endress+Hauser. VEGA suggest that 40 000 such sensors are now in use in the water industry, mainly in Europe. Several of these devices use simple Bluetooth interrogation and programming from a hand-held PDA: E+H demonstrates this at its facility in Maulberg, working on the stream that runs through the factory complex.
Both E+H and VEGA produce further industrial units for use on process vessels, and storage vessels for solids and liquids. Recently, E+H has extended its capability to add long-range units, such as the 80 GHz FMR62, working at up to
80 m range, with an accuracy of 1 mm. Other units work up to 125 m range, at 3 mm accuracy. These units will eventually be aimed at the large petrochemical industry storage tank markets, and specifically are working towards use for custody transfer duties.
Krohne have similarly announced a new range of its 80 GHz Optiwave sensors. Some of these can even operate at up to 700°C, for example for use on molten salt vessels in solar power plants. Lower specification units rated at up to 150°C can be used through a tank roof made of plastic, or similar materials. Suitable for small or narrow tanks, the unit can measure ranges of up to 100 m. Krohne also offers lower frequency Optiwave systems for use on solids and powders, or to electronically monitor the float position in magnetic level indicator columns attached to process vessels.
Nick Denbow spent thirty years as a UK-based process instrumentation marketing manager, and then changed sides – becoming a freelance editor and starting Processingtalk.com. Avoiding retirement, he published the INSIDER automation newsletter for 5 years, and then acted as their European correspondent. He is now a freelance Automation and Control reporter and newsletter publisher, with a blog on www.nickdenbow.com
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