The surface of the material can cause the intensity of the reflected laser beam to degrade when it exhibits poor reflective qualities, such as when the material itself or contaminants cause the laser beam to reflect poorly.

In addition, accuracy can be degraded based upon the surface on which the laser beam is reflected. For example, the laser beam may measure the top of a layer of foam by reflecting off the top of the foam. If the foam is transparent to the laser energy, the beam may reflect from the foam/liquid interface and measure the liquid level. Translucent foam might cause the level measurement to represent a location within the foam. Further, the foam conditions may vary over time and cause erratic level measurements,

Many laser level measurement sensors utilize Class 1 lasers that do not generally pose a hazard under normal operating conditions. Other sensors use Class 3 lasers that can pose a risk of eye injury when viewed with the naked eye for more than a moment.

Laser level measurement sensors can be used for sanitary applications where the laser beam enters and leaves the vessel via a sight glass.

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

Having analyzed the problem without solution, one might have a strong bent towards replacing the flowmeter and using another technology.  Before doing this, let's take a look at the overall operation of the flowmeter. 

The flowmeter is sized for a process flow of 60 gallons per minute in a 2-inch pipe of a fluid that freezes at 20 degrees Celsius with a specific gravity of 1.00 and a viscosity of 5 centipoise.  A quick calculation shows that the Reynolds number is approximately 19,000 at maximum flow that is acceptable for most vortex shedding flowmeters. 

However, further investigation reveals that the viscosity is 5 centipoise at 90 degrees Celsius.  During the winter, steam tracers maintain the pipe temperature above 100 degrees Celsius, so the liquid does not freeze.  The tracers are turned off during the warmer months because process heat keeps the liquid from freezing by maintaining the pipe temperature over 40 degrees Celsius.  However, the liquid viscosity can increase to 20 centipoise at these cooler temperatures so Reynolds number can fall under 5000 at maximum flow.  Operating at (say) 50 percent flow drops the Reynolds number under 2500 --- where most vortex shedders cease to operate. 

In short, turning off the steam tracer reduced the liquid temperature increasing its viscosity and decreasing Reynolds number to the extent that the flowmeter became inoperative.  The vortex shedding flowmeter could be replaced with a flowmeter that uses another technology, but it might be more pragmatic to just leave the tracer on all year.

This article originally appeared in Flow Control magazine. 

For differential pressure flowmeters operating in the turbulent flow regime, the volumetric flow is proportional to the square root of the ratio of the differential pressure divided by the fluid density. The form of this equation indicates that the differential pressure measurement is divided by the gas density and then the square root of the result is taken.  Answer B and Answer C are not correct. 

A closer look reveals that the density of the gas "corrects" or compensates the differential pressure measurement.  Gas density is a function of pressure and temperature and can be calculated using various techniques to include the Gas Laws, density tables, and equations of state.  Afterwards, the square root of the result is taken.  Therefore, Answer C is correct. 

This question surfaces every so often --- especially when the implementation of pressure and temperature compensation is not implemented in a flow computer.  Nonetheless, it is an important question because failure to properly implement the correct calculation can result in flow errors.  Further, operators and engineers may unknowingly use these flawed measurements.  Investigation as to the accuracy of the flawed measurements will likely occur only when the flawed measurements yield unreasonable results in important calculations.  An instrumentation specialist is often required to find the flaw in the calculations. 

Additional Complicating Factors

There are many reasons for instrument users to implement gas flow pressure and temperature compensation in their control system instead of in a flow computer.  However, flow computers should be used in custody transfer applications and considered when the computational technique used to calculate density (such as the Gas Laws or equations of state) introduces significant measurement error.

This article originally appeared in Flow Control magazine.