Some suppliers specify the one-half of the total beam angle (that is the angle from the centerline of the beam).  Other suppliers state the beam angle without explaining whether it is the total beam angle, or the angle measured from the centerline.

The strongest reflections will generally occur due to objects or material within the cone formed by the total beam angle.  To avoid additional (erroneous) reflections, the sensor should be located such that there are no obstructions within or near the cone.  When this is not possible, a level transmitter that contains features that ignores these additional reflections should be considered. 

In general, larger ultrasonic and radar level sensors produce stronger beams that tend to measure longer distances with narrower beam angles than smaller sensors.  In addition, increasing the sensor excitation frequency tends to make the beam angle smaller and allow measurement of shorter distances in addition to increasing resolution. 

At least one transmitter is self-adjusting in the sense that it automatically adjusts itself to modify the beam power, pulse duration, beam width and sensitivity in order to improve reliability and performance at different operating conditions and material levels. 

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

The supplier also informed us that the flowmeter has an accuracy of 0.5 percent of the actual flow rate.  The user can interpret this performance to determine if it is sufficient for the application. 

These accuracy and turndown specifications were likely stated at different times in the (hypothetical) conversation with the supplier.  The instrument user now has two seemingly valid performance specifications in mind --- 0.5 percent of rate accuracy and 1000:1 turndown.  This sounds great!  While they may both be valid statements taken separately, they may not always apply at the same time. 

For example, many magnetic flowmeters exhibit 0.5 percent of rate accuracy between 0.3 and 10 meters per second.  Below 0.3 meters per second, the velocity error could be 0.0015 meters per second, so the accuracy as a percentage of flow rate is degraded at low velocities.  Therefore, the 0.5 percent of rate accuracy does not apply below 0.3 meters per second, but the turndown claim is still valid because the accuracy is within the (wider) 0.0015 meter per second accuracy statement. 

That said, the user should be aware that multiple valid specifications might not be applicable at the same time.  In this case, the flowmeter turndown is 1000:1 and its accuracy is 0.5 percent of rate --- except that it maintains its turndown specification by meeting a degraded accuracy specification below the velocity of 0.3 meters per second. 

Both statements are applicable to this flowmeter over a turndown of approximately 33:1 from 0.3 to 10 meters per second.  Whereas most flowmeters are sized for a velocity of 2-3 meter per second at full scale, the flowmeter turndown at 0.5 percent accuracy would more reasonably be approximately 10:1 --- a far cry from 1000:1.

This article originally appeared in Flow Control magazine. 

From a practical standpoint, it does not matter much which of these technologies has the highest turndown.  Of importance is whether a given flowmeter technology will accurately measure flow over the desired flow range for the application at hand. 

Presuming that the flowmeter will operate over a wide range of flows, it would be helpful to eliminate those technologies with limited turndown by using the magnetic flowmeter as a benchmark.  A typical magnetic flowmeter typically operates accurately at velocities from approximately 0.1 to 10 meters per second.  This corresponds to a turndown of approximately 100:1.  Coriolis, turbine and ultrasonic flowmeters can approach or exceed this turndown. 

Vortex shedding flowmeters typically operate at velocities from approximately 0.3 to 6 meters per second and turn off at low flow rates so turndown is typically less than 20:1.  Differential pressure primary flow elements can operate over wide ranges of flow, however their differential pressure transmitter(s) often limit turndown to approximately 10:1. 

Needless to say that there are exceptions to the above numbers.  Magnetic and ultrasonic flowmeters can increase their turndowns by measuring at velocities over 10 meters per second, turbine flowmeters can measure lubricating fluids at low flow rates and higher turndowns, and differential pressure flowmeters can achieve over 10:1 turndown. 

Which flowmeter technology exhibits the highest turndown boils down finding the specific flowmeter within the appropriate technologies that may be capable of achieving the desired turndown in a specific application.

Additional Complicating Factors

There are hundreds of flowmeters that can potentially be applied to achieve high turndown in a given application.  Only one may meet all of the requirements.  Investigating the possibilities can sometimes present a daunting task.  

This article originally appeared in Flow Control magazine.