Is radar always the answer?
Radar tank gauging systems are becoming commonplace. The technology is ingenious but not always properly understood, which can lead to less than optimum installations
There are several ways of telling how much liquid is in a tank, including hand-dipping, float and tape, magnetostrictive, servo, hydrostatic and radar gauges. Hand dipping is commonly used to check the contents of small tanks at filling stations or on road and rail tankers, but on large tanks it is really only feasible as a means to calibrate or check other systems occasionally.
Float and tape is basically a tape measure attached to a float. It requires no electrical connection and gives a continuous read out of level – providing someone is there to read it. Interfaces can be added for remote reading, but that negates the nopower advantage and seriously increases the cost. The main disadvantage is that these systems have many moving parts which can get stuck.
Magnetostrictive systems have a magnetic float (sometimes more than one) sliding on a vertical tube with a magnetostrictive wire inside the tube. A short electrical pulse down the wire creates a moving magnetic field which interacts with that of the float, causing a strain pulse in the wire which is detected by a strain gauge. The time delay between generating the electrical pulse and detecting the strain pulse is proportional to the distance between the sensor and the float. These systems are very accurate, often used for custody transfer measurements of refined product, but not ideal for viscous or contaminated material that might jam the float. By matching floats to the tank contents it is possible to detect stratification layers, but changes in the density of the product, due to temperature or entrained water, for example, will affect the accuracy.
For petrol stations magnetostrictive technology is a logical choice – it is low cost, low power consumption and sufficiently accurate. High accuracy is not required, as custody transfer is done via PD meter.
Servo systems lower a mass (normally called a displacer) on a wire into the liquid, and continuously monitor the strain on the wire as it descends or ascends. The strain is dependant on the mass used and the buoyancy created by the density of the fluid around it, giving these systems the ability to detect not only the surface of a liquid but also any stratification layers in a product that has settled. A temperature sensor may be incorporated into the mass to monitor temperature at any depth.
Hydrostatic systems measure the pressure of the product at various fixed depths in the tank. The devices may be mounted on the tank side but it is more convenient if they are mounted on a pipe inserted from the tank roof. The multifunction tank gauge system from US-based Gauging Systems consists of transmitter head, a top mounting flange, 2” pipe (typically stainless steel), a footer section, and multiple sensor sections containing a high accuracy pressure transducer and a temperature sensor. By sensing pressure and temperature at many points, and using cunning software algorithms such systems are able to determine not just the level of the product, but also its specific gravity and water content.
Radar systems work by sending a microwave radio beam down from the top of the tank and timing how long it takes to bounce back to the antenna from the liquid surface. The principle sounds simple but modern systems are very sophisticated. As a non-contact system it is versatile, unaffected by the viscosity of the liquid its chemical composition or density. Modern radar systems are extremely accurate and can be certified for custody transfer. It sounds like the perfect solution, and is in many situations, but knowing more about how they work will enable potential buyers to cut through the sales patter and understand what it is they are buying.
