Understanding the physics of how a sensor works is crucial for grasping what the device really is measuring in unusual conditions versus what it’s reporting — and, thus, how you or the control system should interpret the measurement. Let’s look at three common level-measuring technologies, displacer, float and differential pressure (DP), to put this into perspective.
A displacer uses a solid body attached to a torque gauge. As liquid rises up the displacer, the displaced liquid creates a buoyant force. This changes the force on a torque connection. A torque gauge measures the buoyant force. The reading is interpreted as a level.
The conversion of the torque force to a level requires knowing the density both of the displacer body and the liquid in the vessel. If either is wrong, the calculated level will be incorrect. The key point here is that the displacer instrument doesn’t directly measure level. It measures torque (or force).
A float differs dramatically. Its body must have a density between that of the light phase and the heavy phase in the vessel. The float physically sits on the interface level. As the level of the dense fluid changes, the float moves.
Table 1. A float responds differently than a displacer or DP instrument to various situations.
The float location can be measured by different mechanisms. Mechanical options include devices such as gear assemblies. Magnetorestrictive measurement systems use the Villari effect — a magnet on the float induces a stress in an external detector.DP instruments measure the pressure differential between two points. Based on an assumed liquid density, this pressure difference is interpreted as a level.All three types of instruments can measure a liquid/vapor level or a liquid/liquid level. The level may be expressed either in a height (inches, for example) or as a percent of range.The three measurement technologies respond differently to unusual conditions (Table 1).For a level below the range, all give similar results. The reading sits at a minimum value. Depending upon calibration, the minimum reading might not be zero liquid level. Suspect any instrument that continuously reads the same value without changing. This is especially true of level devices. Even stable processes exhibit variations in level.For a level above the range, the displacer and the DP instrument behave similarly. Both turn into density measurement devices but report the density difference as a liquid level. The reported level may be less than 100% if too high a density is assumed. Operating changes that affect the density may change the reported liquid level.Floats always should read a maximum value if the level is above the range.If the density is lower than expected, the displacer and the DP instrument report a level lower than that actually in the vessel.The float will continue to work as long as the liquid density remains higher than the float’s density. If the liquid density becomes lower than the float density, the float sinks and the instrument reads a minimum value.If the density is higher than expected, the displacer and DP instrument report a level higher than that actually in the vessel.Again, the float will continue to work as long as its density remains lower than the liquid density. In liquid/liquid measurements, if the light liquid becomes heavier than the float, the float will jump to the top of the level range.In general, the displacer and the DP instrument will show the same behavior. The displacer measures torque (or force). The DP instrument measures force per unit area. Because the area of the DP instrument remains constant, changes in DP are changes in force.Both the displacer and the float have a device that sits in the liquid. Many people mistakenly think a displacer is a float. This error leads to improper interpretation of readings and can have serious consequences. One contributing factor to the 2005 disaster at BP’s Texas City refinery (see: “Hard Lessons Worth Sharing,” http://goo.gl/m9Drss) was confusing the displacer installed in a tower with a float. The reported liquid level was incorrect because of an inaccurate density. The unit was full of liquid but the level instrument showed a level less than 100%. What was being measured was average liquid density instead of level. Rising operating temperatures were causing the reported liquid level to drop as density decreased. Most errors in understanding won’t have such catastrophic results. Nevertheless, understanding the physics of how an instrument works is key to interpreting its reading correctly.
