Test & measurement
servo gauges are fundamentally influenced by product density. Variations caused by temperature or compositional changes can appear as genuine level movement, compromising accuracy. As operational expectations have tightened around accuracy, safety performance and lifecycle costs, these limitations have become increasingly significant.
Q. WHAT IS THE MODERN APPROACH? Today, non-contacting radar – specifically frequency modulated continuous wave (FMCW) technology – has become the preferred method for level measurement in LNG and LPG tanks. These instruments emit continuous microwave signals towards the surface of the liquid and determine the level from the time delay and frequency shift of the reflected echo. Because no mechanical components contact the product, contamination and wear are eliminated. This design results in exceptionally high availability and long service life. Non-contacting radar gauges routinely operate for 20 years or more without adjustment, with the MTBF of critical components measured in decades. Accuracy typically reaches ±0.5 mm, significantly reducing inventory uncertainty and improving custody transfer confidence. Modern non-contacting radar level gauges can measure at distances exceeding 60 metres – supporting even the largest cryogenic tanks – and perform reliably on products with very low dielectric constants, such as LNG and LPG, where reflected echoes are naturally weaker. The Rosemount™ 5900S Radar Level Gauge from Emerson exemplifies this capability, with a high dynamic range and sophisticated signal processing algorithms ensuring stable and precise measurements under all conditions.
Q. HOW IMPORTANT IS ANTENNA SELECTION?
Antenna selection is critical to achieving accurate and reliable radar measurements in liquefied gas tanks. Specialised antennas are engineered to optimise transmission and reception even at low dielectric constants, withstand cryogenic temperatures, resist vapour condensation or icing, and maintain consistent performance despite changes in vapour pressure and density.
Q. HOW IS SURFACE DISTURBANCE MANAGED?
Turbulence – whether caused by filling, withdrawal or natural boiling – can disrupt the liquid surface. To mitigate this, most radar gauges in LNG and LPG tanks operate within a still-pipe. This pipe guides the radar beam, isolates it from surface agitation and provides a consistent reflection path. The result is a strong, stable echo even when there is surface movement.
A Rosemount 5900S Radar Level Gauge from
Emerson, with an added Rosemount 2051 Pressure Transmitter for use in liquefied gas tanks.
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Q. HAVE THERE BEEN CONCERNS ABOUT RADAR PERFORMANCE IN THESE APPLICATIONS?
Historically, some operators questioned whether vapour or high gas density above the liquid surface
might affect radar accuracy. However, these concerns are unfounded. Modern non-contacting radar level gauges automatically compensate for the effects of vapour on microwave propagation speed. By using a pressure transmitter to measure vapour pressure and combining this data with information about the gas composition and temperature, a radar gauge can dynamically correct for any influence of vapour conditions on signal transmission. The result is stable, accurate measurement even as tank conditions vary. The global installed base of non-contacting radar gauges provides strong evidence of their suitability and reliability. More than 10,000 LNG and LPG tanks worldwide now rely on radar level measurements, and in four decades of service, no Emerson radar installation has experienced reliability or safety problems due to vapour or high gas density. The design of these devices enables their availability to usually reach close to 100 per cent during their long lifespan, which helps to maximise the operational availability of tanks. In the unlikely event of a failure occurring, diagnostic software will typically identify it and take the device to a safe state.
Q. HOW CAN OPERATORS VERIFY RADAR DEVICE PERFORMANCE?
A key differentiator of modern radar gauges is their ability to perform verification without interrupting tank operation. These devices incorporate fixed reference points – a verification pin and a reflective target – within the still-pipe. The gauge continually compares measured echoes against these known distances, enabling early detection of calibration drift, mechanical issues or signal degradation. This non-intrusive verification can be carried out online, with no need to access the tank roof or isolate instrumentation, thereby improving safety and reducing maintenance costs.
Q. SHOULD RADAR THEN BE CONSIDERED THE BEST CHOICE FOR THESE APPLICATIONS?
Yes, because liquefied gas storage demands instrumentation that delivers extreme accuracy, high reliability and minimal maintenance. Servo technology, although once the industry standard, now represents a legacy approach that struggles to meet modern expectations for performance and maintainability. Non-contacting FMCW radar, meanwhile, has demonstrated decades of dependable performance, providing accuracy within fractions of a millimetre and availability that approaches 100 per cent. With proven capability in cryogenic environments and the ability to verify performance online, radar technology has become the clear choice for LNG and LPG tank gauging and overfill prevention.
Emerson
www.Emerson.com/Rosemount5900S February 2026 Instrumentation Monthly
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