Noting that the laser beam travels to and returns from the material, degradation of the beam strength between the sensors and the material can cause laser level measurement systems to fail. Degradation can occur at the sensor, in transit to/from the material, and at the surface of the material.

Dirt or other coatings on laser transmitter/receiver can cause the received laser signal to be weak. When accumulations over time are normal for the process, routine maintenance may be required to keep the transmitter/receiver operating. In many applications, the sensor may be shielded in a tube and/or continuously purged with gas to keep it in operation. Similar concerns exist when the laser beam travels through a sight glass that can become dirty and cause attenuation of the beam.

Excerpted from The Consumer Guide to Non-Contact Level Gauges.

In particular, differential pressure level transmitters with diaphragm seals are often calibrated in the instrument shop --- especially prior to initial installation. To simulate the actual installation in the instrument shop, the diaphragms should be mounted at different elevations that correspond to the actual installation to calibrate the transmitter.  This procedure becomes impractical in many applications due to tank heights that limit this simulation to but a few meters. 

In place of this simulation, differential pressure level transmitters are often calibrated with their diaphragm seals mounted at the same elevation.  The calibration values will be different than those in the actual vessel due to the different elevations of the diaphragm seals in the shop as compared with the field.  Sometimes transmitters are calibrated with their diaphragm seals just "laying around". 

Aside from these potential shop calibration issues, installing the diaphragm seals on the actual vessel may result in minor calibration shifts due to the physical nature of positioning the diaphragm seal and tightening its bolts.  As a minimum, a zero calibration should be performed after the transmitter and its diaphragm seals are permanently installed. 

Performing a shop calibration has its virtues because it can find transmitter and diaphragm seal problems before installation in a field environment.  However, appropriate valves and calibration ports should be installed to allow field calibrations that will improve measurement accuracy.  One such device is a "filler flange" that is a thin wafer spool piece that contains drain and bleed ports that can be used for calibration.  Better yet, it would be more convenient to purchase the diaphragm seals with these features.

This article originally appeared in Flow Control magazine. 

1. 33,700 pounds
2. 134,800 pounds
3. 18 metric tons
4. 72 metric tons
5. None of the above
   

The solutions to this problem are relatively straightforward.  The volume of a full tank of water is pi times the square of the radius times the tank height (3 x 3² x 20) or approximately 540 cubic feet.  The density of water is approximately 62.4 pounds per cubic foot so the water in the tank weighs approximately 33,700 pounds. 

The solution to the example in metric units is (3 x 1² x 6) or approximately 18 tons (18,000 kilograms) because the density of water is approximately 1 metric ton per cubic meter.  Therefore, Answer A and Answer C are correct. 

As mentioned, this problem is relatively straightforward from a technical perspective.  The point of performing the calculation in both the English and metric systems is that the English system is more prone to error.  The metric system is more elegant --- even though both will yield the same answer for the same size tanks.  The metric system is less prone to calculation error because many of its conversion factors are 1 or multiples of 10.  (If you have already attended one of my seminars you know that I am good at multiplying and dividing by 1.) 

This exercise may not seem important, but small errors can make a big difference in results.  I seem to recall that a space probe crashed into a planet (instead of orbiting) due to a misunderstanding regarding the measurement units used during design.

Additional Complicating Factors

The above calculations in the English system can be made even more complicated by using the weight of water per gallon and the number of gallons per cubic foot.  In addition, the liquid could be different from water and have a density that is different from that of water so the tank contents could be heavier or lighter than calculated in either system.

This article originally appeared in Flow Control magazine.