For electrically conductive materials, the sensor typically consists of a metal rod covered by a nonconductive coating. In these sensors, the conductive material and rod are separated by the nonconductive coating and form a capacitor. As the level of a conductive material rises to cover more of the sensor, the effective size of the plates increases because more of the probe assembly is covered. This tends to increase the capacitance of the sensor. A capacitance level transmitter is used to measure the capacitance which is indicative of the level to which the sensor is covered and hence, the level in the vessel.

In electrically nonconductive materials, the sensor often consists of a metal probe surrounded by an electrically conductive pipe. The probe and pipe form the capacitor plates that are separated by a distance. The electrical dielectric constant of the nonconductive material is different than that of the gas/vapor in the vessel. As the level of a nonconductive material rises to cover more of the sensor, the effective distance between the plates decreases because the nonconductive material has a higher dielectric constant than the gas/vapor that it displaced. This tends to increase the capacitance of the sensor. A capacitance level transmitter is used to measure the capacitance which is indicative of the level to which the sensor is covered and hence, the level in the vessel.

Excerpted from The Consumer Guide to Capacitance and Radar Level Gauges

Many flowmeters can withstand a full vacuum --- especially when the flowmeter is constructed entirely of metal.  However vacuum service can be catastrophic to some flowmeter designs.  A case in point is that of magnetic flowmeters where certain liners can actually be sucked into the pipe when a vacuum condition occurs.  If vacuum limitations exist, you can usually find them in the manufacturer specifications --- if you are looking for them.  They often appear as notes at the bottom of tables or in fine print.  As previously stated, be careful because exceeding the vacuum specification can be catastrophic. 

Interestingly, the engineer took designing for vacuum service one step farther by suggesting that all instrumentation be designed for vacuum service --- even when the instrument would seemingly not operate under vacuum.  His rationale was that many instruments that should not operate under vacuum can and do operate under vacuum. 

Consider measuring the pressure at the bottom of a distillation column operating at atmospheric pressure.  The gauge pressure at the bottom of the column might be (say) 0.5 bar.  A 1-meter long impulse tube connects the pressure tap to the pressure transmitter.  The distillation column runs under pressure, has an atmospheric vent, and has no source of vacuum.  Yet the pressure transmitter routinely measures pressures below atmospheric pressure when the distillation column was operating.  In this installation, the process gas condensed in the impulse tube and caused a vacuum condition to occur. 

Similarly, what if a vessel was blocked in while being cleaned with steam?  The condensing steam can create a vacuum that could collapse the tank. 

There is something to be said for conservative design --- despite its political ramifications.

 This article originally appeared in Flow Control magazine.

Answers A, B, C and D were calculated by multiplying last month's answers for water service by the specific gravity of ammonia.  As a reminder, Answer D (1 to 5 meters of water column) was the correct answer for water service. 

However, the physical properties of liquid ammonia are quite different from the physical properties of water.  In particular, liquid ammonia has a high vapor pressure.  This means that if the liquid ammonia tank is at (say) 20 degC, liquid ammonia in impulse tubing will boil if its temperature is above 20 degC.  This means that installing a differential pressure transmitter in a liquid ammonia tank in the same manner as one would install it in a water tank will create havoc with the measurement.  In particular, ammonia will boil out of the impulse tubes when their temperatures increase and condense when they cool down. 

The best answer would be Answer E, but it might be more prudent to select another level technology for this application. 

Additional Complicating Factors

Ammonia presents sealing issues for level transmitters because its vapor pressure is relatively high while the size of its molecule is relatively small.

This article originally appeared in Flow Control magazine.