Capacitance and radar contact level sensors are in direct contact with the material and are exposed to the material's vapors and general environment. The sensors generally contain one or more probes that require a seal where the probe enters the vessel. This seal can come in direct contact with the material when the level of the material is excessively high.

These technologies measure level continuously, however they do so at essentially one point in the vessel. This generally does not pose a problem for liquid applications where the gas/liquid interface is horizontal.

However, in solids applications, material entering and leaving the vessel affect the solid/gas interface. As such, this interface is typically not horizontal, so a single level measurement may not representative if the amount of material in the vessel. In these applications, the level measurement reflects the level at one point in the vessel. Level measurements can change rapidly when the material level that is sensed changes rapidly. In some applications, multiple level measurements may be needed to provide a more accurate indication of the inventory of material in the vessel.

For example, a rat-hole may form as solid material leaves the vessel. If the level measurement reflects a point in the rat-hole, the measured level will decrease (as expected). However, if the material remaining on the sides of the vessel falls and fills in the rat-hole, the level will abruptly (and unexpectedly) increase. Sensors should be located such that the measured level represents the actual level while avoiding rat-hole affects. Multiple sensors may be needed if an appropriate location cannot be found.

In addition, most capacitance and radar contact level sensors cannot accurately measure distances that are close to the sensor itself. Sensors are typically installed to allow the transmitter to disregard measurements at these distances.

Excerpted from The Consumer Guide to Capacitance and Radar Level Gauges

For starters, the fluid should not affect the geometry of the selected flowmeter.  For example, the flowmeter should not have any carbon steel wetted parts if it is found that sodium chloride brine affects carbon steel. 

As for flowmeter technology selection, one possibility is to select a flowmeter that does not require straight run.  This might be possible, but many such flowmeters have limited size available and they can be expensive.   Another possibility is to alter the piping such that the strange shapes are not formed and the inside of the pipe remains straight/smooth. 

In this application, replacing the carbon steel metering runs with plastic pipe suitable for the pressure in a vertical run with flow up would increase the probability of maintaining straight/smooth pipe in the straight run.  However, plastic pipe has limited pressure ratings and strength so it might be more pragmatic to use glass-lined carbon steel pipe.  Stainless steel pipe is a possibility if the grade of stainless steel is compatible with the fluid. 

The ideas described above are related to the flow measurement system.  The operative word is "system".  The entire system must function properly to implement an accurate flow measurement.  The straight run is part of the system.  Changing the geometry of the straight run can affect the flow measurement.  Designing the straight run to be unaffected by the process may not be common, but it is sometimes necessary.

 This article originally appeared in Flow Control magazine.

The low side of the differential pressure transmitter is open to atmosphere at zero pressure (gauge). 

When the water level is at the tap at 1 meter, the impulse tube is full of water and produces 1 meter of water column at the differential pressure transmitter.  When the tank is full, the water level is at 5 meters and produces a pressure of 5 meters of water column on the transmitter.  Therefore, Answer D is correct because the calibration is 1 to 5 meters of water column. 

However, the specified calibration for an analog differential pressure transmitter could be 0 to 4 meters of water column.  This is because the 1-meter offset would be adjusted out electronically.  Microprocessor-based instruments allow the user to set the actual zero and span pressures in actual pressure units.  

Additional Complicating Factors

This transmitter cannot measure levels below the tap located 1 meter above the tank bottom.

This article originally appeared in Flow Control magazine.