Noting that the ultrasonic energy travels to and returns from the material, attenuation between the sensors and the material can cause ultrasonic level measurement systems to fail. Degradation can occur at the sensors, in transit to/from the material, and at the surface of the material.

The design of the ultrasonic sensors is important. In particular, dirt or other coatings on ultrasonic sensing system can cause the emitted and received ultrasonic signals to be weak. As the signals become weaker, the measurement system can become erratic and fail to operate. When accumulations over time are normal for the process, routine maintenance may be required to keep the sensors operating.

Careful analysis of the sensor design should be performed. Some designs inherently tend to maintain the sensor clean while others tend to inherently accumulate material. For example, the sensor system on the right in the figure emits ultrasonic energy upward to a reflector that redirects the energy down to the material. In this design, material can accumulate on the sensors and stick to the reflector. Both of these phenomena can attenuate ultrasonic energy and cause the level measurement system to fail to operate.

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

Flowmeters are often sized such that the liquid velocity at full scale flow is approximately 2-3 meters per second.  A turndown of 10:1 implies that a typical flowmeter will operate properly from (say) 0.25 to 2.5 meters per second.   Increasing the turndown to 100:1 implies that the flowmeter will improve operation from (say) 0.025 to 2.5 meters per second.  This turndown (100:1) is 10 times better, but the actual difference is the coverage of the velocities between 0.025 to 0.25 meters per second.  This is a relatively small flow range, but it could be important in some applications.  Further improving the turndown to 1000:1 would allow improved measurement between 0.0025 and 0.025 meters per second.  This represents another improvement, but it similarly applies to a small part of the flow range. 

Will the process ever operate at the flow rates (velocities) covered by the improved turndown specifications?  In some applications, the answer is absolutely "yes".  However, in most applications, the process does not operate in these relatively small flow ranges of increased turndown... and if it does, flowmeter accuracy is not the prime concern.  This is not to say that turndown is not important --- it is.  But accurate measurement may not be the biggest problem when the process operates in upset conditions only (say) 0.1 percent of the time.

Let's integrate turndown and accuracy next month. 

This article originally appeared in Flow Control magazine. 

The differential pressure flow transmitter should be calibrated for 150 inches of water column.  Even though it might be possible to calibrate Transmitter A to 0-150 inches, the maximum capsule span should be larger than the calibration.  Transmitter A will likely not be accurate. 

The maximum span for Transmitter B matches the primary flow element requirement while Transmitters C and D can be calibrated to 0-150 inches of water column.  Therefore, Transmitters B, C and D could legitimately be selected. 

However, it would be appropriate to gather more information about the application in order to select the best transmitter.  For example, will the process be expanded to (say) double its existing capacity?  If so, Transmitter D might be the best selection so as not to have to purchase a new transmitter during the expansion. 

Given that the maximum capsule span should be higher than the calibrated span and that the transmitters are of similar manufacture and design, superior flowmeter performance is usually achieved by selecting the capsule that is as close as practical to the calibrated span.  In this application, Transmitter B should be selected because it is equal to the calibration. 

Additional Complicating Factors

The above analysis is quite cursory in nature.  More detailed analysis of the accuracy, temperature effect, and pressure effect should be performed to quantify the improved accuracy of Transmitter B.  Further complication will occur when comparing differential pressure transmitters of different design and manufacture. 

This article originally appeared in Flow Control magazine.